The document discusses three types of bending that occur when sheet metal is bent: partial bending, bottoming, and coining. It explains the relationship between bending force and bending angle through an S-curve diagram. Partial bending and bottoming occur through air bending with relatively low force, while coining requires much higher force and eliminates springback for greater precision. Springback occurs due to the material retaining elasticity even after yielding. The document also discusses bottoming as the most common air bending technique, providing a table relating sheet thickness to optimal V-width for the die.

1. 1-1 Kinds of Bending
1-2 Curve of Bending Force Versus Bending Angle
1-3 Air Bending and Coining
1-4 Springback
1-4-1 Why springback occurs
1-4-2 Positive and negative springback
1-5 Bottoming
1-6 Partial bending
1-7 Coining
1-8 Arrangement of Three Types of Bending
2-1 General
2-2 Reading the bending force chart
2-2-1 Explanation of symbols
2-2-2 Minimum flange length
2-2-3 Required tonnage
2-3 Four Relationships
2-3-1 Relationship between F and V
2-3-2 Relationship between F and t
2-3-3 Relationship between F and t
2-3-4 Relationship between F and b
2-4 Calculation of Bending Tonnage
2-5 Complement of the Bending Force Chart
3-1 The Necessity of General Knowledge
3-2 Requirements for Good Tooling
3-3 Tooling Materials, Heat Treatment, and Manufacturing Process
3-3-1 Hardened tooling
3-3-2 Thermally refined tooling
3-4 Length of Tooling
3-5 Sectional Shape of Tooling
3-5-1 Punches
3-5-2 Dies
3-6 Sectionalized Type
3-6-1 Sectionalized punch
3-6-2 Sectionalized die
3-7 Holder
3-7-1 Punch holder
3-7-2 Die holder
THE ABC OF BENDING TOOLS
Three Kinds of Bending
Use of Bending Force Chart
General Information on Bending Tools

2. 3-7-3 Die base
4-1 Process of Selecting Tooling
4-1-1 Selection of V-width
4-1-2 Tooling as a punch and die combination
4-1-3 Machine specifications and setting of tooling
4-2 Bending Pattern
4-3 V-Bending
4-3-1 90' bending tooling
4-3-2 Tooling for acute-angled bending
4-4 Radius Bend
4-4-1 Characteristics of R-bending
4-4-2 Springback in R-bending
4-4-3 Multi-breakage bending phenomenon
4-4-4 Types of R-bending
4-4-5 Summary of R-bending
4-5 Hemming
4-5-1 Type requiring tooling replacement
4-5-2 Double-deck type
4-6 Bending Sequence
4-6-1 Procedure for determining bending sequence
4-6-2 Use of tools
4-6-3 Automatic programming
4-7 Guaranteed Allowable Tonnage of Tooling
5-1 Objective of Labor Economy
5-1-1 Elimination of problem caused by spring-up
5-1-2 Reducing the number of processes
5-1-3 Increasing tooling versatility
5-1-4 Making material handling easy
5-1-5 Reducing man-hours to replace tooling
5-1-6 Bending coated workpieces
5-1-7 Elimination of welding process
5-2 Types of Labor-Saving Tooling
5-2-1 Spring-up-preventive tooling
5-2-2 Tooling that reduces number of processes
5-2-3 Versatile tooling
5-2-4 Tooling that makes material handling easy
5-2-5 Tooling that eliminates tool change
5-2-6 Tooling that produces no marring
5-2-7 Technique which eliminates welding
5-3 Points of Tooling Design
5-3-2 L-bending tooling
5-3-2 Accordion bending tooling
5-3-3 Seaming tooling
Selection of Bending Tools
Labor-Saving Bending Tools

3. 5-3-4 Urethane tooling
6-1 Dispersion of Bending Angles
6.2 Marring
6.3 Cracking
6.4 Warping
Go to Top
Problems in Sheetmetal Bending

4. 1. THREE KINDS OF BENDING
Reference must be made to the peripheral areas of bending in order to give accurate information
concerning bending tools. The peripheral area here refers to bending machines and how to use them
for bending work. As in the saying "You can't see the describe bending tools independently of
bending machines and bending forest for the trees," it is not practical to only work. However, there is
no point in extending the area so broadly as to cover all items related to sheetmetal bending. This
"The ABC of bending tools" will, therefore, deaf with the above-mentioned subjects, concentrating
on the fundamentals necessary bending tools. The fundamentals will be discussed in Chapters 1 and
2. They will be for the reader to understand referred to several times throughout the book.
Such being the case, the reader should bear with us a little further if he feels there is no description
of bending tools. It may be difficult to understand these tools in the future without a thorough
comprehension of the fundamentals. The reader will be convinced that he has not wasted time once
he has read througj this book.
1-1 Kinds of Bending
Many operators encounter inexplicable troubles throughout their daily bending operation. For
example, the intended product was not produced-it lost its shape, it was out of the required
dimensional tolerance, or it had scores and cracks in the bent part. Perplexed with incomprehensible
troubles, some may have uttered that bending is difficult and beyond their comprehension.
Sheet metals such as steel, stainless steel, and aluminum should bend easily when pressed with a die
and punch mounted in a bending machine, as long as the specified requirements are met. This
bending phenomenon is recognized as quite a natural event, due to the fact that one can actually see
the sheet metal bend. Then it does not seem that sheet metal bending is complicated and difficult.
However, sheet metal bending is not as simple as it appears.
This chapter teaches that there are three kinds of bending. The material to be bent (hereinafter called
"work") undergoes three characteristic kinds of bending in accordance with bending force
application. When told there are three kinds of bending, one may wonder, "Why?" or "It can't be
true." Due to the bending phenomenon appearing simple, one cannot possibly observe the work
experiencing three kinds of bending while it is being bent. In fact, here is a clue to understanding the
seemingly incomprehensible characteristics of sheet metal bending. Go to Top
1-2 Curve of Bending Force Versus Bending Angle
As a sheet of flat work is gradually pressed with a punch and die mounted in a bending machine, the
bending of the work begins according to the application of bending force. At that time, a curve is
obtained by plotting the angles to which the work bends under the respective force applied. This
curve shows the relationship between the bending force and the resultant bending angle (Fig. 1-1).
Three Kinds of Bending
Kinds of Bending Bottoming
Bending Force Vs. Bending Angle Partial bending
Air Bending and Coining Coining
Springback Three Types of Bending
Table of Contents

5. The curve, which is also called an "S curve" due to its resemblance to the letter "S," varies greatly in
shape due to the type of material.
Fig. 1-1 shows a curve of the cold rolled carbon steel
(SPCC). The bending force or the tonnage required for
bending a work length of I meter is taken along the Y-
axis, and the bending angle 0 along the X-axis.
Bending angle 0 refers to the angle to which the work
is bent after forming (the product angle).
The work placed between the punch and die does not
immediately bend after a bending force has been
applied and the bending force will increase to a certain
level. In Fig. 1-1, this increase in the bending force
appears as a vertical rise in the curve. The work is
known to have been pressed with approximately 6 tons of force. The work may look bent at a glance,
but restores the previous shape when the bending force applied has been relieved. This is caused by
the elasticity of the material. Elasticity is the quality or tendency to go back to the previous size or
shape after being pulled or pressed.
As the bending force gradually increases, the bending will proceed rapidly and the bending force will
reach a peak (about 10 tons) at a bend of around 130 degrees. However, the bending force begins to
decrease slightly when the bend exceeds 13.0 degrees. As shown in the figure, the bending angle in
this region changes remarkably with a slight difference in bending force. This is called region 1.
The required tonnage increases while the bending angle decreases below 100 degrees in the bending
range. 90° bending can be performed with a bending force of approximately 12.5 tons, which is 25
percent greater than in 130° bending. The bending force that bends a material to 90 degrees is called
the "required tonnage" of the material. If the material continues to be pressed, it will bend at an angle
3 or 4 degrees smaller than 90 degrees, i.e., an acute angle. This is region 2.
The acute-angled bend goes back to a 90° bend if a still greater force is exerted on it. The tonnage
needed at this time corresponds to approximately 75 tons, which is about 6 times the required
tonnage. In Fig. 1-1, the line steadily rising along the Y-axis in the 90° bend indicates a rapid
increase in the bending force. The region in which the bending angle changes very little despite the
rapid increase in bending force, is called region 3.
The events that occur under the bending force in the three regions,1, 2 and 3 , are referred to as
partial bending, bottoming, and coining respectively. These bending force events are the foregoing
three kinds of bending. Further, partial bending and bottoming are both called air bending. Go to
Top
1-3 Air Bending and Coining
In reference to basic nature, bending can be divided into two categories: air bending and coining. Air
bending is a bend where air exists between the work and the die groove. As already stated, partial
bending and bottoming belong to this category. These two methods of bending will be described in
detail later.
Now let us compare air bending with coining in terms of economy and precision. Air bending
provides the following features:
[1] Since this bending method can bend the work with relatively little force, a machine
with a small capacity can be used. The cost of equipment is low, therefore, the method
provides great economy.

6. [2] Having been influenced by springback (refer to subsection 1.4), the bending
precision may not be satisfactory.
The features of coining are as follows:
[1] A machine with a large capacity must be used because this method requires a
bending force which is 5 to 8 times the required tonnage in air bending. The cost of
equipment is high, therefore, the method is inferior in economy.
[2] Extremely high bending precision can be obtained because springback is eliminated.
The use of air bending or coining should be decided upon according to the application and function
of the product (refer to subsection1.8). Go to Top
1-4 Springback
1-4-1 Why springback occurs
Let us examine springback phenomena first. Fig. 1-2 shows a springback in V-bending by which
obtuse-angled, 90°, and acute angled bends are formed. In the figure, the solid lines indicate the
angle during forming (Ø´), and the dotted lines indicate the angle after being formed (Ø).
Here we will consider what causes springback to occur from two points of view. One view is
springback considered from the stress-strain diagram, and the other view is considered from the
displacement of the molecules inside the work.
In the stress-strain diagram in Fig. 1-3, as the exerted external force at point G in the plastic* region
is reduced by low degrees, the amount of strain (deformation) in the work decreases gradually. It
decreases parallel with the line OA (which is in the elastic region where stress is proportional to
strain). When the work is totally removed from the external force, the strain reaches point X. This
indicates that the work retains a certain amount of elasticity in the plastic region. In the diagram, G'X
shows the amount in which the work has returned to its former condition, and OX shows the amount
of permanent deformation.
To summarize the above facts, the elasticity of the work material is not eliminated even after the
stress produced in the work has exceeded the yield point (The yield point is where the work gives

7. way to stress; the plastic region lies beyond that point.). This is a cause of springback.
Fig. 1-4 is an exaggerated illustration of the molecular displacement in flat work when it is bent at an
obtuse or 90 degree angle. This figure shows that the inner side of the work is compressed and the
outer side is stretched. Between these sides, there is a plane which is neither compressed nor
stretched. This plane is called a neutral plane or neutral axis.
When the work is bent, stresses which are opposed to each other act on the inner and outer sides of
it. In general, the compressive strength of the material is far greater than its tensile strength. Exerted
pressure will permanently deform the outer side of the work, but the inner side stress does not reach
the yield point. Therefore, the inner side tends to go back to the former condition. Since stress is a
resisting force that acts in the opposition to the exertion of the external force, a compressive stress
acts outward on the inner side. This compressive stress changes into springback.
1-4-2 Positive and negative springback
You might recall the product angle if you hear the word "bending angle." This is not incorrect, but to
be more exact, it should be called the "bent angle." This is because the bending angle involves the
angle during formation and the already formed angle. When Ø denotes the angle after formation and
Ø´ the angle during formation, we have Ø-Ø´=ØA. The delta (A) is a small quantity. This ØA is the
springback which is the angle of the small amount.
In Fig. 1-1, as already stated, the bent angle becomes 3 or 4 degrees less than
90 degrees in region . This is a phenomenon called spring-go or spring-in,
which is expressed as negative ØA if the foregoing Ø-Ø´=ØA is applied. In
brief, a springback is +ØA and a spring-go or spring-in is -ØA. ØA becomes
either positive or negative, depending on the bending pressure exerted.
Then what are the conditions that cause a negative springback or spring-go?
Let us ex- amine the process of V-bending (Fig. 1-5). (a)
During V-bending, the work inserted between the punch and die changes its
state from (a) to (b) and then to (c) under the bending force. The work at that
time can be regarded as a continuum that contains positive factors
(springback) and negative factors (spring-go).
Additionally, it can be considered that (c) these positive and negative factors
are not constant, but show their qualities of being positive or negative while
changing in accordance to the force applied. Therefore, the (d) springback or
the spring-go will occur on the work, depending on the way bending force is
applied to the work. Go to Top
1-5 Bottoming
The term "bottoming" comes from the verb "bottom" which means "to reach the bottom." This
bending technique is also called "bottom pressing" or "bottom striking" at the production site.
Bottoming, one type of air bending, is most widely used because it can accurately bend the work
with relatively low tonnage.

8. In Fig. 1-6, t stands for the sheet thickness, V for the 'V-width (shoulder width of a V-die), and ir for
the inside radius of the bend. The desirable, proper V-width varies with sheet thickness. Table 1-1
shows the relationship between the sheet thickness and the V-width in bottoming. This table shows
the optimum standards in which the coefficient of the V-width becomes greater as the sheet
thickness increases. However, one should note that the V-width should also be determined by taking
the flange length and inside radius of the product into consideration. The method of determining the
V-width will be described in a later chapter.
The inside radius in bottoming has been experimentally clarified to be approximately 1/6 of the V-
width, i.e., ir=V/6. So ir can be determined by substituting the V-width obtained from Table 1-1 into
this equation. For example, if the V-width is 6 times the sheet
thickness, the inside radius equals the sheet thickness (ir = t). Further, if V=12t, then ir=2t. From
these examples, the inside radius ir is known to vary from It to 2t with the sheet thickness.
When the inside radius (ir) is equal to t (ir=t), it is called the standard ir. For example, in the case of
t=l mm, bending at the standard ir can be accomplished with a die of V=6 mm. Similarly, if t=1.6
mm, then a die of V=10 mm will produce the same result. Many products are bent at the standard ir.
Bending accuracy in bottoming is influenced by springback. The most widely used countermeasure
is to offset springback by bending the work an additional amount equal to the springback. This is the
reason tooling for 90° bending is available in such V-groove angles as 90°, 88°, 85°, and 80°. The
90° V-tooling has no allowance for springback, but in practice the product angle can be finished at
90 degrees by applying bending force while simultaneously keeping good balance between
springback and spring-go.
In the tooling used for bottoming, the punch tip and die V-groove must be of the same angle. This is
an essential condition to obtain satisfactory precision of the product. (This subject will be described
in detail later.) Go to Top
1-6 Partial bending
The name "partial bending" comes from the fact that the work partially contacts the tooling in three
places during bending (A, B and C in Fig. 1-7). Partial bending is typical air bending.
This bending method is characterized by a wide range of bending angles which can be selected
freely. For instance, in partial bending using a 30' punch and a 30' V-die, the work can be bent at any
angle between 30 and 180 degrees. Partial bending is convenient, thus, it is widely used, following
bottoming.
Table 1-1 Relationship between Sheet Thickness (t) and V-width
Sheet thickness 0.5-2.6 mm 3.0-8 mm 9-10 mm 12 mm or more
V-width 6t 8t 10t 12t

9. The V-width in partial bending should be 12 to 15 times the sheet thickness to achieve precision.
Fig. 1-8 explains the reason. Bending precision refers to bending angle variation, which is
theoretically related with the depth of the punch into the die.
In Fig. 1-8, Vb is the V-width used in bottoming and Vp is the V-width used in partial bending; Vp
is approximately two times greater than Vb. When the depth of the punch into the die is constant, Op
(the range in which the angles vary in forming with the partial bending V-width) is small in
comparison to op. This means the range of bending angle variation is smaller in Vp than in Vb.
During partial bending, the use of a larger V-width than that in bottoming results in good precision.
However, even if the V-width is increased, partial bending remains inferior in bending precision to
bottoming. For that reason, a bottoming type of tooling should be used when high precision is
required. The bottoming type of tooling are the punch and die in which springback (AO) is
considered to suit the required angle of the product. Go to Top
1-7 Coining
You might think it is unusual that a bending method is called "coining." Coining comes from the
word coin that means "metal money" and "making metal into coins." In spite of mass production,
each piece of coin is made with almost no variation in shape and size. From this, the name "coining"
seems to have been applied to this bending method whereby accurate bends are obtained.
Coining provides two advantages: (1) very high bending precision and (2) the capability of reducing
the inside radius to as small as possible. Fig. 1-9 shows the work and tooling in the final stage of
coining, from which you can see the punch tip is imbedded into the work. This penetration of the
punch tip, together with a high pressure produced by the punch and die V-groove, eliminates
springback. This is the reason coining requires a bending force 5 to 8 times greater than bottoming.
The V-width in coining is smaller than in bottoming, preferably 5 times sheet thickness. One reason
for this is to reduce the amount of the punch tip penetration into the work by decreasing the ir (you
will recall ir is 1/6 of the V-width). Another reason is to increase the V-die's surface pressure by
making its V-groove area smaller.
If coining is performed in a V-width close to that of bottoming, the punch tip penetration will
increase as much as the ir increases, requiring an additional bending pressure. Also, since the V-
groove area becomes larger, it is inevitable that the surface pressure will drop. It should be noted that

10. these factors will act like a brake upon satisfactory coining.
In coining, the angles of the punch tip and die V-groove should be made equal to the required angle
of the product. For instance, in the case where a 90' bend is to be formed, a 90' punch and 90' die
should be used without considering springback.
As previously explained, high tonnage is needed to perform coining. The limit of the sheet thickness
bendable by coining is governed by the machine's capacity; it also varies with the amount of pressure
the upper beam can withstand. Accordingly, consideration should be given to this point in
determining the bending limit. The tolerable pressure of an upper beam is usually guaranteed by the
tonnage per unit length. Bending pressure for coining a 1.6 mm thick cold rolled carbon steel is 75
tons of bending pressure per meter and a 2 mm thick cold rolled carbon steel is approximately 115
tons per meter. So the bending thickness limit is 2 mm, because, in general, the tolerable pressure of
the upper beam of the press brake is approximately 100 tons per meter. Go to Top
1-8 Arrangement of Three Types of Bending
As stated in subsection 1.3, it is necessary to select the proper bending method in accordance with
the application and function of the product. Do not determine the merits and demerits of the three
bending methods, but use them properly in order to make the most of the advantages of each.
For example, if an NC press brake and acute-angled tooling are used, partial bending will permit
products having acute-angled, 90', and obtuse-angled bends to be formed continuously from the first
to the last bend.
The demand for higher bending accuracy is becoming greater. In the late 1960's in Japan, excellent
sheet-metal working machines, such as shearing machines, punch presses, and press brakes became
popular. The appearance of quality punches and " dies followed. With such background
circumstances, accuracy of finishing made remarkable progress, and sheet-metal working machines
superseded part of the jobs of machine tools such as the milling machine, drilling machine, and lathe.
Speaking of the bending accuracy at that time, it was at an age of "±1 mm" in terms of general,
nominal tolerance. With the present technical level, "±0.2 mm" is an acceptable standard of
tolerance, which tends to rise as high as "±0.l mm." Such being the case, the bending accuracy today
is required to be 10 times as precise as that of twenty years ago. In other words, we have entered an
age in which bending accuracy must equal the accuracy achieved in coining, and in that sense,
coining must be taken into consideration.
As already stated, bottoming is the most widely used method of bending because it achieves
relatively high precision. Due to the high economy in the cost of equipment, bottoming will continue
to play a significant part in material bending. The serious problem in bottoming is the easy
occurrence of springback. However, we have now accumulated so much technical knowledge on
springback and can apply it to tooling designs. That is to say, countermeasures against springback
have advanced so much that bottoming can be performed with ease and confidence.
The present situation of the three types of bending has been outlined. Table 1-2 shows the important
points from the description I comparing these three methods. Each method should be understood
properly through this table and used in bending work.
Table 1-2 A Comparison of Three Types of V-Bending
Type of
bending
V-
width
ir
Bending angle
dispersion
Surface precision Features
Partial
bending
12t-
15t
2t-
2.5t
±45´
Forms surface
with large
curvature radius
Range of bending angle can be
selected freely.

11. Go to Top
Bottoming 6t-12t 1t-2t ±30´ Good
Good precision is obtainable by
using relatively little tonnage.
Coining 5t
0.5t-
0.8t
±15´ Good
Very good precision is obtainable.
The tonnage required is 5-8 times
that required in bottoming
t=thickness

12. 2-1 General
Abending force chart is one of the necessary instruments in bending operations. It can easily be
found as a nameplate on the press brake or in the manufacturer's catalogue and instruction manual.
This chapter explains the use of the force chart for mastery purposes. The operator should understand
the chart completely in order to enhance job efficiency and technical ability. It is recommended that
this chapter be reread if there are any questions concerning its contents.
2-2 Reading the bending force chart
Look at Table 2-1 which is the air bending force chart. The first or leftmost column shows the sheet
thickness (t) of the work. The three rows at the top indicate the V-width of the die (V), minimum
flange length (b), and the inside bending radius (ir) from the top. The figures shown are the required
tonnage per meter of work. The following items can be read from this table:
1. The V-width of the die to be used for bending.
2. The length of the smallest, bendable flange.
3. The required tonnage per meter of work.
4. The inside radius produced.
2-2-1 Explanation of symbols
Symbol (t) in the first column indicates the sheet thickness of the work, ranging from 0.5 mm to 30
mm.
Symbol V in the top row indicates the width of the die V-groove, or V-width. The figures in this row
are standard commercial sizes. However, the standard size of the V-width vary from manufacturer to
manufacturer.
t : Sjeet thickness (mm) (tensile strength
F: Bending force per meter (ton/m)
i: inside bending radius (mm)
b: Minimum flange length (mm)
V: V-die width (MM)
The following can be read from this chart if the Sheet thickness of the material and the inside radius
of the bend are determined.
1. The required force to bend one meter of the material.
2. The V-width of the tooling used in that bending.
Use of Bending Force Chart
General
Reading the bending force chart
Four Relationships
Calculation of Bending Tonnage
Complement of the bending force chart
Table of Contents

13. 3. The minimum flange length that can be bent.
Symbol b in the middle row indicates the minimum flange length.
Symbol ir stands for the inside bending radius of the product. Here let us examine the bending force
chart to see if ir= V/6. When V is 10 mm, ir is 1.6 mm, and when V is 32 mm, ir is 5 mm. Therefore,
ir is approximately 1/6 the V-width.
F indicates the "required tonnage" used to bend one meter of the work.
2-2-2 Minimum flange length
As previously mentioned, if the V-width is known from the sheet thickness, the minimum flange
length can be found from the bending force chart.
Let us examine why the word "minimum" is used for the flange length. Until bending is completed,
the work must be securely supported at the shoulder of the V-die groove. If not, as bending
progresses, the work slides off the shoulder, and the bending line changes irregularly. Good bending
accuracy cannot be obtained in such a bending operation and the operation itself becomes dangerous.
The "minimum flange length" is necessary in order to bend the work with sufficient accuracy and
safety.
Fig. 2-1 shows the minimum flange length calculation. According to the right isosceles triangle
ABC, the length of side b is V2-times V/2. That is, the minimum flange length can be calculated by
b=(2)½( V /2).

14. If a 6 mm V-width die is used to bend work, b is 4 mm according to the chart. Similarly, when the
V-width is 25 mm, then b becomes 17.5 mm. If the flange size in the work drawing is larger than b,
then it can be assumed the V-width is correct. However, if it is less than b, the V-width is inadequate
for the bending and a smaller V-width must be used.
2.2.3 Required tonnage
The statement, "tensile strength: 45-50 kg/mm², is shown in Table 2-1. This 45-50 kg/mm² refers to
mild steel.
As already stated, the value of F (bending force) is the required tonnage per meter of work. It is also
the required tonnage in bottoming. Generally, the bending force charts indicate the required tonnage
needed to bend mild steel by bottoming.
The following steps should be taken to find the necessary F. First, determine the V-width according
to work thickness t. Next, follow the horizontal row of the necessary t to the right, and read the
figure in the position where it meets the vertical row of the determined V. For example, if a V-width
of 12 mm is used to bend work that is 2 mm in thickness, you will find the required tonnage F is
equal to 22 tons. Likewise, when t is 3.2 mm and V is 25 mm, then F should read 27 tons. When t is
6 mm and V is 50 mm, F should be 48 tons. These F values are the required tonnage per meter of
work. Go to Top
2-3 Four Relationships
The four relationships refer to those between the required tonnage and V-width, sheet thickness,
bending length, and tensile strength. In many cases, the bending force chart is not utilized effectively
due to the operator not understanding it.
Effective use of the chart is being able to make the most of these four relationships, or those between
F and V, F and t, F and and F and b.
2-3-1 Relationship between F and V
Table 2-2 shows the required tonnage used per meter when 1 mm and 2 mm mild steel sheets are
bent in different V-widths.
As shown in the chart, if the V-width is doubled, the required tonnage will be reduced to 1/2. In
short, the required tonnage F is inversely proportional to the V-width V. This inverse proportion is
expressed as F=k-11V.
Table 2-2
t=1.0 mm t=2.0 mm
V(mm) 6 12 V(mm) 12 25
F(ton) 11 6 F(ton) 22 11

15. 2-3-2 Relationship between F and t
The relationship between the required tonnage and the sheet thickness is apt to be misunderstood. A
conversation, "The sheet is double in thickness, so the tonnage also becomes double," is often heard
in the production site. This statement is incorrect.
Table 2-3 shows changes in the required tonnage when the V-width was kept constant and the sheet
thickness was varied. When t doubles, F becomes approximately four-fold if the V-width is the
same. That is, bending force F increases in proportion to the square of sheet thickness. This is not a
simple proportional relationship.
For this reason, die designers and expert operators caution by saying "Bending force is affected by
the square of sheet thickness." Therefore, the above conversation on the job site must be corrected.
Operators should know that if sheet thickness doubles, the tonnage will become four-fold. The
relationship between F and t is expressed by F=k.t².
2-3-3 Relationship between F and L
LP is the bending length of work, which is proportional to F. The tonnage shown in the bending
force chart are values based on the unit bending length of one meter. To obtain the tonnage for a
certain length of work, convert the length into meters and multiply by the related tonnage.
The relationship between F and t is given by F=k-t.
2-3-4 Relationship between F and bb
F and tensile strength bb are proportional to each other, just as F and e above. Thus, if only b is
known, the required tonnage to bend materials other than SS41 can be determined by using this
relationship.
Table 2-4 shows the tensile strength of commonly used sheet materials. Since a variety of materials
are available, the bb of the confirmed when calculating F.
The relationship between F and bb is expressed by F=k-b. Go to Top
"
Table 2-3
t=1.0 mm t=2.0 mm
t(mm) 1 2 t(mm) 1.2 2.3
F(ton) 6 22 F(ton) 6 23
Table 2-4 Tensile Strength Of Various Materials
Material
Tensile strength, kg/mm²
Soft Hard
Lead 2.5-4 -
Tin 4-5 -
Aluminum(99.0%) 9.3 171
High-tension aluminum alloy Type 4 23 48
Duralumin 26 48
Zinc 15 25

16. 2-4 Calculation of Bending Tonnage
The bending tonnage serves as the basis for judgement in selecting a new machine, as well as
determining whether an already-installed press brake can achieve a bending operation from the
standpoint of capacity. The required bending tonnage can be calculated by the four relationships,
based on the F value in the bending force chart.
Examples of bending tonnage calculations and their solutions are given below. The reader should try
to answer them in order to familiarize himself with the four relationships.
[Example 1]
How many tons are required to bend 4 meters of a 1.5mm thick SUS304 stainless steel
sheet (tensile strength: 60 kg/mm²)?
[Solution]
The sought V-width is calculated as V=6XI.5 . .10
(6xl.5=9, but V=9 mm is not a standard size so V=10 mm is substituted). In this case,
t=1.5 mm is not in the bending force chart, so the bending force (F=17 tons) should be
read for V=to mm and t = 1. 6 mm (the closest t). The calculating formula, which takes
into consideration the sheet thickness, tensile strength, and bending length, will be given
as follows: F.17X(1.5/1.6)2X60/45 x 4= 80
The sought solution is 80 tons. In this calculation, raise the fractions to units.
[Example 2]
How many tons are necessary to bend a 15 mm thick and 3,100 mm long sheet of rolled
SS41 steel? The flange length is 120 mm.
[Solution]
The sought V-width is V=t2xl5=180.
Copper 22-28 30-40
Brass (70:30) 33 53
Brass (60:40) 38 49
Phosphor bronze 40-50 50-75
Bronze 40-50 50-75
Nickel silver 35-45 55-70
Cold rolled iron sheet 32-38 -
Steel, 0.1%C 32 40
Steel, 0.2%C 40 50
Steel, 0.3%C 45 60
Steel, 0.4%C 56 72
Steel, 0.6%C 72 90
Steel, 0.8%C 90 110
Steel, 1.0%C 100 130
Silicon steel sheet 55 65
Stainless steel sheet 65-70 -
Nickel 44-50 57-63

17. Although a standard V-width of 160 mm or 200 mm is acceptable, a V-width of 200 mm
is not suitable from the viewpoint of length b (flange length). This is because the
minimum flange length for a V-width Of 200 mm is 14o mm, which is greater than the
required 120 mm. Therefore, V is set at 160 mm.
Also, t=15 mm is not shown in the chart, so 107 tons is obtained when the F for t=16
mm and V=160 mm is read. Thus, we will have the following formula: F= 107 x (15/16)
2 x 3. 1=292 Consequently, the sought answer is 292 tons.
[Example 3]
Calculate the required tonnage when bending SPCC of t-3.2 mm and =2,400 mm with a
V- width of 18 mm. 6b is 32 kg/mm².
[Solution]
The sought V-width is V=8x3.2=25, and V=25 mm is adequate. However, when
reducing the V- width for any reason (e.g., when the flange length is short or the inside
radius of the product is small), you may lower by one rank (a blank space where the
tonnage is not stated). We don't recommend lowering more than one rank because
satisfactory precision cannot be obtained with a V-width of 16 mm. Because F=34 tons
can be read as t=3.2 mm and V=20 mm, the required tonnage will be given as follows:
F= 34 x 20/18 x 32/45 x 2.4=65 Thus, the required tonnage is 65 tons.
[Exercises]
1. Determine the tonnage required to bend SPCC of t=1.6 mm and =3,000 mm.
2. How many tons are required to bend an anti-corrosive aluminum alloy sheet with
t=3.0 mm, =2,500 mm, and bb=22 kg/mm²
3. Determine the tonnage required to bend SS4] of t=20 mm and e=600 mm.
4. Calculate the tonnage required to bend soft copper (6b=25 kg/mm²) of t=8 mm and
=2,500 mm.
5. Find the tonnage of F when bending SUS304 (t=8 mm and l=3,000 mm) at V=63 mm.
(See page 25 for the solutions.) Go to Top
2.5 Complement of the bending force chart
So far, we have described in the previous subsections the necessary fundamentals of how to read the
bending force chart, the four relationships, and the calculation of bending tonnage. The information
already given will enable you to properly use the chart in your job site. The inquires we have
received mainly concern the usage of the chart. These inquires seem to suggest the chart is not
always effectively utilized. If so, that is indeed an unfortunate situation. The above fundamentals
may appear complicated, but each of them can be broken down into simpler factors. They are all
made up of correlative simple factors. Just start with the first step, and you will find that it is easy to
master the usage of the bending force chart.
Now, let us consider a certain calculating formula that is mentioned in almost all books regarding
sheet bending. It is the formula,
F=(c x b x l x t²)/(V x 1000)
At a glance, you will find that this formula involves all the factors of the four relationships. The
denominator V is inversely proportional to F, and the numerator b, e and t² is proportional to F.
The major problem with the above formula is knowing how to determine the value of coefficient C.
Usually, the value of C is somewhere between 1.00 and 2.00, varying with V/t. The smaller V/t, the
larger value C becomes. When V is 8t, C is said to be 1.33. However it is the value of just one
datum. Another datum (by the same researcher) shows a difference of 15 percent in the value C.
Such being the case, it causes US to question this calculating formula, and the true value for C

18. varying with V/t. This means the calculated tonnage may turn out to be extremely inaccurate.
Our experience reveals the calculation using the tonnage in the chart (as in the exercises) is more
practical than the use of calculating formula. This account for the fact that we have calculated the
bending tonnage according to the bending force chart , not using the calculating formula.
[Solutions]
1. F=17x32/45x3=37
2. F =24x22/45x2.5=30
3. F=125x(20/19)²x0.6=84
4. F=52x(8/7)²x25/45x2.5=95
5. F=(52(8/7)²x60/45x3=272
Go to Top

19. 3-1 The Necessity of General Knowledge
This chapter provides the necessary background information to enable the reader to understand
punches and dies, the tooling used in bending. The main section of this chapter deals with general
subjects, such as the requirements for good tooling, material, and the manufacturing process.
Usually, operators recognize the importance of tooling knowledge, but only within the limits of their
job. Some operators do not take much interest in the general subjects noted above, and some think it
is unnecessary to learn the extensive process involved in tooling.
However, the precision, material, and hardness of a punch and die are directly related to the
precision of the product which the operator must achieve. The more they know about tooling, the
better chance they will have to attain their objective. Go to Top
3-2 Requirements for Good Tooling
The requirements for good tooling must be examined when judging the quality of a punch and die.
Moreover, we must know how these requirements are related to our bending work.
Generally speaking, the following are essential requirements for good tooling:
1. The length should enable easy installation and removal.
2. Completely heat-treated for greater strength and longer wear resistance.
3. High accuracy of finish.
4. Interchangeable regardless of the equipment.
In summary, good tooling consists of a punch and die which provide ease of handling, a long service
life, high accuracy, and interchangeability. (The relationship between the length of tooling and its
replacement will be described in subsection 3-4.) Go to Top
3-3 Tooling Materials, Heat Treatment, and Manufacturing Process
During the bending operation, the tooling undergoes repetitive loads such as compressive loads and
bending moments. Given the fact that the tooling may perform 3,000 processes a day, the frequency
of repetitive load application could total 70,000 to 80,000 times per month and 800,000 to 1,000,000
times per year. The strength and wear resistance of tooling to withstand such repetitive loads can
only be obtained by using a quality material and proper heat treatment.
There are two methods of heat treatment: "hardening" and "thermal refining." The terms "hardened
tooling" or "thermally refined tooling" comes from these methods of heat treatment. Generally, if it
is called bending tooling, it refers to either hardened tooling or thermally refined tooling.
There is other tooling which is made of so-called raw material which is not heat-treated. In a strict
sense, however, this should not be considered tooling because the steel used for tooling is only
General Information on Bending Tools
The Necessity of General Knowledge Sectional Shape of Tooling
Requirements for Good Tooling Sectionalized Type
Tooling Mat'l, Heat Treatm't, & Mfg Process Holder
Length of Tooling Table of Contents

20. effective when it is heat-treated (though the quality of material must be suitable for heat treatment).
3-3-1 Hardened Tooling
As previously stated, the purpose of heat-treating tooling is to increase its wear resistance and
strength against bending force. The material commonly used in hardened tooling is alloy tool steel,
though it varies with the tool manufacturer. The paragraphs that follow will describe chromium
molybdenum steel (type 4) and DM.
(i) Chromium molybdenum steel, type 4 (SCM4)
Chromium molybdenum steel is one of the structural alloy steels. It is a pearlitic steel containing
chromium steel and a small amount of molybdenum. This steel, which is suited for tooling material,
features toughness (tenacity), great wear resistance and superior hardenability by heat treatment.
This tooling material is either completely or locally hardened (= overall or local hardening*') to
obtain HRC43 to 48. HRC indicates the Rockwell hardness C scale. The Rockwell hardness test is
used to measure the hardness of mechanical parts in many job sites because it is a simple method of
measurement.
(ii) DM
DM is the trade name for "Yasuki Steel" made by Hitachi Metal. This tooling material is equivalent
to JIS alloy tool steel, SKT4. However, it contains more nickel for greater toughness. Like chromium
molybdenum steel, DM provides excellent toughness and wear resistance, and its hardening strain
(the tendency of bending, warping, or twisting after being hardened) is less than that of other
material. Therefore, while SCM4 is used in general tooling, DM is utilized specifically for tooling
whose sectional shapes have a greater hardening strain* 2 . Its hardness is identical to that of SCM4.
Table 3-1 compares the heat treatment of tooling materials.
*1. Local hardening and overall hardening: Local hardening means heat-treating only part of the tooling
such as a punch tip and a die V groove. This does not produce much hardening strain and the portion
requiring grinding is small. However, sufficient strength cannot be obtained in the essential part of the
tooling (for example, in the part of a punch where the maximum bending moment acts). Amada's tooling is
treated by )overall hardening.
*2. Sectional shapes having a large hardening strain: This refers to the sectional shapes of a gooseneck
punch, straight sword punch, sash punch, and sash die. Shapes with large relief, slender shapes, and
shapes with great changes in thickness are subject to develop hardening strain.
(iii) Manufacturing process
Fig. 3-1 shows the manufacturing process of hardened tooling. Hardened tooling is generally
manufactured in the order shown below.
Material => Primary forming => Hardening => Correcting =>
Finishing
The formed workpiece, which has been hardened, is corrected prior to finishing. Then the workpiece
Table 3-1 Heat Treatment and Tooling Materials
Operation Hardening Tempering Hardness (HRC)
SCM4 850° 4.5H Oil cooled 420° 5H Air cooled 43-48
DM 860° -880° 1.5H Oil Air cooled 850° 2H Air cooled 43-48
S45C 850° 4.5H Oil cooled 680° 5H Air cooled 23-28

21. is finished by grinding with a grinding machine.
Finishing work can best be performed by "profile grinding." Photo 3-1 shows a 2V die being ground
by profile grinding. As seen from the photo, the contour including the V-grooves (each consists of
groove face, shoulder R, and groove bottom R) and surfaces are ground simultaneously when using
profile grinding.
The grinding wheel used in profile grinding is dressed with a diamond dresser and finished with high
accuracy by a crushing roll (Fig. 3-2). The job of the crushing roll is to smooth any roughness on the
grinding wheel after dressing. This enables the wheel to function property.
3-3-2 Thermally Refined Tooling
(i) Purpose of thermal refining

22. The purpose of thermal refining is to unify the internal structure of the tooling material so that it will
remain operable during use. The difference between thermal refining and hardening is that the
tempering temperature in thermal refining is higher than in hardening. As indicated in Table 3-1,
there is no difference between the two in hardening temperature (thermal refining may be considered
a type of hardening). Thermal refining increases wear resistance and strength in its own way. Carbon
steel (S45C) is used as thermally refined tooling material.
(ii) Application of thermal refining
In general, thermal refining is employed in the following cases:
1. Large, hard-to-harden tooling (This is related to the size of the electric furnace.)
2. Complicated sectional shape, hard-to-grind tooling (This is often the case with
special tooling.)
3. Long, one-piece, hard-to-grind tooling (This is related to the workable length of
the grinding machine.)
The hardness of thermally refined tooling is lower than that of hardened tooling, and is somewhere
between HRC23 and 28. HRC28 is generally the maximum hardness that is workable with a bit (toot
used with a lathe, shaper, and planer).
(iii) Manufacturing process
The manufacturing process of thermally refined tooling differs between large and small tooling. As
shown below, large tooling is produced in the same process as hardened tooling.
Material => Primary forming => Refining => Correcting => Finishing
Small tooling is fabricated of already-refined material (commercially available) in the following
process.
Refined material => Primary forming => finishing
The difference in the manufacturing process of large and small tooling originates from the "mass
effect" of heat treatment on each type of material. Generally, when a thin steel bar or a thin steel
sheet is heat-treated, the effect of heat treatment penetrates completely through it. In thick steel bars
or sheets, however, the outer part can be heat-treated to the desired hardness, but the inner part is not
sufficiently hardened. This is because the cooling speed of the outer part is different from that of the
inner part. Thus, the inner part is less hard than the outer part. This phenomenon is the mass effect.
Since large tooling requires much working, the inner part with less hardness is exposed as a result of
machining. For this reason, it is preferred that the tooling be refined after primary forming.
Thermally refined tooling is finished with a planer or a profiling planer. A slow machining speed and
a small amount of cut-in provide a high-precision surface finish. When working a complicated curve
or arc such as those found in special tooling, an accurate template (profiling plate) should be
prepared. The material is profiled by using the template and profiling planer .

23. Go to Top
3-4 Length of Tooling
With regard to length, tooling used in bending can be divided into one-piece tooling and split type
tooling. One-piece tooling is simple tooling made in conformance with the required bending length.
Split type tooling is tooling that can be split into individual sections.
The length of the tooling greatly influences the efficiency of bending work. Now, we will examine
how the length of tooling influences job efficiency in connection with tooling replacement, which is
frequently performed in bending.
The frequency of tooling replacement in a given day is between 5 and 20 times, with 10 being the
average. Tooling replacement involves a series of preliminary operations as shown below.
Carrying => Mounting => Aligning => Accuracy adjusting => Removing =>
Storing
Table 3-2 compares both types of tooling with respect to this series of operations. The reader will be
able to easily determine which type of tooling is more efficient.
Next, let us focus on how the type of tooling influences bending accuracy. The primary factor that
affects bending accuracy is the parallelism of the tooling (tooling height). A comparison of
parallelism between one-piece tooling and split type tooling is shown in Fig. 3-3. The limit of the
working accuracy that a planer can achieve is 0.05 mm per meter of tooling. It is 0.02 mm in
grinding work. In this example, the working accuracy of the one-piece tooling is 0.1 mm for an
overall length of two meters, while the split type tooling is 0.02 mm. Thus, the split type is five times
more accurate than the one-
piece type.
Table 3-2 A Comparison of Tooling Replacement Operations
Operation One-piece type Split type
Carrying A forklift or more than one person is needed Can be carried by one person
Mounting, Aligning Must be done by two persons while signaling. Can be done by one person
Accuracy adjusting Must be adjusted with shims after test bending. Unnecessary if alignment is correct.
Removing More than one person is needed. Can be done by one person
Storing A forklift or more than one person is needed. Can be done by one person

24. So far, we have seen how the length of tooling influences job efficiency and bending accuracy. From
the above facts, the reader will readily discover that split type tooling is superior.
Fig. 3-4 shows the length of each tooling that can be mounted on various Amada machines. The
tooling is available in two lengths: L (Long size) and S (Short size). L size is 835 mm (32.87") and S
size is 415 mm (16.34").
Go to Top
3-5 Sectional Shape of Tooling
A long shape which has an identical cross section from end to end is characteristic of tooling.
Therefore, it is important for operators to remember the sectional shape and size of individual tooling
for more accurate and speedy performance. It may be difficult for operators to remember them all
because there are so many types. Some hints will be provided to help the reader remember.
3-5-1 Punches
Fig. 3-5 shows an example of a punch sectional shape. The sectional shape of a punch can be
classified by (1) tip angle, (2) relief shape, and (3) installed height. 'These should be memorized.
(i) Tip angle
There are punches with a tip angle of 30°, 45°, 60°, 88°, or 90°. Punches with a 30° or 45° tip are used
for acute-angled bending, and those with 60°, 88°, and 90° tips are used for 90° bending. The punch
tip for 90° bending has a "punch tip R," which corresponds to the "punch tip flat width" in the 30°
and 45° punches. In Fig. 3-6, (a) illustrates a 90° punch and (b) the tip of an acute-angled punch.

25. (ii) Relief shape
Punches have a "relief" necessary for various bending. The shape of the relief is determined by the
shape of the product. The relief shapes of major punches are shown in Fig. 3-7. As shown, punches
can be divided into two types. One type has a relief at the rear. The other type has relieves of one or
different shapes at the front and rear. Punches (a), (b), and (c) belong to the first type and (d) and (e)
belong to the latter type.
In Fig. 3-7, (a) is a bottoming punch for 90' bending and is the most commonly used type; (b) is a
punch for acute-angled bending or air bending; (c) is a gooseneck punch; (d) is used for products
having a symmetric shape and is called a straight-sword punch; and (e) is a sash punch that is widely
used in the sash industry.

26. (iii) Installed height
The height of a punch refers to its installed height (see Fig. 3-5). In the case of the radius ruler (R
punch) or the coining ruler, note that the height E of the punch holder plus the ruler's height is the
installed height (Fig. 3-8). There are seven different installed height: 65, 67, 70, 90, 95, 104, and 105
3-5-2 Dies
Fig. 3-9 shows an example of a die sectional shape.
Dies are classified by (1) the shape of the groove and (2) the number of grooves. A die with one V-
groove is called a IV die and a die with two V-grooves is called a 2V die. Further, a die with three
rectangular grooves is called a 3U die.
(i) IV die
The IV dies having one V-groove can be divided into two types. One type is for bending work over 4
mm in thickness and the other type is used for making sashes. The first type has "working relief*" on
each bank of the V-groove, as shown in Fig. 3-10. The sash die does not have such relief. As
indicated in Fig. 3-11(a), the sash die is installed to thg sash die 'holder and used in combination with
the sash,h punch.
*The working relief reduces forming time of the die and improves the surface precision of the groove. (;generally, dies
with a large V-width are provided with working relief, but small V width dies are not.
(ii) 2V die

27. This is a bottoming die that can bend a maximum sheet thickness of 3.2 mm. It has two different V-
grooves that have different V-widths but have identical V-groove angles. The 2V die is installed to a
die holder when it is used [Fig. 3-11(b)]. Like sash dies, it has no working relief in the V-groove.
(iii) 3U die
As shown in Fig. 3-12, the 3U die consists of three different rectangular grooves and one flat surface.
The die is used for partial bending in combination with a 45' punch. It is directly attached to the
lower beam without using a die holder.
Go to Top
3-6 Sectionalized Type
The sectionalized type mentioned here is different from the split type tooling we have discussed in
the previous subsections. Split type tooling is a general term. The name is a classification term
indicating that split tooling is distinct from one-piece type tooling. The term "sectionalized type"
indicates a die or punch that is split into small, specific sizes.
3-6-1 Sectionalized Punch
Photo 3-3 shows an example of a sectionalized punch. The shape of the sectionalized punch
coincides with those of the L and S sizes. Accordingly, if sectionalized punches are used in a line,
various combinations of sizes, including the total length of the beam can be created according to the
operating requirements. This method is effective in making box-shaped products.
The sectionalized sizes are 10, 15, 20, 40, 50, 100, 200, and 300 mm. There are two types of the 100
mm punch; an overhang called a horn at the left end and the other has a horn at the right end. The
shape of the horn is identical among all Amada standard punches. They are ideal for box + pan
forming when a return flange is involved. Amada offers a variety of sectionalized punches enabling
you to purchase only those which best suit your needs.

28. 3-6-2 Sectionalized Die
The sectionalized die is attached to the die holder on the rail, as shown in Fig. 3-13. The size of H
conforms to the flange height of the workpiece that can be bent with the tooling. That is, H = 10 mm
indicates bending work can be performed up to a flange height of 10 mm, and similarly, H = 40 mm
up to a flange height of 40 mm.
The sectionalized dies which have an H size of 10 mm can be utilized in a row because they are 26
mm or the same height as the L and S sized 2V dies. With regard to a sectionalized die with the
flange height of 40 mm, it is necessary to prepare several units of the same die or make a 2V die with
an installed height of 56 mm as special tooling.
Sectionalized dies are divided into eight lengths: 10, 15, 20, 40, 50, 100, 200, and 400 mm.
Go to Top
3-7 Holder
There are two methods of attaching the tooling to the machine. One type fixes the punch or die
directly on the upper or lower beam. The other type locks them by using holders. When using tooling
holders, attach the punch to the punch holder and the die to the die holder.
3-7-1 Punch Holder
As shown in Fig. 3-8, the use of a punch holder permits the desired shape punch to be attached as
necessary, just as radius rulers of different radii or coining rulers can be attached thereto.
3-7-2 Die Holder
Two types of die holders are utilized according to the type of die.
One type is the die holder used in the bending of sashes [Fig. 3-11(a)]. The other type is the die
holder used with 2V dies [Fig. 3-11(b)]. The die holder must be selected in conformance with the
shape of the product. A detailed description of die holder selection is provided in subsection 4.3.

29. 3-7-3 Die Base
The lack of bending stroke commonly occurs in cases where a IV die (type not for sashes) is used
with a machine having a large open height. The die base (Fig. 3-14) is set under the IV die to
increase die height. (For the open height of the machine and the die height, refer to subsection 4. 1
Go to Top

30. 4 SELECTION OF BENDING TOOLS
This chapter describes the various types of bending tools used in V-bending, R-bending, and
hemming. It explains the requirements of adequate tooling selection, as well as operational problems
and care required with these three types of bending. Special shapes that cannot be formed with the
above tooling must be made by using special tooling. This special tooling will be discussed in
chapter 5.
4-1 Process of Selecting Tooling
You have probably heard that synthetic judgment is required to select tooling. "Synthetic judgment"
means there is a process for properly selecting the tooling, and that the tooling selection must be
made in accordance with this process. Problems that occur during operation usually result from the
wrong tooling selection.
The process of selecting the proper tooling involves a V-width decision. Also, the combination of a
punch and die, the machine's specifications, the bending sequence and return bends, and the
guaranteed pressure tolerance of the tooling must be considered. If the process is observed, the
likelihood of making a mistake in selecting proper tooling is greatly reduced. Now, we will describe
the selection process.
4-1-1 Selection of V-Width
The V-width has already been described in subsection 1.5, "Bottoming." Furthermore, in chapter 2,
"Use of Bending Force Chart," you have tried some exercises using given V-widths.
As previously noted, the starting point in selecting the proper tooling is to determine the V-Width.
The V-width must be determined on the basis of relationship between sheet thickness and V-width.
However, the coefficient 6 of 6t or 8 of 8t in the table is only a standard. Therefore, the V-width
provided by the table must be reviewed in accordance with the actual requirements of the product
(such as minimum flange size: b, inside bending radius: ir that are indicated in the drawing, and
required tonnage: F). Such a review frequently leads to a reduction of the V-width or a change in
product design.
Selection of Bending Tools
Process of Selecton Tooling Hemming
Bending Pattern Bending Sequence
V-Bending Guaranteed Allowable Tonnage
R-Bending (radius bending) Table of Contents

32. In conclusion, the V-width obtained from Table 1-1 should be considered a provisional V-width. It
should be noted that the "true V-width" can only be determined when b, ir, and F are carefully
examined.
4-1-2 Tooling as a punch and die combination
One of the essentials of proper tooling selection is to understand that bending tooling is a
combination of a punch and a die. The combination varies with sheet thickness and shape of the
workpiece, and occasionally with the type of material. Table 4-1 shows typical combinations of
punches and dies from which an optimum combination with regard to bending pattern, sheet
thickness, and shape can be selected. Operators should make it a practice to consider punch-die
combinations. Improper attention to the selection of a punch or die may be due to a lack of
recognition that a bending tool is a combination.

33. 4-1-3 Machine specifications and setting of tooling
The fundamental machine specifications which require careful consideration are capacity, beam
length, open height, and stroke. The manufacturer's catalog and instruction manual should be
reviewed prior to determining the necessary tooling for these major elements.
Fig. 4-1 illustrates the attachment of bending tooling to the machine. In the figure, F is the open
height, B is the punch height, D is the die height (including the die holder height E), G is the stroke,
and C is the distance from the punch tip to the die.
The distance between the punch tip and the die should allow the work to be freely loaded or
unloaded in the fore-and-aft direction during operation. Some work, such as sash products, must be
removed sideways because of their shapes. This method of unloading, called "sideways pull-out," is
to be avoided if possible because it reduces operating efficiency. From the viewpoint of operating
efficiency, it is best to load and unload the work from machine.
The information provided above teaches that sufficient space bgtween the punch tip and die is
advantageous in terms of operational efficiency. Generally, a large punch tip-to-die distance is
required when the punch and die are high or the product is bulky. However, the stroke of the
machine must be considered if a low punch and die are used; otherwise, proper bending cannot be
performed because of the lack of stroke.
Useful formulas which compare the distance between the punch tip and die C, and stroke G are as
follows:
C = F-(A+B+D)............................................ (1)
C+V/2 < G.................................................... (2)
where A is the height of the distance piece. As shown from formula (2), the stroke will be sufficient
if the sum of the punch tip-to-die distance and 1/2 of the V-width is less than the stroke.
Go to Top
4-2 Bending Pattern
Table 4-2 shows the bending patterns of a variety of products classified according to their shapes.
The bending tooling is named to correspond with the bending pattern, such as V-bending tooling and
R-bending tooling.

34. There are three patterns which can be formed with standard Commercial tooling. These patterns are
V-bending, R-bending, and hemming. No other pattern can be formed without using special tooling
designed for the particular shape of each product.
The following subsections will deal with V-bending, R-bending, and hemming. Go to Top
4-3 V-Bending
V-bending produces a representative pattern of all the bending techniques and is the most widely
used. V-bending has a wide range of applications, from simple one-stage bending of angular shapes
to complicated multi-stage bending used in forming building materials and sashes. The thickness of
the work ranges from very thin sheets of approximately 0.5 mm to thick sheets of 25 mm.
V-bending can be divided into bottoming, partial bending, and coining. Also, as seen from the
bending shape, it can be divided into 90° bending and acute-angled bending. (Obtuse-angled bending
can be accomplished by using either 90° or acute-angled tooling.)
4-3-1 90° bending tooling
(i) Combination of 90° bending tooling
The 90' bending tooling must be selected according to the sheet thickness of the work to be bent. In
Table 4-1, "Typical Combinations of Bending Tooling," Nos. I through 3 are applicable to work up
to 3.2 mm thick. Nos. 5 and 6 are applicable to sheets thicker than 3.2 mm.
Sheets in the first category are subdivided into two thickness ranges: one range is from 0.5 to 2 mm
and the other from 2 to 3.2 mm. Use a 90° combination (punch and die) for sheets 0.5 to 2 mm, and
an 88° combination for those 2 to 3.2 mm.
(ii) Graph of return bend limits
Table 4-3 shows the size limits of bending shapes. "Size A" is called a step difference and "size H" a
down bend. The step difference is indicated by the minimum value at a certain V-width, and the
down bend by the maximum value.
Sizes A and H can be obtained in the manner shown in Fig. 4-2(a) and (b). Size H is determined by
the height of the die holder. In Table 4-3, (a) indicates the numerical values that apply to V-bending

35. using a 39 mm high die holder, while (b) indicates those applicable to V-bending using an 81.5 mm
high die holder for sashes.
The limit sizes include "W" and "L" in addition to A and H. The
Table 4-3 Limit Sizes of Bendable Shapes
relationship between W (expressed as "inside-inside" size) and L (expressed as "inside-outside" size)
can be read from a graph of return bend limits.
Fig. 4-3 is an example of a graph of return bend limits. To read the graph, ensure the inside line of
the work in the second stage of bending agrees with the reference lines of L and W; extend a line
from size W along the axis L until it meets the profile of the relief part of the tooling; take a margin
approximately 1 mm from that point in the directions of L and W and read the size by using the
graphic scales. The dimensions shown in the drawing are usually outside measurements which
include sheet thickness. If so, subtract sheet thickness from the dimensions in the drawing.
There are three types of bending tool used with sizes W and L. One type is designed for bending
with a narrow W. A second design allows bending with a large L. The third design can be used for
both kinds of bending. The gooseneck punch, which has a large relief shape, is the type capable of
bending with a large L size. Amada's standard gooseneck punch is available in two types: one for an
L size of 50 mm and the other for 60 mm. The 50 mm punch is also available for use with a narrow
W.
In cases where a punch with a tip angle of 88° is used, the bending angle is 88°; therefore, sizes W
and L in the return bend are somewhat smaller than those in 90° bending. Size L must be carefully
selected when using a gooseneck punch because it is large.
(a) Punch No. 4 or 16 with 39 mm high die holder.
V 4 6 7 8 10 12 14 16 18 20 25
t 0.7 1 1 1.2 1.6 2 2.3 2.6 3 3.2 3.2
W (minimum) 9.7 10 10.1 10.2 10.6 11 11.3 11.6 14 14.6 18.6
A (minimum) 4.9 6.5 7.1 8 9.7 11.5 13.2 14.7 15.5 18.5 21.7
H (maximum) 44.5 mm
(b) Punch No. 200 or 201 with 81.5 mm
high die holder for sash.
V 6 8 10 12 14
t 1 1.2 1.6 2 2.3
W (minimum) 9 9.3 9.6 10 10.3
A (minimum) 9 9.5 10.7 12.5 13.6
H (maximum) 91 mm

36. (iii) Effect of punch tip radius (R)
T he No. 5 or No. 6 combination in Table 4-1 should be selected when bending sheets thicker than
3.2 mm. Use No. 5 to bend sheets 4 to 10 mm thick. Use No. 6 to bend sheets 12 to 15 mm thick.
The punch used in bending sheets 4 to 10 mm thick is called a "heavy punch" or "thick work punch."
Its tip R is 6 mm. Further, an R punch called a radius ruler is used to bend sheets 10 to 15 mm thick.
The size of R, as stated above, differs with the thickness of the work to be bent.
It is usually necessary to use a punch with a large tip R when 90' bending thick sheets. As already
noted, ir of the product is basically determined by the V-width, but the punch tip R also influences ir.
When the punch tip R is large, the value of ir = V/6 turns out to be slightly larger, and vice versa. To
say that ir is determined by the punch R is incorrect. This is evident from a close review of the
relationship between the V-width and ir in the bending force chart based on measured values.
Therefore, the punch should have a tip R somewhat smaller than the ir determine by the V-width. If
the punch does not meet this specification, a proper inside radius cannot be obtained and cracks may
develop in the outside radius of the product. Cracks cause a considerable decline in product
precision. Deeply cracked products become useless rejects. This is why radius rulers of R20 and R25

37. are used with a IV die of V=125 mm and V=160 mm, respectively.
(iv) IV die with V-width of 200 mm
Amada offers a semi-standard IV die with a V-width of 200 mm for use with a special R punch (R: 3
(l mm). Fig. 4-4 illustrates the attachment of the die to the machine.
This 200 mm V-width die is 100 mm long. When used, several dies are arranged in line according to
the length of the workpiece. Usually, they are not installed close together but rather at intervals of
0.5t to it. For this reason, the interval can be increased for thicker work sheets. Accordingly, the
number of dies required, relative to the bending length of the work can be reduced.
The main feature of this die ties in the hardened rollers provided at the shoulders of the V-groove.
The work is bent with the help of the rollers and therefore the bending force can be decreased. This
type of tooling is capable of bending 25 mm thick sheets at maximum.
4-3-2 Tooling for acute-angled bending
(i) Tooling combination for acute-angled bending
Acute-angled bending ks a type of V-bending used to bend work into a final design or for use during
the preliminary stage of hemming work. Nos. 9 through 12 in the table of combinations correspond
to punch and die combinations for this bending.
The die V-width in acute-angle bending may be determined using the same standard as for 90°
bending. Amada's standard sizes are 8, 12, 18, 25, 32, 40 mm, etc. The minimum flange length for
acute-angled bending is shown in 'table 4-4. It is longer than that for 90° bending because the
minimum flange length is related to the side length of the V-groove.
(Dash indicates unworkable) table
(ii) Punch tip angle
Two punch tip angles are used: 30° and 45°. A punch with a 30° tip is preferable when acute-angled
bending is performed in the preliminary stage of hemming. A 30° tip should also be used for sheets
less than 3.2 mm thick, and a 45° tip should be used for thicker sheets.
(iii) Flat width of punch tip
The punch for acute-angled bending is flat at the extremity of its tip. This corresponds to the R tip of
Table 4-4 Minimum Flange Length in Acute-Angled Bending
Sheet thickness (mm) 1.6 2.0 2.3 2.6 3.0 3.2 3.5 4.0 4.5 5.0
Minimum flange length (mm)
30° 10 16 24 24 24 35 35 - - -
45° 10 10 19 19 19 - - 35 35 41

38. the 90° bending punch. Fig. 4-5 shows the flat tips of typical acute-angled bending punches. In
acute-angled bending, fissures and cracks sometimes appear on the outside R surface of a product.
The width of the flat area on the punch tip greatly influences the occurrence of cracks. When
bending easily cracked aluminum sheets or thick sheets of other materials, punches with large flat
areas on the tip will prevent cracking.
When bending a 2 mm thick sheet of corrosive-resistant aluminum to 30' and then hemming it,
cracks formed along the bend line. Our investigation revealed that the cracks developed during the
30' bending and became more severe during hemming.
The 300 punch used (No. 13 combination in Table 4-1) has a tip with a flat width of 0.5 mm. The flat
area was increased to a width of 3 mm, and the modified punch produced satisfactory bends without
any cracks. This is a good example to show how the flat portion of the punch tip influences bending
work. Go to Top
4-4 R-Bending (radius bending)
4-4-1 Characteristics of R-bending
R-bending is a bending process in which the ratio of inside radius (ir) to sheet thickness (t) of the
product is large. As stated in subsection 4.3, the inside radius of a product can be obtained by
ordinary V-bending with 90' or acute-angled tooling. However, air bending and coining differ from
R-bending because of the small value of ir/t.
Generally, in R-bending where the value of ir is large, the amount of springback is also large. This
factor must be taken into consideration when radius bending.
Another factor that should be taken into consideration is the multi-breakage bending phenomenon
that sometimes occurs in R-bending, depending on bending conditions. These two factors, namely, a
large amount of springback and the occurrence of multi-breakage bends can characterize R-bending.
4-4-2 Springback in R-bending
It is necessary to know the amount of springback (AO) in advance in order to properly perform R-
bending. Table 4-5 shows the amount of springback of SPCC relative to the punch R. The table also
shows that springback AO increases with an increase in ir/t.
If you apply the requirements of t and ir provided in the drawing to this table, you will find that the
springback (AO) can be determined. For example, suppose a 1 mm thick sheet of SPCC is to be bent
to a shape with an inside radius of 20 mm. You can see that the springback (At)) is 9° and the punch
R (rp) is 18 mm.
In addition to the ratio of inside radius to sheet thickness, the tensile strength of the work and the
type of tooling also greatly influence springback. Generally, the amount of springback is
proportional to the tensile strength of the work. The springback values for SUS are 1.5 times the

39. values of Table 4-5 (i.e., for SPCC). Thus, judging from the tensile strength, it may appear that the
springback of
aluminum and copper is approximately 0.5 times as large. In fact, however, the springback of these
nonferrous materials is much greater than that of SPCC. This example clearly shows that there is a
need to experimentally confirm the amount of springback of each material.
Commonly, R-bending is performed by air bending, bottoming, or using urethane tooling, Table 4-5
tabulates the data on R-bending by air bending. If the amount of springback in air bending is taken as
1, the amount of springback in bottoming is half that of air bending, and that of urethane tooling is
2/3 as large.
[Exercise]
Calculate the springback AO when a I mm thick stainless steel sheet is bent to a shape with an inside
radius of 20 mm using urethane tooling.
4-4-3 Multi-breakage bending phenomenon
Multi-breakage bending is a phenomenon that occurs in R-bending. The cause of this phenomenon is
as follows:
In air bending, when a radius ruler and IV die are used in combination, a phenomenon occurs in
which the work separates from the R-punch tip in the course of bending, as shown in Fig. 4-6. This is
an advanced bending phenomenon in which the work bends in advance of the tool's motion. As
bending progresses with the application of bending pressure, the advanced bending spreads left and
right and consequently, a polygonal, angular R surface is formed. This is the multi-breakage bending
Table 4-5 Punch R and Springback of SPCC (Air bending)
ø: Amount of spring (degree)
ir: Inside R of product (mm)
rp: Punch R (mm)
t: Sheet thickness (mm)
t 10 15 20 25 30 35 40 45 50 55 60 ir
0.8
9.3 13.5 17.6 21.4 25.0 28.8 32.0 35.0 36.1 37.9 40.0 rp
6 9 11 13 15 16 18 20 25 28 30 ø
1.0
9.4 13.8 18.0 21.9 25.7 29.2 32.9 36.0 38.9 41.6 43.3 rp
5 7 9 11 13 15 16 18 20 22 25 ø
1.2
9.6 14.0 18.2 22.5 26.3 29.9 33.8 37.5 41.1 42.0 46.0 rp
4 6 8 9 11 13 14 15 16 18 20 ø
1.6
9.6 14.3 18.7 23.1 27.0 31.1 35.1 38.8 42.2 45.8 49.7 rp
3.5 4.5 6 7 9 10 11 12.5 14 15 15.5 ø
2.3
9.7 14.4 19.0 23.5 27.8 32.3 36.4 40.5 44.4 48.3 52.0 rp
2.5 3.5 4.5 5.5 6.5 7 8 9 0 11 12 ø
2.6
9.8 14.5 19.1 23.6 28.2 32.5 36.7 41.0 44.7 48.9 53.0 rp
2 3 4 5 5.5 6.5 7.5 8 9.5 10 10.5 ø
3.2
9.8 14.6 19.3 23.9 28.5 32.9 37.3 41.5 45.8 50.1 54 rp
2 2.5 3 4 4.5 5.5 6 7 7.5 8 9 ø

40. phenomenon.
Although multi-breakage bending occurs only in R-bending, it does not occur every time. The larger
the ratio of sheet thickness to the inside radius, the more readily it develops. Also, multi-breakage
bending occurs less frequently in materials with high tensile strength and ductility. In other words, it
is less likely to occur in stainless steel than in cold-rolled steel; and in SPCE than in SPCC.
In table 4-5, the bold line indicates the region in which multi-breakage bending occurs. As a
judgment standard, multi-breakage bending usually occurs when the ratio of sheet thickness to inside
radius is 30 or greater (in the case of SPCC). Accordingly, counter-measures must be taken if the
bending conditions fall within the above region. As already stated, multi-breakage bending is caused
by advanced bending of work that has separated from the punch tip. Thus, in order to prevent it, the
work should be kept in contact with the punch by applying counter-pressure from the bottom. Use of
a spring-loaded counter-holder type of tooling [see Fig. 4-7(c)i and an elastic body, such as a
urethane pad, is an effective means of preventing the work from multi-breakage bending.
4-4-4 Types of R-bending
Fig. 4-7 illustrates the types of R-bending generally used; (a) air bending type, (b) cam type, (c)
counter-holder type that utilizes a 1V die, (d) bottoming type using formed tooling, and (e) the type
using urethane tooling. This subsection will discuss the air bending and urethane tooling types of R-
bending.
(i)Air bending
Photo 4-1 shows the air bending type of R-bending being performed with a radius ruler (R punch)
and 1V die. In this type of R-bending, standard 85' die can be used if the amount of springback AØ
is 5° or less. In the case of SPCC with AØ=5°, ir/t is known to be approximately 10 from Table 4-5.
If this is applied to I mm thick SPCC, the inside radius will be 10 mm. If AØ is greater than 5
degrees, the V-groove relief must be modified so that the work can be bent to a smaller angle.
Because the actual V-width changes to the distance between points A and B (Fig. 4-8), the bent
portion of the work reaches the bottom of the die. Thus the work cannot be bent to an angle
absorbing AØ, or more than 5° in this example.

42. The V-width of the R-bending die is determined by the following formulas:
If AØ=50, V-2.2x(rp+t)
If AØ=50, V-2.5x(rp+t)
As is clearly seen from Table 4-5, the punch R should be smaller than the inside radius of the
product. When selecting a punch, it is common practice to select one with a standard R which is
closest to the inside radius of the product. The inside radius to which the product is finished with a
commercial radius ruler is calculated by the following formula. ir = rp x90/ (90-AØ)
In R-bent products, the dispersion of their inside radii is questioned more frequently than their
conformance with the absolute value of ir.Therefore, standard tooling should be utilized even if
design modification of the product is required. If this is not possible, a new punch and die must be
designed and produced. The amount of time required for delivery and the cost of the tooling must be
considered beforehand.
Because the hatched portion interferes with the work in R bending, it is advisable to cut it out of the
V-bending die.
Fig. 4-8 Modification of standard die
(ii) Urethane tooling
Recently, urethane tooling is being used more often for R-bending. The reasons why urethane
tooling is suitable for R-bending are: (1) the work is not scratched because no slippage occurs
between the work and die, (2) it can serve as a counter-holder, and (3) springback can be absorbed
by overforming (excess bending).
No. 8 in Table 4-1 shows combinations used in R-bending. Fig. 4-9 shows the shapes and sizes of
representative retainer boxes. The urethane pads, which are placed in their respective retainer box,
come in five sizes: 25x25 mm, 30X25 mm, 50X50 mm, 80X30 mm, and l00x45 mm. Some
operators attempt R-bending using only a urethane pad. This is because they don't understand the
principle of bending when using urethane pads, and such attempts will not produce satisfactory
results. The reader should remember that a urethane pad must be used with a retainer box in bending
operations.
When R-bending is to be performed using urethane pads, special attention should be paid to the
following points: (1) the tonnage required for the R-bending, (2) the size and hardness of the pad,
and (3) the bending shape. The paragraphs that follow will discuss these three points.

43. Table 4-6 tabulates the tonnage required for 90° R-bending under various conditions. The tonnage
required in R-bending using air bending tooling is usually very small because the V-width ratio to
sheet thickness is large. On the other hand, in R-bending with urethane tooling, the required tonnage
increases as the product's radius increases. The reason for this is that the required tonnage is
proportional to the volume of the urethane pad forced into the retainer box. Therefore, it is necessary
to determine whether the machine's capacity is sufficient to achieve the R-bending intended. In a
sense, one drawback of bending with a urethane pad is that it requires extremely great force.
A rectangle is more suitable than a square in regards to the sectional shape of the pad used in R-
bending. Table 4-7 shows applicable die pads relative to the punch R and sheet thickness. Further,
the two urethanes used in R-bending differ in hardness: "Rubber Shore A 80° "and "Rubber Shore A
90°." Use of a pad with Rubber Shore A 80° can reduce the tonnage required with a Rubber Shore A
90° pad by 20 percent.
One disadvantage of urethane tooling is that it is not very durable to sharp edges. If the edges of the
work (after shearing) come into contact with the upper surface of the pad while force is being
applied, the life of the pad will be shortened remarkably.
Also, the use of a retainer box makes urethane tooling unsuitable for bending shapes with short
flanges and a small step differences.
4-4-5 Summary of R-bending
The air bending type of tooling best serves R-bending. In order vo eliminate scarring caused by
slipping, a coated work material must be utilized. Otherwise, a urethane sheet should be spread on
the IV die. A certakn degree of multi-breakage bending is unavoidable with the air bending method.
Therefore, urethane tooling should be used if you don't want polygonal-bent products. In any case,
you should know the advantages and disadvantages of urethane tooling in order to use it properly.
Table 4-6 Required Tonnage for 90° R-Sending
(Rubber Shore A 90°)
Punch R (mm) 10 15 17.5 20 25
t (mm) (unit: ton)
0.4 13 16 18 21 26

44. Go to Top
4-5 Hemming
Hemming comes from the word "hem" which means to turn and sew down a border or edge of cloth.
It is one of the conventional bending techniques.
Hemming requires great bending force. Table 4-8 shows the tonnage required for hemming. As
indicated in the table, extreme force must be applied in the second stage of the work process, when
the folded edge is pressed. When selecting a machine for hemming operations, special consideration
should be given to ensure adequate tonnage.
A thrust load is produced during the hemming operation. The thrust load is a force acting in the fore-
and-aft direction of the machine. It exerts an adverse influence on the machine body. The thrust load
occurs in the crushing stage of hemming and increases proportionally with the required tonnage.
0.6 15 18 21 24 29
0.8 17 21 23 27 33
1.0 19 23 27 30 36
1.2 22 25 29 34 39
1.6 26 30 34 40 46
2.3 34 39 43 (50) (58)
2.6 37 43 (47) (55) (63)
3.2 46 (54) (60) - -
Note: SPCC - 80x30 mm urethane pad - No. 4 retainer box
Table 4-7 Die Pad Application (Rubber Shore A 90°)
Punch R (mm) 10 15 17.5 20 25 30
t (mm) (unit: mm)
0.4
0.6 Die pad 50x50
0.8 Die pad 80x30
1.0
1.2
1.6
2.3
2.6 Die pad 110x45
3.2
Note:
-Use 1 rank smaller t for Rubber Shore A 80° pad.
-Large pad includes the applicable rane of smaller pad.

45. Preventing the occurrence of a thrust load can be considered from two aspects: the type of tooling
and the working method.
As stated above, the characteristics of hemming are a high tonnage requirement and the occurrence
of a thrust load.
Hemming is performed in two stages; acute-angled bending is the first stage. Two types of tooling
are used. One type requires the use of two sets of tools, one for 30' bending in the first stage and one
for flattening in the second stage. The other is a double-deck type that enables hemming without
tooling replacement. The type that requires tooling replacement is further divided into two types for
light and heavy work according to the sheet thickness of the work.
4-5-1 Type requiring tooling replacement
There are two tooling combinations for this type: a combination of No. 9 or No. 12 (both for the first
stage) and No. 14 (for the second stage), and a combination of No. 10 or No. 11 (the first stage) and
No. 15 (the second stage). The former can bend a maximum sheet thickness of 2 mm, and the latter
can bend sheets from 2.3 to 4.5 mm in thickness.
Material: SPCC, SS41
Sheet thickness
t (mm)
Bending shape
Required tonnage C Required tonnage 2t
0.6 9 3 23 1.2
0.8 12 3 32 1.6
1.0 15 3.5 40 2.0
1.2 17 3.5 50 2.4
1.6 22 4.6 63 3.2
2.0 30 5.5 80 4.0
2.6 55 6.5 90 5.2
3.2 70 8.0 100 6.4
*4.5 105 11.3 200 9.0
Material: SUS
Sheet thickness
t (mm)
Bending shape
Required tonnage C Required tonnage 2t
0.6 15 3 35 1.2
0.8 20 3 50 1.6
1.0 25 3.5 60 2.0
1.2 26 3.5 80 2.4

46. The tooling discussed above is widely used because it is simple and easy. However, after the first
stage, the partially finished products must be stacked near the machine and the second stage
performed after tooling replacement. Therefore, use of this tooling requires an extensive working
space and often stagnates production flow.
In order to prevent the undesirable effects of the thrust load, hemming must be performed with a
small bending force. Press the work by slowly applying a smaller force in degrees while movkng the
work back and forth. Then use the proper force. The type of tooling (combination of No. 9 or No. 12
and No. 14) mentioned above requires that bending force be applied in several steps.
The die used with No. 15 in Table 4-1 is a sliding-type hemming die. The top of the die slides
backward to absorb the thrust load. This type of tooling permits the hemming of SPCC sheets which
have a thickness of 2.3 to 4.5 mm.
According to Table 4-8, heavy spacers must always be used if the required tonnage per meter
exceeds 100 tons. They protect the upper beam face and the face of the punch. However, when
performing a bending operation in which the required tonnage exceeds 100 tons per meter, special
consideration must be taken.
4-5-2 Double-deck type
This bending technique uses double-deck type No. 103 punch and Table 4-1. The tooling
combination consists of the he die selection the No. 104 or No. 1046 die (see Photo 5-9). The die
selection depends upon the sheet thickness of work and the hemming size. The double-deck type is
applicable to a sheet thickness range of 0.6 to 1.6 mm (t=1.2 mm at maximum for SUS). Sheets
having a thickness excess of this range must be formed by using the tooling described in subsection
4.5.1.
This type of tooling is advantageous because 30º bending and crushing can be achieved continuously
without any extra semi-finished products. Go to Top
4-6 Bending Sequence
Bending work must be performed according to a predetermined bending sequence. The bending
sequence determines whether or not products are properly formed. It also makes bending work more
difficult, by causing an increase in tooling types and attachments (especially backgauges). The
requirements of precision and job efficiency, however, dictate that a proper bending sequence be
used.
Repeated practice is required to be able to quickly determine the bending sequence. As in the saying
"Practice makes perfect," one can learn the various bending sequences from repeated practice,
enabling one to handle more complicated bending sequences.
Next, we will briefly describe the procedure for determining the bending sequence, the tools
available for determining the bending sequence, and a new technique of drawing using automatic
programming.
1.5 38 4.6 95 3.0
2.0 50 5.5 130 4.0
*2.5 90 6.5 180 5.0
*3.0 100 8.0 210 6.0
*4.0 140 11.3 280 8.0
* Use a heavy spacer for sheets marked with an asterisk.

47. 4-6-1 Procedure for determining bending sequence
The procedure for determining the bending sequence involves investigation of the following:
1. The bending shape required
2. The first and final stages of bending
3. Important dimensions of the product
4. Efficiency of the work handling
5. The relationship between bending angle and backgauge position
6. Examination of "pre-bending"
In general, the bending sequence is determined by processing some of the above items together.
4-6-2 Use of tools
Determination of bending sequence using only brain work is inefficient and often ends in failure.
Since the bending sequence is not theoretical but concrete, it can be determined best by using various
tools.
The major tools include (1) a graph of return bend limits (Fig. 4-3), (2) wire (thin copper wire is
preferable), (3) a profile model of the tooling (Photo 4-2), (4) a full-size drawing of the punch and
die, and (5) an attachment plan of the tooling.
When determining the bending sequence, it is a common practice to investigate the factors listed
above using these tools. It is possible to decide which type of tooling should be used after the
bending sequence has been determined- There are three types of tooling: a standard type, a standard
type requiring modification, and a special type that must be designed and manufactured.
4-6-3 Automatic programming
Automatic programming is the newest technique which
determines the necessary tooling. Those items described in
subsections 4.6.1 are programmed. Upon inputting the required
shape, a suitable type of tooling is indicated. Then the bending
sequence with the designated tooling and the figure to be bent
is plotted. In addition to selecting tooling and determining the
bending sequence, automatic programming can output various
data. Examples of data are the NC instructions (L value, D
value, pullback value, and stopper position) , work handling
(turning of the work), tact time (time required for bending a
product), and weight calculations.
Automatic programming is very convenient but it is not always
perfect and trouble free. For instance, the bending sequence
determined by automatic programming is not necessarily an
optimum working order [(3), (4), 1 and (5) in item 4.6.1 may
be problems] because it can only be determined after many

48. factors have been analyzed and unified. Go to Top
4-7 Guaranteed Allowable Tonnage of Tooling
The guaranteed allowable tonnage is usually indicated by the amount of tonnage that can be applied
per unit length to the tooling. The tooling should be operated under a pressure less than the allowable
tonnage.
Table 4-9 lists the guaranteed allowable tonnage of standard tooling, Prior to operation, the operator
should know the guaranteed allowable tonnage of the tooling in order to prevent accidents and insure
a long service life. Applying a bending force which exceeds the allowable tonnage is dangerous for
both the operator and machine because the excessive tonnage damages the tooling and sometimes
breaks or bends it.
Adequate bending force must be applied when bending a work-piece. This means that pressure
adjustments must be made in accordance with the respective bending conditions (work material,
work thickness, bending length, tooling allowable tonnage, V-width, etc.). most tooling breakage
results front failure to observe this principle.
Go to Top
Table 4-9 Guaranteed Allowable Tonnage of Standard Tooling
Punch Die
Punch No. ton/m Die ton/m
4, 16 100 121 60
147, 148 45 123 70
116 20 124 80
45, 46 50 125, 126 100
50,51 45 127 70
452, 462 70 301 60
150, 151 45 302, 303 70
453, 463 50 304, 305, 306 100
155, 156 30 13, 14, 18, 35 100
108, 109 50 5, 36, 37, 38, 39 100
200, 201 100 337 30
103, 210 100 340 80
8 30 341 60
49 20 342 40
215 100 343 70
3 100 320, 321, 322, 323 95
230, 231 100 324, 325, 326 100
220 100 12 100
311 80
314 100
104, 1046 65
Note: Apply the value for the punch or the die, whichever is smaller.

49. 5 LABOR-SAVING BENDING TOOLS
This chapter compliments chapters 3 and 4, which outline the fundamentals of standard tooling. It
describes some special tooling and discusses this tooling from the viewpoint of saving labor. Note
that the title "labor-saving bending tools" is adopted for convenience and is not a general term.
Three aspects of labor economization in sheet metal bending must be investigated: the machine, the
peripherals, and the bending tools. More detailed investigations in this field will eventually lead to
the development of a production system of compound lines. This type of investigation, however, is
not a future prospect; but there is an urgent need for it.
5-1 Objective of Labor Economy
How can we economize on labor in sheet metal bending work? What are the objectives of labor
economy? Here we will consider labor-saving techniques. Overall, the objectives of labor economy
depend on the needs which arise on the job site. Hints for labor economy are hidden in the
difficulties and problems that operators encounter daily .
In this subsection, the following seven items will be discussed from the viewpoint of labor economy:
(1) elimination of spring-up, (2) reducing the number of processes, (3) increasing tooling versatility,
(4) making material handling easy, (5) reducing man-hours to replace tooling, (6) bending coated
workpieces, and (7) elimination of the need for welding.
5-1-1 Elimination of problem caused by spring-up
When bending small flanges on large work (such as switchboard doors, console boxes, partition
boards, and steel doors), "buckling" occurs when the work springs up. Buckling is a phenomenon in
which the work bends slightly due to its own weight in a portion off the V-groove of the die.
In addition to buckling, the work springs up during bending and drops after completion of bending.
This is dangerous for the operators. It takes several operators to perform this operation safety and it
is a very fatiguing job.
Bending with a press brake is a type of spring-up bending (spring-up also is a problem when
planning automated bending).
Bending is divided into spring-up bending and folding. Folding is suitable for the above examples of
product bending. A folding machine is a special machine designed specifically for folding work. Fig.
5-1(a) illustrates spring-up bending and 5-1(b) folding.
A sheet follower/support system (Photo 5-1) provides a means of eliminating problems caused by
spring-up. There are several types of sheet follower/support systems that correspond to the weight of
the work. Each permits the operator to easily and quickly produce accurate products.
Labor-Saving Bending Tools
Objective of Labor Economy
Types of Labor-Saving Tool
Points of Tooling Design
Table of Contents

50. As stated above, problems caused by spring-up can be eliminated by using a folding machine or
sheet follower. Also, spring-up can be reduced significantly by the use of special dies. There are two
types of tooling design. One type produces no spring-up and the other type produces a slight spring-
up (refer to item 5.2.1, "Spring-up-preventive tooling")
5-1-2 Reducing the number of processes
The shapes of bent products are becoming more and more complicated. This is primarily due to the
existence of NC press brakes which permit easy setting of the backgauge dimension and bend angle.
At the same time, there is a growing tendency to take positive attitude, "Let's bend everything." This
is a desirable trend because the experience gained by bending complicated shapes is expected to help
eliminate or reduce the need for welding. Bending is definitely changing due to the advent of NC.
Hopefully, the number of processes can be reduced to one, bending shapes that now require two or
more bending processes. However, considering the prevailing NC applications, it does not seem as
advantageous as before to design special tooling (For example, tooling that forms a hat shape
requiring four bends in one process). In this case, adopting special hat tooling should be avoided in
consideration of dimensional changes. The following are three common characteristics of special
tooling:
1. Special tooling is designed to serve a specific purpose; therefore, it is not versatile
enough to cope with dimensional changes. This means that new tooling must be
designed and produced each time a change in product dimensions 'is desired.
2. Special tooling is expensive and delivery takes a long time in comparison with
standard types.
3. The tonnage required for bending with special tooling is greater than that with
standard tooling.

51. Accordingly, special tooling should be employed for shapes that cannot be formed by V-
bending and for mass-production goods. It is not recommended for the sole purpose of
reducing the number of bending processes.
5-1-3 Increasing tooling versatility
Boxes and panels consisting of two short and long sides are considered identical bending patterns,
even though they differ in size. The purpose of increasing tooling versatility is to cope with various
product sizes by making adequate combinations of punches and dies. Whether or not tooling
versatility can be increased is dependent upon product standardization (keeping the variety of sheet
thickness and sizes of products to a minimum).
5-1-4 Making material handling easy
Bending work involves "turning," "reversing," and "holding" of the material. During loading and
unloading operations, the material is handled either manually or by some mechanical means. A
loading device, unloading device, turning table, sheet follower, reversing table, and robot are utilized
in handling bending work.
An effective means of using tooling more efficiently is to eliminate the necessity of reversing the
work. An example is shown in Fig. 5-2. Continuous wavy folds can be formed without reversing the
work. This is done by changing the conditions of the punch and die. It is best suited for forming
shapes with long bending lengths or hard-to-handle thin sheets.
5-1-5 Reducing man-hours to replace tooling
Labor economization in bending work refers to making material handling easy and reducing man-
hours required for tooling replacement. Tooling is changed 20 times a day at most and 10 times on
an average. The number of man-hours for tooling replacement is expected to increase in the future. A
flexible manufacturing system (FMS) must be both "adaptable" and "versatile." A versatile
manufacturing system is another name for a many-type, small-quantity production system.
Production efficiency in such a system depends upon how quickly and frequently the tooling can be
changed daily. In this sense, the FMS or a many-type, small-quantity production system is closely
related to sheet metal processing.
In order to accomplish as many daily tooling replacements as possible, it is necessary to reduce to a
minimum the man-hours required per change of tooling (in other words, challenging to reduce the
man-hours required for each tooling replacement to zero). An automatic tool changer (ATC) and a
one-touch tooling clamp serve this purpose.

52. The automatic tool changer can accommodate several types of punches and dies (the tool changer
shown in Photo 5-2 accommodates three types of punches and four types of dies). This tool changer,
when interlocked with NC, can reduce the number of man-hours for bending if the product shape is
within the applicable range of the tooling. An interesting case has been reported where the
production time was shortened from 9 to 2.5 minutes per panel in the bending of industrial
refrigerator panels. There are two types of ATC: (1) one is designed to cope with changes in product
shape and (2) the other is designed to cope with changes in length. Although type (1) prevails at
present, a new tool changer which would provide both functions is desired. ATC can keep pace with
the requirements of the FMS age only if it can accommodate changes in the shape and size of the
product. For information, the elimination of work reversal described above can be effected by using
type (1).
The one-touch clamps for punches and dies help reduce the man-hours required per tooling change.
The clamps are mechanisms that cope with "single job preparation." In the future, it is desired that
such clamps be provided as attachments to standard machines.
Deck-type tooling is designed to reduce the number of tooling changes. This type of tooling includes
the double-deck type for hemming and a triple-deck type used to bend the side panels of
refrigerators. Use of deck-type tooling together with NC functions (control of L-axis, D-axis,
bending speed, etc.) will enhance the economy of labor and facilities.
5-1-6 Bending coated workpieces
Usually, sheet metal fabrication is performed in the following sequence:
Shearing => Punching => Bending => Welding and assembling =>
Painting
Some changes in this sequence have been noted with the increased use of coated sheets such as
vinyl-chloride steel sheets and colored iron plates.
In comparison to the conventional production process, a considerable reduction in man-hours can be
obtained by the shearing, punching, and bending of coated steel sheets. The difference in total man-
hours is determined from the conditional difference between the case where cubic products formed
by bending are painted and the case where commercially coated materials are formed. (Of course,
the man-hours required for painting is zero in this case.)
As stated above, there is a great advantage in painting prior to bending because the man-hours
required for painting can be reduced. Another advantage is that it will most likely reduce the cost of
the painting facilities.

53. However, the use of coated steel sheets or painting prior to bending causes the following problems:
(1) The need to prevent marring (scoring, scars made by slipping, creases, etc.) and (2) Development
of a manufacturing method that supersedes welding. Presently, urethane tooling and seaming tools
are utilized as countermeasures against (1) and (2).
5-1-7 Elimination of welding process
Welding is necessary in sheet metal fabrication. Since welding distortions occur, many man-hours
are required to smooth them. A workshop campaign, to get rid of "three evils" is carried out in many
factories. The "three evils" refer to (1) marking-off, (2) hammering, and (3) finishing with a sander.
Needless to say, hammering and finishing with a sander is the post-process of welding.
Reducing the man-hours required for welding, smoothing, and finishing can be an effective means of
saving labor in sheet metal processing. The following are useful methods to achieve the above:
1. Improvement in bending accuracy
2. Using bending instead of welding as much as possible
3. Utilization of seaming and locking
4. Use of fasteners and adhesive
5. Altering product design so welding can be eliminated
Next, we will introduce a couple examples of boxes built without welding.
Photo 5-3 shows a box made of I mm thick vinyl-chloride steel without welding. In this box, welding
is replaced by caulking, fasteners are substituted for hinges, and a new device is used for the door
stopper.
Photo 5-4 is a switch box made by bending only. It becomes a complete product simply by screwing
the bottom plate. Both are the direct result of designs created to eliminate the welding process.
Go to Top
5-2 Types of Labor-Saving Tooling
In subsection 5.1, we outlined seven methods of saving labor in sheet metal processing. We have
provided customers with a great number of tools designed to fit individual needs, but they have not
been satisfactory in all respects. This merely confirms that the problems involved in bending tools
are complicated and extensive. The reader should remember this when reading this subsection. The
following pages describe types of tooling designed and manufactured for the purpose of "labor
economy."
5-2-1 Spring-up-preventive tooling
This bending tool can be divided into two types: the first produces no spring-up and the other
produces slight spring-up.

54. (i) Tooling producing no spring-up
The tooling used in L-bending, D-bending, and Z-bending are of this type. Each tool can bend in one
process and is usable with a single machine or in an integrated production system.
a) Tooling for L-bending
There are two types of L-bending tooling: a counter-holder type and a cam type.
When the counterholder type is used, the work is formed by "wiping" after being clamped. Thus,
accurate, uniform-quality products can be obtained. If a punch with a relief is used as shown in Fig.
5-3, it is possible to form ] " shapes in two stages. When bending box-like shapes such as panels, the
punch may be divided into any optional length. The maximum bendable sheet thickness are 2.3 mm
for SPCC and 2 mm for SUS. The obtainable inside radius of the product is 1.2 to 1.5 times the sheet
thickness. Replacing the shoulder piece allows limited processing of sheets of a different thickness.
The counterholder prevents the work from swelling or sliding by providing counterpressure. The
cam tooling in Fig. 5-4 is a folding type. This bending method features light or little marring; unlike
wiping. Furthermore, bending angle precision is easily obtained because the cam permits
overforming (over-bending to compensate for springback). On the other hand, in comparison to the
counterholder type, the inside radius of the producv is somewhat large (about two times sheet
thickness). Also, there is a tendency for the flat surface to swell. Cam tooling requires a number of
manufacturing processes; therefore, the tooling cost is high. The sheet thickness which can be used
with the cam type tooling are the same as with the counterholder type.
(b)Tooling for ] bending
]-shapes can be formed in two processes by using L-bending tooling (either counterholder or cam
type). The ] bending tool (Fig. 5-5) is capable of forming :]-shapes in one process. The tooling
contains a cam mechanism in its counterholder, so it can also perform Z-bending, depending on the
usage of the cam.
When forming shelf sheets, the punch is divided into suitable lengths and ears (horns) are attached at
both ends to prevent interference with the flange.

55. The ] bending tool is capable of bending work that is held horizontal. Thus, a production line which
incorporates several press brakes equipped with this tooling and material loaders and autoloaders can
provide continuous unmanned operation. Also, even with a single machine, this tooling satisfactorily
serves as a labor-saving device in general bending operations.
c)Tooling for Z-bending
There are two types of Z-bending tools: a single-hat type and a type similar to the counterholder type
with a cam mentioned in the previous paragraph.
The single-hat type (Fig. 5-7) features low cost and is high in utility value. The tooling in Fig. 5-7 is
designed to bend the short sides (at a fixed length) after the long sides, enabling it to handle
long sides of any desired length.

56. (ii) Tooling producing slight spring-up
The tooling used for beading is of this type. The principal sectional shapes after processing are
semicircular, triangular, quadrilateral, trapezoidal, etc. The quadrilateral shape is called a hat bend.
Beading is effective in fabricating containers, structures, and reinforcements because a flat sheet
increases in strength when beaded.
There are two types of beading tools: the air bending type and the counterholder type.
Fig. 5-8 shows a counterholder (trapezoid) type tool used for forming containers, This tooling is
suitable for processing a large lot of products requiring uniform precision, and also for use in a
production line. The tooling is profitable though high in cost.
An automated production line for containers is illustrated in Fig. 5-9. This is an unmanned
production line consisting of a feeder, loading device, press brake with backgauges, feed device, two
shearing machines, and an inverting stacker.

57. The beading tool (Fig. 5-8) and the backgauge which determine the bead pitch, are set in the press
brake. The work carried by the feed conveyor runs against the first stopper and stops. Then the beam
rises under the control of the current signal passing through the work.
After the first bending stage has been completed, the beam descends, the conveyor operates, and the
work moves until it strikes the second stopper. In this way, the work advances to the fifth stopper
where the final stage of forming is completed. The work can be made to pass any of the five stoppers
by giving instructions. Then, the work is forwarded and trimmed at both ends by shearing machines.
Finally, it is turned over and stacked. Checking the pitch size completes the entire process of this
line.
5-2-2 Tooling that reduces number of processes
U-bending, offset bending, or beading tooling is useful in reducing the number of processes. Many
of these shapes, however, can be formed by the multi-stage V-bending method. As already noted, if
V-bending with NC can be used, there is no need to provide U-bending, offset bending, or beading
tooling. These tools are best suited for forming (1) a large number of products requiring a uniform
finish and (2) shapes that cannot utilize the multi-stage V-bending method.
(i) Tooling for U-bending
U-bending is a process whereby work is bent into a U-shape in one stage. The same U-shape can be
obtained in two stages by using V-bending tooling but this is not called U-bending. U-bending
tooling reduces the two stages required in V-bending to one stage. It can also form a U-shape which
cannot be accomplished by V-bending; for instance, a shape whose flange is long in comparison to
its width.
The tooling used in U-bending includes a wedge type, roller type, counterholder type, cam type, and
types using urethane. The proper tooling should be selected considering (1) parallelism, (2) bottom
finish, (3) friction marring, and (4) tooling cost (in relation to the number of products).

58. a) Wedge type (see Fig. 5-10)
Wedge type tooling is characterized by the following: (1) it is widely used regardless of the number
of products, (2) the bottom is almost flat or concave and the degree of parallelism is satisfactory, and
(3) it is inexpensive. Therefore, it is widely used as an economical tooling.
b) Roller type (see Fig. 5-11)
Of all U-bending tooling, the roller type is the simplest and lowest in cost. It features: (1) Low
friction marring, (2) the bottom is almost flat (though sometimes convex or concave), (3) the degree
of parallelism is good. Roller type tooling can bend a maximum sheet thickness of 6 mm.
c) Counterholder type
The counterholder type (Fig. 5-12) provides a high level of flatness and parallelism. It is suitable for
the mass production of products requiring uniform, high precision. The tooling cost is high but it is
sufficiently profitable if the number of products is large.
d)Cam type (see Fig. 5-13)
The cam type of tooling contains cams. The work can be overformed (bent excessively in

59. anticipation of springback) by means of these cams. Thus, post-forming parallelism is quite
satisfactory. Further, the degree of flatness is relatively good. However, this tooling is comparatively
expensive because its manufacturing process requires too many man-hours. If sheet thickness is 3.2
mm or greater and the machine is lacking in stroke, this cam type should be used to yield good
results.
e) Urethane tooling (Photo 5-5)
U-bending with urethane tooling provides results similar to those of the counterholder type of metal
die. Urethane tooling will (1) prevent die adhesion, and (2)"assure flange parallelism. With the
exception of bottom flatness which tends to swell, the use of urethane yields satisfactory results in
respecv to (1) and (2) above. It is also advantageous because it can form products without scratches.
In addition, the tooling is inexpensive.
U-bending with urethane tooling Step bending/Spring-up
(ii) Offset bending tooling
Offset bending is a process whereby work is formed into a Z-Iike shape in one stage. The same
shape can be obtained in two stages by using V-bending, but offset bending tooling eliminates one
stage. A shape that has a step difference equal to the sheet thickness cannot be formed by V-bending.
The tooling used in offset bending is roughly divided into spring-up and horizontal types. The
spring-up type allows the work to spring up during bending. Three types of tooling used are: a
balanced type, an unbalanced type, and a versatile type. The horizontal type, which is generally
called a non-spring-up type, is used in bending offset shapes and is suitable for bending the ends of
large sheets.

60. a) Spring-up type (Fig. 5-14)
This is the most commonly used type of offset bending. It is capable of bending shapes with step
differences 3 to 10 times sheet thickness. This type of bending tool can be either a balanced type or
an unbalanced type (the 90° punch tip angle is not equally divided into 45° angles, as shown in Fig.
5-14). The unbalanced type tooling produces a small thrust load but is relatively expensive.
b) Horizontal type (Photo 5-6)
This type is best suited to bending offset shapes in which the step difference is equal to the sheet
thickness; or forming where the step difference is less than 3 times the sheet thickness. Since this
tooling is simpler than the spring-up type, its mechanical processing and hardening is easier and
therefore less expensive.
(iii) Beading type
For details concerning beading tooling, refer to (ii) "Tooling producing slight spring-up" in the
description regarding spring-up-preventive tooling in item 5.2.1.
5-2-3 Versatile tooling
The versatile type tooling can form products of the same pattern in different sizes using a punch and
die combination. The sectionalized punch, sectionalized die and offset bending tools are examples of
this type of tooling.
(i) Sectionalized punch
As previously explained in item 3.6.1, the sectionalized punch is used when forming boxes with both
short and long sides. It used to be common practice to use a punch cut to the required dimensions of
the product. This practice has been replaced by using a combination of punches of different lengths.
Sectionalized punches are designed to
handle dimensional changes by means of a sectionalized punch combination (see Photo 3-3).
(ii) Sectionalized die
As shown in Photo 5-7, the die must be cut each time to conform to the inside dimensions when a
workpiece having opposing flanges is bent. The sectionalized die is designed to handle the necessary
dimensions by means of sectionalized die combination (refer to item 3.6.2, "Sectionalized die").
(iii) Offset bending tooling (Fig. 5-15)
The versatile type tooling can produce several kinds of step differences by using a punch and die
combination. The special-use type should be used if the number of products is large. If the number of
products is small and includes several kinds of step differences, the versatile type will be much more
effective.

61. 5-2-4 Tooling that makes material handling easy
Accordion bending tooling and the punch-die replacement type eliminate the need for reversing
work during bending. The punch-die replacement type can be built into an automatic tool changer
(refer to Fig. 5-2, "Elimination of work reversal").
(i) Accordion bending (Fig. 5-16)
Accordion bending is a process whereby consecutive bends of various shapes, such as a hat, U-shape
with R, and triangle, are formed. There are two types of accordion bending tools: the nonreversible
type and the reversible type. Nonreversible bending can be accomplished with one set of tools.
However, reverse bending requires separate tooling for the preliminary stage (and occasionally for a
stage prior to the preliminary stage) as well as for the following stage. Many of the work materials
used in accordion bending are long and narrow and it takes many man-hours to reverse the work.
This can fatigue and frustrate the operator. Therefore, it is desirable to eliminate the reversing
operation. Fig. 5-16 shows an example of nonreversible type tooling.
Working conditions
Product Radiating plates for console boxes
Material Galvanized iron sheet
Sheet thickness 0.3 mm

62. (ii) Hinged-type sectionalized punch (Photo 5-8)
Flanged shapes are formed by bending the short sides in the initial stage and then the long sides.
After bending the long sides, the work must be removed from the punch by moving it backwards and
tilting it slightly. This requires time and reduces work efficiency. As shown in Photo 5-8, the hinged-
type tooling has a hinge at both ends so they can be moved easily. This arrangement enables the
flange to be released without moving the work and considerably reduces the number of stages.
Dimensional changes in the long or short side can be dealt with by means of a combination of
various punch lengths.
5-2-5 Tooling that eliminates tool change
The combined forming tooling is capable of forming two or three kinds of bends without tooling
replacement. One type with an upper and lower working position is called double-deck (two-stage)
type. Another design with three working positions is called triple-deck type.
(i) Double-deck type (Photo 5-9)
This type of tooling performs 30° bending with the upper stage and hemming with the lower stage. It
saves working space because work need not be piled up when shifting from the first to second stage.
Furthermore, it saves a considerable amount of loading and unloading time and also assures smooth
production flow.
Length 915 mm
Required tonnage 20 tons

63. Fig. 5-17 illustrates a triple-deck type tooling with three working positions. This tooling performs
four stages of bending in each position: (1) step bending, (2) hemming, (3) 90° foldback bending,
and (4) simple 90° bending.
Generally, the bending work shown above is performed using five machines and ten operators
positioned in a line. However, use of the special tooling, coupled with a numerical-control machine,
enables two operators to accomplish the same job, resulting in considerable savings of labor and
equipment.
5-2-6 Tooling that produces no marring
The utilization of urethane tooling is probably the only available means of eliminating marring.
Unbalanced ] bending with urethane in Fig. 5-18 is an example of making the most of urethane
tooling. When the appearance of the product is a prime consideration, use vinyl-coated work or bend
the work with a sheet placed between the die and work. Usually, a two-step method which uses
different tooling for each step is employed. The advantages of urethane tooling can be summarized
as follows:
1. It absorbs thrust load caused by unbalanced bending.
2. It functions as a counterholder in R-bending and U-bending to prevent advanced
bending.
3. It can accomplish overforming which takes the place of the cam type tooling.
4. A good-looking product can be obtained because no marring and scratching
occurs during operation.

64. 5-2-7 Technique which eliminates welding
Seaming, which is similar to hemming, is a useful bending technique which
eliminates the need to weld. In this process, two sheets or members are
joined together without riveting or welding (welding or screw clamping may
occasionally be used). Seaming is also called locking. The Pittsburgh lock
used in the U.S.A. is an example of seaming (see Fig. 5-19). The tooling
utilized in seaming is either a wedge or a crushing type.
The wedge type shown in Fig. 5-20 is a single type with a wedge on one side. A double type with
wedges on both sides is another wedge type. Seaming requires that the work be formed into an easy-
to-seam shape in the preliminary stage, as shown in Fig. 5-21.
(ii) Crushing type
Fig. 5-22 shows an example of a crushing type of seaming tool. A core must be used to seam such
hollow shapes.

65. Go to Top
5-3 Points of Tooling Design
In subsections 5.1 and 5.2 above, we described the objectives of labor saving and labor-saving
tooling respectively. Now we will describe the main aspects of tooling. Because of limited space, we
will discuss only some of the tooling described in subsection 5.2, namely, the L-bending type,
accordion bending type, seaming type, and urethane tooling. For further details, the reader should
refer to "Bending Technique" published by Amada Sheet Metal Working Research Association.
5-3-1 L-bending tooling
The clearance rate of the work to the sheet thickness, the depth of the bend, the punch shoulder R,
and the die shoulder R are major points in tooling design.
It is not advisable to initially work the punch shoulder R to the prescribed R. Failure can be avoided
if it is initially worked to about R=0.5 mm and then gradually increased through trials.
The most common problems encountered with L-bending tooling are friction marring and the
inability to obtain the proper bending angle of 90'. Marring is affected by the size of the die shoulder
R, and the bending angle is influenced by the clearance and punch shoulder R. A frequently used
method of decreasing marring is to increase the die shoulder R. Even though an obtuse angle is
formed with L-bending tooling, possible solutions are to decrease either the clearance or punch
shoulder R; or increase the bending force.
5-3-2 Accordion bending tooling
The open height and stroke of the press brake determines whether or not nonreversible type tooling
requiring no preliminary bending can be utilized. The stroke length must be greater than 2.2 times
the height of the product.
The punch angle or die groove angle (angle 0 in Fig. 5-16) should be the maximum angle in the
range where it absorbs springback. Also, with regard to the punch shoulder and die shoulder, the key
point in the design of this tooling is to keep the edge. This is because the desired shape is obtained
by producing a spring-go effect with the edge.
When forming accordion-bent products, an accurate pitch gauge is required for determining the
pitch. The pitch gauge should be structurally designed to suit the shape of each product. Further,
consideration should be given to combining the use of a pitch gauge and side gauge to enhance

66. effectiveness.
For your reference, the maximum sheet thickness this tooling is 0-5 mm (material: galvanized iron
rlate). it is essential that the height and width of the product be increased in proportion to the
thickness when bending sheets thicker than the"maximum limit.
5-3-3 Seaming tooling
Fig. 5-22 shows the tooling used for seaming an angular copper flume. This tooling incorporates
several devices and has a wide range of applications despite its being a special-use die. A total of six
preliminary stages are required: two stages of 30' bending and four stages of 90° bending. Seaming is
performed by crushing as shown in the above figure.
When seaming hollow shapes like this product, the following should be considered in the tooling
design: (1) use of a core, (2) a structure in which the product is extracted by moving the core, and (3)
ensuring smooth extraction of the product.
Next, we will briefly describe the core and die which are major components of this tooling. As
shown in Fig. 5-23, the following parts are installed to the core; a hinge pin hole used to turn the
core; a slide needle bearing which supports the force that lifts the dead weight of the core; a stopper
for positioning; a leaf spring to hold the product against the stopper; and an ejector (Fig. 5-22) for
product ejection. The stopper and ejector are both actuated by a counterspring to facilitate product
extraction.
The block-shaped die contains a core stopper, a coil spring that floats the core stopper and hinge pin,
and a block that forms a bearing sliding surface. The surfaces of the die and core which come into
contact with the work must have a ground finish when working with easily marred products
5-3-4 Urethane tooling
At first, urethane was used as a replacement for rubber to protect the work. However, as research
progressed, it was discovered that urethane outperforms conventional metal dies in some of the
difficult bending operations. Therefore, future bending operations cannot be discussed without
considering urethane. The reason urethane is referred to as "urethane tooling" is based on this
significance. The advantages and disadvantages of urethane tooling are as follows:
1. (i) Advantages of urethane tooling
(1) Work preparation time is reduced because tool alignment is not required.
(2) Minimal springback occurs.
(3) Alterations are not necessary when the bending angle or work thickness changes.
(4) It absorbs side thrust.
(5) It may also be used as a stripper or spring, or to hold the work.
(6) It does not scratch or mar the work.
(7) Overforming can be performed.

67. (8) It functions as a counterholder that prevents advanced bending.
(9) It possesses excellent physical properties for easier forming.
(10) Special operations such as drawing, embossing and bulging can be accomplished.
2. (ii) Disadvantages of urethane tooling
(1) The tonnage required is several times greater than that for metal dies.
(2) It is not durable to edges; therefore, it is not suitable for products with short flanges.
(3) It has a short service life in comparison to a metal die.
(4) There is a limitation on bendable sheet thickness.
(5) It is not as accurate as a metal die in partial bending.
(6) There is a limitation on bendable work shapes.
(7) Cost is somewhat high because a retainer box is required.
Go To Top

68. PROBLEMS IN SHEETMETAL BENDING
Problems will always occur in sheetmetal bending because they are individually or intricately
related to the machine, tooling, work material, welding, etc. It would take a great amount of time to
describe all the problems; therefore, we will only discuss some which are directly related to the
tooling.
6-1 Dispersion of Bending Angles
Most of the complaints we receive concern the dispersion of bending angles. The dispersion of
bending angles is classified as either "tendencious" or "non-tendencious". "Middle bulging" (a
phenomenon in which the bent angle is larger in the middle than at either end) is a tendencious
instance, which is greatly influenced by the quality of the machine. The dispersion of bending angles
caused by variations in material quality and different lengths of work falls within this group, even
though they show different tendencies.
The bending angle is greatly affected by qualitative variations in the work (variations in sheet
thickness and internal structure). Since these are not tendencious, they belong to the latter non-
tendencious group.
Needless to say, tooling manufacturers guarantee the bending angles of their tooling as having
synthetic precision; regardless of whether the dispersion is tendencious or non-tendencious.
Generally, the angular tolerance that manufacturers guarantee is +/-30', though it depends upon the
kind of bending operation. Accordingly, it is desirable that precision be maintained at +/-30' at all
times. This is necessary to enhance proper maintenance of the machine and tooling.
Deformation and wear of the bending tool adversely affects the maintenance of bending precision.
With regard to tooling deformation, care must be taken properly to adjust the pressure setting of the
machine and consideration must be given to the tonnage tolerance of the tooling. If the tooling is
deformed, there is no choice other than to partially cut it or discard it. Worn tooling can be refinished
by grinding, but special precautions must be observed. Go to Top
6-2 Marring
The work can be marred by slippage or scraping at the tooling joints during bending. Marring is a
serious problem with stainless steel, aluminum, vinyl-coated steel, and painted steel sheets which are
easily scratched. Easily scratched material is usually coated with vinyl before bending; however, the
vinyl coating sometimes breaks, leaving scratches on the base metal.
The shoulder R of the die V-groove is closely related to scratches made by slippage. If other working
conditions (such as the size of the step difference) are ignored, scratches made by slippage become
shallower as the shoulder R increases. It is recommended that dies with a large shoulder R be used.
Problems In Sheetmetal Bending
Dispersion of Bending Angle
Marring
Cracking
Warping
Table of Contents

69. They are available from some manufacturers.
When such a die cannot be used, it is advisable to use the die in combination with a urethane sheet.
Select a urethane sheet of adequate thickness in accordance with the work sheet thickness and place
it on the die. The same effect will be obtainable as with vinyl coating. If this method is applied to R-
bending, it will be more efficient than use of a urethane pad because a large bending force is not
required and unmarred products can be obtained. Go to Top
6-3 Cracking
Fig. 6-1 shows cracks and fissures in bent products. A bad crack is called a fissure. Cracking occurs
when the inside radius is too small in relation to sheet thickness. Cracking is influenced by the
tensile strength of the work material, the directions in which it was rolled and bent, the condition of
its cut face, etc.
The following precautions must be observed to prevent cracking.
1. Cut the work so the end face is neat and smooth. Shearing is preferable to fusion
cutting and machining is preferable to shearing. Furthermore, it is advisable to
bend work with the sheared side (lustrous side) facing outward.
2. Pay attention to the direction the work material is rolled. The work is apt to crack
when it is bent in parallel with the direction of roll.
3. Pay attention to the punch tip R. In the case of aluminum, the punch tip R should
be greater than the sheet thickness.
4. Material with great extensibility is hard to crack.
5. Use a slow bending speed.
Go to Top
6-4 Warping
Fig. 6-2 illustrates a phenomenon (called camber) in which the bent work warps. When a material is
bent into another shape, its texture changes. It becomes thinner on the outer surface as it is pulled in
the "a" direction; so part of the material flows from the longitudinal direction, as shown by "b", to
fill the thinner part. As a result, the material contracts in the longitudinal direction. The material
becomes thicker on the inner surface as it is compressed in the "c" direction. Thus, material flows in
the "d" direction to offset the thicker part. This event occurring on the inner surface, together with
the force on the outer surface, causes the bent work to warp in the longitudinal direction, as shown in
Fig. 6-2. The warp is often called a saddle camber because it looks like a horse saddle.
"
This phenomenon varies with the type, quality, sheet thickness and inside radius of the work.
Generally, the ratio of h to l is somewhere between 1/1000 and 511000. Warping is more severe with
longer bending lengths. If the bending length is shorter, it can be corrected more easily. In the case
of long products, such corrections are made by hand in most factories.

70. Go to Top