This document provides guidelines and considerations for designing sheet metal parts that can be efficiently manufactured. It discusses various sheet metal forming processes like blanking, piercing, bending, deep drawing and provides examples of different press tools. It also outlines guidelines for blank and hole design to enable economical manufacturing like minimum section sizes, radii on corners, hole diameters relative to material thickness. Process details covered include principles of plastic deformation and shearing, effects of cutting clearance and calculating flat blank length for bending.

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Design For Manufacturability
Sheet Metal Part Design
By
Suresh Sunnam

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Table of contents
Process details :
•Types of sheet metal operations
•Press working principles
Tool details :
•Types of press tools
•Types of Press Machines
•Guidelines :
•Part design considerations.
•Heat treatment requirements.
Tolerancing :
Case studies :

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Process details
Types of sheet metal operations
Sheet metal operations can be classified in to two types.
• Shearing operations
• Non Shearing operations.
•Shearing operations
Blanking : Shearing out a closed contour of a plane stock-strip to make a component
is called blanking.The cut-out piece is the blank or component.
BlankScrap
Piercing : Cutting out holes in a blank strip or a semi-finished component using a
press tool is called piercing.The cut out piece is scrap or slug.
ScrapPart

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Process details
Notching : In notching,the punch does not cut on all the sides.It may cut on two or
three sides making a change in the contour of the blank.The cut-out material is scrap.
Cropping : When the shape of the component is such that a single line shearing
produces a component from a strip already sized to the component width,the
operation is called cropping.Cropping is a scrap-less operation.

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Process details
Parting : When contour is complicated and notching is employed in obtaining the
contour,the final separation (cutting-off) of the component is achieved by parting.It
can also be employed on simple works where width of strip and component are same.

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Process details
Lancing : It is the combination of shearing and bending.A tongue is created by
shearing three sides and the same is bent on the fourth side downwards using the
same punch.
Louvering : Louvering is a combination of shearing and
forming operation carried out in a single stroke of the punch.

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Process details
Trimming : Trimming is an operation carried out to cut off the excess material such as
flashes,fins etc.In forged components.Truing the edges of deep-drawn parts also may be
called trimming.

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Process details
•Non-Shearing operations
Bending : Metals take a permanent deformation if they are stressed beyond their elastic
limits.In bending,deformation is made to sheet metal in a straight line along,across or in an
angle to its sides to obtain a new configuration.Bending done on the edges contributes
strength and rigidity to the component.
Flanging : When the width of bent down portion is small
compared to the width of the component,it is called
flanging.It may be in a straight line or curved.As the name
indicates,it is done to create a flange on the part.

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Process details
Coining : As the name indicates,it is an operation carried out to make coins and similar
components by cold-forming a slug of equal volume.The opposite side of the component may
have different markings as per the engravings made on the top and bottom punches.
Semi-Piercing : It is done to create a projection of a lug on the face of a component to
facilitate location,Spot-welding etc.The amount of displacement of the material is controlled to
avoid shearing of the portion.

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Process details
Embossing : It is done to produce shallow depressions or projections in sheet metals in
various shapes & Designs.The thickness of the material should remain the same as the stock
in the embossed area also.
Curling : Curling is forming an edge of circular cross-section along the edge of a sheet metal
part.Normally done to strengthen the edge often by keeping a wire inside for higher rigidity.

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Process details
Crimping : It is normally done to close a seam or for setting down.The components are pre-
bent.Sometimes crimping is done to fix eye-lets to cable-ends.
Deep-Drawing : Deep hollow components such as cups,containers,shells etc are produced
from sheet-metals by stretching the sheet-metal into a die with a punch when the sheet is
held between a pressure-pad and die-face,

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Process details
Collar-Drawing : In order to provide adequate length for threads,the component is first
pierced and subsequently drawn,maintaining hole size equal to core-diameter of the
thread.This operation is called collar-Drawing.
Forming : Forming in general terms refers to all operations that impart a change of shape to
a blank or component without causing change in its thickness.In forming,unlike in bending,the
deformation may be in two or more directions including closed contours.

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Process details
Press Working Principles
First Stage – Plastic deformation.
The stock material has been placed
on the die,the press has been tripped, and the
punch is being driven toward the die. The punch
contacts the stock material and exerts pressure
upon it.When the elastic limit of the stock
material is exceeded,plastic deformation takes
place.
Shearing operation :
Second Stage – Penetration.
As the driving force of the ram
continues,the punch is forced to penetrates the
stock material, and the blank or slug is displaced
into the die opening a corresponding
amount.This is the true shearing portion of the
cutting cycle,from which the term “Shearing
action” is derived.

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Process details
Third Stage – Fracture.
Further continuation of the punching
pressure then causes fracture to start at the
cutting edges of the punch and the die.These
are the points of greatest stress
concentration.Under proper cutting
conditions,the fracture extend toward each other
and meet.When this occurs,the fracture is
complete and the blank or slug is separated
from the original stock material.
Closer Look at the shear portion

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Process details
Importance of cutting clearance on the punch and die :
Proper cutting clearance is necessary to the life of the die and the quality of the piece part.Excessive
cutting clearance results in objectionable piece-part.Insufficient cutting clearance causes undue stress
and wear on the cutting members of the tool because of the greater punching effort required.
Effect of less cutting clearance

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Process details
Effect of larger cutting clearance

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Process details
Bending operation :
In bending operations,the material is formed around a straight axis which extends completely across the
material at the bend lines.A bending operation produces a plane surface which is at an angle to the
original plane of the work piece or stock strip.
Some silent observations in bending :
1.The bend radius is tangent to the inner plane surfaces of the piece part.
2.The bend lines occur at the tangency of the bend radius with the inner plane surfaces.
3.At the bend lines,the bend radius is perpendicular to the inner plane surfaces.
4.The bend axis is located at the center of the bend radius.

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Process details
Bend elements:

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Process details
Calculation guide lines for Flat-blank length:
As a general principle the length of the neutral plane of a sheet is equal to the length of a
blank.Estimating the required flat work piece length is a matter of determining the theoretical length of the
neutral plane.To determine the theoretical length,it is necessary to calculate the bend allowance “A” for each
bend.The estimated blank length will then be the sum of the lengths “L” of the bend legs and the
allowances.Then,where “B” is the length of the flat blank.
Assumptions :
T= Material thickness.
R=Bend radius.
N=Bend angle (Degrees)
L=Length of flat plane (Legs)
A=Bend allowance.
C=Distance of inner surface to neutral plane.
B = L1 + A + L2.
To find “A” :
A= (R+C)*(2*PI*N)/360 ( OR ) A= (R+C) * 0.01745 N.

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Process details
Distance “C” from the inner surface to the neutral plane is a variable factor depending largely
upon the ratio of the stock thickness “T” to the bend radius “R”. Optimum value for “C” may be considered to
be.
Where
R < 2 T C= 0.33T
R = 2T to 4T C=0.4T
R > 4T C=0.5T
Thus,for the bend of any angle
Where
R < 2T A = (R+ 0.33T) 0.01745 * N
R=2T to 4T A= (R+0.4T) 0.01745 * N
R > 4T A= (R+0.5T) 0.01745 * N

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Process details
Types of Press Tools :
Blanking Tool :

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Process details
Piercing Tool : Perforating Tool :

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Process details
Progressive Tool : Compound Die :

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Process details
Bending Tool : Forming Tool :

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Process details
Types of Press Machines :
Press Break :

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Part Design Guidelines
Sheet Metal Part Design Guidelines :
Blank Design :
A. Minimum Practical Section should never be less than material thickness or .060". A minimum section must
be one and one half times material thickness for high shear strength material for the most practical stamping.
B. Radii on Blank Corners - Corners can be sharp if material thickness is 1/16" or less - over 1/16 " allow corner
radii (R) equal to 1/2 material thickness. See illustration.
C. Practical Design For Economy Manufacture.
•W = .060 minimum for materials thinner than .060" wider if possible.
•W1 = Never less than material thickness, wider if possible.
•L = 5 x W is maximum depth, should be less if possible.
•L1 = 5 x W is maximum length, should be less if possible.
The above rules (a and b) apply for maximum economy. If followed, all blank periphery can be included in the
blanking die, eliminating secondary tooling and operations.

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Part Design Guidelines
Piercing Round Hole Design :
To pierce holes with economical tools and operations, the hole diameter must not be less than the
sock thickness. If the hole diameter is less than the material thickness (or less than .060") it usually must be
drilled and deburred and each of these operations is slower than punching.

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Part Design Guidelines
Illustration "B" indicates a hole diameter with a tolerance of plus or minus .002". We can pierce a
hole within these limits on the punch side for approximately 25% to 30% of the material thickness as indicated
in Illustration "C". The percent of thickness varies with the shear strength of the materials.
On holes where a machine finish is required, they can be punched undersized, redrilled and
reamed to size as shown. (See Illustration "E".)
If the web (distance between the hole and edge of material) is a minimum of the stock thickness,
the hole can be punched which is less expensive than drilling and deburring. (See Illustration D.)
A web that is less than the stock thickness will result in a bulge on the blank. Budge conditions
would increase progressively as the web decreases, until there would be a complete break through. However,
the bulge is hardly visible until the web is reduced to less than 1/2 the stock thickness. These examples would
also apply to a web between holes. (See Illustration F.)
If a measurable bulge is not permitted, a drilling and deburring operation may be necessary.
As a suggestion, if the web is too narrow, the profile of the blank could be changed by adding an
ear of sufficient dimensions and shape to eliminate the problem. (See Illustration G.)
Another alternate suggestion would be to change the contour of the blank to include the hole as a
notch. (See Illustration H.) The notch could either be pierced or be wide enough so it could be included in the
blank without a piercing or notching operation .(See Blank Design)
Caution: The addition of the word "thru" to any hole diameter, regardless of tolerances, indicates the
requirement of the hole to be reamed. Reaming and the additional chamfer to remove burr add two extra
operations to the cost of the part.

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Part Design Guidelines
Minimum Ratios of Hole Diameter to Stock Thickness :
Limitations are established in common practice for most economical production.
Recommended ratios are applicable to all common metals.
P = Punched Hole Diameter (0.062 min. dia.)
T = Stock Thickness
Material Ultimate Tensile Strength (PSI) - Ratio P to T
32,000 - P = 1.0T
50,000 - P = 1.5T
95,000 - P = 2T

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Part Design Guidelines
Piercing holes adjacent to Bends :
Illustration "A" indicates that the minimum inside distance required from the edge of a hole to a bend
is 1-1/2 times the material thickness (T) plus the bend radius (R)
Otherwise, distortion will occur as indicated in Illustration "B" - or piercing after form must be
considered.
Illustration "C" indicates a similar condition to "A", except for openings with an edge parallel to bend. In
this case the following requirements apply for economical tooling and production:
•When "L" = up to 1" - 2T + R (minimum).
•When "L" = 1" to 2" - 2-1/2T + R (minimum).
•When "L" = 2" or more - 3T to 3-1/2T + R (minimum).

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Part Design Guidelines
Limits of Extruded Holes:
R = Outer radius
H = Flange height
D = Inner diameter
T = Material thickness
Specifications and Measurement of Formed Parts:
Preferred dimensioning and points to measure:
L = Linear dimensions; corner radius
R = Radii;
R1 = Typical inside bend or
R2 = Radius in flat blank
T = Material thickness
2 - Typical examples - additional views as needed.

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Part Design Guidelines
Position of the Form :
"A" illustrates a design that is not desirable for quality or economy. When the form is inside the
blank profile, as shown, the material must be torn through the stock thickness and the bend radius. If the part
is under stress, this tear will likely cause fatigue failures. In addition, stock tooling cannot be adapted
because the flat area adjacent to the form must be held in position during forming, which means extra tooling
expense.
"B" illustrates a similar condition, but with the form just outside the blank profile. In this case, the
tear extends to the center of the required bend radius.
"C" and "D" illustrates a possible solution by changing the blank profile to provide relief for the
bend. Besides eliminating the chance of fatigue under stress, there is a possibility of using stock 90 degree
vee punches and dies. The results are better quality and less expensive engineering charges.
If the relief notches in illustration "D" are wide enough compared to the material thickness and
shear strength, or are designed like the relief in illustration "E", they can be included in the blanking operation
for very little engineering cost and no extra operation.

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Part Design Guidelines

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Part Design Guidelines
Height of Form :
"A" illustrates a 90 degree bend with
insufficient height (h) to form properly.
Consequently, stock must be added so the
form is high enough (H), stock is then cutoff, which
means additional tooling and an additional operation.
If "h" is not high enough, the cutoff tool
may not have sufficient strength to stand up for a
particular material or thickness. This may result in a
higher cost secondary operation such as milling.
Illustration "B" indicates how to determine
the minimum inside height "H", which in this case
equals 2-1/2 times the material thickness (T) plus the
required bend radius (R).
The concept illustrated by "B" above is
converted to a chart form below for your convenience.
These recommended minimum formed height
dimensions are general to cover most variables of
design, size, material types, tempers and thicknesses
but which will permit the most economical tooling and
production. Proper design, small parts and easily
formed material, such as Aluminum, Brass, Copper
and Mild Steel may be formed with a slightly lower
minimum inside formed height (roughly 20% less).

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Part Design Guidelines
Specifications and Dimensions of Embossed Parts :
Preferred dimensioning and points to measure on embossed parts.

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Part Design Guidelines
Recommended Limits of Embossed Parts :
Limits for depths of embossments to minimize fracturing.
FLAT V-BENDS - L (MAX) = 3T*
OFFSETS - L (MAX) = R1 + R2**
*Reduce to 2T for commercial grades of steel, one-quarter hard tempers, and alloys of aluminum.
**Reduce to .5(R1 + R2) for commercial grades of steel, one-quarter hard tempers, and alloys of
aluminum.

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Part Design Guidelines
Specification and Characteristics of Drawn Parts :
The specification should show the form of the part, state the material, specify dimensions and
condition of symmetry.
D1 or D2 (not both) D3
R1
R2
T
L1
L2
R3

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Part Design Guidelines
Recommended bending radii :
Stainless Steel :
MATERIAL SHEET THICKNESS
-- .012 .016 .020 .025 .032 .036 .040 .045 .050 .063 .080 .090 .112 .125 .160 .190
302 Annealed .06 .06 .06 .06 .06 .06 .09 .09 .09 .09 .12 .12 .16 .19 .22 .25
347-1A .06 .06 .06 .09 .09 .06 .06 .09 .09 .09 .12 .12 .16 .19 .22 .25
1/4 Hard Cres .06 .06 .06 .06 .06 .09 .09 .09 .12 .12 .16 .19 .22 .25 .31 .38
1/2 Hard Cres .06 .06 .06 .09 .09 .12 .12 .12 .16 .16 .25 .25 .31 .38 .50 .62
Full Hard Cres .06 .06 .09 .12 .12 .16 .16 .19 .22 .25 .31 .38 .44 .50 .62 .87

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Part Design Guidelines
Recommended bending radii :
Aluminum :
MATERIAL SHEET THICKNESS
-- .012 .016 .020 .025 .032 .040 .050 .063 .071 .080 .090 .100 .125 .160 .190
2024-0 & W .06 .06 .06 .06 .06 .06 .09 .09 .12 .12 .16 .19 .22 .31 .36
2024-T3 .06 .06 .06 .09 .09 .12 .16 .22 .25 .31 .38 .44 .62 .75 1.00
2024-T36 .06 .09 .09 .09 .12 .16 .19 .25 .31 .38 .44 .50 .75 1.00 1.25
3003-0 .06 .06 .06 .06 .06 .06 .06 .06 .09 .09 .09 .12 .12 .16 .19
3003-H14 .06 .06 .06 .06 .06 .09 .09 .12 .12 .16 .19 .22 .31 .38 .44
5052-0 .06 .06 .06 .06 .06 .06 .06 .09 .09 .09 .12 .12 .16 .19 .22
6061-0 & W .06 .06 .06 .06 .06 .06 .06 .09 .09 .09 .12 .12 .16 .19 .22
6061-T4 & T6 .06 .06 .06 .06 .06 .06 .09 .09 .12 .12 .16 .19 .22 .31 .38
7075-0 & W .06 .06 .06 .06 .09 .09 .12 .16 .19 .22 .25 .31 .38 .50 .62
7075-T6 .06 .09 .12 .12 .16 .22 .25 .31 .41 .44 .50 .69 .87 1.00 1.25
7178-0 & W .06 .06 .06 .06 .09 .09 .12 .19 .22 .25 .31 .38 .50 .75 -
7178-T6 .06 .09 .16 .19 .22 .31 .38 .50 .56 .62 .62 .75 1.00 1.25 -

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Part Design Guidelines
Specification and Characteristics of Drawn Parts :

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Feasible operations
• What are the feasible manufacturing operations that can be performed in this process ( bending
upto 180 degrees / injection molding of components as thin as 0.02 inches in thickness)
• What are the general manufacturing operations
• What are the easier and common operations
• What are difficult operations
• What are not possible operations

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Part design Guidelines
• Provide the guidelines of each design operations

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Case studies
• some case studies of part designs…..

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Dos and donts