Design For Manufacturing Module 3

Design For ManufacturingModule3
FUNCTIONAL DATUM
• A Datum feature – a locating or a positioning
feature. It can be a face (a surface) or the
centerline of a hole.
• A functional datum feature is a face or a hole in a
component, which is of importance to the
function of the component in the machine.
• In the preparation of component detail drawing
from assembly drawing, it is necessary to identify
necessary functional datum faces or holes which
are critical in functioning of component.
Sapthagiri College of Engineering

Machining Sequence
Considering the stud example, the machining processes required are
of two types.
1. Turning for two diameters and
2. End face Groove.
The turning can be done on any lathe irrespective of type and groove
cutting can be done by milling, planing, slotting or broaching.
The sequence is fixed:
First Turning and
Second, Groove cutting
The limits for turning dimension needs to be obtained which cannot be
obtained by adding functional dimensions of turning length.

The MANUFACTURING DATUM face for the turning operation is
the right hand end face of the component – a change of the datum
face. When the datum face is changed there is, inevitably, a
reduction in tolerance in one or more dimensions.

CHANGING DATUM
1) Decide the required manufacturing datum face.
2) Decide the required manufacturing dimension(s)
[which also involves deciding the omitted
dimension(s)].
3) Determine the tolerance for each of the (new)
manufacturing dimension.
4) Set suitable limits for all, but one of the required
dimensions.
5) Determine the limits for the final dimensions

Then applying this procedure for the stud
1. The manufacturing datum face, fro both
processes, is the right hand end face.
2. Dimension L for turning; dimension G for
groove depth, the 35.3~35.0 dimension omitted..
3.Tolerance of omitted dimension is 0.3. Let
tolerance (limits) for G remain as 0.15. Therefore,
tolerance L is 0.15.
4. Limits for G remain unaltered, i.e.15.00, 14.85.
5. Determine limits for L as follows.

Design For ManufacturingModule3To ensure that the limits of omitted dimension 0 are not
exceeded, use is made of a diagrammatic representation of the
limits of L and G in terms of the limits of 0 where it is seen that
a. When 0 is minimum, then L is minimum and G is maximum
b. When 0 is maximum, then L is maximum and G is minimum
Hence, L min = 35.0 + 15.0 = 50.0 mm
and L max = 35.3 + 14.85 = 50.15 mm
Verification that the limits of dimension 0, although now
omitted from the drawing, will not be exceeded, is shown in
diagrammatic representation.
Fig. Shows the stud re-dimensioned to suit the requirements of
the operation sequence. The tolerance reduction that occurs
when the datum face is changed is shown in the new dimension
for the turning operation, where the tolerance of 0.15mm is half
the tolerance of the omitted dimension.

COMPONENT DESIGN-
MACHINING CONSIDERATIONS
Two aspects of designing.
1) Designing for function.
2) Designing for production.

Twist Drills – Standard length.
Standard twist drill –
• the jobber series,
• the stub series,
• the Morse taper shank series and
• the long series.
The machining consideration for twist drills
is length – the length of the flutes, the need
to ensure that the length of a standard drill
is sufficient for an intended drilling
operation.
long series
taper shank
stub

Even with half of shank edged in drill chuck, it is incapable
of drilling,
Long drill cannot be used because of torsion effect

Alternate design:
Holes are replaced by grooves.
Grooves produced by machining operation by
special form of cutter

End Milling Cutters

The shaft in Fig has two 8 mm keyways in the
flange face for two keys, which are to be a tight fit.
Normally, an end milling cutter or a key-seating
cutter produces such a key but because the cutter
holder cannot approach nearer than the end of the
shaft, a cutter length of at least 180 mm would be
needed.
An 8 mm diameter of such a length would be
completely impracticable, and therefore an
alternative machining process is required, namely
shaping or slotting.
For such reciprocating cutting tool action a run out
for the tool is required. To enable the keyways to be
machined, run-out holes are drilled. The holes are
drilled from the left hand side of the shaft.

Drilling – Entry and Run-out
• Twist drills should enter, and break through,
normal to the surface to be drilled.
• Drilling at an angle to a surface can use
deflection of the drill, possibly resulting in drill
breakage.
• A modified design is necessary

• Equally undesirable is the required drilling process
for the screw holes in the housing is shown in fig.
• The entry of the drill is normal to the surface but,
on leaving, the drill would “break out”, if in metal-
half in space. The drill would deflect away from the
metal.
ALTERNATE DESIGN:
• First, an obvious feature to change is the pitch circle
diameter of the screw holes: a smaller P.C.D. to
bring the holes wholly into body material-
• a longer P.C.D. to position the holes solely in flange
thickness.
• If, for good reason, neither of these changes
possible Sapthagiri College of Engineering

1. If the housing is produced from casting, with the
outside surfaces unmachined, and the body wall
thickness is relatively thin, then four local bosses may
be added to obviate drill breakout.
2. If the housing is produced from a similar casting but
having a suitably thick body wall, then four local
pockets may be cast to provide a clear drill run-out.Sapthagiri College of Engineering

1. If the housing is to be machined all over, and as a
suitably thick body wall, an undercut may be
machined (in the turning process) to provide a clear
drill run-out.
2. If the housing is machined all over but as a thin body
wall ( too thin for under cutting ) , then a circular shaft
material may be left to obviate drill break out. The
component of course, requires a second turning
process to remove the surplus materialSapthagiri College of Engineering

Keyways-Sunken and Run-out
• Sunken keyways in a shaft can be machined only with a key seating or slot drilling
cutter. These cutters have two or sometimes three teeth. An open end keyway, at
the end of a shaft, can be machined by an end-milling cutter, which can have six
or more teeth.

The rate of metal removal for these cutters is in terms of the
number of cutter teeth. More teeth allow a faster machining
time, and, therefore when possible, open-end key ways should be
shown. The rigid hand keyway in Fig can be altered to open end
–the key will be retained by the axial clamping washer and screw

Dowels and Dowelling Procedure
• Dowels are used to obtain location between two or
more mating parts. Usually the requirement is to obtain
a precise location between the mating parts although
sometimes dowels are used for an approximate
location. The machining procedure for each of these
requirements can differ.
• For a precise location, when the two parts must
assembly together repeatedly with a complete absence
of play or shake between them, the machining, which
consists of drilling and reaming, is individual to each
unit –as distinct from mass production method.

Design For ManufacturingModule3• The dowels are made a press fit in one component –in this design, the cover
plate –and a tight push fit in the mating component. When the dowels
holes in one of the components are blind holes, as in the gear box, means
of avoiding an air lock should provided: in this design a slight flat ,either
filed or ground, along the dowel length satisfactory.
• A further design feature which facilitates the segment of the dowels into
the dowel holes during Assembly is to one dowel longer, by approximately
one diameter; it is much easier to engage one tight push fit dowel than two
dowels simultaneously.

To achieve the required locational precision of
the dowel holes a both components, the reaming to
size must be done when both components are
screwed together and the usual practice is to drill
holes 0.5 mm. Less than final size in the upper or
outer component only (in this example the holes
are drilled in the cover plate ).
Then , with the upper (outer )component secured in
the working position and the previously drilled
holes serving as guidance, the same size holes are
drilled into the lower component, followed by the
final reaming to size.

Countersunk Head Screws
The basic requirement for countersunk head screws is that the
head should fit into the countersunk hole with as great a degree of
flushness as possible… to achieve this it is necessary for the dimensions
of both the head of the screw and the countersunk hole to be
controlled within prescribed limits.

• Such a degree of accuracy in the fitting of the
screw head must surely be a requirement for very
special conditions only because, normally, the
function of a screw is to fasten two or more
components surely together – the “flushness” of
the screw head is of no importance. When two or
more countersunk head screws are involved the
centers of screwed holes, along with the mating
centers of the countersunk holes, must be exactly
identical, otherwise the heads of all the screws
will not fit correctly. But all dimensions must have
a tolerance – no matter how small – and therefore
the hole centers cannot be guaranteed ‘EXACT’.Sapthagiri College of Engineering

Because of these uncertainties the use of
countersunk head screws should be restricted to
design conditions where there is a space restriction
– prohibiting the use of to other types of screws -
and where only ONE screw is involved, as in the
bearing clamping washer.

An alternative design for securing the plate with the
body is to use cheesehead screws with the screw
heads fitting in a counterbore (Fig.). Precise lateral
location of the holes is not necessary because the plate
holes give clearance with the screw body and head
diameters.

Reduction of Machined Areas
• In the interest of economy in manufacture,
component design should keep machining time
to a minimum.
• This designing consideration can results in a
reduction of machined areas (in the case of
castings / stamping) or reduction in the amount
of small tolerance, smooth surface finish
machining . The relieving of the base by a cast
step will facilitate the machining process.

The machined base of cast iron bearing bracket
assuming that the machining is by milling, then very
large diameter milling cutter is required or several
passes of smaller cutter is necessary.
Alternate design: Relieving of base by casting itself

Same observationis made in Screw Jack
body part.
The body part is casted for required
shape insteadof using machining
operations In plummer block body part also
Rectangularslot is designed in casting
process itself

The bore of the bearing bracket is machined, the full length,
limits to receive a bearing bush, which is to be a press fit in
the bore. since small tolerance machining takes longer –
because of the accuracy involved – a reduction in the length
of machined surface is economically desirable.
Alternate design: A cast relief in the central area of the hole
achieves such a reduction.

Relief in the central areas of both hole and outside diameter of the
bearing bush reduces the machining time.
The relief for machining the bearing bush from solid bar and from a
casting, are shown in (a)and(b) respectively

In producing the
housing bush,
there are two
machining
processes:
1.Bore & Turning,
2.Cut square
Simplification by Separation

Procedure for slotting square hole by drilling
access for 11mm dia and then using square
slotting tool for quick and quantity production.
Machining
groove by
slotting
:8.5minutes

The required machining is simplified by separation.
By producing component from two separate places and finally
joining two pieces.
Redesign: End milling the square slot
Machining
groove by end
mill cutter:
1.9minutes Sapthagiri College of Engineering

Two parts joined by spot welding
Spot welding can
be done in
seconds

Simplification by Amalgamation
Design assemblies and sub-assemblies should be critically
analyzed with regard to the possibility of achieving
economy by amalgamating two or more components into
a one piece unit for example the gear shaft assembly

The central gear is keyed with the shaft, and
serves, along with the two distance pieces, to help
in the axial clamping of the inner rings of the ball
bearings against the abutment face provided by
the shaft flange.
By an amalgamation of the shaft with the gear and
the two distance pieces, an appreciable amount of
machining is eliminated : gear bore and keyway, all
surfaces of the distance pieces, shaft key fit
diameter and keyway.

Provision for Holding the Workpiece
For turning operations the component may be
produced from lengths of bright or black bar stock
which would be held in a collet or a three jaw
chuck. Or the work piece may be mounted between
centers and therefore would require center holes at
each end.
Or components machined from castings or forgings
would be held in a three-jaw or a four-jaw chuck,
depending upon the shape of the component.

For the turning and facing of the 170mm. Diameter the cover
plate will be held in a three-jaw chuck, but it is not possible to
grip on the 200mm diameter and keep the chuck jaws clear of
the cutting tool.

Because of the fine sandblasting of the convex surface, which has some
importance regarding the finished appearance, it is not acceptable to
add some projection to this surface for the purpose of holding.
Therefore a suitable holding device will have to be devised in the
concave area – the inside.
Design modification: to provide a “grip” medium for the chuck jaws is
shown in Fig. a recess, incorporated in the casting, permits an internal
grip by the chuck jaws, and the cast contour provides free access for
the turning and facing tools.

Surface Grinding
The larger sizes of reciprocating table or rotary table surface grinding
machines are used for the machining of flat surfaces on a large
component or a large number of small components. The preferred type
of components for this process are those which lie flat and are stable
without the need for packing or supporting.

The lever is manufactured in batches of 100 and is eminently suitable for a surface
grinding process – if it can be laid flat on lever surface. A circular boss added to
the 38mm diameter end of lever will achieve the requirements for surface
grinding both faces, and this added boss will be removed automatically during the
milling process.
The milling fixtures are not now required. All the components will be placed on
the table of even a medium size machine and only one component need be
checked for size during grinding. The time for machining the boss faces is now
greatly reduced. Sapthagiri College of Engineering

Design For Manufacturing Module 3

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