Design for Manufacturing-Module 4

DESIGN FOR MANUFACTURING–MODULE-4DESIGN FOR MANUFACTURING–MODULE-5
Design for Manufacture
Module 4
Design of componentswith castingconsiderations:
Pattern, mould, and parting line. Cored holes and machined holes.
Identifying the possible and probableparting lines. Castings
requiring special sand cores. Designing to obviatesand cores.
Welding considerations:requirementsand rules, redesign of
componentsfor welding; case studies.
Text book: Harry Peck

DESIGN FOR MANUFACTURING–MODULE-4
• One of the shortest routes from raw material to finished
part is a casting process. In casting, a molten metal is
poured into a mold or cavity that approximates the shape
of the finished part.
• Heat is extracted through the mold (in this case a sand
mold), and the molten metal solidifies into the final solid
shape.
• The chief design issues for the mold are to provide an
entry for the molten metal into the mold that creates
continuous laminar flow through the sprue and runner,
and to provide a source of molten metal, suitably located
in the mold so that it stays molten until all of the casting
has been solidified.
• Cores are placed to provide hollow features for the part.
• Liquid metal shrinks on solidification. Thus, the casting
and mold must be designed so that a supply of molten
metal is available to compensate for the shrinkage
Dr.Basavaraju.S, Departmentof MechanicalEngineering, SapthagiriCollege of Engineering, Bangalore

PATTERN
• a Pattern is a replica of the object to be cast, used to
prepare the cavity into which molten material will be
poured during the casting process.
• Patterns used in sand casting may be made of wood,
metal, plastics or other materials. Patterns are made to
exacting standards of construction, so that they can
last for a reasonable length of time, according to the
quality grade of the pattern being built, and so that
they will repeatably provide a dimensionally
acceptable casting
• Made of wood, metal or plastics(Rapid
Prototyping),Wax and Plaster of Paris.

• For a Cast Iron Pulley
component,
following
considerationsare
made:
• Mold parting Line
• Machined Surfaces
• Shrinkage
• Draft angle

The Mold

Parting Line
It is decided on following parameters:
• Position of pattern in mold
• Length –width-breadth relation of pattern
• Sizes of mold box available
• Pouring and venting points

• For Bush component,with the allowance of
3mm,
• Casting length will be 146mm, 106mm and
hole 64mm

• For Bush component, possible parting lines are

cavity moulded by
pattern with sand
core, after casting
sand core is removed
by chipping or
sandblasting.
For hole, bush is placed
horizontally, which require
special sand cores.
So, extra extensions are
provided called as core
prints.

CAST Holes
• Holes produced by casting directlt as

Cored Holes
• Holes produced with the help of Cores

Machined Holes
• Round machined holes can be produced by drilling,
reaming or Boring. Depending upon size of hole,
Operations and required tolling is decided.
Upto 30mm diameter drilling is carried out.
More than 30mm diameter, Boring Operation is carried out.
• Some times casting holes will be imprecise. To avoid
misalignment of drill and drill breakage when cast holes are
out of position, then the process or providing holes is
eliminated and design is done as follows:
Machined Holes up to 30mm dia will be casted as solid and
more than 30mm will be casted/ cored approximately 6mm
below amchined size.

Identifying Parting Line
• Designer has to assess mould parting line for a
componentto recognize where sand cores are needed.
• For Lever made of cast iron,

parting line without need
of sand cores or minimum
sand cores.

Without sand cores holes in casting will be

In a Housing
component
Casting will be solid,
as diameter is less
than 30mm.
Similarly parting line
is mentioned.

Casting with
machined bore size
35mm, as diameter is
more than 30mm.
Similarly parting line
is mentioned.

Casting requiring Special Sand Cores
• For a Double Flange bush, Sand core is needed for each of two
parting lines.
• External sand core is required for 65mm body diameter- hole is now
cast. The parting Line is sand column height is kept minimum.

Designing to Obviate Sand Cores
• By suitable design Modifications, economy in casting process is
achieved b y suitable design modification. Then the need of sand
cores is eliminated. With minimum alteration, size and weight, the
component must not be affected.
• Ex: Connecting Bracket

Parting
Line
The parting line is top face.
It requires 6 sand cores for casting.

Alternate Design
• Open the relief features in end faces in direction of
withdrawal of pattern from the mould. The bosses on the
inside faces are changed from a circular to D-shape and
therefore need of sand core is eliminated.
• The additional weight is also reduced by enlarging reliefs.

• Cast Iron Pedestal has parting line and uses
sand cores for 50mm diameter hole and six
pockets between webs.

• Alternate Design:
• Change four web position
• Simple one sand core is used at center.

Bearing Housing
Indicateparting line and necessary sand cores.
Suggest new design which eliminateduse of cores.

New Design
60mm dia H8 hole can be cast to required allowance
for machining by mould sand.
Relief in base requires sand core.
Base relief rotated90 to cast

Forked Lever

Change arms from U section to H section
eliminated need of external sand core
whilst maintaining strength of costing
with the minimum change in weight.
New Design

Pedestal housing

• Modify web
design
makes it
possible to
use parting
line
transverse to
hole axis and
so tocast the
required
shape no
cores are
required.
New
Design

V-Belt Pulley

• Joiningbosses eliminatesuse of
sand core. Long cast bore is relieved
by central area by rough machined
recess.
New Design

Fulcrum Lever

• Bosses will be cast solid. Webs are cast and machined jaw has sand core.
• H section webs rotated 90 so as to cast along with the jaw.
• A + section fro the webs gives same casting facility
New Design

The location of the parting plane can affect each
of the following:-
1. The number of cores,
2. The use of effective and economical gating, 3.
The weight of the final casting,
4. The method of supporting the cores,
5. The final dimensionalaccuracy, and
6. The ease of molding.

Welding considerations
For successful cast-weld construction the methods of
welding must be examined for their capability of
producing the desired chemical composition, physical
and mechanical properties, as well as ease and rate of
welding. The weld engineer has several processes that
can be considered. These include:
1. shielded metal-arc welding(SMAW)
2. submerged arc welding(SAW)
3. Gas metal-arc welding(GMAW)
4. Gas tungsten-arc welding(GTAW)
5. Electro slag welding(ESW)

• The rate of welding as related to the size of the
weld to be made, determines to a great degree
the economics of the process.
• The manual arc process is most versatile, but for
large cast-weld construction it is limited in
application because changing electrodes and
other interruptions typically limit the weld
deposit rate to an average of 0.9kg/h.
• The submerged arc process, on the other hand,
due to its continuous operation with wire
electrodes, deposits metal at much higher rates,
as do gas shielded methods.
• Electroslag welding is accomplished at rates of
approximately 14(kg/h) per electrode.

• When joining components of different composition,
choosing the correct method is very important.
• Example: stainless steel joined to carbon or low alloy
steel is desired in certain applications to achieve
specific properties.
• The manual arc welding process is often used in such
instances with electrodes of the 300 series. However
care must be taken to have a minimum of penetration
as carbide formation with mild or low alloy steels will
give an excessively brittle layer, which can result in
early failure.
• Distortion of an assembly is another major
consideration in selecting a welding process. Single
pass procedures, such as electroslag welding , produce
less distortion than multipass processes in which each
pass results in warpage.

Design for Manufacturing-Module 4

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