2casting pattern.ppt

8/10/2019 2CASTING PATTERN.ppt

1/10

CASTINGS

Dr. Mohd Hasbullah Idris

Dept. of Manufacturing and Industrial

Engineering

Faculty of Mechanical Engineering


2/10

Pattern Allowance

Castings are always smaller in dimensionsthan

the pattern from which they are made. The excess in

dimensions is referred to pattern allowance.

There are 3 allowances included in a pattern:

1. Shrinkage allowance

2. Machining allowance3. Draft allowance


3/10

Shrinkage Allowance

Shrinkage allowance is provided to compensate the

contractions of a casting. The contraction of a castingtakes place in 3 stages.

1. The contraction of the liquid from the pouringtemperature to the freezing temperature

2. The contraction associated with change of phasefrom liquid to solid

3. The contraction of the solid casting from freezingtemperature to the room temperature


4/10

Shrinkage Allowance (cont)

However, only the last stage of the contraction

will be considered in pattern making. Incorporation

of contraction allowance was done by using aspecially engraved rules known as the contractionrules.

Contraction allowance: 1 in 75 for aluminium alloys1 in 96 for iron castings


5/10


Predicting casting contraction can never be precise.Examples;

1. Rigidity of the mould. When iron castings are

made in green sand moulds, the mould walls moveunder pressure of the liquid metal, causing theexpansion of the mould cavity, thus compensating forsome of the metal contraction

2. Restriction by the core. Cored castings may not

contract as much as expected, because the presenceof a strong core may restrict movement of the castingas it is cooling.


6/10


3. Expansion of core binder. Some core binders

expand with the heat of the cast metal causing the

casting to be larger than otherwise expected.

For these reasons, and others, it is only possible topredict contractions approximately. Precise

contraction allowances can be built into the patternsbased on experience.


7/10

Machining Allowance

Normally, a cast surfaceis rough to be used for the

final product. Machining is therefore required to

obtain the final surface texture or dimensions.

The total machining allowance depends on thematerialand the overall dimensionof the job.


8/10

Machining Allowance (cont)


9/10

Draft Allowance

At the time of withdrawing the pattern from the sand

mould, thevertical facesof the pattern are in

continual contact with the sand, which may damagethe mould cavity. To reduce the chances of thishappening, vertical faces of the pattern are alwaystapered from the parting line. This provision is calleddraft allowance.

In general, inner details of the pattern require higherdraft than outer surfaces


10/10

Draft Allowance (cont)

2casting pattern.ppt

Documents