A pattern is replica of casting but it has slightly large dimensions. This change in pattern in casting due to various reasons is known as pattern allowances in casting.
When the cast solidify, it shrink at some extent due to metal shrinkage property at the time of cooling. So a pattern is made slightly larger to compensate it. This is an example of pattern allowance. There are many other reasons for pattern allowances which will be discussed below.
These are the types of pattern allowances :-
i) Shrinkage Allowances:
Shrinkage is defined as reduction is the dimension of the cast during the cooling or solidification process. This is a general property of all materials. The magnitude of shrinkage varies from material to material but every material shrinks.
Shrinkage is of three types:-
a) Liquid Shrinkage
b) Solidification Shrinkage.
c) Solid Shrinkage
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ii) Draft Allowances:
When the pattern is removed from the mold, the parallel surface to the direction at which the pattern is withdrawn gets damaged slightly and gets converted into slightly tapered surfaces.
For compensation of these changes, these parallel surfaces on pattern are made slightly tapered (nearly 1 -2 degrees). This allow easy removal of pattern from the mold and does not effect the casting by anyway. These changes in pattern surface to prevent it from damages are called draft allowances.
The magnitude of taper depends upon:
a) Molding Methods.
b) Mold materials.
c) Shape and size of pattern depth direction in contact with mold cavity.
iii) Machining allowances:
As we know that the product of a casting process gives very poor surface finish, hence the surface of final product of casting will be rough. But we need a product which is polished and have good surface finish.
So to obtain good surface finish, the final product of casting is machined with the processes like turning or grinding to improve the surface finish.
This machining after the casting process causes a significant loss of mrtal from the mold part. To compensate this loss, machining or finishing allowances are given in the pattern of casting. This allowance is added together with basic size of pattern. It ranges from lower than 2 mm to 15 mm based on size and material of the part.
The amount of allowance added to the pattern is based on,
a) Material used
b) Machining processes used
c) Requirement of accuracy.
d) Molding methods used.
iv) Shake or rapping allowances:
When the pattern is to be removed from the sand of casting , the pattern will have to be shaken slightly to remove it from the sand and this will cause a slight increase in dimension of casting.
To compensate this increase in dimension of casting, the patterns are made slightly smaller from casting. This change in dimension of pattern is known as shaking or rapping allowances.
v) Distortion or camber allowances:
When the metal is in cooling process, stress is developed in the solid metal due to uneven metal thickness in the casting process. This stress may cause distortion or bending in the casting.
To avoid this bending or distortion in casting, camber is provided in the opposite direction so that when bending occurs due to uneven thickness of metal, casting becomes straight. This change in pattern shape to compensate bending while casting is known as Bending Allowances.