Today we will learn about different types of allowance used
in pattern making. The pattern is replica of casting but it has slightly large
dimension. This change in dimensions in
pattern and casting are due to various allowance used in casting. When
the cast solidify, it shrink at some extent due to metal shrinkage property
during cooling, so a pattern make slightly larger to compensate it. There are another reasons which are due to poor surface finish and casting
limitations, casting make slightly larger so it can machine or polish further. So the pattern hence cavity for casting forms slightly larger which
can compensate all these drawbacks of casting. The change in dimensions of
pattern and casting are known as allowance.
Types of Pattern Allowance:
Allowance can be classified into following types.
Shrinkage allowance:
Shrinkage is defined as reduce the dimension of casting
during solidification or during cooling. This is general property of all
materials. Some metal shrinks more, some less but every material shrinks. There
are three types of shrinkage.
1. Liquid Shrinkage
2. Solidification Shrinkage
3. Solid Shrinkage
The liquid shrinkage and solidification shrinkage are
compensated by suitable riser but solid shrinkage does not compensated by it so
the pattern is made slightly larger to compensate shrinkage. This is known as
shrinkage allowance.
Draft Allowance:
When the pattern is removed from mould, the parallel surface
to the direction at which pattern is withdrawn, damaged slightly and convert
into slightly tapered surfaces. To compensate these changes, these parallel
surfaces on patterns are made slightly tapered (1-2 degree). This allow easy
removal of pattern from mold and does not affect the actual dimension of
casting. These are known as draft allowance.
Machining Allowance:
As we known the casting gives poor surface finish and rough
surfaces. Also, the Pattern is made manually which does not fixed accurate angles and
dimensions of mating parts. But we need a proper finished and dimensionaly
accurate casting. To avoid these problems, casting made slightly larger and
after solidification machining is done on it for better surface finish and
accurate dimensions. This change in dimension of casting is known as machining
allowance.
Distortion Allowance:
When casting of very thin surfaces like V Shape, U shape
etc. it will distort or damage during
pattern removal or during casting. To avoid this problem, a chamber is provided on pattern to compensate distortion during pattern removal. This change in casting dimension
is known as distortion allowance.
Rapping Allowance:
When the pattern is removed from casting, it will slightly
increase the dimension of casting. So to compensate this changes, the pattern is
made slightly smaller from casting. This change in dimension is known as
rapping allowance.
These are all main types of allowance which are considered
in pattern making. If you have any query regarding this article, ask by
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