Pattern- Making, Allowances and its Types of Pattern

A pattern may be defined as a model of the desired casting which when molded in sand forms an impression called mold. The mold when filled with the molten metal forms casting after solidification of the poured metal. The quality and accuracy of casting depend upon the pattern making. The pattern may be made of wood, metal(cast iron, brass, aluminum, and alloy steel.), plaster, plastics, and wax. 

Pattern Allowances:
A pattern is always made larger than the required size of the casting considering the various allowances. These are the allowances that are usually provided in a pattern.

1: shrinkage or contraction allowance:
The various metals used for casting contract after solidification in the mold. Since the contraction is different for different materials, therefore it will also differ with the form or type of metal.

2: Draft allowance
It is a taper that is given to all the vertical walls of the pattern for easy and clean withdraw of the pattern from the sand without damaging the mould cavity. It may be expressed in millimeters on a side or in degrees. The amount of taper varies with the type of patterns. The wooden patterns require more taper than metal patterns because of the greater frictional resistance of the wooden surfaces.

3: Finish or machining allowance
The allowance is provided on the pattern if the casting is to be machined. This allowance is given in addition to the shrinkage allowance. The amount of this allowance varies from 1.6 to 12.5 mm which depends upon the type of the casting metal, size, and the shape of the casting. Ferrous metals require more machining allowance than non-ferrous metals.

4: Distortion or camber allowance
This allowance is provided on patterns used for casting of such design in which the contraction is not uniform throughout.

5: Rapping or shaking allowance
This allowance is provided in the pattern to compensate for the rapping of mould because the pattern is to be rapped before removing it from the mould.

Types of Patterns:

The common types of patterns are as follows:

1.  solid or single piece patterns
2.  split or two/multiple piece patterns
3.  match plate pattern
4.   cope and drag pattern
5.   loose piece pattern
6.   gated patterns
7.   sweep pattern
8.   skeleton pattern
9.   shell pattern
10. segmental pattern
11. follow board pattern
12. lagged up pattern
13. left and right-hand pattern

A pattern is a 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.

For Example:
When the cast solidifies, it shrinks at some extent due to the 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 as 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

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 for these changes, these parallel surfaces on the pattern are made slightly tapered (nearly 1 -2 degrees). This allows easy removal of the pattern from the mold and does not affect the casting in 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 for this increase in dimension of casting, the patterns are made slightly smaller from casting. This change in dimension of the pattern is known as shaking or rapping allowances.

v) Distortion or camber allowances:
  • When the metal is in the cooling process, stress is developed in the solid metal due to uneven metal thickness in the casting process. This stress may cause distortion or bend 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 for bending while casting is known as Bending Allowances.

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