Pattern Allowances

Patterns are not made into the exact size of the castings to be produced. Patterns are made slightly larger than the required castings. This extra size given on the pattern is called pattern allowance. The pattern allowances are given for the purpose of compensating the metal shrinkage to provide extra metal which is to be removed in machining, to avoid metal distortion, for easy withdrawal of pattern from mould and for rapping. If allowances are not given on the pattern, the casting will become smaller than the required size.

The various types of allowances normally provided on the pattern are

1. Shrinkage allowance

2. Machining or finish allowance

3. Draft or taper allowance

4. Distortion or camber allowance

5. Rapping or shake allowance

1. Shrinkage Allowance

The metals shrink on solidification and contracts further on cooling to room temperature. To compensate it, the pattern is made larger than the required casting. This extra size provided on the pattern for metal shrinkage is called shrinkage allowance. If it is not given, the casting will become smaller after it is cooled. The typical values of shrinkage allowance for various metals are given in Table 1.1.




2. Machining or Finishing Allowance

All castings are to be machined to get the required surface finish on the metal. During machining, some of the metals are removed from the casting. For this purpose, the pattern is made larger than the required casting. This extra size given to the pattern for machining purpose is called machining or finishing allowance.

The amount of finish allowance depends on the material of the casting, size of casting, volume of production, method of moulding, configuration of the casting, method of machining, degree of finishing etc. Machining allowance is always larger for hand moulding when compared to machine moulding.

Table 1.2 gives an idea of the machining allowance for various materials.




3. Draft or Taper Allowance

If the vertical faces of pattern are perpendicular to the parting line, the edges of mould may be damaged when the pattern is removed from the sand. Hence, the vertical faces are made into taper for easy removal of pattern. This slight taper provided on the vertical sides of pattern is called draft allowance.




The amount of taper depends upon the following factors.

(a) Height and size of pattern

(b) Moulding method, and

(c) Mould materials

The common draft provided on the pattern is 1° to 3°. Otherwise,

For taper on external surface, 10 to 25 mm / m is provided.

For taper on internal surface, 40 to 65 mm/m is provided.

4. Distortion or Camber Allowance

The casting may distort or wrap during cooling if it is an irregular shape, flat long casting surface and U or V shape. All surfaces do not shrink uniformly. The arms having unequal thickness are also the reason for distortion. Due to distortion, the casting will not get the required shape. It may bend. To avoid this, the shape of the pattern is slightly bent to the opposite direction. So, the casting neutralizes the initial distortion given on the pattern and gets the correct shape after cooling.

For example, a casting of U shape may distort and legs become divergent instead of being parallel. To avoid this, the legs are made convergent as shown in Figure 1.11 instead of being parallel. Se, the legs are become parallel after cooling.




5. Rapping or Shake Allowance

To remove the pattern out of mould cavity, it is slightly rapped or shaked to detach it from the mould cavity. This is called rapping. Due to rapping, the mould cavity may become large. To avoid this, the pattern is made slightly smaller than the required casting. This allowance given on the pattern is called rapping or shake allowance. As the allowance is subtracted from pattern dimensions, it is known as negative allowance.