Pattern Allowances

The surface finish of the casting product may not be as good as required, therefore, extra dimensions in the pattern are provided. The extra dimensions or extra materials provided for the pattern are known as allowances. The following allowances are provided for pattern making:

• Draft allowance/Taper allowance.
• Machining allowance.
• Shrinkage allowance.
• Distortion allowance.
• Shaking allowance/Rapping allowance.

Draft Allowance: To exit out the pattern from the mould easily, the surfaces of the pattern are made taper. The larger dimension side of the pattern is at the parting line. The taper provided may be 1°– 3°. When small jerk is given to pattern to exit out from the mould, air enters into the small clearance created due to the jerk and breaks the contact between pattern and mould surfaces. The inner side surface of the pattern is provided more taper angle because during solidification metal shrink towards the core. The amount of draft allowance depends on the material used for mould making shape, size of the pattern, etc.

Shaking/Rapping Allowance: To remove the pattern from the mould, pattern is rapped with the help of draw spike so that they can be detached from the mould. But due to rapping, the cavity in the mould gets enlarged. Therefore, the pattern is made smaller than the casting, which is known as shaking allowance. This is negative allowance.

Machining Allowance: The dimensional accuracy and surface finish of the casting (especially sand casting) is poor. Therefore, machining is required for good surface finish and dimensional accuracy; to compensate the removal of unwanted materials, extra materials are provided to the pattern, which is known as machining allowance. Machining allowance depends on type of casting process, for example, machining allowance in die casting is very small in comparison to sand casting.

Shrinkage/Contraction Allowance: Most of the metals occupy more volume in molten state in comparison to solid state. When molten metal is poured into a mould cavity there is shrinkage in metal during solidification. When metal is transferred from molten state to solid state there is shrinkage and from hot solid state to room temperature solid state, there is additional shrinkage. So the volume of pattern is larger than the casting. The extra dimension provided to the pattern to compensate the shrinkage is known as shrinkage allowance.

Distortion Allowance: Distortion in casting occurs in the process of cooling. It occurs due to differential solidification and setup of thermal stress. It applies to casting of irregular shape. To eliminate this defect, an opposite allowance of equal amount is provided in the pattern, which is known as distortion allowance.