PATTERN ALLOWANCES

PATTERN ALLOWANCES:

Since most metals shrink in volume, when solidifying from liquid state and again on cooling, it is obvious, that the pattern should be made slightly larger than the size of finished casting. This difference in size of the pattern is called shrinkage allowance. For cast iron, this allowance is 1% and for aluminium, it is about 1.6%.

On many occasions, castings produced in the foundry shop are machined subsequently. The object of machining is to get exact sizes and better surface finish on the component. If such is the case, a layer of 1.5–2.5 mm thick material has to be provided all round the casting. This is done by making the pattern suitably bigger than the casting. This increase in size of pattern is called “machining allowance”.

Another important allowance provided on patterns is called draft allowance. It facilitates withdrawal of pattern from the mould. It is provided on vertical surfaces. The idea is to give an inclination of 2–3 degrees to vertical surfaces, so that while lifting the pattern, the upper surface is wider and withdrawing the pattern with draft provided will not damage the sand mould. On inner vertical surfaces, draft is provided in such a way that top surface is narrower and bottom portion of pattern is wider.

Apart from the above allowances, some other allowances are sometimes given to compensate for inherent distortion or bending of castings. Sharp corners and bends are also radiusized while making a pattern.

Patterns are usually made of good quality wood. Wood is easy to work, acquires good smooth surface and properly seasoned wood retains its size. It is also relatively cheap and abundant. However, if a very large number of castings are required, metal patterns may be used. Usually, they are made of aluminium-magnesium alloys.