The patterns are not made the exact size as the desired casting because such a pattern would produce undersize casting. When a pattern is prepared, certain allowances are given on the sizes specified in the drawing so that the finished and machined casting produced from the pattern will conform to the specified sizes. While designing pattern, the allowances commonly considered are discussed below. 1. Shrinkage Allowance
Generally metals shrink in size during solidification and cooling in the mould. So casting becomes smaller than the pattern and the mould cavity. Therefore, to compensate for this, mould and the pattern should be made larger than the casting by the amount of shrinkage. The amount of compensation for shrinkage is called the shrinkage allowance. Generally shrinkage of casting varies not only with material but also with shape, thickness, casting temperature, mould temperature, and mould strength. Therefore, it is better to determine the amount of shrinkage according to the past record obtained from many experiences. Table 1.2 shows an average amount of shrinkage for important cast metals. Table 1.2 Typical shrinkage allowances for important casting metals Type of metal Amount of shrinkage (%) Grey cast irons 0.55-1.00 White cast irons 2.10 Malleable cast irons 1.00 Steels 2.00 Manganese steel 2.60 Magnesium 1.80 Type of metal Zinc Brasses Bronzes Aluminium Aluminium alloys Tin Amount of shrinkage (%) 2.60 1.30-1.55 1.05-2.10 1.65 1.30-1.60 2.00
In practice, pattern makers use a special rule or scale, called the “pattern maker’s contraction rule”, which, after providing with necessary allowance, is slightly longer than the ordinary rule of the same length. The graduations are oversized by a proportionate amount, for example, when constructing a pattern for aluminium alloys, the pattern maker uses a contraction rule measuring about 12 mm longer per metre than the conventional rule because aluminium alloys shrinks 10 mm per...
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