Distortion or Camber Allowance
The distortion in casting can also happen due to its typical shape. For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end causing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical (Fig. 6.3.2). This allowance is known as distortion allowance.
The distortion in the casting, because of the shape of the casting may occur due to internal stresses, which are developed due to unequal cooling of different section of the casting and hindered contraction. Measure taken to prevent the distortion in casting include:
i. Modification of casting design
ii. Providing sufficient machining allowance to cover the distortion affect
iii. Providing suitable allowance on the pattern, called camber or distortion allowance
Rapping Allowance
When a pattern is removed from the molding sand aggregate, the pattern is rapped all around its vertical sides. This rapping of the pattern is done to enlarge the mould cavity on its vertical sides so that the pattern removal from the mould becomes smooth. Since it enlarges the final casting made, it is desirable that the original pattern dimension should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. It is a negative allowance and is to be applied only to those dimensions that are parallel to the parting plane.
