PROBLEMS IN PATTERN-MAKING 385 



428. Allowance for Finish. The allowance for finish is 

 the amount added to a pattern to allow for machining in the 

 machine-shop. As all castings are more or less rough when 

 they come from the mold sand, and as they warp or spring 

 in cooling as well as contract or shrink unequally, it is neces- 

 sary to allow for their being machined to the required size 

 and shape. No set rule can be laid down to cover the allow- 

 ance for these irregularities. On small iron or brass castings, 

 an allowance of -^ to J/g m - is usually sufficient. Under the 

 modern method of mounting small patterns on a plate and 

 molding them by machine, this allowance may be reduced 

 to a minimum. 



The allowance for finish on steel castings must be greater in 

 all cases than for iron castings because steel has a greater 

 shrinkage, contracts unequally, and warps. The allowance 

 should be from M to J^ in., but in many cases more is allowed. 

 In every instance, however, much of the allowance for finish 

 is made necessary by the distortion of the casting in cooling. 



429. The Match or Odd Side. A device to aid the mold- 

 er to obtain quickly and easily a "parting line" in the mold 

 is called a match. It supports the pattern and consists of 

 a false part of the flask on which the drag is rammed. The 

 match is about 3 in. deep and is made of some material con- 

 fined within a wooden box or part of a flask to which a bottom 

 board is securely fastened, the pattern or patterns being 

 partly embedded. 



Many different compositions are used to make up the hard 

 support for the pattern. Sand, mixed with linseed oil and 

 baked after shaping, is a very good mixture. Sand mixed 

 with litharge also gives a very hard match, as does plaster 

 of Paris. While this latter composition is more brittle than 



