PATTERN MAKING 



67 



the cutting tool of the finishing machine to cut well below the 

 surface so that it shall not become (lulled by the sand and hard 

 scale on the outside. 



A pattern for the plain cast-iron bar illustrated in Fig. 136 

 will afford a good example of the allowance necessary for finish 

 and for draft. This bar is to be finished all over, the finished 

 sizes being 36 inches long, 1 inch wide, and 1 inch thick. 



O * _ 



o O 



Fig. 136. 



A slender bar of this length is liable to warp or bend when 

 cooling in the mould, and for this reason the bar should have an al- 

 lowance of at least inch all over for finish, thus requiring a pattern 

 36} inches long, li inches wide, and 1J inches 



ft rt 



thick. Moreover, to enable the moulder to 

 remove the pattern from the sand without 

 injury to the mould, we must add on two of 

 the opposite sides a draft of about \ inch to 

 the foot, making a cross-section through the 

 pattern of the shape and dimensions as shown 

 in Fig. 137. 



When accuracy is required in testing bars 

 36 inches X 1 inch X 1 inch (which are 



_i 



Fig. 137. 



seldom finished), they are often moulded 

 partly in the cope and partly in the 

 drag, as shown in Fig. 138, the parting 

 being 011 the line a b. in this position 

 the inclination of the sides of the pat- 

 tern in the mould is so great that no 

 draft is required, the pattern being 

 simply a square bar of wood of dimensions of 36 inches X 1 

 X 1 inch, measured with the shrinkage rule. 



Fig. 138. 



inch 



