PATTERN MAKING 



135 



that it fits perfectly the outer rim of the tooth block. Next make 

 a thin blade of hard wood and screw to the head A, using the 

 greatest care to have one edge of the blade coincide exactly with 

 the line c. After screwing the blade to the head, its accuracy may 

 be tested by placing a try square against the edge a. The result 

 will be as shown in Fig. 249, in which the edge c is radial to the 

 arc x y. This edge will describe the center lines of the teeth 

 radially as required. 



Fig. 247. 



i 



This temporary square can be used up to a certain limit, on 

 wheels of larger diameter than that to which it has been fitted, but 

 cannot be used for smaller wheels. For larger gears the position 

 will be as shown in Fig. 250, which will give the correct per- 

 pendicular if the angles at x and y are carefully made. By using 

 in this way, only a few squares will be needed for a great number 



