134 



PATTERN MAKING 



Drawing. The arms, partly shown in Fig. 246, in place in Fig. 

 244, are next fitted and fastened to the rim. It is well to glue a 

 small disc on each side of the web of the arms as shown in Fig. 

 246, which not only strengthens the arms, but serves as a fillet 

 around the hub of the wheel. 



The blocks for the teeth are next fitted in place, either as 

 illustrated in Fig, 247, or in the form of alternate blocks and 

 strips as was shown for the spur gear, Fig. 237. After all the 

 blocks are in place, the wheel must be put in the lathe and 

 turned to the sizes and angles required for laying out the teeth. 

 A sharp line must be drawn on the face of the blocks, while 

 in the lathe, to serve as a guide for the dividers while spacing the 

 teeth. 



a 



Fie. 246. 



Fig. 248. 



To obtain the center lines for the tooth faces after spacing on 

 the blocks, it will be readily seen that the ordinary try square 

 cannot be used as in the case of the spur gears. A teihporary 

 square must be made for this purpose as follows: 



Take a piece of hard wood, about 6 inches long and 3| inches 

 wide and \ inch in thickness. Dress the two edges perfectly 

 parallel and from the upper edge , Fig. 248, with a try square and 

 a sharp pointed knife, draw the line r, equally distant from each 

 end of A, and at right angles to the edge a. Lay the edge l> 

 of A. against another board J5, of the same thickness, and continue 

 the line c OL this board as shown by the dotted line. With the 

 dividers set to a radius equal to the longest radius of the outside 

 ends of the tooth blocks, from the extended line < on the board '15. 

 describe the arc. x // on A. Cut the edge A, to this are. and 



