PATTERN MAKING 99 



For the beginner the following formula is safe to follow: 



in which 



6 = the breadth of the arm at the outer end, 

 d = the outside diameter of the pulley, 

 w = the width of the rim, 

 n = the number of arms, 



all dimensions being taken in inches. Thus, for a pulley 24 inches 

 in diameter with a rim 6 inches wide and fitted with 5 arms, the 

 formula becomes- 



5X8 



Hence, 5 = 1.53 inches or 1^ inches. 



The width of the arm should be one-fourth greater at the hub 

 than at the rim. The thickness at the hub and rim should be one- 

 half the width, and the section should be elliptical. The arm just 

 calculated then becomes, 



1^4 inches wide at rim, 



% inch thick at rim, 

 1^ inches wide at hub, 

 1 inch thick at hub. 



For the skeleton pattern last described, the common method 

 of constructing the pattern for the arms, is to make each arm of a 

 separate piece of wood with the grain running in the general direc- 

 tion of the arm, and to fasten them together at the center with glue 

 and a flat plate or disc, which can also be used as a rapping plate. 

 This pattern need be parted only in the case of very large and 

 heavy wheels. For all ordinary work it can be made in one piece 

 and moulded as directed in connection with the hand wheel, 

 Fig. 167. 



HUBS. 



An ordinary rule is to make the outside diameter of the hub 

 twice the diameter of the shaft. The two half hubs (one on each 

 side of the arms) are usually loose and are held central by a single 

 dowel pin. Their diameters are adapted to the size of the shaft 

 upon which the pulley is to run, and the length is proportioned to 



