100 PATTERN MAKING 



the width of the rim as well as its diameter. The length of the 

 hub should be about two-thirds the width of the rim except in the 

 cases of tight and loose pulleys, where the hub should be a trine 

 longer than the width of the rim. It may then project about fa inch 

 on the sides in contact, and ^ inch on the outside. 



RAPPING PLATES. 



In the description of the making of the pulley pattern, the 

 ring serving as a binder for the hub is spoken of as a rapping plate. 

 When a pattern is imbedded in the sand, the latter is closely com- 

 pressed all about it, and slightly adheres. The moulder is, there- 

 fore, in the habit of rapping the pattern gently in order to loosen 

 it in the sand before attempting to draw it. If the pattern is not 

 provided with a metal plate, the moulder will drive the sharp point 

 of a lifter into the wood and strike it alternately on opposite sides 

 and at the same time use it to lift the pattern from the sand. This 

 mars the pattern and will in time ruin it. The rapping plate, 

 shown in the engraving, is a piece of thin metal 

 i to -j%- inch thick, inserted so that it is nush 

 with the parting face of the pattern and is held 

 by wood screws with countersunk heads. These 

 plates are drilled and tapped for a -inch screw 



l and should be the same for all patterns in the 



Plate. foundry so that one set of rods can be used 

 interchangeably. The method of using is to screw the rod into 

 the plate and rap it gently to and fro until the pattern has been 

 loosened, when it may be lifted. For small patterns, one rapping 

 plate will be sufficient and this should be so placed that the hole 

 for the lifting rod comes directly over the center of gravity of the 

 piece. This w r ill prevent tilting of the pattern as it is lifted from 

 the sand. For medium sized patterns, two rapping plates should 

 be provided, 8O that the pattern can be raised from two opposite 

 sides. For still larger patterns three or four rapping plates are 

 used; the object being to give such perfect control when drawing 

 that there can be no tearing away of the sand. 



METAL PATTERNS. 



M'-ial patterns are extensively used where either one of two 

 conditions prevail: first, where the character of the work is so li^ht 



