PATTERN MAKING 



101 



and delicate thaf a wooden pattern could not hold together, as in 

 ornamental castings ; and second, where such a large number of 

 castings are to be made that the wooden pattern would not last 

 long eiioiigh to complete the work. Metal patterns may be made 

 of iron, brass, or aluminum. The latter metal is corning into great 

 favor because of its light weight and its freedom from corrosion by 

 the moist sand. These patterns should be given a light coating of 

 shellac varnish, but it is not absolutely necessary. Where iron is 

 used, some preservative must be put upon the surfaces to protect 

 them from rust. The best method is to warm the metal and rub it 

 with a rag dipped in melted beeswax. This excludes the air arid 

 leaves a smooth surface so that it is easily drawn out of the sand. 

 This, however, is not a very durable protection ; the more common 

 method is to use a shellac varnish. In order that the varnish may 

 adhere, the metal should first be wet with a solution of sal ammo- 

 niac, and, when dry, sand-papered and shellaced. 



In the small 12-inch crank pattern shown in Fig. 176, is shown 

 a very simple one-piece pattern. In spite of its simplicity it requires 

 considerable skill in shaping 

 and in obtaining the neces- 

 sary draft. The parting of 

 the mould will be on the line 

 C D. The piece e, for the 

 main body of the pattern, O~ 

 should be made rectangular in 

 form, and laid off with center 

 lines from which the positions Fig. 176. 



of the bosses c, b, and d, and the core print x may be drawn on the upper 

 and lower sides respectively. The bosses are turned on the lathe 

 to the required form, and given a draft of \ inch to 12 inches. After 

 b and c have been glued on, the part e is sawed to shape, sawing 

 close to, but not touching J and c with the saw. The thin boss d 

 is next glued in place, after which e is filed and dressed to the 

 required shape and even with 5 and c, giving it the same draft, 

 \ inch to 12 inches, but in the opposite direction from the parting 

 line C D. The rib a is next turned on the lathe, and one side split 

 off on the band saw, after which it is fitted between ?> and c. The 

 core prints may be turned with a small tenon on one end to fit into 



Section 

 throuqh AB 



