64 FARM MECHANICS 



piece of work may be turned out. Such work is valu- 

 able because of the instruction. A good test of skill at 

 blacksmithing is making an octagon punch that tapers 

 true to the eye when finished. 



Set-Screws. It is customary to fasten a good many 

 gear wheels, cranks and pulleys to machinery shafts 

 by set-screws. There are two kinds of set-screws ; one 

 has a cone point, the other a cup end. Both screws are 

 hardened to sink into the shaft. A cup is supposed to 

 cut a ring and the point is supposed to sink into the 

 shaft to make a small hole sufficient to keep the wheel 



Figure 94. Machine-Bolt and Set-Screw. The bolt to the left is 

 used to clamp cylinder heads in place. The set-screw to the right is 

 the cup variety. The end is countersunk to form a cup with a 

 sharp rim. 



from slipping. However, unless the cone-pointed screw 

 is countersunk into the shaft, it will not hold much of 

 a strain. The point is so small it will slip and cut a 

 groove around -the shaft. To prevent this, the set-screw 

 may be countersunk by first marking the shaft with an 

 indentation of the point of the screw. Then the wheel ' 

 or crank or collar may be removed and a hole drilled 

 into the shaft with a .twist-drill the same size, or a 

 sixty-fourth smaller, than the set-screw. Then by 

 forcing the end of the set-screw into the drill hole, the 

 wheel is held solid. 



The principal objection to set-screws is that they are 

 dangerous. The heads always project and are ready 

 to catch a coat sleeve when the shaft is revolving. In 

 all cases, set-screws should be as large as the hub will 



