428 APPLIED SCIENCE 



The latest machine of this class instead of permitting only 

 one bar of stock at a time to be fed into the machine, and 

 the tools advanced in order, allows five bars to be fed at once, 

 five operations being performed at the same time. The only 

 skill required to operate the machine is in the setting up of 

 the tools and in the adjustment of the machine. The man 

 who does this is not necessarily a machinist, but he must 

 understand the screw machine and give close attention to the 

 details of its operation. 



494. Drilling. A different cutting speed is necessary for 

 nearly every material, and drills are constructed so as to meet 

 the various requirements. A large drill must run slower than 

 a small one, the turns per minute becoming less as the size of 

 the drill increases. For example, a drill J^ in. in diameter 

 should make twice as many revolutions per minute as a 1 in. 

 drill, and four times as many as one 2 in. in diameter. For 

 this reason, there must be a number of different feeds, as well 

 as speeds. Hand-feeding is to be preferred to machine-feeding 

 for small or delicate work, since it is possible to "feel" the drill 

 and keep it going through the work at a varying rate accord- 

 ing as the metal is hard or soft or contains blow-holes and 

 imperfections. A heavy power-driven feed will sometimes 

 break quickly into a blow-hole and spoil the drill or the work. 



The cone pulley shown in Fig. 143b (page 288) gives three 

 different speeds to the drill spindle and the drill. There are 

 two cone pulleys, an upper and a lower; when the belt is 

 on the smallest step of the upper cone pulley, the fastest 

 speed is obtained. 



The drill table generally swings about the post or upright 

 frame of the machine, thus allowing the work to be adjusted 

 under the spindle. Some machines are provided with tables 



