272 APPLIED SCIENCE 



have a tendency to " draw in " when breaking the hole through. 

 The twist drill is the most rapid cutter of the three, and is 

 especially desirable for work on very hard woods or heavy 

 metals, and for work where a deep hole is to be drilled. The 

 twist drill, besides presenting a cutting edge at the point, carries 

 the chips up to the surface, and thus prevents clogging. It is, 

 therefore, unnecessary to remove a twist drill from the hole 

 to clear it of chips. 



311. Drills and Drilling. Most drills are made from 

 round bars of tool steel hardened and tempered to suit the 

 work to be performed, generally to a dark straw color. The 

 flat drill is made in the shop and is used because it is cheap. 

 It is impossible to drill a hole accurately with a flat drill, 

 although such a drill does very well for rough work in the 

 boiler or smithshop. The flat drill was the first form of 

 drill made, but later it was found to wear out quickly and 

 require frequent grinding. Thus the cutting edge or lip 

 wore away so rapidly that the drill soon had to be redressed 

 (made over) by the tool-maker. To overcome this fault the 

 lips were twisted into a curve or spiral. This improvement 

 was found to give a cutting edge which did not change its 

 shape when the drill was reground. Thus it is evident that 

 the twisted flat drill led to the fluted twist drill and later to 

 the flat twist drill. The former drill is a round bar of tool 

 steel having two straight grooves or flutes cut on opposite 

 sides. This form of drill is used for drilling in brass, copper, 

 or Babbitt metal. 



312. Mechanism of Drill Points. To understand the 

 principle of drilling efficiently, it is necessary to study the 

 mechanism of a drill point. Drills are used to separate small 



