274 APPLIED SCIENCE 



they will not drill the initial hole. Some twist drills are 

 made with a small groove cut around the outside, which 

 contains a tube for carrying oil to the drill point. 



313. Operation of Reaming. It is difficult, if not quite 

 impossible, to drill a hole to an exact diameter. For most 

 work, however, a variation of a few hundredths of an inch 

 is of no account, but when greater accuracy is required the 

 hole must be reamed. Holes to be reamed are first drilled 

 a little smaller than the desired size ( a V in. or even T io in.), 

 and then reamed out to exact size. They should never 

 be drilled over ^ in. smaller than the size of the reamer. 

 Reaming is especially necessary where two or more parts 

 are to be bolted together, since the drill in passing through 

 them will often cut more out of one part than another because 

 of the variation in the structure of the metal. 



Reaming may be done by hand or with a drilling machine; 

 or the reamer may be held in the drilling machine, or in the 

 drill spindle socket and turned by hand with a wrench. 

 Reaming should be done very carefully, and it may be neces- 

 sary to tap the reamer gently with a hammer or wrench 

 to feed it. There should be no "wobbling" or irregular mo- 

 tion, but a very steady and slow motion under light pressure. 

 In some cases the weight of the reamer and the wrench is 

 sufficient to feed the tool through the hole. 



In reaming with a drill press a power feed may be used 

 in some cases, but great care must then be exercised to see 

 that the reamer does not stick and break. Some reamers 

 have a shallow screw thread cut on the small end which 

 makes them self-feeding. Oil, drilling compound, or some 

 other lubricant should always be used when reaming wrought 

 iron or steel, but not when reaming cast iron or brass. 



