The cutting of cylindrical holes of greater or less size in metals is a very varied requirement in machine-shop practice and many methods are employed for accomplishing this work, depending upon the diameter and length of the hole. Small holes are drilled on the drilling machine, very long holes, as in gun barrels or propeller shafting, are drilled in the boring lathe, and cylindrical holes of large diameter, as in a steam cylinder, are bored on the boring mill. Usually a hole is cast in a casting to reduce to a minimum the work of boring, and in a less degree forgings are made hollow, when so required, for the same purpose. Generally the term drilling is applied to the cutting of small holes by a drill, and boring is applied to the cutting of larger holes which may or may not have been previously made in casting or forgings.
Fig. 187. - Hand Chasers.
There are many different styles of drilling machines. The principal styles among these are (1) the vertical drill; (2) the radial drill; (3) the gang drill, and (4) the multiple spindle drill. There are many sizes of each of these styles designed for particular kinds of work.