These are used to advantage in chucking machines, for enlarging cored holes or holes that have been drilled smaller than the required size. Large holes in solid stock are often made below size, as most manufacturers consider it more economical to use a smaller drill and a roughing reamer to bring them to proper size for the final reamer. Fig. 60 shows a reamer of this description.
The instructions already given for making the various reamers may be followed for this form, with the exception of cutting the grooves, which should be of a sufficient size to hold the chips. The small groove cut in the center of the lands is to feed oil to the cutting edges when cutting steel. When cast iron is the material to be operated on, the grooves are cut straight and the oil groove omitted.
If a finish reamer is to be used in sizing the holes, it is customary to make the roughing reamer 1/54 inch smaller than finish size. On account of the rough usage, great care should be exercised in hardening. While satisfactory results may be obtained by heating them to a low red, plunging them into a bath of brine, and drawing the temper to a light straw, the tools will do a great deal more if they are pack hardened.