Small shops that do a variety of work in sheet metal have frequent use for circle cutters of the type that can be driven by a drill press. Home craftsmen also find use for these cutters in toymaking and ornamental metalwork. Pictured are two designs, one made from aluminum alloy especially for light work in wood, hardboard, plastics and soft metals, the other for the heavy-duty jobs in metals. The latter type, shown in Fig. 1 and pictured in use in Fig. 2, is fully detailed in Fig. 6. The shank is machined with a No. 2 Morse taper to fit a drill-press spindle, or sleeve, and it also is turned with an integral pilot. The lightweight job detailed in Figs. 3 and 5 and pictured in operation in Fig. 4, clamps directly to the taper spindle of the drill press into which the pilot drill, of the tapered-shank type, is fitted.
Although the best workmanship would require that the square holes in the shank and tool-holder arm, Fig. 6, be broached, it is possible to drill holes and square them with a file with sufficient accuracy. However, the tool-holder arm must be a smooth, sliding fit in the squared hole in the shank to prevent chattering in heavy cuts. The tool-holder arm, Figs. 1 and 6. is threaded its full length and flats are milled on the four sides, bringing the dimension across the flats to 5/8 in. The length of the arm gives a maximum radius adjustment of approximately 5 in. The lightweight cutter, Figs. 3. 4 and 5, does an exceptionally smooth, clean job on a variety of materials. When carefully made, it is practically chatterproof. Both the cutting tool and the tool-holder arm are held in place with binding clamps of special design and these two parts are attached to the taper sleeve with a binding clamp of sufficient length to hold the parts in exact alignment. Note that the cutting bit is supported in a groove milled across the face of the binding-clamp seat and that the groove is slightly less than 1/4 in. deep, Fig. 3. This allowance is necessary to permit the clamp to seat the bit firmly. The upper details in Fig. 5 show how to grind the bit for cutting holes and blanks in thin material. When ground for hole cutting, the bit leaves the outer edge smooth. When ground for blank cutting, the inner edge of the cut is smooth. For work in metals, the speed of the cutter should not exceed 100 r.p.m.