When grooves must be formed deep within a hole their shape, size and concentricity often can be improved through the use of a revolving recess milling attachment.

The holder itself mounts to a wedge-type drawback recessing attachment in which the tool is held in a spindle mounted on precision bearings in such a manner that it can be driven by a splined shaft from the screw machine's gear box, as an accelerated drill, or die head would be.

The tool generally is a one piece item resembling a Woodruff key cutter. Using this combination, close limit recesses six diameters back from the face of the hole have been produced routinely. In one application, a ganged high speed steel cutter with 5/8 shank is producing four grooves simultaneously. The rearmost groove is 3-3/8 inches from the outer edge of the hole. Largest of the four diameters recessed is 3/4 inch.

This tool is by no means a cure-all. If a conventional recessing tool will work as well it should be used. Recess milling appears to pay off best when (1) recesses are wide or deep in relation to shank diameter of the tool, (2) when close concentricity is needed between a recess and the spindle, or (3) when the shape or shapes to be recessed are complex and can be ground in a milling cutter more easily than in a single point tool. It follows that the method also applies best for long run operations, particularly in the more difficult materials.

From a feed and speed standpoint, the mathematics of recess milling are a bit more involved than for other conventional spline driven tools. As an example, assume a 3/4 inch diameter recessing cutter is working in a one inch hole, and is being driven at 1:1 ratio counter to a machine spindle running at 508 rpm:

If, instead of a revolving cutter, a standard recess tool were used, cutting speed would be 135 sfm (one-inch at 508rpm). If the work were stationary and the cutter revolved, sfm would be 100 (3/4 inch at 508 rpm). Hence, the effective cutting speed for this operationis the sum of the two sfm's, or about 235. Feed will be the per-revolution sidewise motion of the recessing holder divided by number of teeth in the cutter.

From this the mechanical advantage of a revolving tool will be apparent; a lightly fed, high speed cut, which is less apt to deflect the tool's shank, results. Field reports indicate users are holding recessed dimensions to within .003 inch total deviation by this means, including the very slight scallop effect which is sometimes noted. The extremely light per tooth feed yields fine chips.