Cutting Angles of Tool Bits

Out in the Tool Shed

by H.J. Coventry

The Miniature Locomotive, November-December 1953

The table of angles to which to grind lathe tools may assist the new recruit or model maker unfamiliar with machine shop work. The angles given are measured from the horizontal base of the bit if this surface is level with tool post slide. If on the other hand any of the standard tool holders are used which lock the bit at an angle to the slide, then the TOP RAKE viz angle "e" will either be already set by the holder or even a negative rake will be required. Put in another term what is needed is to establish the angles in relation to the vertical and horizontal center lines of the work piece. The angles are those most used for the various materials and the cutting speeds are reasonable for model work using high speed tool bits. If carbon tools are used, reduce the speed to half.

For those who do not have any form of protractor, the table of tangents may be used to make templates. Lay out a base line 10" long, and at one end erect a vertical equal to the value of "y" as given for each angle. Join the extremities.

The tool shape shown to right of sketch is probably more used than any other, and is particularly useful for any interrupted cut on castings as the actual cutting is first done back from the tip, thus avoiding breakage of the point. It also enables the tool slide to clear the work.

After the tool is ground, the faces should be well oil-stoned, the smoother the cutting edges and surfaces, the better the quality of the work with properly shaped tools there should be no excuse for using file or abrasive cloth. For example say we are using tool shown at right sketch, on a steel shaft of low carbon or soft steel quality, rough the job to say within 10 thousandths of finish by feeding towards lathe headstock, then at the end of cut, feed tool forward 5 thousandths and trail the tool back toward the tailstock. With lard oil or soluble oil solution dripped on the tool point, you will have a finish that would be spoiled if emeryed.