Thread Grinding
Diamond Tools and Crushing Equipment for Form-Dressing Wheels
Prepared by the Tool Technical Panel of the Diamond Die and Tool Control, Ministry of Supply.

The grinding of screw threads is now a regular manufacturing operation that finds a wide application in engineering production. It is not so well understood as it deserves to be, and the purpose of this article is to give a short review of current practice and particularly to direct attention to points that will ensure the most economical and effective use of diamond tools.

Grinding is applied to the production of practically all types of thread, single- and multi-start, with a range of pitches between approximately 2 and 6o threads per inch (tpi). The range of work being treated covers thread gauges, many types of thread-producing tools, such as taps, thread-milling hobs and chasers, as well as components of machine tools, aircraft engines and the like. The process shows to particular advantage in the case of hardened pieces where threads finer than about 12 tpi can generally be ground on the solid blank, thus eliminating thread-cutting operations before hardening and possible ill effects of heat treatment, such as distortion and over- or de-carburisation of the surface of the blanks. Grinding is also very usefully applied to screw threads on tubes or similar thin-walled components to overcome the difficulties mentioned above in case of heat-treated parts and to avoid the stresses to which they would be subjected during ordinary thread-cutting operations.

Essentially, thread-grinding consists in using a grinding wheel in place of a single-point tool, chaser or thread-milling hob, giving the superior precision and finish usually associated with grinding. The grinding wheel is formed with one rib, or a series of ribs, around its periphery having the exact form and pitch of the thread to be ground. These ribs are formed either by traversing with a suitably shaped diamond tool or by crushing the form into the wheel by means of a hardened steel roller. Usually the grinding wheel is set over to the exact helix angle of the thread to be ground, although the wheel may be dressed to a specially compensated form that will produce the correct form on the work from a vertical wheel.

One method of form dressing with a diamond consists of a pantograph mechanism employing an enlarged master thread form cut from a steel plate and a stylus having the same form as the diamond, but equally enlarged; the movement of the stylus over the thread form is copied and reduced by the mechanism, then applied to the diamond, which reproduces the form on the grinding wheel. This mechanism is usually mounted at the rear of the grinding wheel. In another method, the diamond is traversed across the face of the wheel by the leadscrew, while interchangeable cams, geared to the leadscrew, are employed to feed the diamond into and out of the wheel in the correct manner to form the desired thread shape. In some cases two diamonds are used, each dressing one half of the thread form, and on one machine the diamond used for finish dressing is cone shaped, with a spherical radius at its apex. The form-dressing mechanism has been simplified on several machines by using a diamond having its nose formed to the exact radius as the root of the thread.