"Polygon" Attachments are used on a cross slide to produce flats, polygons, threads and certain slots in round material without stopping the spindle.

These operations are best suited for non-ferrous materials because of the very high surface cutting speed developed by the cutter.

Usually the flats are done with a plunging operation using a form tool cam.  On machines or attachments with axial movement, the flats can be machined by longitudinal turning, which facilitates extra wide flats or flats in difficult materials.

Some companies do flat generation in mild steels, but they are willing to accept very low insert life.  Thread milling will not work in steel.

Polygon cutters use carbide inserts, either single edge re-sharpenable or multi-edge throw-away types.

Thread Milling cutters are HSS and have the thread form ground on the O.D.  This way you can make virtually any thread in non-ferrous materials, even tapered or close to a shoulder.  No shaving is required before thread milling.