This is a method of grinding external and internal cylindrical surfaces to a finer degree of accuracy (1/10000 inch) than can be obtained with certainty in the use of a grinding wheel. It is under more certain control and is employed to finish wheelground work when the highest attainable degree of accuracy is required.

The process consists of revolving the work in a grinding lathe while it is lightly held in a lead-lined clamp on which is smeared a thin coating of oil and very fine grains of grinding material. For internal grinding, a lead-covered iron plug, or a plug with lead ribs cast in longitudinal grooves, is used to carry the grinding material. The hard grains of grinding material become unbedded in the soft lead and their grinding action is guided by the contact of the lead itself with the surface to be ground.

Fig. 268. - Armor Plate Hardening.