Report 9: Measurement of Transient Surface Temperatures in Metal Cutting

This report is concerned with measuring the varying thermal stresses in a metal-cutting tool, due to the temperature variations occurring at the tool-chip interface, in the course of intermittent cutting operations. During cutting, the tool is rapidly heated, developing compressive thermal stresses in addition to the mechanical stresses due to the cutting action, but during the non-cutting, or cooling, part of the cycle these latter stresses are absent, and a tensile stress develops on the tool surface. It is believed that these rapidly alternating stresses lead to tool failure by eventual chipping of the cutting edge. The temperature at the interface is measured by means of the tool-work thermocouple technique, in which a tungsten carbide cutting tool contacts a steel workpiece, thereby forming the hot junction of a thermocouple system. The technique is well established for temperature measurements during continuous cutting operations, but care must be taken if it is to provide reliable results. The extension of the technique to include intermittent processes is described, and results are shown for processes having a frequency of 15 c/s. Cutting times as small as 0·026 s have been used and temperature records for this, and greater cutting times, are presented. A disadvantage of this method is that a temperature record is obtained only whilst the tool and work are in contact, and a method to overcome this is outlined.