Feed Choice

Feed Choice

Feed Choice - When a finishing cut is to be taken, the appropriate feed will be that which will give an acceptable surface finish. In this case the choice of feed is in the hands of the designer who will have specified the surface finish requirements. To explain how the appropriate feed is chosen in a roughing operation, it is first necessary to consider the relative effects of cutting speed and feed on tool life.

It is known that if changes in cutting speed affect neither the geometry of the cutting operation nor the specific energy consumption, the tool temperatures are a function of the thermal number, ρcvac /k, where k is thermal conductivity, c is specific heat capacity, ρ is density, v is the cutting speed and ac is the underformed chip thickness.

Thus equal changes in speed or feed should affect the tool temperatures by the same amount. However, an increase in feed will not affect the relative speed of sliding at the wearing surface of the tool, whereas the speed of sliding will change in proportion to the cutting speed. Since tool wear is a function of both temperature and relative speed of sliding, it can be appreciated that increases in cutting speed will result in a greater reduction in tool life than similar increases in feed.

This conclusion agrees with practical experience and means that if an increased production rate is required in rough machining, it will always pay to increase the feed rather than increase the speed. Of course, this procedure will not always be practical since, in general, an increase in feed will increase the tool forces whereas an increase in cutting speed will not.

A limit on feed increase will therefore exist and will depend on the maximum tool force the machine tool is able to withstand. The guiding principle in choosing optimum cutting conditions in a roughing operation is that the feed should always be set at the maximum possible.