SECTION 4.4
The effects of controlling factors on tool wear

When we consider the controlling factors upon tool wear, we are concerned either with modifications that influence the cutting process directly for given tool and workpiece materials, or with inherent properties of materials that resist or promote wear. For a given tool and workpiece material combination, cutting temperatures are influenced most by cutting speed, and to a lesser extent, feed and depth of cut. Adjustments in speeds or feeds, or both, will affect tool wear. It may be possible to substitute another tool material or a coated tool material that has inherently better temperature-resistant properties to maintain original or even higher production rates with less sensitivity to temperature failure. The cost of the second material may be higher than that of the first, but it may be more than justified by higher production rates at increased operating temperatures.

Changes in tool geometry that result in higher shear angles, less chip distortion, lower frictional resistance, and thinner chips will lower cutting forces and decrease cutting temperatures, and thus contribute to a reduction in the rate of tool wear for given cutting conditions. Within practical design limitations, rake, relief, radii, etc. should be matched to the application providing the most free-cutting strong geometry that directs the cutting forces in the most rigid section of the workpiece. Heat transfer characteristics may also be adversely affected if the point (cutting edge) of the tool is too thin as a result of high back-off and rake (hook) angles. The heat at the cutting edge does not dissipate as rapidly, and higher temperatures prevail.

Workpiece materials that have relatively high hardness, high shear strength, high coefficient of friction, high work-hardening capacities, and contain hard constituents, promote more rapid wear for given cutting conditions. Materials such as titanium, stainless steel, low-carbon steel, etc. which have poor thermal conductivity, do not dissipate heat from the cutting zone as rapidly as others, and temperature failures are much more common.