Materials and Processes in Manufacturing

E. Paul Degarmo, J T. Black, Ronald A. Kohser

Chapter 22 Cutting Tools for Machining

Wear Mechanisms and Wear Regions on the Cutting Tool

The movement of the workpiece past the clearance face of the tool causes flank wear, primarily by abrasion. Tool-chip friction on the tool rake face results in high temperature and adhesion and diffusion wear mechanisms are active. This can result in the formation of a crater and is called crater wear. The maximum temperature on the rake face occurs some distance away from the cutting edge. The maximum crater depth is also removed from the cutting edge.

There are other tool wear mechanisms which may be important in special circumstances. For example, in the sawing of green wood with carbide tools, chemical action may degrade the tool binder exposing the carbide skeleton to mechanical action and so wear.

Flank and rake face wear regions along with a rough estimate of the dependence of wear rate on cutting conditions is shown in this exercise.

A Tool Wear Model

In mechanical processes almost all energy is dissipated as heat with only a small part going to material structural changes. The implication for tool wear is that the heat generated causes a temperature rise that decreases tool material strength. So, wear rate is expected to increase with cutting conditions which produce higher temperatures in the cutting zone.

The power input to the chip formation process, P, is primarily due to the cutting force and cutting speed.

P = Fc V
The implication is that temperature in the chip formation zone depends critically on cutting speed. If V is held constant and feed rate and/or depth of cut is increased, cutting force will increase. The increase in machining forces with feed rate and depth of cut also increases power dissipated and so increasing temperature and increasing wear rate.

The process parameters of cutting speed, feed rate and depth of cut can be set in operations and are used in machining process models. The dependent variable, which is critically dependent on chip formation zone temperature, is tool life, T.

A very general tool life model is

V T n Fm dp = K'

The following exercise is aimed at adding meaning to this equation by providing a graphical representation of it.

Relative Effects of Cutting Conditions

Another exercise is provided to point out that the cutting conditions included in the tool wear model have different effects on tool life.

© 2001 by Barney E. Klamecki. All rights reserved.