SECTION 4.2
Tool wear

Tool failure is usually associated with some form of breakdown of the cutting edge. Under proper operating conditions, this breakdown takes place gradually over a period of time. In the absence of rigidity, or because of improper tool geometry that gives inadequate support to the cutting edge, the tool may fail by mechanical fractures or chipping under the load of the cutting forces. This is not truly a wear phenomenon for it can be eliminated or at least minimized by proper design and application.

As a direct result of contact with the work material, there are two major regions on the tool where wear can take place: (1) the face, or front of the tooth, and (2) the land, or top of the tooth.

SECTION 4.2.1
Face wear

The face of the broach tooth is the surface over which the chip passes during its formation. Wear takes the form of a cavity or crater which has its origin not along the cutting edge but at some distance away from it, yet still within the chip contact area. As wear progresses with time, the crater gets wider, longer, and deeper and approaches the outside edges of the tooth.

This form of wear is usually associated with ductile materials which give rise to continuous, tightly curled chips. If crater wear is allowed to proceed too far, the cutting edge becomes weak as it thins out, and breaks down suddenly. Usually, there is some preliminary breakthrough of the crater at the cutting edge. These smaller, prefatory breaks serve as focal points for the development of notches and nicks on the land. In general, crater wear develops faster than land wear on ductile materials and is the limiting factor in the determination of premature tool failure.

SECTION 4.2.2
Land wear

Although crater wear is the most prominent in the machining of ductile materials, land (or top) wear is always present regardless of work and tool material, or even of cutting conditions. The land is the clearance face of the cutting tool, along which the major cutting edge is located. It is the portion of the broach that is in contact with the work at the chip separation point and resists the machining velocity forces. Because of the clearance, initial contact is made along the cutting edge. Land wear begins at the cutting edge and develops into a wider and wider flat of increasing contact area, called a wear land.

Materials that do not form tightly curled, continuous chips promote little if any crater wear on the tooth face. Land wear then becomes the dominant factor in tool failure. In the case of nearly all broach cutting tools, the wear land is in direct contact with the finished surface, and usually becomes the basis for failure, even on ductile materials, particularly if surface finish specifications are controlling factors in the process. Quite often, land wear is accompanied by a rounding of the cutting edge, particularly in the broaching of abrasive materials. The rounding of the cutting edge is significantly greater on form broaches, especially on sharp corners, where corner wear occurs much more rapidly than wear at the land-face intersection (see Figure 4-1). This results in large increases of cutting forces which, if allowed to accelerate unchecked, could lead to tooth fractures.

SECTION 4.2.3
The Causes of Tool Wear
All of the evidence indicates that tool wear is a complex phenomenon and is influenced by many factors. The causes of wear do not always behave in the same manner, nor do they always affect wear to the same degree under similar cutting conditions. The causes of wear are not fully understood. In recent years, great strides have been made by various researchers. Even though there is some disagreement regarding the true mechanisms by which wear actually takes place, most studies agree that there are at least five basic causes of wear:

1. Abrasive action of hard particles contained in the work material.
2. Plastic deformation of the cutting edge.
3. Chemical decomposition of the cutting tool contact surfaces.
4. Diffusion between the work and tool material.
5. Welding of asperities between the work and the tool (attrition).

The relative effects of these causes are a function of cutting velocity or cutting temperatures. Investigations have also been made on other possible causes such as oxidation and electrochemical reactions in the tool work contact zone.

The most important factor influencing tool wear is cutting temperature. Of the five basic causes of wear, temperature has considerable effect in all but one. Cutting temperatures are important for two basic reasons: (1) most tool materials show rapid loss of strength, hardness, and resistance to abrasion above some critical temperature, and (2) the rate of diffusion between work and tool materials rises very rapidly as temperature increases past the critical point.

Analytical and experimental methods have been used to show that the average peak temperatures at the tool-chip interface occur near the point where the chip leaves the tool surface on the tooth face. Crater wear appears greatest at this point. The rate of wear increases very rapidly beyond a critical temperature. High-speed steel tools begin to lose their properties very rapidly at approximately 1100oF. Chemical decomposition and diffusion will not occur at any appreciable extent until the critical temperatures are reached.