SECTION 4.3
Identifying the types of tool wear

There are no known materials or coolants that can completely resist or prevent tool wear. The contact and rubbing at high temperatures and at high pressures will change the original tool contour over a period of time. As a result of direct contact with the work material, we discovered in Section 4.2.3 that there are five major types of wear patterns usually found on a broach.

SECTION 4.3.1
Abrasive wear

Abrasive wear requires the presence of particles in the workpiece that are harder than the matrix of the tool. This type of wear may be explained by the fact that hard particles (sand, inclusions, carbides, etc.) in the part material literally gouge or dislodge particles from the tool, causing continuous wear under any cutting condition. The particle track, or scratch, alters the geometry of the cutting edge and is not considered a normal cutting condition. At higher cutting speeds, even some of the softer constituents may contribute to the gouging action as a result of higher impact values and reduced tool resistance to abrasion.

SECTION 4.3.2
Plastic deformation

This wear mechanism is believed to take place at all ranges of cutting temperatures; it arises from the high unit pressures imposed on the tool. This results in a slight depression and bulging of the edge. The net effect is greater tool pressure and increased cutting temperature resulting in further deformation and concluding in edge wipe out. This mode of failure is common when machining hardened materials at high speeds.

SECTION 4.3.3
Chemical distortion

Chemical distortion occurs through localized chemical reactions at the tool-workpiece interface. These reactions are temperature dependent and result in weakening the bond between minute tool segments and the segments surrounding them. This may occur either through formation of weaker compounds, or in the case of carbide tools, by a dissolving action of the bond between the binder and the individual carbide particles. As a result of this weakening effect, the particles are pulled out from the main body of the tool by the chip or work as it moves past the contact surfaces. Once the critical temperature for this chemical action is reached, the rate of wear is relatively rapid.

SECTION 4.3.4
Diffusive wear

Diffusive wear is a complex work phenomenon between the work and the tool and results in a rapid breakdown of tool material once critical temperature is reached. There is an alloying effect which weakens the bond for the tool particles and permits them to be pulled out by the chip as it sloughs off. Carbon transfer from the tool material to the workpiece is enhanced at higher temperatures and this greatly contributes to premature tool failure.

Diffusion is dependent on the ability of the tool material to be dissolved into the metal flowing over the cutting surface, rather than on the hardness of the tool material itself. The wear appears to be very smooth and accounts for the formation of craters at speeds below those at which plastic deformation of the tool begins. Rates of diffusion increase rapidly with temperature, the rate typically doubling for every 20oC [78oF] increase. Diffusive wear is the most important wear process responsible for top (land) wear and is reduced by proper coolant choice and application.

SECTION 4.3.5
Attrition wear

Attrition wear occurs at low temperatures and low speeds where a large built-up edge may be formed. Larger fragments of microscopic size may be torn intermittently from the tool surface, leaving a very uneven worn surface. A built-up edge forms because of a high resistance to chip flow along the tool face, which causes a portion of the chip to shear off as it moves past the tool. With continuous cutting operations using high-speed tools, attrition is usually a slow form of wear, but more rapid destruction of the tool edge occurs in operations involving interruptions of cut, or where vibration is severe due to a lack of rigidity in the machine tool. Adhering metal often completely conceals the attrition-worn surface and, under these conditions, visual measurements of wear may be misleading.