Metal cutting load analysis

IN a metal cutting operation, a tool deforms the workpiece material and causes it to shear away in the form of chips. The deformation process generates heat and pressure, loads that eventually cause the tool to wear out or fail. Traditional wear theory says the failures result from friction between the chip and tool, which are in contact but not attached to one another.

Recent research into cutting tool failure mechanisms, however, has determined that the pressures and temperatures in metal cutting, especially those generated when machining high-performance workpiece materials, are such that traditional wear theory does not fully describe what occurs at the chip/tool interface.

Tribological research has determined that the cutting process does not simply involve a single shearing event and subsequent disconnection of chip and tool. In fact, secondary and tertiary connections and disconnections also occur. The chip shears away, adheres to the rake face and then shears away again before finally sliding off the tool. The main wear mechanism is repeated shearing, not friction.

Figures 1 and 2 illustrate the metal cutting process as described via tribology. Figure 1 shows preliminary deformation of the workpiece material in Zone 5. Zone 3 is the separation zone, also called the stagnation point, because the relative movement of the workpiece material and tool at that area is essentially zero. Initial shearing takes place in Primary Shearing Zone 1, where the material shears off, and the chip forms. Then, in Secondary Shearing Zone 2, the chip is in contact with the rake face. The high pressures cause the chip to adhere to the rake face of the tool.

Built-up edge
Adhering of workpiece material to the tool rake face begins in thin layers and builds as further layers accumulate. This process can lead to a negative phenomenon known as built-up edge. If a significant amount of material accumulates on the tool, it can change the profile of the cutting edge. The built-up material can also break off and damage the edge. In perhaps the worst case, the edge build-up may be deposited on to the workpiece. In any or all of these situations, edge build-up makes the cutting process unpredictable and uncontrollable. The main focus of tribology is learning what causes built-up edge and what can be done to minimise the problem.

Two aspects of the cutting process contribute to attachment of the chip to the rake face. One factor is the very high pressures and temperatures that exist in the cutting zone. The other factor is the relatively slow speed of the chip across the tool rake face, beginning with zero motion at the stagnation point. When two materials are in contact with each other under high pressure and temperature, and move slowly, the conditions are prime for them to adhere to one another and for built-up edge to form.

Minimising adherence and the chances for forming built-up edge involves reducing the contact time between the chip and the rake face. The most straightforward solution is to increase the cutting speed and apply a sharper tool. Faster cutting speeds reduce the time the tool and workpiece material are in contact with each other. The resulting higher process temperatures can also reduce the strength of any edge build-up or eliminate it entirely. The sharper tool has a higher approach angle that forces the chip to travel a longer distance over a set period of time, i.e. move more quickly.

Material tendencies
Tribology has gained attention recently because the possibility of built-up edge formation is much greater in workpiece materials that were not commonly machined 20 years ago. For example, the phenomenon of built-up edge occurs but has not been a critical problem in familiar materials such as higher-carbon steels. Application of correct machining parameters generally eliminates adhesion and prevents built-up edge. Further, there is no issue in extremely short-chipping materials such as cast iron. Long-chipping materials, on the other hand, automatically produce longer contact time between the chip and tool, creating more risk for adherence between them. When machining materials such as low carbon steels and aluminium, the possibility of built-up edge is greater.

Built-up edge is most prevalent when machining materials with high ductility, high adhesion tendencies, and abrasiveness. A prime example is the family of aerospace and energy industry materials encompassing titanium, nickel-based alloys, and heat-resistant metals. Additional factors promoting edge build-up are the high pressure and temperatures that are generated when machining these tough alloys that have poor thermal conductivity. And in general, cutting speeds for these materials are usually slower than average.

In addition to maximising cutting speeds and tool sharpness, there are approaches to controlling built-up edge that focus on the surface condition of the tool. Somewhat surprisingly, there are two essentially opposing schools of thought on the subject. One approach says that if the surface of the tool is smoother, there will be less energy generated as the chip glides over the tool face. Lower temperatures and less contact reduce tendencies for built-up edge. Contrary to that theory is the concept that a rougher tool surface, formed with ridges or features on the scale of microns, will result in less contact between the chip and rake face and thereby reduce the chance for adhesion. Both approach is fully proven, and in some circumstances either can be effective.