There is one simple fact about carbide cutting tools — they eventually fail during use, no matter how high their quality. The extreme cutting forces and high temperatures generated by a machining operation take a tremendous toll on the cutting tool.
The key is to recognize the type of failure so preventive measures can be taken to maximize productivity and minimize tooling costs. Oftentimes, more than one type of failure is the culprit, making it difficult to accurately diagnosis the cause of the failure. Therefore, a good understanding of the different types of cutting tool failure is essential. An incorrect diagnosis may lead to control actions that worsen the problem instead of improve it.
The eight most common types of cutting tool failure are discussed below. They are abrasive flank wear, cratering, built-up edge, chipping, thermal cracking, plastic deformation, notching, and fracture.
Abrasive Flank Wear
Of all cutting tool failures, abrasive flank wear is the most desirable and predictable, because it means the insert simply wore out over a period of time. Abrasive flank wear is caused by the abrading action of the workpiece against the cutting edge of the insert. Although this abrasive action is a normal part of the machining process, it causes a "wear land" to appear on the flank of the cutting tool, as shown in the figure above. The degree to which a wear land develops is directly related to the time of the cut. As shown in the chart below, there are basically three "zones" of wear land development.
Zone "A" is referred to as the "break-in" period and is characterized by rapid flank wear. Zone "B" makes up the largest part of the cutting tool's life. In this zone, wear is constant and very predictable. In Zone "C," the cutting forces and high temperatures begin to exceed what the cutting tool can withstand. These extreme conditions accelerate flank wear and ultimately cause the cutting tool to fail. Therefore, cutting tools should be indexed at the beginning of Zone "C," before unpredictable flank wear causes problems with the finish of the workpiece.
Cratering
The type of failure known as "cratering" is characterized by a concave wear pattern on the rake surface of an insert, as shown in the figure. This wear pattern occurs when small particles of the cutting tool are worn away by the chip as it passes over the rake surface. If not corrected, this erosion process will continue until the crater breaks through to the cutting edge causing complete failure of the insert. Although elimination of the crater is not always possible, the growth of the crater can often be controlled so that normal flank wear preempts a failure caused by the crater.
Suggested Control Actions
- Use TiC or Al 2O3 coated grades. These types of grades have high hardness values and exhibit excellent crater wear resistance.
- Use TiC-bearing grades. Uncoated grades that have TiC in their composition also exhibit the same crater wear resistance as coated grades containing TiC or Al 2O3.
- Use coolant. If coolant is not being used but is an option, it can sometimes help in the control of crater failure. The lubricating and heat-reducing effect of the coolant may suppress the conditions necessary for crater formation.
- Reduce operating conditions. If the above suggestions do not control the cratering problem, the operating conditions may need to be reduced. This will result in lower productivity and is the least desirable of the suggested control actions, but reducing the operating conditions may be the best solution to the cratering problem.
Built-Up Edge
The failure mode known as "built-up edge" occurs when the extreme forces at the point of contact between the cutting tool and the workpiece weld small particles of the workpiece to the cutting tool, as illustrated in the figure. This type of failure is common when machining a soft, malleable workpiece at slower than recommended cutting speeds. The built-up edge reduces the efficiency of the cutting tool, which increases the cutting forces on the insert and often leads to chipping of the cutting tool.
Suggested Control Actions
- Increase feed rate. If the conditions that cause the built-up edge condition can be reduced, the built-up edge can be controlled or eliminated. Increasing the feed rate (surface feet per minute) reduces the cutting time and, consequently, reduces the opportunity for any welding action to form a built-up edge. This solution may also increase productivity and improve the surface finish of the workpiece.
- Use coolant. Many types of coolant interfere with any welding action by "contaminating" the surfaces of the workpiece and the cutting tool. Since welding requires a clean surface, the residue of the used coolant on the workpiece and the cutting tool helps prevent the formation of a weld.
- Remove coolant. If coolant is being used in the operation, it may be cooling the cutting edge to a temperature suitable for the welding action can take place. Removing the coolant will increase the temperature of the cutting edge, reducing the likelihood that workpiece materials can weld to the cutting edge.
- Use an insert with a positive rake angle. Cutting tools with positive rake angles help reduce the cutting forces necessary for the built-up edge to form.
- Use TiC or Al2O3 coated grades. These types of grades have anti-welding characteristics and higher hardness values that help to impede the formation of a built-up edge.
Chipping
The failure mechanism called "chipping" occurs when small pieces of the carbide insert are chipped away from the cutting edge during the machining process, as shown in the figure. Eventually, increased cutting forces at the chipped cutting edge cause the cutting edge to become inefficient, leading quickly to catastrophic failure. Chipping may not always be obvious. Some chipping occurs microscopically, the appearance of which may be confused with normal flank wear unless examined closely. Chipping can result from a variety of conditions poor rigidity in the tooling set-up, weak cutting edge, deflecting workpiece, inadequate machine tool, varying cutting loads.
Suggested Control Actions
- Minimize deflections. Deflection can originate in the tooling set-up, the workpiece, chucking or fixturing of the machine tool, or in the tool blocks, tail stock, live centers, carriages, cross slides, and rests. Any such deflection causes varying cutting loads on the insert that can lead to chipping. To minimize deflections, check the machine tool for excessive clearance in the spindle bearings and the gibs, check the toolholder or boring bar for excessive overhang and secure clamping, and use large boring bars with low length-to-diameter ratios.
- Increase edge preparation on inserts. The cutting edge of an insert is honed to increase its strength. Honing helps evenly distribute the cutting forces along the cutting edge, thereby making it stronger. The amount of hone required depends on the cutting forces to be encountered during the machining process. Greater shock loads require heavier hones. Although most cutting tools are purchased with honed cutting edges, a heavier hone or a "T"-land may be required in extreme applications.
- Use an insert with a stronger geometry. Negative rake inserts are stronger than positive rake inserts and are capable of handling greater shock loads. If negative rake inserts aren't available, use positive rake inserts with smaller relief angles. Also, inserts with large nose radii are stronger than inserts with small nose radii.
- Use a tougher carbide grade. If deflections have been minimized and cutting edges have been honed and chipping still occurs, it may be necessary to change to a more shock-resistant carbide grade with a higher cobalt content. However, the feed rate will probably have to be reduced to avoid other types of failure, which will decrease productivity. This solution should be the last alternative chosen.
Thermal/Mechanical Failure
Thermal/mechanical failure appears on the cutting tool as cracks that are generally perpendicular to the cutting edge, as seen in the figure. This type of failure is usually caused by the inability of the cutting tool to withstand the extreme temperature variations of interrupted cutting operations. The heat produced in such machining operations tends to remain at the cutting edge instead of being transferred to the rest of the insert, because carbide is a poor conductor of heat. This causes extreme thermal stress on the overheated cutting edge causing it to crack.
Suggested Control Actions
- Use coolant correctly or don't use it at all. If coolant is applied intermittently or in insufficient volume, the thermal cracking problem will be worsened. If coolants cannot be applied correctly, the operation should be performed without coolant at reduced speed, feed, and depth of cut.
- Use a stronger carbide grade. Stronger grades of carbide with higher cobalt content have greater tolerance to extreme temperature changes. Grades with TaC also possess heat-resistant characteristics.
Plastic Deformation
The type of failure known as "plastic deformation" occurs when the carbide at the cutting edge is softened by the high temperatures produced during machining operations. The softened carbide is deformed from its original shape by the cutting forces, as shown in the figure. When a cutting edge appears to have developed a large wear land after a very short time, plastic deformation should be suspected.
Suggested Control Actions
- Use coolant. The use of coolant to reduce the temperature of the insert at the cutting edge will prevent plastic deformation by allowing the cutting tool to maintain its hardness and better withstand the cutting forces.
- Reduce operating conditions. The reduction in feed, speed, or depth of cut will reduce the heat and cutting forces generated at the cutting edge. This will result in lower productivity and is the least desirable of the suggested control actions, but reducing the operating conditions will correct the problem.
- Use a more wear-resistant grade. Carbide grades that are wear-resistant also resist plastic deformation. These grades generally have lower cobalt content and higher hardness values and may contain TiC or TaC.
Notching
The failure mechanism called "notching" appears as a severe "notch-shaped" abrasive wear pattern that is localized at the depth-of-cut line, as illustrated in the figure. Notching generally occurs during the machining of high temperature alloys and work-hardened materials, where "scale" material on the surface of the workpiece is very hard and causes accelerated abrasive wear on the insert at the depth-of-cut line.
Suggested Control Actions
- Use tooling that provides a large cutting edge angle to the workpiece. The large cutting edge angle distributes the cut over a larger section of the insert, which weakens the chip and reduces the abrasive effect on the insert's cutting edge.
- Increase the hone at the depth-of-cut line area of the cutting edge. A stronger edge at the depth-of-cut line achieved by additional honing will improve the insert's resistance to the abrasive action of the scale on the workpiece.
- Reduce the feed rate. If the application of a larger cutting edge angle does not solve the problem, a reduction in feed rate may be necessary to eliminate notching. However, this will decrease productivity and should only be used if other attempted solutions fail.
Fracturing
"Fracturing" occurs when the cutting forces exceed the strength of the insert's cutting edge causing catastrophic removal of a large piece of the cutting tool, as shown in the figure. Several circumstances can cause fracturing, including excessive flank wear land, shock loading during interrupted cutting operations, improper carbide grade selection, and improper insert size selection. Fracturing may result in injury to the operator, the inability to use the remaining unused cutting edges of the insert, and damage to the workpiece, toolholder, or machine tool. Suggested Control Actions
- Index the insert before the wear land reaches its end-point. Fracturing will occur when the wear land reaches its end-point, or the point at which it can wear no more. Therefore, the insert should be indexed before this occurs.
- Use a tougher carbide grade. Carbide grades with higher cobalt content are generally tougher and more resistant to the shock loads that can cause fracturing.
- Use an insert of appropriate size. If fracturing occurs, the insert is not able to withstand the cutting forces of the machining operation. A larger or thicker insert will absorb more shock forces.
- Use an insert with a stronger geometry. Negative rake inserts are stronger than positive rake inserts and are capable of handling greater shock loads. Also, inserts with large nose radii are stronger than inserts with small nose radii.
- Use honed inserts. The cutting edge of an insert is honed to increase its strength. Honing helps evenly distribute the cutting forces along the cutting edge, thereby making it stronger. The amount of hone required depends on the cutting forces to be encountered during the machining process. Greater shock loads require heavier hones. Although most cutting tools are purchased with honed cutting edges, a heavier hone may be required in extreme applications.
- Reduce operating conditions. The reduction in feed, speed, or depth of cut will reduce the cutting forces generated at the cutting edge. This will result in lower productivity and is the least desirable of the suggested control actions, but reducing the operating conditions may be the only solution.
- Use a toolholder with better support. If an insert does not have sufficient support in the toolholder, it may fracture. A toolholder with proper support must be used or even the best inserts will fracture.