Failure modes – how they can improve your machining processes

Failure analysis as it relates to cutting tools is an often-overlooked path to improving a machining process. Of course, eventually all cutting tools will fail, but failure analysis will help to optimize the output of the operation. Failure analysis works by checking the condition of the insert at a predetermined number of pieces. The condition of the insert would be analyzed at these checkpoints. The condition of the insert would be compared to typical failure modes, a diagnosis would be determined and adjustments would be made to maximize productivity. All too often a machine operator will bring a fractured insert to the tool crib and request more inserts. Once the insert has fractured, there are no clues as to the reason for the failure.Often in North America, tool life is simply measured by the amount of cut time, the number of parts or the distance the tool travelled while removing material. The determinant to indicate when the tool has reached its life expectancy is often the load on the spindle, the surface finish on the part or in some cases catastrophic failure. This is unfortunate, as the performance of the tool could be greatly enhanced with the proper diagnosis and alterations.In Japan, it is common to set predetermined checkpoints to inspect the insert. This is helpful, because one can see what is happening to the insert and make adjustments to optimize the tool’s output. Once the other failure modes have been corrected, measuring flank wear can then be the determinant of tool life.As mentioned earlier all inserts will wear out. However the ideal wear pattern is flank wear. By checking the insert at predetermined intervals, it is possible to see how the insert is wearing. There are many different types of wear patterns classified in failure analysis. Ultimately these failure modes – flank wear, notch wear, crater wear, plastic deformation, built up edge, thermal cracking, chipping and edge fracture – are symptoms of a shortened tool life. If the symptoms are recognized early, adjustments may be made to reach the desired outcome of flank wear.Some of the symptoms of tool wear are:

• Visible signs on the insert
• Load Meter
• Part Size
• Work-piece Finish
• Burr Generated
• Work-piece Temperature
• Vibrations
•     Sound
•     Chatter
• Color of Chip
• Chip Control

It is useful to observe the visible signs on the insert. This illuminates common failure modes, and helps to correct those failure modes that lead to under-performing cutting tools. The desired outcome is standard flank wear on the insert.Flank WearFlank wear is normal. All inserts will eventually wear out. The best situation is to have the insert wear through flank wear.  However, premature flank wear is not desired. The ideal situation is when it is possible to extend the life of the insert before flank wear causes failure, thus minimizing cycle time.The cause of flank wear is generally abrasive wear caused by the cutting speed, the hardness of the part, or a hard skin on the material being removed. Flank wear occurs when abrasion erodes the coating and eventually the substrate. The ideal flank wear is an even progression, making the process predictable.Some remedies to reduce flank wear are to select a grade that has more wear resistance. As a quick fix, one could simply slow the cutting speed, thereby increasing tool life but sacrificing cycle time. Of course, the best solution is to keep cycle time as short as possible. The long-term solution is to select a better grade.Notch WearNotch wear appears as a notch on the insert, generally at the depth of cut level. It is an oxidation at the depth of cut. This oxidation is caused by high temperatures and the presence of air. Tungsten and cobalt, the main ingredients in carbide inserts, form weak oxide films during oxidation.To remedy notch wear, select a more wear resistant grade. Generally inserts coated with Al2O3 which can help prevent oxidation of the substrate. As a quick fix one could increase the lead angle on the tool. This would ease the insert into the material and would also provide “chip thinning”. Another remedy would be to vary the depth of cut. In this way, the insert is not always cutting at the same depth of cut point. A variation of that method would be to make your first pass a taper and your next path a straight cut which would produce a constantly changing depth of cut.Crater WearCrater wear appears as a diffusion between the insert and the workpiece material. It occurs when enough thermal energy is created to enable the exchange of atoms across the tool-workpiece interface. The amount of diffusion depends on the chemical affinity between the two. For instance, tungsten and steel have a high chemical affinity. Effective remedies include using a lower cutting speed, or choosing a grade with more wear resistance. For instance, an Al2O3 coating helps to prevent diffusion and reduces crater wear.If you begin to lose chip control, check for crater wear because excessive types of this wear tend to change the edge geometry of the insert. Plastic DeformationCompression and heat, combined with high feeds and hard work materials, cause deformation of the insert edge. As edge deformation increases from the pressure, temperatures continue to rise, contributing to further deformation.Look for the edge to lose sharpness and bulge out when this happens.A quick fix is to reduce the speed or feed rate. But for long-term productivity, a more wear-resistant grade is key. Built-up edgeA built-up edge is when the workpiece material gets welded to the cutting edge. This usually occurs when cutting temperatures are too low. Additional chip flow is needed to avoid it. The remedies to this are to increase the cutting speed, and select a more positive insert geometry. Another remedy would be to increase the coolant flow to add lubricity. In the case of aluminum, use a polished insert to increase lubricity. In gummy steels, use a smooth, slippery coating.Thermal CrackingThermal Cracking is caused by abrupt temperature changes. A quick remedy is to turn the coolant off completely, or by drenching the tool in coolant to stabilize the temperature. Using a tougher, more wear-resistant grade is also an effective solution. When doing interrupted cutting, always run without coolant. ChippingChipping occurs when the insert edge breaks rather than wears. The fatigue associated with chipping is generally caused by temperature and force fluctuations. It usually means that the application is too demanding for the selected insert.To remedy this, it is necessary to select an insert with a stronger cutting edge. Also, try a tougher, more shock-resistant grade of carbideEdge FractureEdge fracture happens as a result of the other wear types. Remedy check all the cutting data and setup to consider what it might be. Once the edge fractures, the reason is unknown. Ideally, you want to check the insert before a fracture occurs to determine why such an incident might happen. The likely remedies for this are to reduce the feed rate, reduce the cutting speed, or select a tougher grade.John Mitchell, MBA, is the General Manager of Tungaloy Canada.