Don Graham looks at eight common insert failure modes and explains that, using simple tools, manufacturers can analyse used tooling to predict usage and achieve maximum tool life.
Insert failure and its negative impact on manufacturing equipment is similar to an athlete exhausting a good pair of running shoes.
Much like a shoe under the weight of the runner wearing it, an insert endures tremendous stress over and over again, creating wear and tear. If not addressed, wear can cause pain for an athlete and inaccurate processes or poor productivity for a manufacturer.
Manufacturers, however, can analyse used tooling to achieve maximum tool life and predict tool usage, thereby maintaining part accuracies and reducing equipment deterioration. Early insert examination is important in determining the root cause of its failure as is careful observation and reporting. By not taking these important steps, it’s possible to become confused between the different types of failure modes.
To assist in the insert examination process, a stereoscope with good optics, good lighting and a magnification of at least 20X, can pay great dividends in identifying these eight common failure modes that contribute to premature insert wear.
An insert will fail due to normal wear in any type of material. Normal flank wear is the most desired wear mechanism because it is the most predictable form of tool failure.
Flank wear occurs uniformly and happens over time as the work material wears the cutting edge, similar to the dulling of a knife blade.
Normal flank wear begins when hard microscopic inclusions or work-hardened material in the workpiece cut into the insert.
Causes of such wear include abrasion at low cutting speeds and chemical reactions at high cutting speeds. In identifying normal flank wear, a relatively uniform wear scar will form along the insert´s cutting edge. Occasionally, metal from the workpiece smears over the cutting edge and exaggerates the apparent size of the wear scar on the insert.
To help slow down normal flank wear, it’s important to employ the hardest insert grade that does not chip, as well as use the freest cutting edge to reduce cutting forces and friction.
Rapid flank wear, on the other hand, is not desirable, as it reduces tool life and the normally desired 15 minutes of time in cut will not be achieved.
Rapid wear often occurs when cutting abrasive materials such as ductile irons, silicon-aluminium alloys, high temp alloys, heat-treated PH stainless steels, beryllium copper alloy and tungsten carbide alloys, as well as non-metallic materials such as fibreglass, epoxy, reinforced plastics and ceramic.
The signs of rapid flank wear look the same as normal wear. In correcting for rapid flank wear, it becomes key to select a more wear resistant, harder or coated carbide insert grade, as well as make sure coolant is being applied properly. Reducing cutting is also very effective, but counterproductive as it negatively affects cycle time.
Often occurring during the high speed machining of iron or titanium-based alloys, cratering is a heat/chemical problem where the insert essentially dissolves into the workpiece chips.
A combination of diffusion and abrasive wear causes cratering. In the presence of iron or titanium, the heat in the workpiece chip allows components of the cemented carbide to dissolve and diffuse into the chip, creating a ‘crater’ on the top of the insert.
The crater will eventually grow large enough to cause the insert flank to chip, deform or possibly result in rapid flank wear.
Built-up edge occurs when fragments of the workpiece are pressure-welded to the cutting edge, resulting from chemical affinity, high pressure and sufficient temperature in the cutting zone.
Eventually, the built-up edge breaks off and sometimes takes pieces of the insert with it, leading to chipping and rapid flank wear.
This failure mechanism commonly occurs with gummy materials, low speeds, high-temperature alloys, stainless steels and nonferrous materials, and threading and drilling operations.
Built-up edge is identifiable through erratic changes in a part´s size or finish, as well as shiny material showing up on the top or the flank of the insert edge.
Built-up edge is controllable by increasing cutting speeds and feeds, using nitride (TiN) coated inserts, applying coolant properly (e.g. increasing the concentration), and selecting inserts with force-reducing geometries and/or smoother surfaces.
Chipping originates from mechanical instability often created by non-rigid setups, bad bearings or worn spindles, hard spots in work materials or an interrupted cut.
Sometimes this occurs in unexpected places such as during the machine of powder metallurgical (PM) materials where porosity is deliberately left in the components.
Hard inclusions in the surface of the material being cut and interrupted cuts result in local stress concentrations and can cause chipping.
With this type of failure mode, chips along the edge of the insert are highly noticeable. Ensuring proper machine tool set up, minimising deflection, using honed inserts, controlling built-up edge, and employing tougher insert grades and/or stronger cutting-edge geometries will deter chipping.
Thermal mechanical failure
A combination of rapid temperature fluctuations and mechanical shock can cause thermal mechanical failure. Stress cracks form along the insert edge, eventually causing sections of the insert’s carbide to pull out and appear to be chipping.
Thermal mechanical failure is most often experienced in milling and sometimes during interrupted-cut turning, facing operations on a large number of parts, and operations with intermittent coolant flow. Signs of thermal mechanical failure include multiple cracks perpendicular to the cutting edge. It is important to identify this failure mode before chipping begins.
It´s possible to prevent thermal mechanical failure by applying coolant correctly or, better yet, removing it from the process completely, employing a more shock-resistant grade, using a heat-reducing geometry and reducing feed rate.
Excessive heat combined with mechanical loading are sources of edge deformation. High heat is often encountered at high speeds and feeds or when machining hard steels, work-hardened surfaces and high-temperature alloys.
Excessive heat causes the carbide binder, or cobalt, in the insert to soften. Mechanical loading happens when the pressure of the insert against the workpiece makes the insert deform or sag at the tip, eventually breaking it off or leading to rapid flank wear.
Signs of edge deformation include deformation at the cutting edge and finished workpiece dimensions not meeting the required specifications.
Edge deformation is controllable by properly applying coolant, using a more wear-resistant grade with a lower binder content, reducing speeds and feeds, and employing a force-reducing geometry.
Notching occurs when an abrasive workpiece surface abrades or chips the depth of cut area on a cutting tool. Cast surfaces, oxidised surfaces, work hardened surfaces or irregular surfaces all can cause notching.
While abrasion is the most common culprit, chipping in this area can also occur. The depth of cut line on an insert is often in tensile stress, making it sensitive to impact.
This failure mode becomes noticeable when notching and chipping starts showing up in the depth-of-cut area on the insert.
To prevent notching, it’s important to vary the depth of cut when using multiple passes, use a tool with a larger lead angle, increase cutting speeds when machining high-temperature alloys, reduce feed rates, carefully increase the hone in the depth-of-cut area, and prevent build-up, especially in stainless steel and high-temperature alloys.
Mechanical fracturing of an insert occurs when the imposed force overcomes the inherent strength of the cutting edge. Any of the failure modes discussed in this article can contribute to fracturing.
It´s possible to avoid mechanical fracturing by correcting for all other failure modes besides normal flank wear. Utilising a more shock-resistant grade, selecting a stronger insert geometry, using a thicker insert, reducing feed rates and/or depth of cut, verifying set-up rigidity and checking the workpiece for hard inclusions or difficult entry are all effective corrective actions.
By understanding these eight common failure modes and developing failure analysis skills, manufacturers stand to gain a lot. Increased productivity, improved tool life and tool life consistency, improved part tolerance and appearance, less wear and tear on equipment, as well as a decreased chance of catastrophic insert failure that shuts down production and damages an important job are all important benefits.
[Don Graham is Manager of Education and Technical Services at Seco Tools.]