Don GrahamClick image to enlargeby Don Graham

Spotting and preventing common insert failure modes

 

Through early insert examination, careful observation and reporting, shops can better determine the root causes of eight different types of insert failure modes: flank wear, cratering, build-up edge, chipping, thermal mechanical wear, edge deformation, notching and mechanical fracturing.

Flank Wear: Flank wear occurs uniformly and happens over time as the work material wears the cutting edge, similar to the dulling of a knife blade.

With normal flank wear, a 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. Rapid flank wear, which looks the same as normal wear, is undesirable because it reduces tool life and time in the cut. Rapid wear often occurs when cutting abrasive materials such as ductile irons, silicon-aluminum alloys, high temp alloys, heat-treated PH stainless steels, beryllium copper alloy and tungsten carbide alloys, as well as non-metallic materials such as fiberglass, epoxy, reinforced plastics and ceramic.

To correct this, select a more wear resistant, harder or coated carbide insert grade and make sure coolant is applied properly. Reducing cutting is also effective, but counterproductive, as it negatively affects cycle time.

Cratering:Often occurring during the high speed machining of iron or titanium-based alloys, a combination of diffusion and abrasive wear causes cratering. 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.

Craters or pits on the top of the insert identify this failure mode, and coated inserts will help correct cratering issues. However, aluminum oxide coatings work best. Also, when combating cratering, it's important to use coolant to reduce heat, as well as decrease speeds and feeds, with speed reductions being the most effective.

Built-Up Edge:Built-up edge occurs when fragments of the workpiece are pressure-welded to the cutting edge. Eventually, the built-up edge breaks off and sometimes takes pieces of the insert with it, leading to chippage and rapid flank wear.

This failure mechanism commonly occurs with gummy materials, low speeds, high temperature alloys, stainless steels and non-ferrous materials, and threading and drilling operations. Built-up edge is identifiable through erratic changes in a part size or finish, as well as shiny material showing up on the top or at 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:Chipping originates from mechanical instability. Hard inclusions in the surface of the material being cut and interrupted cuts can cause chipping.

Chips along the edge of the insert are noticeable. Ensuring proper machine tool set up, minimizing deflection, using honed inserts, controlling built-up edge and employing tougher insert grades 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.

Signs of thermal mechanical failure include multiple cracks perpendicular to the cutting edge. It is important to identify this failure mode before chipping begins.

To address this failure, apply coolant correctly or remove it from the process completely, employ a more shock-resistant grade, use a heat-reducing geometry and slower feedrates.

Edge Deformation:Excessive heat−combined with mechanical loading–is a source of edge deformation. The 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 include deformation at the cutting edge and finished workpiece dimensions not meeting the required specifications. Edge deformation is controllable by applying coolant, using a wear-resistant grade with a lower binder content, reducing speeds and feeds, and employing a force-reducing geometry.

Notching:Notching occurs when an abrasive workpiece surface abrades or chips the depth of cut area on a cutting tool. Cast surfaces, oxidized surfaces, work hardened surfaces or irregular surfaces can all cause notching.

This failure mode becomes noticeable when notching and chipping starts showing up in the depth-of-cut area on the insert. To prevent notching, vary the depths of cut when using multiple passes, use a tool with a larger lead angle, increase cutting speeds when machining high temperature alloys, reduce feedrates, increase the hone in the depth-of-cut area, and prevent build-up, especially in stainless steel and high temperature alloys.

Mechanical Fracturing:Mechanical fracturing occurs when the imposed force overcomes the inherent strength of the cutting edge. Any of the other failure modes can contribute to fracturing.

Utilizing a more shock-resistant grade, selecting a stronger insert geometry, using a thicker insert, reducing feedrates and/or depth of cut, verifying set-up rigidity and checking the workpiece for hard inclusions or difficult entry are corrective actions. SMT

Don Graham is manager of Education and Technical Services, Seco Tools, Troy, MI

Seco Tools

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