COMMON INSERT FAILURES

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.

Flank 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.

Cratering

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

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. Builtup 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