Why mould makers can’t treat cutting tool choice in isolation
There are eight common insert failure modes. Of these, flank wear, thermal cracking and chipping are the ones to watch out for when machining moulds. However, using unsuitable inserts during the process can cause the other five modes to occur.
- Flank wear occurs uniformly and happens over time due to the insert’s cutting edge becoming dull or worn. With normal flank wear, a relatively uniform wear scar will form along the insert’s cutting edge. Occasionally, metal from the work-piece smears over the cutting edge and exaggerates the apparent size of the wear scar on the insert. To slow down normal flank wear it is important to employ the hardest insert grades as well as using the positive cutting edge to reduce cutting forces and friction. On the other hand, rapid flank wear often occurs when cutting abrasive materials such as ductile irons, silicon-aluminium alloys, high temperature alloys, heat-treated PH stainless steels, beryllium copper alloy and tungsten carbide alloys. The signs of rapid flank wear look the same as normal wear, and avoiding rapid flank wear requires a more wear-resistant, harder or coated carbide insert grade be used.
- Cratering is caused by a combination of diffusion and abrasive wear in inserts. Heat build-up in the workpiece chip causes elements used in 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 and deform, or possibly result in rapid flank wear. While common coatings will provide crater resistance, an aluminium oxide type is recommended.
- Built-up edges occur when fragments of the workpiece are pressure-welded to the insert cutting edge. Eventually, the built-up edge breaks off and sometimes takes pieces of the insert with it, leading to chipping and rapid flank wear. Built-up edges are identifiable by erratic changes in part accuracies or finish, as well as by shiny material appearing on the top or the flank of the insert edge. Built-up edges are controlled by increasing cutting speeds and feeds, using nitride (TiN)-coated inserts, and selecting inserts with force-reducing geometries and/or smoother surfaces.
- Chipping of insert cutting edges originates from mechanical instability often created by non-rigid setups, bearing wear, worn spindles, hard spots in work materials, or interrupted cutting operations. Ensuring proper machine tool setup, minimising deflection, using honed inserts, controlling the creation of built-up edges, and employing tougher insert grades and/or stronger cutting-edge geometries will reduce chipping.
- Thermal mechanical insert failure is a combination of rapid temperature fluctuations and mechanical shock. Stress cracks form along the insert edge, eventually causing sections of the insert’s carbide to pull out and appear to be chipping. A sign of thermal mechanical failure is multiple cracks occurring perpendicular to the cutting edge. Thermal mechanical failure can be addressed through the correct use of coolant or to remove its incidence completely, by employing a more shock-resistant grade and using a heat-reducing geometry.
- Edge deformation arises from excessive heat combined with mechanical loading, as is often the case with mould machining. High temperatures can occur when machining with high speeds and feeds or when machining hard steels, work-hardened surfaces and high-temperature alloys. This causes the carbide binder or cobalt in the insert to soften. Edge deformation can be controlled by using a more wear-resistant insert grade with lower binder content, as well as by reducing speeds and feeds and employing a force-reducing insert geometry. Notching becomes noticeable when chips (or notches) appear in the depth-of-cut area on an insert. To prevent notching the following should assist: 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 insert cutting edge. Any of the seven previously mentioned failure modes can contribute to fracturing and this phenomenon can be avoided by using a more shock-resistant grade, selecting a stronger insert geometry, using a thicker insert, reducing feed rates and/or depth-of- cut, verifying setup rigidity and checking the work-piece for hard inclusions as well as difficult entry.