Don Graham, Manager of Education and Technical Services at Seco Tools explains eight common failure modes associated with tool inserts in manufacturing processes.
Insert failure is a common occurrence in manufacturing caused by tremendous and repeated stress, creating wear and tear on the insert. If left unaddressed, this problem can impact manufacturing equipment, resulting in inaccurate processes or poor productivity for the manufacturer.
Manufacturers need to analyse used tooling to achieve maximum tool life and predict tool usage, thereby maintaining part accuracy and reducing equipment deterioration. Early insert examination is important in determining the root cause of tool failure as is careful observation and reporting.
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.
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 caused by abrasion at low cutting speeds and chemical reactions at high cutting speeds. Normal flank wear can be identified by a relatively uniform wear scar that will form along the insert’s cutting edge.
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.
However, rapid flank wear 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.
Similar in appearance to normal flank wear, rapid flank wear can be corrected by selecting a more wear-resistant, harder or coated carbide insert grade, as well as making sure coolant is being applied properly.
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.
Cratering is caused by a combination of diffusion and abrasive wear. 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.
A typical occurrence 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 can be controlled by increasing cutting speeds and feeds, using nitride (TiN) coated inserts, applying coolant properly, and selecting inserts with force-reducing geometries and/or smoother surfaces.
Arising out of mechanical instability often created by non-rigid setups, bad bearings or worn spindles, hard spots in work materials or an interrupted cut, chipping also occurs in unexpected places such as during the machining 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.
This problem is identifiable by the appearance of chips along the edge of the insert. Chipping can be minimised by 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.
Thermal mechanical failure
Thermal mechanical failure is caused by a combination of rapid temperature fluctuations and mechanical shock, with stress cracks forming 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 and 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.
Thermal mechanical failure can be prevented by applying coolant correctly or, removing it from the process completely, employing a more shock-resistant grade, using a heat-reducing geometry and reducing feed rate.
Edge deformation is the consequence of excessive heat combined with mechanical loading. 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 controlled by properly applying coolant, using a more wear-resistant grade with lower binder content, reducing speeds and feeds, and employing a force-reducing geometry.
Notching is the result of an abrasive workpiece surface abrading or chipping the depth of cut area on a cutting tool. Cast surfaces, oxidised surfaces, work hardened surfaces or irregular surfaces can all 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 start 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. Therefore, mechanical fracturing can be avoided by taking corrective action for all other failure modes. Corrective actions include utilising a more shock-resistant grade, selecting 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.
Manufacturers stand to gain significantly by understanding these eight common failure modes and developing failure analysis skills. Key benefits include 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.
Headquartered in Fagersta, Sweden and present in more than 50 countries, Seco Tools is a leading global provider of metal cutting solutions for milling, turning, holemaking and toolholding. Seco Tools Australia represents the company in Australia.