Careful observation and analysis can help operators to define and correct common tooling and insert failures. According to Don Graham, Manager - Education & Technical Services, Seco, the basic question to ask is, "Is this a heat problem or a mechanical problem?" Graham identified eight common failure mechanisms: • Normal flank wear • Cratering • Built-up edge • Chipping • Thermal/mechanical failure • Edge deformation • Notching • Mechanical fracture. Flank Wear Flank wear is normally a problem with abrasive metals and non-metallics. Abrasive metals include ductile irons, silicon-aluminum alloys, high temperature alloys, heat treated PH stainless steels, beryllium copper alloys and tungsten carbide alloys. Non-metallics include fiberglass, epoxy, reinforced plastics and ceramic. Normal flank wear will typically be uniform and predictable. However, jagged disparities at the bottom of the wear scar suggests chippage. According to Graham, of the three operating conditions (cutting speed, feed and depth of cut), cutting speed has the greatest effect on tool life. Operators should choose the ideal operating conditions for each type of workpiece material used. Cratering Cratering is a heat/chemical problem that occurs when the tool is dissolved into the chip. This can take place when machining iron or titanium alloys. Graham suggests the following to control cratering: • Use a coated grade: coatings containing aluminum oxide are best; TiAlN is the most crater resistant PVD coating • Apply coolant • Use an uncoated grade with higher crater resistance (TiC and TaC content) if using uncoated carbide • Use a freer cutting geometry to reduce heat • Increase lead angle (small effect) • Reduce cutting speed • Reduce feed rate. Built-Up Edge Built-up edge may occur from gummy materials, low speeds, high temperature alloys and stainless steels, threading operations, drilling or non-ferrous materials. To control built-up edge, Graham suggests the following: • Increase cutting speed • Use nitride (TiN) coating (any coating will be better than uncoated—nitride coatings are preferred) • Apply coolant (especially with high solids content) • Select force-reducing geometries • Use a smoother surface, such as a polished surface or PVD coatings. Chipping Chipping occurs when small pieces of the insert are broken off. Graham suggests the following as possible sources of chipping: • Non-rigid set-ups • Bad bearings, worn spindles, etc. - Look for regular small chips on edge - Look for harmonic marks on workpiece: f = V/(5 X WL) f = vibration frequency (cycles per second) V = cutting (sfpm) WL = wavelength of vibrations (inches) • Hard spots in work material • PM materials. To control chipping, Graham recammends the following: • Ensure proper machine tool setup • Minimize deflection • Use honed inserts • Control built-up edge • Use a stronger insert geometry • Use a more shock resistant grade. Thermal/Mechanical Failure Thermal cracking is a result of rapid temperature fluctuations or a heat/shock combination and leads to chippage. According to Graham, thermal cracking can be a problem during milling, with facing operations when a largenumber of parts are machined, or in operations with intermittent coolant flow. Graham offers the following guidelines to prevent thermal/mechanical failure: • Apply coolant correctly or remove completely • Use a more shock resistant grade, such as one with higher binder content • Use a heat reducing geometry. Edge Deformation High heat and high pressures can result in deformation of the cutting edge. According to Graham, heat sources can include high speeds and high temperature alloys, and pressure sources may include high feed rates, high temperature alloys, hard steels and work hardened surfaces. To control edge deformation, Graham suggests the following: • Apply coolant • Use a more wear resistant grade, such as one with a lower binder content • Reduce cutting speeds • Reduce feed rate • Use force reducing geometry (marginally effective). Notching Notching can result from a variety of sources, including surface scale, work hardened surfaces, build-up, cast surfaces, irregular surfaces and oxidation. According to Graham, the following guidelines can help control notching: • Vary depth of cut when using multiple passes • Use a tool with a larger lead angle • Increase speed if machining a high temperature alloy (this will generate more flank wear) • Reduce feed rate • Carefully increase the hone in the DOC area • Prevent build-up, especially in stainless and high temperature alloys. Mechanical Fracture Fracturing is the separation of a large section of the insert. According to Graham, the following can control mechanical fracturing: • Correct for all other failure mechanisms besides normal flank wear • Use a more shock resistant grade, such as a grade with a higher binder content • Use a stronger insert geometry • Use a thicker insert • Reduce feed rate and/or depth of cut.