larger radius causes more deflection on the tool and, at a certain point, can lead to chatter. The trick is finding the largest radius that produces chatter-free results. A way around this is to use a wiper geometry, which is now offered for some PCD and CBN inserts. A wiper is a secondary, smaller radius or series of smaller radii that follow the main corner radius to additionally smooth the scalloped profile of the main corner’s cut. It’s a way to get a better feed-to-surface-finish ratio. Q: Should coolant be used with PCD and CBN inserts? A: Use either a lot of coolant or none at all. Thermal fracture of these materials happens with large temperature fluctuations. If coolant is used effectively, it helps increase tool life and improve chip evacuation. If coolant can’t be directed at the cutting action with consistent flow, it will be detrimental to the operation. Q: Does edge preparation matter with PCD and CBN inserts? A: The extreme processes used to create these very hard cutter materials make edge preparation important for both CBN and PCD tools, both for reinforcement and chip control. What’s more, you’ll always find microscopic edge defects on nearly every tool before edge preparation. When high performance is required, as is often the case with these cutters, preparation eliminates defects that cause erratic performance. PCD and CBN inserts now are offered with almost as many edge preparation styles as carbide inserts. They are categorized by “land” types, and each type gives advantages for specific materials and cutting conditions. Q: How do you match a land type with an application or material? A: Most edge preps still consist of a radius hone, and a smaller radius typically is preferred so more material is left at the cutting edge, giving longer tool life. A waterfall hone skews the radius to leave more material under the cutting edge to strengthen it for heavier cuts or interrupted-cut applications, but this style also leads to higher tool pressures. Q: Are there minimum and maximum speeds for PCD and CBN inserts? A: Contrary to most perceptions, PCD has a wide range of allowable surface feet per minute (SFM) settings for aluminum, from as low as 200 SFM to 6,000 SFM,
and it typically outlasts carbide inserts by at least 10 times, allowing tools to remain in tolerance without adjustments for very long periods of cutting time. With CBN inserts, feeds as low as 200 SFM often are used in hardened steel applications. Speed also depends on the tool assembly as a whole. For boring tools, for example, it often is necessary to consider the tool assembly and workpiece setup rigidity when making the decision to use high-speed machining with these types of inserts. Often a boring head with built-in counterweights for balance is required to be able to run the speeds that these inserts are capable of.
Because of their extreme hardness, polycrystalline diamond tools provide a very long tool life in non-ferrous materials.
Q: What are the common wear modalities for these inserts?
A: Crater wear occurs in CBN machining of iron- based and titanium-based alloys when the heat from the workpiece’s chips dissolves and disperses the cemented carbide binding agents and forms craters on the top of the tool. Once the craters grow large enough to cause microcollapsing of the crystal structure, deformation and rapid flank wear occur. Built-up edge (BUE) happens when fragments of the workpiece are bonded to the cutting edge by hot- pressure welding (a combination of high pressure and temperature along with chemical affinity in the cutting zone). This buildup eventually will fail, and when it does, it also can take some of the cutting edge with it. This issue can be identified by abnormal changes in the workpiece’s feature size or surface finish and shiny areas on the cutting edge’s surface. BUE often can be controlled by increasing the tool’s cutting load (higher cutting speed and/or feed) or by altering the coolant flow or concentration in the cutting zone.
CONTRIBUTOR Matt Tegelman is the Senior Product Specialist at BIG DAISHOWA. matt.tegelman@us.bigdaishowa.com
33
Powered by FlippingBook