b'\x1b\x1a\x19 \x1f\x18\x1d\x17\x16Ask the Grinding DocGRINDING CERMETS & SPARK MYTHSBy Jeffrey Badger diamond. In an attempt to cope with the difficulty of grinding cermet, the company kept slowing D ear Doc: We grind mostly carbide but occa- down the feed rateeventually arriving at a pa-sionally have to grind cermets. Theyre killingthetic Q-prime of 0.05 mm 2 /s. (A respectable value us. Why? And is there something we can do? would be Q-prime = 0.5 mm 2 /s.) What specific en-The Doc replies: Both carbide and cermets areergy was obtained? SE = 1,604 J/mm 3 . If you have ground with diamond wheels. Both materials load/ to slow something down, slow down the wheel clog the wheel. But heres the difference: Cermetspeed, not the feed rate.dulls the diamond grits. Carbide doesnt. And thatCermets will never be as easy to grind as car-dulling effect is a killer. bide. But follow these three rules and youll be Why do cermets dull the wheel? Because theyable to grind them reasonably well.contain hard particulatesSiC, Al O , TiC, TiN, 2 3etc.that are close to the hardness of the dia-mond. Tungsten carbide, albeit hard, is not hardDear Doc: I read somewhere on social media that if you dont see sparks, youre probably not burning, and that if you do see sparks, youre We need those diamondsprobably burning. Is that true?to fracture while grinding. The Doc replies: Absolutely not. Ive worked on many spark-free grinding operations in which the workpiece was burned to death. And Ive worked enough to dull the diamond. Sticking the wheelon many sparking operations that had no work-reduces loading in both cermets and tungstenpiece burn.carbide, but it doesnt sharpen dull diamonds. Sparks are simply hot chips coming off the Therefore, we need those diamonds to fracturegrinding zone and oxidizing in the atmosphere. while grinding. Heres how to do it: 1) Use smallerIn a sense, theyre metal burning in the air. And grits. Youll find that 150 mesh is too large, so 250things that burn give off heat and lightthat is, mesh or even 320 mesh is better. 2) Use friable (notsparks.tough) grits. Youll have to talk to your wheel sup- If you have good cooling, you quench those plier about this. 3) Grind aggressively using fastersparks before they have a chance to burn in the feed rates and/or slower wheel speeds. atmosphere. But does that mean you didnt burn The grinding-specific energy is the energy re- the workpiece? No way. Maybe you did, maybe quired to grind away 1 mm 3of material (in Joules/ you didnt. But judging that based on the sparks is mm 3 ). A well-optimized carbide-grinding processa risky venture.CTE will operate in the range of SE = 50-100 Joules/mm 3 . A well-optimized cermet-grinding process will be SE = 75-175 Joules/mm 3 . about the author Heres the biggest mistake made with cermets: As soon as problems arise, people slow down theJeffrey Badger, Ph.D., is an feed rate. That means that the diamonds are noindependent grinding consultant longer penetrating deep into the workpiece who is offering his popular three-theyre just tickling the surface. The diamondsday High Intensity Grinding dont self-sharpen; they just dull. Course in numerous locations in the United States and Europe. The worst case Ive seen was a cermet-tippedFor more information, visit www.TheGrindingsawblademanufacturerusing150-meshDoc.com.ctemag.com/cteguide.com11GrindingDoc.indd 11 11/16/22 10:34 AM'