Dear Doc: We grind mostly carbide but occasionally have to grind cermets. They’re killing us. Why? And is there something we can do?
The Doc replies: Both carbide and cermets are ground with diamond wheels. Both materials load/clog the wheel. But here’s the difference: Cermet dulls the diamond grits. Carbide doesn’t. And that dulling effect is a killer.
Why do cermets dull the wheel? Because they contain hard particulates — SiC, Al2O3, TiC, TiN, etc. — that are close to the hardness of the diamond. Tungsten carbide, albeit hard, is not hard enough to dull the diamond. Sticking the wheel reduces loading in both cermets and tungsten carbide, but it doesn’t sharpen dull diamonds.
Therefore, we need those diamonds to fracture while grinding. Here’s how to do it: 1) Use smaller grits. You’ll find that 150 mesh is too large, so 250 mesh or even 320 mesh is better. 2) Use friable (not tough) grits. You’ll have to talk to your wheel supplier about this. 3) Grind aggressively using faster feed rates and/or slower wheel speeds.
The grinding-specific energy is the energy required to grind away 1 mm3 of material (in Joules/mm3). A well-optimized carbide-grinding process will operate in the range of SE = 50-100 Joules/mm3. A well-optimized cermet-grinding process will be SE = 75-175 Joules/mm3.
Here’s the biggest mistake made with cermets: As soon as problems arise, people slow down the feed rate. That means that the diamonds are no longer penetrating deep into the workpiece — they’re just tickling the surface. The diamonds don’t self-sharpen; they just dull.
The worst case I’ve seen was a cermet-tipped saw blade manufacturer using 150-mesh diamond. In an attempt to cope with the difficulty of grinding cermet, the company kept slowing down the feed rate — eventually arriving at a pathetic Q-prime of 0.05 mm2/s. (A respectable value would be Q-prime = 0.5 mm2/s.) What specific energy was obtained? SE = 1,604 J/mm3. If you have to slow something down, slow down the wheel speed, not the feed rate.
Cermets will never be as easy to grind as carbide. But follow these three rules and you’ll be able to grind them reasonably well.
Dear Doc: I read somewhere on social media that if you don’t see sparks, you’re probably not burning, and that if you do see sparks, you’re probably burning. Is that true?
The Doc replies: Absolutely not. I’ve worked on many spark-free grinding operations in which the workpiece was burned to death. And I’ve worked on many sparking operations that had no workpiece burn.
Sparks are simply hot chips coming off the grinding zone and oxidizing in the atmosphere. In a sense, they’re metal burning in the air. And things that burn give off heat and light — that is, sparks.
If you have good cooling, you quench those sparks before they have a chance to burn in the atmosphere. But does that mean you didn’t burn the workpiece? No way. Maybe you did, maybe you didn’t. But judging that based on the sparks is a risky venture.
Related Glossary Terms
- burning
burning
Rotary tool that removes hard or soft materials similar to a rotary file. A bur’s teeth, or flutes, have a negative rake.
- cermets
cermets
Cutting tool materials based mostly on titanium carbonitride with nickel and/or cobalt binder. Cermets are characterized by high wear resistance due to their chemical and thermal stability. Cermets are able to hold a sharp edge at high cutting speeds and temperatures, which results in exceptional surface finish when machining most types of steels.
- feed
feed
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
- grinding
grinding
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- hardness
hardness
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- sawing machine ( saw)
sawing machine ( saw)
Machine designed to use a serrated-tooth blade to cut metal or other material. Comes in a wide variety of styles but takes one of four basic forms: hacksaw (a simple, rugged machine that uses a reciprocating motion to part metal or other material); cold or circular saw (powers a circular blade that cuts structural materials); bandsaw (runs an endless band; the two basic types are cutoff and contour band machines, which cut intricate contours and shapes); and abrasive cutoff saw (similar in appearance to the cold saw, but uses an abrasive disc that rotates at high speeds rather than a blade with serrated teeth).
- titanium carbide ( TiC)
titanium carbide ( TiC)
Extremely hard material added to tungsten carbide to reduce cratering and built-up edge. Also used as a tool coating. See coated tools.
- titanium nitride ( TiN)
titanium nitride ( TiN)
Added to titanium-carbide tooling to permit machining of hard metals at high speeds. Also used as a tool coating. See coated tools.
- tungsten carbide ( WC)
tungsten carbide ( WC)
Intermetallic compound consisting of equal parts, by atomic weight, of tungsten and carbon. Sometimes tungsten carbide is used in reference to the cemented tungsten carbide material with cobalt added and/or with titanium carbide or tantalum carbide added. Thus, the tungsten carbide may be used to refer to pure tungsten carbide as well as co-bonded tungsten carbide, which may or may not contain added titanium carbide and/or tantalum carbide.
Author
Jeffrey A. Badger, Ph.D., is an independent grinding expert. He will be giving his 3-day High Intensity Grinding Course from Sept. 26-28 in Columbia, S.C., hosted by S.L. Munson. Visit www.thegrindingdoc.com for details. He can be reached by phone at 512-934-1857 or via email at JB@TheGrindingDoc.com.
ARTICLES
