Off topic Q about grinding carbide

C

catspa

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Oct 25, 2009
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Hey y’all, hope .22 is being good to you so far. Got a weird question about grinding carbide tool bits with a diamond wheel. Basically, slow or fast, and why?

TLDR version: In the not too distant future, I will have an opportunity to grind fresh edges on about 2 dozen carbide chipping hammers, using a 400 grit diamond wheel from McMaster Carr. I think a coarser wheel (like 220) would probably do the job, but I already have this one, so I’m going with it.

The hammers (1-1/2 to 3lbs, inch to inch and a half wide) are used by professional sculptors to shape/chip stone and glass in various forms. Eventually, that dulls them to about a 3/64-1/16 radius. They have carbide inserts brazed into a notch in a steel body about 1/8” proud. The edges typically meet at a 60-ish degree angle, although I have seen them as steep as 80. To my knowledge, I’ll only be asked to sharpen the original geometry, i.e. no regrind to a different angle.

There’s a kinda weak time constraint, such that I hope to spend about 2-4 minutes on each hammer.

I have time to set up a machine in advance. I’m not too concerned about overheating, as I can crudely mist cool the work and grind with a light touch. I doubt that carbide loses temper in the same temperature range as steel anyway. My question is, should I mount the wheel on a standard 3450 rpm grinder, a slower 1725 motor, or build a belt and pulley reduction down to say, 600 rpm?

Thanks for your advice.

Parker
 
Depends on whether you're going to hit the steel at all. You don't want to do that at high speed with a diamond wheel or wheel life will go down drastically. If that may happen, grind the steel back first if possible with a normal AlOx grinding wheel, then use the diamond wheel only on the tungsten carbide. If you can manage that, high speeds will be fine.

Also, tungsten carbide does not "lose temper." It's not steel. Totally different material, totally different material properties. You can heat the carbide red hot and it won't lose hardness. However, you do need to take care not to get it very hot then hit it with water. If you do that it's likely to crack on a micro level, causing the material to be greatly weakened. Either keep it flooded with water and nice and cool while grinding or let it get hot then allow it to air cool slowly. A half measure here will not likely have good results.

You're also going to want to watch out on that geometry. As I recall some of these aren't ground to a sharp edge. Be certain to get that specified by the customer.
 
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Y’know, what you said yesterday reminded me of something today.

Concrete drill bits. I always cool mine in water after every 10 seconds of drilling or so. It extends their life quite a bit. They’re not getting anywhere near red hot, though.

Once in a while, some knucklehead will say, “You don’t need to cool those, just keep on drilling.” Or just knock over the water cup with his foot. I just shrug and say, “Your bits, then. My bits get cooled.”

Parker
 
Update on hammer grinding: I didn’t use the 400 grit diamond wheel. Instead, I took the half worn (120-ish grit) wheel off my tile saw and mounted it on a 1725 rpm motor shaft. Worked well, although after about 20 pieces it seemed to grind a little slower. So I flipped it 180 degrees on the arbor and used the other side. Got all the grinding done in under 3 hours.

Parker
 
I'm glad your project went well. I was a toolmaker working on progressive carbide stamping dies and a 400 grit would load up pretty quickly and need redressed or at least cleaned or it would not cut any more. A 100 grit would be better. You definitely don't want to cut much steel with the diamond wheel. When we sharpened carbide it was on a precision grinder taking .0001 to .001 per cutting pass.
 
I was going to mention that too but spaced it. 100'ish grit is good if you need to do any real material removal. 400 is more of a fine finishing grit for carbide. If it's a resin bond wheel you can use a stick of aluminum oxide to open up the wheel if (when) it loads. Just press the stick straight into the diamond wheel several times. If it's a metal bond or vitrified bond wheel things are a little trickier.
 
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