hollow grinding question

I am looking to do some experimenting with very thin hollow grinds, the question of critical importance is how to do the grinds on hardned steel without effecting the temper. The problem comes from the fact that I want the grinds to be thin, 0.01' or less, basically to the limit at which the strength of the edge is so low it deforms instead of holding a straight line when cutting.

I was thinking of a simple variable speed water cooled grinder. I have seen them from 2000 to 4000 rpms. If I go very light, and it is water cooled, will a normal grinder wheel be able to craft that geometry and not fry the steel, assuming plain carbon steel and thus a low temper? Is there a difference class of grinding wheel which would be better, something which would be softer and fall apart easier.

Doug®

Sure is the way they grind straight razors- to the point of submerging the bottom of the grinding wheel in water, so it wouldn't surprise me if some of the grinding itself took place under water. I saw a plan one time for such a set-up, and the only thing unusual was a splash shield made of *screen*- they postulated that screen didn't wet up and obscure your vision, and still caught most of the water. It angled towards the source of spray, and that's about it. It was just a motor driven shaft with the stone in a bath of water.

Both the pink and the white aluminum oxide stones have more a friable matrix so they keep exposing sharp abrasive and generate less heat. But there is no substitute for a wet wheel grinder.

Thanks for the info Steve.

Yes, unfortunately you can't get them locally, and some of them can have problems with balance and such and I would rather not have to deal with returns out of province.

What does give me a little apprehension is that the true wet wheel grinders turn really slow, usually like 100 rpms or so, which is obviously much slower than even a slow bench grinder (~2000 rpms). Now the wheels are usually larger on the wet wheel grinders, but this only raises the velocity by a factor of ~2, not 20.

The speeds you are considering are quite fast. You need a slow bench grinder 1650 rpm, or slower if you can find one, with a continuous wet spray / misting set up. FineWoodworking magazine has had some artilcles on making spray apparatus. The other problem you face is to go that thin you will likely need jigs or someway to keep control of the movement. These grind stones cut fast.

You might also condider a knife maker's grinding rig set up that uses a sanding belt rather than a grind stone.

you can always add a trico mistor to your bench grinder

I don't know what that is for sure, but if it's a water spraying method... I tried that too, got wetter than hell, couldn't see what I was doing since my glasses got wet. I tried dripping water and squirting water in a steady stream about like a squirt gun. A mist might work better but can't 'see' it working very long. Have you tried it?

Dry grinder works and with all the dipping you get kinda wet anyway but doesn't end up on your face or glasses.

I also tried a setup where to identical stones were face to face with water squirting into the gap. When I'd put a piece of steel in there, it acted as if I put the stinkin thing in from the wrong side... it'd kick the blade out or jam up when I got determined with it. The small imperfections in the two stones was what was causing it, I figured at the time. Couldn't see it working without water since both sides were getting cut at the same time.

I've tried all kinds of stuff to cheat my way into a hollow grind and all it did was reasure me the best way was the 'brain damaged copper miner' way is the best and in the long run, the easiest too.

When grinding annealed steel it ok to get it warm and dip every few passes. Getting it real hot has the problem of the steel having a memory and can warp even more than normal if you were heavy handed with it before the heat treat. Besides it gets too hot to hold since you aren't wearing gloves. Bare handed is necesary to feel the blade lay down in it's hollow to fit the stone.

After the heat treat and draw then it's 'one light pass and dip'. I can see where that could be frustrating as anything to a new guy wanting quick results. But it's just one reason hand made knives aren't cheap.

As for getting everything exactly the same on both sides and making it look like a machine made it... forget that crap! You're making something that's hand-made, not machine-made, remember? It's ok if it looks a little sloppy if it's in an artistic way.

I guess the 'artistic way' is a reproducable character of your hand?

Not just willy nilly like you weren't at least trying to get a nice result. On the knife-list years ago there were several knifemakers on there that figured they would quit if they ever made what they thought was a knife with -no- mistakes in it. Where would they go, what would they do from there? Was the question. :/ Quit was the answer.

Something like a stockman with all the part fitting it involves I could point out 6 obvious mistakes and the recipient couldn't see them as mistakes. Then there were the dozen others that I knew were there that wasn't so obvious. :/ And they'd brag on it anyway. :/

If you're looking for guys being interested in the knife you made it's easy and proven in my circles anyway... make it from a file and leave a -small- amount of evidence that it was a file. The sloppiest ugliest piece of crap will draw a crowd of non-knife collectors. All kinds of questions about how you did that and stories about old guys they knew or heard about that did that and speculation about how great it'll hold and edge etc.

If you won't settle for less than perfect, what's your answer? Don't make nuthin? :/ That's how it is tho, the people I've give them to have managed to live with my mistakes why can't I? It's pretty tough to accept sometimes, when you were working hard and looking forward to a better job than it turned out to be from my side of it.

Jump in there, you can do it at least as good as I can, no kidding. (maybe not the -first- day, but neither could I the first day

Alvin in AZ

I was under the impression that you should almost always grind barehanded, precisely so as to feel the heat of the piece and ovoid overheating it, as well as to feel the positioning of the piece on the

Please correct me if I'm wrong, but shouldn't we be discussing the *peripheral* speed, rather than RPM, especially when comparing wheels which are almost certainly of different diameters?

Circumference = Pi * Diameter.

For a 4' diameter wheel, circ. = 12.6'. At 2000rpm this is 2095 feet per minute peripheral speed.

For a 12' diameter wheel, cir. = 37.7'. At 200 rpm this is 628 feet per minute peripheral speed.

The peripheral speed of the 12' wheel is 3.34 times that of the 4' wheel, for only one tenth the rpm.

Yep. Works for me. I like a pinch of soap in the water, just enough to make it sheet some, bead some. Enough to make it sheet out completely makes the blade too slick.

Alvin in AZ