I'm seeking advice about die polishing stones such as those found here:
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I recently heard of their use from another knifemaker, who uses them to polish grinding belt scratches out of the flats of his blades. Apparently, they keep grind lines crisp and sharp, and help avoid rolled edges, both difficult to do with wet & dry papers and wooden sanding blocks.
I've never used these stones before, although I've read the FAQ for them here:
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There are several different types of polishing stones made for different purposes. Which ones (brand & material) are best for carbon and/or stainless steels? Before heat treat? After HT?
What grits do I need (which grits are available)? What lubricants? Do you store the stones in lubricant? What is the expected life of a typical stone when it's used correctly? Any special techniques to using them?
Lots of questions, I know, but these things do sound intriguing and I'm interested in anything that can help me build a hand-made knife better and faster.
At work we use red 40g AlO stones for roughing. The ones we use are loosly bound so they break down quickly (exposing new/sharp crystals quickly, and conforming to the contours of the die) Mind out, we have to polish down milled surfaces so you may not require such an aggressive stone.
Next is a 60g SiC stone, and then an 80g SiC stone. These are used to remove the scratch marks from the 40g (and 60g). You'll always need a good assortment of stones as you must completely remove the scratch marks from the previous stone. This becomes very slow (and stoning is slow enough) when you jump grits.
For semi-finishing, we use a 100g grey AlO stone. Once this is done, 220g,
320g, 400g and 600g sandpaper is used. While this series results in a
*smooth* surface, they aren't typically cosmetically smooth - you can still see scratch marks.
For final polishing of draw surfaces (nearly mirror), we use while AlOx stones, although I don't know what grit (320g comes to mind) as I have never had to do the final polish on a draw die. This is slow, typically taking more than two weeks for a couple of guys on two shifts.
As for technique, dies require consistant surfaces. Stoning in only one direction will cause a surface, that should be flat, to become warped. We call these low spots "holes" although they are only thousandths or ten-thousandths of an inch deep. To prevent these holes, a crosshatch pattern is used when possible (not usually possible on smaller rads) where a surface is stoned at a certain angle, then the angle of motion is changed - typically between 30º and 45º. This obviously gives a crosshatched pattern, and a flat surface. Change directions consistantly. I typically give the surface a healthy once-over in one direction, then change (this cycle may need to be repeated tens of times).
Of course, use lube if the stone requires it. Oil stones don't come apart easily so they load quickly and will stop cutting if used dry. Dry stones can be tapped flatly on a surface to unload them, although good technique should stop the loading. You should be able to feel it when the stone is cutting as opposed to when it is rubbing (due to being loaded). You should not be affriad to push reasonably hard.
Now, we polish surfaces that usually exceed 2 square metres so polishing little knives will be slightly different.
The above is worth exactly what you paid for it. HTH.
That's great info, Robin. Worth way more than I paid :)
I typically take my blades to 320 or 400 grit on the belt grinder, then finish by hand. I often find myself going back a few grits with wet & dry papers, to 180 or so, when I discover deep scratches that the belt didn't get out. This often affects the character of my grind lines and edges. And, rather than creating "holes," I get "hills," or flats that should be perfectly flat but are actually slightly convex. I'm thinking a set of stones from 150g up to about 600 would reduce some of these problems.
The stones come in various sizes. Which do you prefer? I.e., 1/4" x
1/4", 1/4" x 1/2", etc.
They are used exclusively by hand? No holders, handles and so on? Are die grinders a completely different thing?
Any light machine oil good for lube? Kerosene (the Engnath site mentions that) okay? 3-in-1 oil? Tap-magic?
Our stones are typically 1x2x8" or so, but of course our dies are slightly larger than your knives.
As I understand, there are stoning machines. Something like a hand-held power scraper, although I've never seen one. Pausing for too long with a powered tool could cause some major issues for us. Not sure about knife making though.
A die grinder is something like a drill, but it spins in excess of 20,000 RPM (drills typically max out at about 3,000). You use either a mounted stone (mounted to a 1/4" steel post) or a carbide burr. You can google these items to see what they are.
While we regularily use die grinders to create flat surfaces (using the crosshatching pattern), this technique cannot be explained. I'd have to show you (which I cannot). If you become handy with a die grinder, you can do some very impressive work. You may also want a disc grinder, although they are typically more aggressive.
We use WD-40. Kerosene would be good also. I use EDM fluid where I am now as we don't have WD-40. However, we only use lube with "oil stones". The others I mentioned do not require such lubrication...
You should probably at least try what the manufacturer suggests to use, unless it's really expensive.
Well, I might as well put those weeks of polishing dies for good use... See:
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(not us, but we do very similar work. The top right-hand pics are of a body side draw die)
Your website is very impressive. Your knives look beautiful.
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