Precision stones ...

[Jake M]

I understand the cleaning aspect and having two is fine. But “you don’t need three to keep them in good condition” assumes neither stone wears with use. I find that somewhat unlikely. If there is any wear in one of the stones, using one other stone will not correct it.

Now maybe your point is that the stones cannot abrade each other, they can only abrade a protrusion. Maybe so, and if so, then refreshing on the surface grinder is the only way to correct wear.

These don't work like regular stones in this aspect. They really don't "wear" tangibly. They do wear of course, but they don't wear holes like a "normal" stone. Provided that is, you don't use them like a "normal" stone. If you're sharpening knives with them, or rounding off corners, of course you're gonna wear them funny, but you're not going to do that much, because you'd bee there all day long. Because in normal use there is some significant portion of the stone's surface laid on something very flat, they ONLY pick on the one high point. (Or I guess if you're scraping maybe they'd lay on a bunch.....). But if they're laid flat, working on specific damage points, any wear that did occur would be local, not global, which won't affect their global flatness, nor will it affect their function.

When you rub the two stones together (which is VERY often), they can't and don't cut each other. They dig out/remove/shave off any metal particles that have become embedded.

I don't pretend to be an expert here, but honestly I can't see any tangible shape being worn into these stones before the flattened abrasive particles at the surface become "dull", and it just won't cut any more. I believe this is why you don't see sets of three for sale. Because I believe that they'll need replacement or regrinding for sharpness long before they need it for taking on any quantifiable shape at any global scale..

 

[davidpbest]

I understand the cleaning aspect and having two is fine. But “you don’t need three to keep them in good condition” assumes neither stone wears with use. I find that somewhat unlikely. If there is any wear in one of the stones, using one other stone will not correct it.

Now maybe your point is that the stones cannot abrade each other, they can only abrade a protrusion. Maybe so, and if so, then refreshing on the surface grinder is the only way to correct wear.

If properly ground, the abrasive particles in both stones are ground flat - like cleaving off the top of a mountain range - to present a flat surface where all all abrasive particles are coplaner to each other. After true precision grinding, what remains is a surface that is over 50-percent flat with minute cavities between the flat-topped abrasive that cleaves off any protrusions above what would otherwise be material with a flat surface. If used properly, you can't put enough pressure on the stones for the abrasive particles to actually abrade flat surfaces, and thus the stones do not "wear" in the traditional sense. The second stone is used to remove the material cleaved off, and does not abrade the surface of the other stone since well over 50 percent of the surface of both stones is flat. This is all explained in the video and basic material science. The video illustrates these concepts this using an optical flat.

These type of stones are not intended to remove material from flat surfaces like a conventional abrasive. Robins' video demonstrates this starting at 10:45 in the timeline. If you wanted to remove material from flat surfaces, you would need a a different (mountain-top) topography on the stone to abrade the surfaces which would exhibit wear on surface of the material as well as the abrasive stones themselves, thus requiring 3 units to bring them back to total flatness. That's not what's going on here with this type of precision stone.

The entire rationale behind buying a pair of truly precision ground stones is to alleviate the need for a surface grinder in the shop that wouldn't otherwise need a grinder (such as my shop).

 

[jwmelvin]

If the stones were only used on an actually flat surface, there wouldn’t be much point. They are used to remove protrusions that rise above a flat surface. Those protrusions will wear the stone when you use it to remove them.

 

[davidpbest]

If the stones were only used on an actually flat surface, there wouldn’t be much point. They are used to remove protrusions that rise above a flat surface. Those protrusions will wear the stone when you use it to remove them.

Not so. At least in practice due to the large surface area of the stone that is actually flat.

 

[WobblyHand]

Think this all is like arguing about how many angels can fit on a pin... There's some difference of opinion.

The stones are initially flat. They are an abrasive material, they do remove burrs. They may also remove their own material on the other stone when we rub them together. Not saying it is at a huge rate, but they do remove material - or we wouldn't use them. Rubbing two flat surfaces together eventually makes a non flat surface, and it approaches a spherical surface with a little bit of work - that's how telescope mirrors are ground.
I suppose we are merely discussing material removal rates. So technically, one would need 3 stones to keep them (optically) flat, but using two stones is ok for quite a while, unless you use them everyday, multiple times a day and scrub them with excess vigor.

 

[Flynth]

So, this talk about precision ground stones made me finish a spacer and an adapter so I can use an edge cutting diamond wheel on my surface grinder. It is a 120/100 grit resin bonded wheel.

There was time when I bought every single not horribly overpriced bench stone in the correct grit(160~200 and 400~600) available from sellers around here(in EU) in hope of eventually finding good quality stones. Unfortunately almost every single stone with grits under 400 was very bad with uneven grit,as if not mixed properly. They looked, felt to touch and "cut" not like aluminium oxide, but mostly sand.

However, during this time I did find two stones I used as "normal" bench stones for a bit that seemed pretty good. So I decided to grind them.

I haven't got proper magnetic hold downs so I just used a grinding vice, a set of parallels and some copper shim material. It took almost as long to dress true the diamond wheel than the grind itself. Diamond seems to cut those Chinese stones like butter.


Once I was done I decided to try the stones on my surface grinder's chuck. The finer (400) grit works great. It behaves exactly as it should. It didn't cut the chuck's surface, but burnished and polished it making it almost mirror like. Amazing This is despite not giving it enough time during grinding resulting in very slight patterning of the surface. This patterning was not visible, nor able to get felt, but it became apparent after use as horizontal lines matching my auto feed width.. . I ground the stone so after few "spark out" passes I could draw very light pencil grid and a single pass of thd wheel without feeding lower would remove all of the pencil marks in one pass so I know those lines are very shallow. Still next time I'll vary the cross feed. During the cutting edge of the wheel wears faster so no matter how true the wheel is initially, it will become lower on edges.

But, the coarse grit is completely useless. I was shocked that it basically scored the surface (thankfully not very deep) as if there were rougue protruding grains on the stone (despite not feeling any when rubbing stones together). Upon closer inspection it became apparent there was not just sand in the grit, but also what looked like red brick dust, and even hardened metal shavings... (yes, they were embedded in the material, I've dug some out after removing entire surface layer during grinding.

This experience made me appreciate the price those stones go for better. First, just finding good quality abrasives in the correct grits seems to be extremely hard for bench stones. Every single stone sold now seems to be for "knife sharpening". The smaller the grit the better quality. (800 plus especially).

Then grinding them properly takes quite a bit of time and produces lots of mud all over the surface grinder. I think if I were to sell some on ebay (after finding good quality starting stones and getting rid of cross feed lines) I would have to charge $200 per pair which seemed rather expensive before. That's no $500 each, so probably some would be interested. I certainly am interested in making good ones for myself.

 

[mmcmdl]

Whatever they are , I got a $hit pot full of them !

 

[Flynth]

Whatever they are , I got a $hit pot full of them !

Exactly. I have all the bad ones too. I'm not sure what to use them for. They can't be used for opening diamond resin wheels because they aren't even aluminium oxide, but some sort of "pretend silicon carbide" (behaves more like sand than anything else).

It is weird, because my tool cutter grinder came with a Chinese made good quality aluminium oxide cup wheel. So they definitely are able to make decent quality abrasives...

I wonder, how would such a precision ground stone work if it was made from... Let say 60 grit (very large grit). The key point of those stones is that all their grain tops are flat so in theory such stone should still not cut at all unless something protrudes. The main difference would be the number of grains I think. Smaller grit woukd have lots of grain edges, large grit would have a lot less.

An experiment may be needed I'm thinking about cutting a rectangular shape from a broken 60K surface grinder wheel. Grinding it flat and seeing if it works well...