Precision stones ...

[jwmelvin]

You do need a matched pair so they can be properly cleaned.

I’ve been thinking that it would make more sense to use them in threes, so they they tend towards flatness. I suppose it’s easy enough to touch them up on the surface grinder but doesn’t that seem like a preferable approach?

 

[Flynth]

An India stone is also appropriate, I have used one for removing scraping burrs for many years, my thinking is that the so called "precision stones" are more faddish than necessary for anything that most of us do.

It is worth pointing out an India stone is just Norton's name for an aluminium oxide abrasive stones (I didn't realise it initially). Unfortunately Norton's products are hard to come by around here and it's good to know one can use any other good quality aluminium oxide stone instead.

Personally, making a set of such precision ground stones has been on my lisf for quite a while. I do have a surface grinder with a diamond wheel, but I haven't got any way to hold the stones on my magnetic chuck yet. I have to make some magnetic hold-downs as shown by Rob Renzetti.

Area they necessary? It depends. Are you trying to hit the best tolerances using high quality precision ground tools? If you do, they sure are handy from what I saw. However, if you don't have them a set of reasonably flat bench stones is a good replacement providing you're aware of their limitations.

For example, I use cheap non precision ground stones flattened "by hand" on a diamond stone flattening tool to stone my surface grinder chuck as well as parts that are surface ground up until the final surface. But I'm aware a non-precision ground stone actually removes some material (as evidenced by it getting packed in the middle). I tend to use my hand to detect dings or burrs, then I run the stone very gently over the surface looking for any signs of raised areas. Finally if burrs or dings are present I run the stone only over area affected.

Also, I would never run a "normal stone" the way one can do with a precision ground one on my really good tools (for example my best sine plate).

So in summary, are they necessary? Sometimes yes, but one can substitute "normal" good quality stones in many uses. It is important to use the correct grit and good quality (consistent grit throughout the stone) however.

In general finding good quality bench stones in grits we need (~200/400~600) has been really hard recently. It seems most such stones are sold for "sharpening" and for this they tend to use grits like 160, then 600, then 1000. If one never had a good quality stone in correct grit size in their hand it may be difficult to judge quality.

If I ever find good quality bench stones for a reasonable price I can get here in EU I might precision grind them and I may try selling some on ebay.

 

[davidpbest]

I’ve been thinking that it would make more sense to use them in threes, so they they tend towards flatness. I suppose it’s easy enough to touch them up on the surface grinder but doesn’t that seem like a preferable approach?

If you watch the following video you'll discover that you don't need three to keep them in good condition, but you do need two for cleaning of the stones. If the two stones are dead-flat to begin with (which they have to be to serve their intended purpose), you aren't trying to re-flatten them by rubbing them together. Skip to 5:50 in the timeline to get the details on "why two stones?".

 

[ErichKeane]

It is worth pointing out an India stone is just Norton's name for an aluminium oxide abrasive stones (I didn't realise it initially). Unfortunately Norton's products are hard to come by around here and it's good to know one can use any other good quality aluminium oxide stone instead.

Personally, making a set of such precision ground stones has been on my lisf for quite a while. I do have a surface grinder with a diamond wheel, but I haven't got any way to hold the stones on my magnetic chuck yet. I have to make some magnetic hold-downs as shown by Rob Renzetti.

Area they necessary? It depends. Are you trying to hit the best tolerances using high quality precision ground tools? If you do, they sure are handy from what I saw. However, if you don't have them a set of reasonably flat bench stones is a good replacement providing you're aware of their limitations.

For example, I use cheap non precision ground stones flattened "by hand" on a diamond stone flattening tool to stone my surface grinder chuck as well as parts that are surface ground up until the final surface. But I'm aware a non-precision ground stone actually removes some material (as evidenced by it getting packed in the middle). I tend to use my hand to detect dings or burrs, then I run the stone very gently over the surface looking for any signs of raised areas. Finally if burrs or dings are present I run the stone only over area affected.

Also, I would never run a "normal stone" the way one can do with a precision ground one on my really good tools (for example my best sine plate).

So in summary, are they necessary? Sometimes yes, but one can substitute "normal" good quality stones in many uses. It is important to use the correct grit and good quality (consistent grit throughout the stone) however.

In general finding good quality bench stones in grits we need (~200/400~600) has been really hard recently. It seems most such stones are sold for "sharpening" and for this they tend to use grits like 160, then 600, then 1000. If one never had a good quality stone in correct grit size in their hand it may be difficult to judge quality.

If I ever find good quality bench stones for a reasonable price I can get here in EU I might precision grind them and I may try selling some on ebay.

I just used those two part parallel clamps to hold the stones, and msgnetted them down.

 

[cathead]

Take a look at the 2x6 inch diamond hones on E-bay. Mine are 400 grit on one side and 1000 grit on the other side.
They are perfect for small dings and work to touch up carbides too. I use the 400 grit side mostly. They are also
great for a quick pass over the mill table.

 

[Flynth]

Take a look at the 2x6 inch diamond hones on E-bay. Mine are 400 grit on one side and 1000 grit on the other side.
They are perfect for small dings and work to touch up carbides too. I use the 400 grit side mostly. They are also
great for a quick pass over the mill table.

Diamond hones are nice for many tasks, but they are not even in the same league as precision ground aluminium oxide stones.

Those precision stones have to be made of material that can be surface ground with diamond. One of the key advantages is that in those stones tops of abrasive grains are flat and dull. It's not that just the whole stone is flat, individual grains have flat tops. This guarantees you're not going to mess your expensive precision lapped surface with it. It cuts only stuff that protrudes above your surface (burrs, dings). Diamond is unsuitable as such abrasive, because it is practically impossible to grind/cut it. Diamond grains fracture do tops of individual grains remain sharp. It is true when the hone I'd sufficiently flat the force distributes along many individual diamond grains and not a lot of material is removed, but the possibility for material removal exists...

Rob Renzetti makes a great demonstration of it in his "precision ground toolroom stones" video.

As for aluminium oxide stones I believe it is good to have the coarse side be around 200 and the fine 400 up to 600 grit, but that's just a personal preference.

I just used those two part parallel clamps to hold the stones, and msgnetted them down.

Great idea i wish I had parallel clamps that big. I might just put one in a grinding vice... (held through some copper foil, sitting on parallels)

 

[MyLilMule]

At the point in my Acra mill rehab where I will bee wanting to address any burrs or high spots on critical surfaces. My question is are precision stones really necessary, and if so where can I acquire them at a reasonable price. Thanks, Mike

The stones you buy out of the box are not even close to flat. Not even close.

 

[jwmelvin]

If you watch the following video you'll discover that you don't need three to keep them in good condition, but you do need two for cleaning of the stones. If the two stones are dead-flat to begin with (which they have to be to serve their intended purpose), you aren't trying to re-flatten them by rubbing them together. Skip to 5:50 in the timeline to get the details on "why two stones?".

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.

 

[WobblyHand]

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.

To keep them flat, I'd agree you need 3 of them, without a surface grinder. With only two (and no other flat reference) the two stones will adapt to each other's wear pattern and eventually become not flat, but matching. Three stones would prevent that.

Edit: common way to make an optical flat is to use three blanks. This comes from the days of grinding them yourself, with hand tools. This is what I was thinking when I said one would need 3 stones to keep them flat. Two stones would eventually make a spherical surface.

 

[FOMOGO]

Ordered a set off e-bay after watching the video. May try the flattened file thing also. Thanks for all the input. Mike