How flat does an Atoma need to be?

[Ty Williams]

I bought an Atoma #140 to use to flatten my Shapton Pro stones. It's not perfectly flat. Laying it diamond-face down onto my granite surface plate, the 0.0015" feeler gauge can slip under the middle of the Atoma without any resistance. The 0.0020" feeler gauge however drags just enough that the diamonds leave scratch marks on it even as it roughly slips past them. I interpert that to mean that the Atoma is hollow by somewhere between 0.0015" and 0.0020". Is that flat enough to flatten the Shaptons since every single thing I sharpened will, by proxy, be referenced against the Atoma?

Thanks as usual.

[Jessica de Boer]

Believe it or not but a slightly convex stone makes it easier to keep your tools flat. A friend machined 2 aluminium plates for my Atoma 600 and 1200 to be slightly convex.

[Andrew Hughes]

No way would I accept that for dressing my stones.
Did you mean .020 it’s easy to add too many 0s

[Ty Williams]

No way would I accept that for dressing my stones.
Did you mean .020 it’s easy to add too many 0s

No, I meant what I wrote. Somewhere between 15 and 20 ten-thousandths of an inch.

[Brian Holcombe]

Seems a touch excessive. Try flattening your stones and see how it works.

[Tom M King]

Can you send it back?

I bought the Atoma replacement sheets, and put them on a granite surface plate that stays on the drainboard of my sharpening sink. The plate came from Woodcraft, back when they sold the 9x12's several times a year for around 25 bucks. I didn't want a small stone to move around, and have running water on that sink with a double swivel spout that can cover a large area on the sink drainboard. It works fine for me.

15 thousandths is the thickness of a matchbook cover. That, or more, sounds like way too much out of flat for this job, especially being concave.

Back when we ground telescope mirrors, when I was in High School, it was much harder to make an optical flat, than any kind of curved surface mirror.

[Andrew Hughes]

No, I meant what I wrote. Somewhere between 15 and 20 ten-thousandths of an inch.
Ok just making sure 15 ten thousands is pretty small number. So I change my stand it’s good. :)
Curious how do you measure tenths?

[Tom M King]

Oops. I missed the "ten" thousandth's. I was off by a factor of ten. My feeler gauges only measure in thousandths.

[Andrew Hughes]

Oops. I missed the "ten" thousandth's. I was off by a factor of ten. My feeler gauges only measure in thousandths.
Same here Tom.
Im impressed he can measure in the 10ths that’s amazing ;)

[Ty Williams]

To be fair, the feeler gauge set is in multiples of 5 tenths, not single tenths (and only that precisely up to 2 thousandths). I do have a dial indicator that reads in half tenths though (aka 5 one-hundred-thousandths). At that precision, you start worrying about temperature and how long you've touched the tool. You can put your finger against the thing and watch the dial wind as the heat from your hand changes the size of the thing.

[Andrew Hughes]

Sounds great Ty.
Have you seen Stanley’s Nano hones? Certain to satisfy the perfectionist


https://nanohone.com/pages/about

[Brian Holcombe]

A dial test indicator?

Tenths are probably not very useful in everyday woodworking, but can be useful in other aspects of the setup work. If you want something really flat you might be better off making your own plate and going from there, or doing what Tom does.

[William Fretwell]

Don’t trust the granite too much. My 3’x2’ granite was dished 4.5 microns in the middle, rather disappointing.

[Edwin Santos]

No, I meant what I wrote. Somewhere between 15 and 20 ten-thousandths of an inch.

That is really negligible. I would bet it will flatten your stones. At least adequately enough for the most fastidious woodworking shop.
The deviation you are reporting is a little less than a THIRD of the thickness of a human hair.

[Richard Jones]

Don’t trust the granite too much. My 3’x2’ granite was dished 4.5 microns in the middle, rather disappointing.

4.5 microns? I don't think I'd be disappointed in that.......at least not for what I use a granite plate for.......Maybe you were being facetious, hard to tell on here. By my calcs, that's 2x flatter than an AA.

[Tom M King]

With the cheap 9x12 Chinese granite surface plates that Woodcraft sells, you don't get to see exactly how flat they are until you open the wooden box. Inside there is a map of the surface. Just by chance, the one I have the Atoma sheets on has a .0001mm crown in the middle. That's .1 microns, if my math is right. Since the 100mm x200mm sheets are on either side of that crown, I expect they are as flat as they can make the Atoma sheets. I don't use it for flattening backs-only for stones.

I originally bought one just to see if I liked it, then bought another the next time they were on sale. Sometime later, I was at an auction, and bought a AA 12 x 18 for 35 bucks that had two bruised corners. I like the way you don't have to use adhesive to get sandpaper to lay flat on them. A splash of water holds a sheet just fine. I sharpen jointer knives on sandpaper. On the other small one I had bought previously, I keep diamond lapping film stuck to it.

[Ty Williams]

I asked to exchange the Atoma out of curiosity and the new one arrived today. It's flatter, but still not flat. There's a MUCH smaller section of the face that's concave by right about 0.0015" (15 ten thousandths). I tried some googling. Lee Valley says the Atoma is guaranteed flat to +-0.04mm, which is 0.0016", so my plate is just barely in spec. DMT says the DiaSharp plates are flat to within 0.0010", so they're flatter than my 2nd Atoma. DMT also says their DiaFlat line is flat to within 0.0005" (5 ten thousandths) so about 3x flatter than my 2nd Atoma but also about 3x more expensive.

Those numbers are meaningless, of course, without knowing what precision is required for our uses. My gut feeling is that the edge of the tool needs to be flat/planar to within the size of the scratch pattern the abrasive is leaving to maximize the edge quality at that abrasive size. With the Shapton Pro 5k I finish on, that means 3 microns or about 1 ten-thousandth of an inch. What I have no idea about is how flat the start of the chain (the Atoma in this case) needs to be to ensure that the final product is as well sharpened as it can be.

[Reinis Kanders]

I think you are overthinking this.:)

[Jessica de Boer]

Those numbers are meaningless, of course, without knowing what precision is required for our uses.

So the Atoma is a tiny bit concave, that means any stone you flatten with it will be a tiny but convex. That's absolutely no problem. Stop overthinking this and just use the damn thing.

And since you like numbers so much, a chisel never touches the entire stone. If you sharpen a 36mm chisel sideways on a 210mm long stone stone that's convex by 0.0381mm, the flatness for the stone area the chisel touches is 0.0065mm. If you really think that will ever be a problem in any quantum reality you need some professional help.

[James Pallas]

A very accomplished and wise woodworker, read period furniture maker, made this statement to me, “If you can’t see it or feel it, it ain’t there.” A good practice to follow in my opinion.
Jim

[Brian Holcombe]

Convexity of the finished stone will be a benefit for two reasons, the first being that water stones dish by their nature and the second is that it will make it much easier to make a flat bevel.

Technique weighs into this considerably.

I see no harm in wanting things flat, after all I like to pursue tight tolerances myself, but there is a workable reality that must also be considered. A stone simply won't stay super flat in the process of using it, but it can be maintained fairly flat if you sharpen using the whole stone.

[Dan Carroll]

Good, practical advise.
:)

[Jens Hoffmann]

I’d argue that up until a few years ago almost no woodworker had flattening stones that were that level of flat, and they have been okay with it. It’ll be just fine...

[J. Greg Jones]

The first ‘flattening stone’ that I used was an ordinary cinder (concrete) block. Worked fine for me for years.

[Edwin Santos]

The first ‘flattening stone’ that I used was an ordinary cinder (concrete) block. Worked fine for me for years.

The horror! :eek:

[Tom M King]

My first flattening stone was also a cinder block, but on top of that went scrap window panes, and Carborundum (silicon carbide) grit left over from grinding telescope mirrors, when I was a teenager. I'd just toss the window pane after use. It wasn't that long ago that I stopped using that setup for coarse stones.

[david charlesworth]

I agree with Jessica.

Slight convexity of stone is desirable.

Hollowness of any kind should be avoided like the plague, as this is one cause of belly.

Best wishes,
David

[Andrew Seemann]

I asked to exchange the Atoma out of curiosity and the new one arrived today. It's flatter, but still not flat. There's a MUCH smaller section of the face that's concave by right about 0.0015" (15 ten thousandths). I tried some googling. Lee Valley says the Atoma is guaranteed flat to +-0.04mm, which is 0.0016", so my plate is just barely in spec. DMT says the DiaSharp plates are flat to within 0.0010", so they're flatter than my 2nd Atoma. DMT also says their DiaFlat line is flat to within 0.0005" (5 ten thousandths) so about 3x flatter than my 2nd Atoma but also about 3x more expensive.

Those numbers are meaningless, of course, without knowing what precision is required for our uses. My gut feeling is that the edge of the tool needs to be flat/planar to within the size of the scratch pattern the abrasive is leaving to maximize the edge quality at that abrasive size. With the Shapton Pro 5k I finish on, that means 3 microns or about 1 ten-thousandth of an inch. What I have no idea about is how flat the start of the chain (the Atoma in this case) needs to be to ensure that the final product is as well sharpened as it can be.

The stone is just fine. Nothing in woodworking requires the level of accuracy you are worried about. In fact, most machining doesn't require that level of accuracy. I worked in a research lab as a machinist, and most of the stuff we built rarely required a tolerance of less than +/- 2 thousandths of an inch. I only remember working to ten thousands of an inch once, and that was for a laser mount. Heck, most of the lathes and mills machines we had weren't even that accurate. You are almost at the point where you need to worry if the temperature of what you are measuring is even across the material. Actually without some machinist training and a certified stone, most people would be hard pressed to accurately measure something to a half thousand of a inch.

[James Pallas]

The stone is just fine. Nothing in woodworking requires the level of accuracy you are worried about. In fact, most machining doesn't require that level of accuracy. I worked in a research lab as a machinist, and most of the stuff we built rarely required a tolerance of less than +/- 2 thousandths of an inch. I only remember working to ten thousands of an inch once, and that was for a laser mount. Heck, most of the lathes and mills machines we had weren't even that accurate. You are almost at the point where you need to worry if the temperature of what you are measuring is even across the material. Actually without some machinist training and a certified stone, most people would be hard pressed to accurately measure something to a half thousand of a inch.

Hoorah!, sanity
Jim