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Thread: How flat does an Atoma need to be?

  1. #16
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    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.

  2. #17
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    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.

  3. #18
    I think you are overthinking this.

  4. Quote Originally Posted by Ty Williams View Post
    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.
    Last edited by Jessica de Boer; 03-07-2019 at 2:17 PM.

  5. #20
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    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

  6. #21
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    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.
    Bumbling forward into the unknown.

  7. #22
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    Mar 2007
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    Walkersville, Maryland
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    Good, practical advise.

  8. #23
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    Feb 2018
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    Minneapolis, MN
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    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...

  9. #24
    The first ‘flattening stone’ that I used was an ordinary cinder (concrete) block. Worked fine for me for years.

  10. #25
    Quote Originally Posted by J. Greg Jones View Post
    The first ‘flattening stone’ that I used was an ordinary cinder (concrete) block. Worked fine for me for years.
    The horror!

  11. #26
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    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.

  12. #27
    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

  13. #28
    Quote Originally Posted by Ty Williams View Post
    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.

  14. #29
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    Quote Originally Posted by Andrew Seemann View Post
    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

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