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  1. #61
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    Quote Originally Posted by kenlip View Post
    Bryan_machine and Paolo, you last couple of postings have helped a lot to clarify some of the technicalities. As for the concept of 3-plate method, I think I understand the idea of the iterative lapping of the plates with the various sequences and rotations.

    However, there are still some points that I can't work out. For example, in The Whitworth Three Plates Method — Eric Weinhoffer , in step #6, he says, "Finally, the single convex plate is lapped against the Blue control plate"

    Attachment 257881

    I can understand how, if one was utilising scraping, one could selectively work on the convex plate, with the flatter plate as the reference, to get the convex plate to match the reference plate. However, I can't understand why, if lapping the plates, the convexity of the upper plate doesn't introduce a concavity or a series of waves into the lower plate.

    Both plates are made of the same material, so one shouldn't get the same wear pattern one does when lapping a granite block with am iron plate, where the harder material is the one that gets worn down by the abrasive. Both plates should undergo change.
    The plate on top will tend to go concave and the plate on the bottom tends to go convex due to the upper plate's overhang during lapping. This is due to the unsupported weight of the overhang (mentioned in an earlier post) and the fact that, on average, the top plate's overhang is getting lapped less as it's not in contact with the lower plate for part of the lapping stroke.

    So for the blue/green example shown above, the blue plate will get dished at first as that's where the green contacts it, but then the overhanging stroke will begin to take effect and at some point the two plates will be flat and in full contact. Lapping it further will make the green go concave and the blue go convex, so you really need a spherometer test to know when to stop lapping.

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  3. #62
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    Ken you need to just do it to understand it. So many Brainy people think to hard and confuse themselves and by just doing it is all you need to do. Google it and put Professor Alex Slocum MIT and he writes a lot about 3 plates, Kinematics and couplings.

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    Quote Originally Posted by SAG 180 View Post
    The plate on top will tend to go concave and the plate on the bottom tends to go convex due to the upper plate's overhang during lapping. This is due to the unsupported weight of the overhang (mentioned in an earlier post) and the fact that, on average, the top plate's overhang is getting lapped less as it's not in contact with the lower plate for part of the lapping stroke.

    So for the blue/green example shown above, the blue plate will get dished at first as that's where the green contacts it, but then the overhanging stroke will begin to take effect and at some point the two plate will be flat and in full contact. Lapping it further will make the green go concave and the blue go convex, so you really need a spherometer test to know when to stop lapping.
    That's interesting. Thank you.

    I was discussing this with my brother and he felt the top will go concave and the bottom convex, as described in the first paragraph above. I understood where he was coming from, but pointed out that the opposite would happen, because the convexity of the upper plate would create a concavity in the lower. We both ended up seeing both points of view and were totally confused.

    What you say makes perfect sense and adds credence to both my and my brother's anticipation of what would happen. We both were right and wrong.

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    As described in the Moore book, there is no lapping going on anywhere. This is spotting and hand
    scraping. I don't think gravity is a critical component in their process, other than it holds the top plate
    in contact with the bottom plate. There's no overhang anywhere in the way Moore describes it. In
    fact, they go to considerable lengths to point out that square plates are ideal because there's no
    overhang when you rotate one plate 90 degrees with respect to another.

  7. #65
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    Quote Originally Posted by jim rozen View Post
    As described in the Moore book, there is no lapping going on anywhere. This is spotting and hand
    scraping. I don't think gravity is a critical component in their process, other than it holds the top plate
    in contact with the bottom plate. There's no overhang anywhere in the way Moore describes it. In
    fact, they go to considerable lengths to point out that square plates are ideal because there's no
    overhang when you rotate one plate 90 degrees with respect to another.


    The example referred to by kenlip is using lapping rather than scraping, so even with square or circular plates there is overhang from the sideways motion of the top lap over the fixed bottom lap.
    Last edited by SAG 180; 06-02-2019 at 05:06 AM.

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    Quote Originally Posted by kenlip View Post
    ...
    However, there are still some points that I can't work out. For example, in The Whitworth Three Plates Method — Eric Weinhoffer , in step #6, he says, "Finally, the single convex plate is lapped against the Blue control plate"

    Attachment 257881

    I can understand how, if one was utilising scraping, one could selectively work on the convex plate, with the flatter plate as the reference, to get the convex plate to match the reference plate. However, I can't understand why, if lapping the plates, the convexity of the upper plate doesn't introduce a concavity or a series of waves into the lower plate.

    Both plates are made of the same material, so one shouldn't get the same wear pattern one does when lapping a granite block with am iron plate, where the harder material is the one that gets worn down by the abrasive. Both plates should undergo change.
    Kenlip,
    If you thing in terms of a single cycle of six steps, you're perfectly right. The key is the following sentence:
    Quote Originally Posted by Eric Weinhoffer
    The process can be completed in six steps, and then repeated until the desired level of flatness is achieved.
    And, at the end of the six steps explanation, there is another important paragraph:
    Quote Originally Posted by Eric Weinhoffer
    It’s also worth noting that, although this yields great precision “from nothing”, Joseph Whitworth later improved upon the technique by utilizing engineer’s blue and hand scraping, as mentioned previously. Engineer’s blue alone would be an instrumental improvement over using no indicator - with the blue, it’s easy to see which areas have, and haven’t been, scraped.

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