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  1. #16
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    Apr 2019
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    Adelaide
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    What resolution is your gauge? I bought a 1um Millimess but am concerned it will not have sufficient resolution. 1um = 0.0000394" = 39.4 u"??? Is that maths correct?

    My granite plate is (from memory) 36" x 72" x 8" so according to a Mitutoyo table I found, an AA grade plate should have 60 u" local flatness (repeat-o-meter) and 300 u" overall flatness (autocollimator). If this is all correct then my Millimess will be good to go?

  2. #17
    Join Date
    Aug 2019
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    Revesby - Sydney Australia
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    57
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    Quote Originally Posted by Mk1_Oz View Post
    1um = 0.0000394" = 39.4 u"??? Is that maths correct?
    1thou = 25.4µm

    so 25.4 * 0.00000394 = 0.00100076"

    Your maths is correct.

    As for the amount of patience and cleanliness you will need?
    (to physically clock a 3ftx6ft surface plate to tens of microns)
    That is beyond my calculations!
    Nigel, from a cave FULL of unfinished projects and lost tools.

  3. #18
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
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    849

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    Hardened and ground silver steel feet.
    IMG_6398.jpg

    IMG_6404.jpg

  4. #19
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    Apr 2019
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    Adelaide
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    Quote Originally Posted by nigelpearson View Post
    ...Your maths is correct.

    As for the amount of patience and cleanliness you will need?
    (to physically clock a 3ftx6ft surface plate to tens of microns)
    That is beyond my calculations!
    Thank you and lol.....my hope is that the autocollimator and soon (well, in the future) to be built repeat-o-meter will reveal an A grade plate that I will not need to re-lap. It came from John Heine so it could either have been abused by workers who did not GAS or might equally have been well looked after.....will have to wait to find out.

  5. #20
    Join Date
    Nov 2017
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    Geelong, Australia
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    57
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    Looking great! Almost time to cut the flexure.

    Is there a particular diameter for the foot on the moving part? I assume that for checking a scraped surface it needs to be large enough that it spans across the shallow hollows between single scrapes to avoid “chatter” in the readings, but not so large that it reduces the practical practical sensitivity?

    Steve

  6. #21
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
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    849

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    The U.S. Federal specification calls up 3/8" diameter feet. I managed to cut the slot today and then try it out on 3 surface plates with good results, the maximun error I found was a low spot of about 4 microns on a 600 x 600mm granite plate.

    repeat o meter 12.jpg

    repeat o meter 13.jpg

  7. #22
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    Apr 2019
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    Adelaide
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    598

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    Is there any way to test whether the flexure is 'weak' enough to move quickly and sufficiently?

  8. #23
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    Nov 2017
    Location
    Geelong, Australia
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    Quote Originally Posted by Mk1_Oz View Post
    Is there any way to test whether the flexure is 'weak' enough to move quickly and sufficiently?
    I’d expect that if it drops against the restraining link under its own weight when picked up by the top - then it’s good.

    Steve

  9. #24
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
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    That's it exactly Steve, the links only have about .005" diametral clearance on the shoulder screws.

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