diy surface plate lapping - Page 5
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  1. #81
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    Much has been said here about "proper", Airy point suspension. While I am sure that this is probably a good, perhaps the best method of supporting a plate, it is more important that it be supported in the same manner that it was supported when it was originally made. The plate will sag in one way or another with any type of suspension. The Airy points are said to minimize that sag, but they do not eliminate it. And it will sag differently with different methods. So, if it was originally manufactured with the present suspension points, they will be the best points to suspend it for use. They will duplicate the original sag and minimize any error from that source.

    Since you say it is not marked and you can not tell where the intended mounting points are, you can not be sure if you are suspending it in the best way or not.

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    Quote Originally Posted by EPAIII View Post
    Much has been said here about "proper", Airy point suspension. While I am sure that this is probably a good, perhaps the best method of supporting a plate, it is more important that it be supported in the same manner that it was supported when it was originally made. The plate will sag in one way or another with any type of suspension. The Airy points are said to minimize that sag, but they do not eliminate it. And it will sag differently with different methods. So, if it was originally manufactured with the present suspension points, they will be the best points to suspend it for use. They will duplicate the original sag and minimize any error from that source.

    Since you say it is not marked and you can not tell where the intended mounting points are, you can not be sure if you are suspending it in the best way or not.
    Agreed. Youre right about the mounting points, i considered them unknown as they werent marked in any way.

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    The wyler way


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    If ever you want to know how HeathRobinson your efforts really are, watch a video by Robin


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    That's 3/4 or an hour I won't get back .

    It'll be interesting to see how he gets on, if he posts follow-up videos.

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    Quote Originally Posted by Mark Rand View Post
    That's 3/4 or an hour I won't get back .

    It'll be interesting to see how he gets on, if he posts follow-up videos.
    I nearly died when he started checking with a single bubble level. One for the team!!

    After watching some of his other videos, Robins several levels beyond super sharp imo. He wont quit till its dead nuts and back again I bet.

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    With a 2 tenths in 10" level, one should be able to get to a grade II plate without interpolating. So call it grade I overall with a bit of interpolating. But that's pushing things to their limit.

    Still haven't started re-lapping my 36x48" table. Hope to do it this summer...

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    Answering some questions from the previous. Good solid views and a brief showing of the Renzometer.


    Apparently a couple guys got into a bun fight about the use of bubble levels, no prizes for guessing who main protagonist was .

    Quote Originally Posted by Mark Rand View Post
    With a 2 tenths in 10" level, one should be able to get to a grade II plate without interpolating. So call it grade I overall with a bit of interpolating. But that's pushing things to their limit.

    Still haven't started re-lapping my 36x48" table. Hope to do it this summer...
    How are you off for laps an such?

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    Quote Originally Posted by Demon73 View Post
    How are you off for laps an such?
    I've got a 24"x18" and an 18"x12" cast iron surface plate to use as laps, along with 500 carats of diamonds in grades from 60 to 6 microns (It was sold by a guy on PM. Later turned out to be stolen, but when I contacted the original owners, they did not want it back).

    I was planning to verify and possibly re-scrape the cast iron plates before using them as laps.

  14. #92
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    Quote Originally Posted by Mark Rand View Post
    I've got a 24"x18" and an 18"x12" cast iron surface plate to use as laps, along with 500 carats of diamonds in grades from 60 to 6 microns (It was sold by a guy on PM. Later turned out to be stolen, but when I contacted the original owners, they did not want it back).

    I was planning to verify and possibly re-scrape the cast iron plates before using them as laps.
    Lol, at least they have a history to them! 24"x18" and 18"x12" sounds nice for a plate your size, small one to rough in and the big one to finish up im guessin.

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    Why are soft pitch type laps not used on surface plates?
    The astronomical mirror makers use pitch and/or tile grinding tools.

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    Quote Originally Posted by dazz View Post
    Why are soft pitch type laps not used on surface plates?
    The astronomical mirror makers use pitch and/or tile grinding tools.
    Based on my single experience grinding a Newtonian mirror 40 years ago, pitch was used only for the very final polishing and figuring phase of the process. Glass-on-glass was used for the general shaping and refining of the mirror. The lapping of a surface plate would be analogous to rather coarse grinding of a mirror and certainly would be done glass on glass in the case of mirror production.

    Using a hard grinding blank drug over the the mirror blank ROLLED the grit between the grinding blank and the mirror. This pressed the sharp edges of the grit against the glass with high local pressure and caused the grit to split out tiny chips of glass similar to the way knapping flakes out bits of hard stone. The cutting action was thus much more rapid than that used in polishing where the grit is embedded in the pitch and drug over the surface etching tiny grooves.

    Denis

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    OK so what about a tile grinding tool?
    or alternatively another smaller granite surface plate?

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    In the spirit of making do with what you have I'm hoping to make a surface plate out of a chunk of granite I have on hand. Here's what I have in mind. Comments welcome.

    The granite is 13"x25"x2". That's half the thickness it should be but I want to be able to lift it without a fork lift so I would rather not have anything thicker. Instead I think I can arrange counter weights underneath so any significant loads are roughly neutralized.

    Rather than three supports I will use a whippletree at one end giving four supports.

    Everyone talks about using airy points for support but when I looked it up it seems airy points are used when you want to support a bar while keeping the ends parallel. That isn't what we want for a surface plate.

    The bessel points are used when you want to minimize the deviation in the length of a beam supported at two points. This is also not what we are interested in.

    I expect we are interested in the least surface deviation. From Wikipedia

    Other sets of support points, even closer than the Bessel points, which may be wanted in some applications are:

    - The points for minimum sag, 0.5536 times the length. Minimum sag occurs when the centre of the rod sags the same amount as the end points, which is not quite the same thing as minimum horizontal motion of the ends.

    - The nodes of free vibration, 0.5516 times the length.

    - The points for zero central sag (any closer and the beam rises between the support points): 0.5228 times the length.


    I suspect it doesn't matter much where the support points are, within reason, as long as they are the same in use as they were when the plate was figured.

    I hope to use a water surface reference interferometer and a set of 4" cast iron laps to figure the surface plate. I am hoping I can get close to the 19nm/m curvature of the earth as an approximation of flat. I will post updates if there is interest here.

    Has anyone tried making an I beam style surface plate using 1" counter top granite and epoxy? I'm not suggesting it would work better than a solid slab but it should weigh less and equalize faster if that matters.

    Cheers. Mike

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    Quote Originally Posted by Wet Coast Canada View Post
    In the spirit of making do with what you have I'm hoping to make a surface plate out of a chunk of granite I have on hand. Here's what I have in mind. Comments welcome.

    The granite is 13"x25"x2". That's half the thickness it should be but I want to be able to lift it without a fork lift so I would rather not have anything thicker. Instead I think I can arrange counter weights underneath so any significant loads are roughly neutralized.

    Rather than three supports I will use a whippletree at one end giving four supports.

    Everyone talks about using airy points for support but when I looked it up it seems airy points are used when you want to support a bar while keeping the ends parallel. That isn't what we want for a surface plate.

    The bessel points are used when you want to minimize the deviation in the length of a beam supported at two points. This is also not what we are interested in.

    I expect we are interested in the least surface deviation. From Wikipedia

    Other sets of support points, even closer than the Bessel points, which may be wanted in some applications are:

    - The points for minimum sag, 0.5536 times the length. Minimum sag occurs when the centre of the rod sags the same amount as the end points, which is not quite the same thing as minimum horizontal motion of the ends.

    - The nodes of free vibration, 0.5516 times the length.

    - The points for zero central sag (any closer and the beam rises between the support points): 0.5228 times the length.


    I suspect it doesn't matter much where the support points are, within reason, as long as they are the same in use as they were when the plate was figured.

    I hope to use a water surface reference interferometer and a set of 4" cast iron laps to figure the surface plate. I am hoping I can get close to the 19nm/m curvature of the earth as an approximation of flat. I will post updates if there is interest here.

    Has anyone tried making an I beam style surface plate using 1" counter top granite and epoxy? I'm not suggesting it would work better than a solid slab but it should weigh less and equalize faster if that matters.

    Cheers. Mike
    Mike i think you are wasting your time. Just buy an import plate. Unless you have a larger True Stone (Starrett) or Stanridge, or Rahn or somethign similar quality reconditioning is not really cost effective. As a hobby effort or a learning excercise, by all mans play around with stones

    dee
    ;_D

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    West coast Canada-

    Check out a guy named Dan Gelbart on youtube. He made a precision lathe using granite parallels for the ways. Pretty neat.

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    Dam cool!

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    Quote Originally Posted by Demon73 View Post
    Dam cool!
    That one is downright brilliant. I hate him for this, now I have to make one.

    dee
    ;-D
    Last edited by dcsipo; 04-11-2018 at 09:28 PM.

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