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Thread: Cast iron plate

  1. #41
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    Quote Originally Posted by J Tiers View Post
    That procedure is almost guaranteed to produce a non-flat top. On both. One will be convex, the other will be concave. Just depends how long you do that.

    Maybe it will no be bad enough to bother you.
    Although I fully understand the concept of grinding a lens, we are talking about two large plates that are starting out flat that are too heavy to lift. No pressure will likely be needed as the top plate is heavy and the weight will be uniformly distributed. It would take a very long time to remove any appreciable metal. Polishing and smoothing the surfaces this way in a single session will not change a flatness enough to measure with any home shop equipment I'll wager.

  2. #42
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    Quote Originally Posted by 754 View Post
    I bet the 36 by 96 doesnot have 3 point mount , or trunnions .
    But I bet the ones show do..
    Well you got me there. My work plate is 1.5"x36"x96 hot rolled mounted to the floor of my garage with four 1/4" four by fours (and feets at the floor mounting area). JR
    My old yahoo group. Bridgeport Mill Group

    https://groups.yahoo.com/neo/groups/...port_mill/info

  3. #43
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    The OP stated that the larger plate was too heavy to lift, so he could try a little lapping with the smaller on top. As for lapping compound, I recon that valve grinding compound on such a large surface would be too stiff to move, I would go for something like 600 grit in some light oil. A few minutes lapping would not change the surfaces in a measurable way, but would leave indications as to whether they are close to flat or not.
    A straight edge and a torch used at multiple angles would certainly tell you if they are flat within 0.001". I would be delighted to have a table that flat, I have to make do with a piece of float glass 12" square.

  4. #44
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    Quote Originally Posted by Gary Paine View Post
    Although I fully understand the concept of grinding a lens, we are talking about two large plates that are starting out flat that are too heavy to lift. No pressure will likely be needed as the top plate is heavy and the weight will be uniformly distributed. It would take a very long time to remove any appreciable metal. Polishing and smoothing the surfaces this way in a single session will not change a flatness enough to measure with any home shop equipment I'll wager.
    Well, yes and no.... .You originally suggested keeping at it until he has an even finish on both across their faces. And that means more than a light duty working over if one or both are worn.

    And if one happens to be nicely flat and the other is cupped or crowned then BOTH will wear to a less than flat condition if he keeps at it until both have an even finish across the whole face. Hence JTiers' post about using three surfaces worked in the proper order to arrive at three truly flat surfaces. He can't get there with just two with any certainty.

    If he really did just do a light duty short term job and the edges of both wear to a consistent finish then what is the situation? Is one concave? Are both concave? And if one is flat, other than the burrs, then it just became ruined around the edges.

    All in all I agree with filing or stoning the burrs from the impacts down. But at that point I think it is best to stop there and evaluate the situation for each in a separate manner. And if that means doing a bit of homework and even making or buying some tooling to allow such checks then it's not a bad thing.

  5. #45
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    Gambler, do you have a granite plate at all that you can trust as a reference? A known really good and rigid straight edge? And I don't mean an 18 or 24" rule. I mean a good wide precision straight edge that will have a high degree of stiffness.

  6. #46
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    You all realise that lapping the plates together for a couple of hours will result in less than 0.00005" being removed from both faces.

  7. #47
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    Quote Originally Posted by old mart View Post
    You all realise that lapping the plates together for a couple of hours will result in less than 0.00005" being removed from both faces.
    And, then it probably will do nothing useful as well as nothing damaging.... But, that is going to depend on grit, abrasive type, technique, etc. How many eons would be required to make a telescope mirror at that rate? Right... too many, and yet they are made all the time by hand with abrasives.

    Essentially, working the plates with abrasives long enough to wear them each down to an "even" surface (not necessarily flat) means that enough material has been removed to alter the flatness significantly. Significantly as far as use as a surface plate is concerned. They might be plenty flat for many purposes, even if left as they are now.

    If there is no need for a surface plate, then doing that grinding, using them as-is, or just getting a piece of stone countertop instead, would probably all be equally useful. Not doing the grinding would be less work......
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  8. #48
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    Quote Originally Posted by BCRider View Post
    Gambler, do you have a granite plate at all that you can trust as a reference? A known really good and rigid straight edge? And I don't mean an 18 or 24" rule. I mean a good wide precision straight edge that will have a high degree of stiffness.
    I have a 12 inch square granite surface plate.

  9. #49
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    What grade and how worn ?

  10. #50
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    Quote Originally Posted by 754 View Post
    What grade and how worn ?
    I have no idea.

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