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  1. #16
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    Quote Originally Posted by Michael G View Post
    That would be the hard part - keeping those surfaces absolutely square. It would be nice if they could be rotated but the key thing is that they are processed until a perfect match, one of the surfaces swapped out processed again, swapped out and so on. The end result is that the surfaces are flatted out by average. From memory, I think Connolly has the method in there.

    I may be being pedantic, but my understanding of the 3 plate technique is that one plate is blued, rubbed on another and then the high spots scraped off. Clean and repeat...
    Lapping involves embedding an abrasive into a lap, and then rubbing the other part on it.

    Back on topic, if you wanted to try the technique out (and I would not mind doing so myself one day), there is nothing wrong with just doing a couple of small cast iron plates. I have a 4"x10" Wing surface plate - just the right size for a small part and a height gauge. I'd be pretty sure that if you scraped in 3 plates of around that size in there would be takers for the left overs as one of the big unknowns of surface plates is how flat are they? A plate freshly made with the 3 plate method would be guaranteed flat.

    Michael

    Just re reading all this , and it takes a while to absorb all the new stuff Hypothetical Lapping Question.

    Michael, what is a Wing surface plate ?
    Or is that just a brand as opposed to just a normal Surface plate, off which I have one but in Ballarat tho it’s only A4 size( Carbatec brand)?

    Also 4/10 inch plate,tho not big is still a bit or am I getting something wrong.

    Cheers Matt.

  2. #17
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    Wing is an old Australian brand, made of CI. The one I have is 4"x10" or for those of a metric mind, 100mm by 250mm.
    P1040892.JPG P1040893.JPG

    Michael

  3. #18
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    Quote Originally Posted by Michael G View Post
    Yeah but...

    If the plates are scraped together, only two will twist one way. The other will be mirror image.

    Michael
    No. You have three plates that match, they also match when rotated 180 degrees. So you can't see any twist. Of course there may not be any twist, but you cant know that.
    I think


    You can get non embedding lapping compound, which would seem to be the go as I believe you are meant to take the high spots off both parts(not scrap one part to match the other)

  4. #19
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    [QUOTE=Stustoys;1973321]No. You have three plates that match, they also match when rotated 180 degrees. So you can't see any twist. Of course there may not be any twist, but you cant know that.
    I think

    No they don't

    Quote Originally Posted by Michael G View Post
    ... (To put it a way that can be visualised, if 1 is concave then scraping 2 to it will make 2 convex. Scraping 3 to 2 will make 3 concave. Scraping 1 with 3 will be concave to concave.)

    Michael

  5. #20
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    Quote Originally Posted by Michael G View Post
    No they don't
    Do to

    As I'm seeing it, you are correct with simple concave/convex shapes. i.e one plate is concave the other is convex
    With a twist I believe you have both concave and convex on the same surface.(it might not be strictly speaking concave/convex)


    I think this may demonstrate what I am trying to say.
    Take two pieces of paper.(ok strictly three pieces but as they are all the same two will do)
    Label the top left and bottom right corners "High"
    Label the top right and bottom left corners "Low"
    These are your plates with a twist.
    Flip one over on top of the other. H and L match up so they appear flat
    Rotate 180', H and L still match so things appear as flat as the first test.
    Adding a third doesn't help.

    Rotate 90', H and H match and you have a problem.

    I think

  6. #21
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    Think of the twist as concavity (or convexity) along a diagonal. H and H don't match with a 90 degree rotation.

    Michael

  7. #22
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    Quote Originally Posted by Michael G View Post
    H and H don't match with a 90 degree rotation.
    You mean in my example? or that three twisted plates that spot flat will not match with a 90 degree rotation?

  8. #23
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    proof.jpg

    Yours.

    Michael

  9. #24
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    Quote Originally Posted by Michael G View Post
    Yours.
    They match in the same sense that H and L matched in the preceding sentences.(they are one on top of the other)
    Not that the surfaces matched.

    Doesn't your drawing show that
    1. 90 rotation is essential to detect twist?
    2. Three twisted plates will happily spot each other in any order if limited to rotation of 180 degree?

  10. #25
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    Hi Guys,

    Flat Plane.jpg

    Does this help ? Rotating the plates 90 degrees as you progress will help ensure that there are no hollows, lifted corners or convex surfaces. Eventually all three plates will become flat planes.
    Best Regards:
    Baron J.

  11. #26
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    Just to tip my head in ,

    I’m still following all this, but just trying to keep up.


    Cheers Matt.

  12. #27
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    Quote Originally Posted by BaronJ View Post
    Hi Guys,

    Does this help ? Rotating the plates 90 degrees as you progress will help ensure that there are no hollows, lifted corners or convex surfaces. Eventually all three plates will become flat planes.
    Its really starts on page 20 with more on 24

  13. #28
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    Hi Stuart,

    Thank you for your post.

    Quote Originally Posted by Stustoys View Post
    Its really starts on page 20 with more on 24
    I think that its a very informative paper ! It really helps with understanding what is happening and why.
    Best Regards:
    Baron J.

  14. #29
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    I reckon 90 and 180 degree rotation is needed.
    I have cut some aluminium as suggested in post 13, and taken some pics.
    We are interested in 4 of the different ways to position plate 2.
    2 stacked.jpgRotated.jpgflipped.jpgFlipped and rotated.jpg

    Counter-intuitively all the faces match in their twist direction, in all four placements.
    It follows that these 2 “plates” (plus one identical extra) could theoretically each have one surface lapped or scraped to 40 points, eliminating concavity/convexity, using the 3 plate method. However the twist would still be there and not detectable by bluing unless tested with 90 degree rotation, which won’t work well on Matt’s 3 or 10mm wide straight edges. A fourth truly flat surface would be needed.
    Neil

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