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Thread: Hypothetical Lapping Question
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19th Aug 2020, 07:53 PM #16
Just re reading all this , and it takes a while to absorb all the new stuff .
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.
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19th Aug 2020, 09:44 PM #17Philomath in training
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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
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20th Aug 2020, 11:42 AM #18Most Valued Member
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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)
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20th Aug 2020, 07:15 PM #19Philomath in training
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20th Aug 2020, 09:59 PM #20Most Valued Member
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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
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21st Aug 2020, 07:55 AM #21Philomath in training
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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
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21st Aug 2020, 08:44 PM #22Most Valued Member
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21st Aug 2020, 09:02 PM #23Philomath in training
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proof.jpg
Yours.
Michael
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21st Aug 2020, 09:45 PM #24Most Valued Member
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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?
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22nd Aug 2020, 01:28 AM #25
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.
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22nd Aug 2020, 11:37 AM #26
Just to tip my head in ,
I’m still following all this, but just trying to keep up.
Cheers Matt.
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22nd Aug 2020, 07:41 PM #27Most Valued Member
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23rd Aug 2020, 01:11 AM #28
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26th Aug 2020, 12:18 PM #29Senior Member
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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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