Flattening a Lapping Plate

I've got a used iron or steel lapping plate 10 inches diam. by 1 1/2 inches thick which has roughly concave faces - I measure the dip in the middle of each face to be 0.25mm and 0.1 mm. I was hoping to use it as both a surface plate and lapping plate (lapping mainly by using it to support lapping film to avoid wearing it) but it obviously needs work. Can I fix this myself by hand? Thanks,
Alan

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Yes! You need two more plates! Lap them against each other until all three are dead flat. Optical flats are often made that way.
Steve R.

Thanks Steve, What about when you only have one plate? The techniques I have in mind is grinding the lap on a sheet of float glass first with silicon carbide grit to get rid of the worst of the error and then scraping the surface whilst using a sheet of float glass as a reference with engineers blue to spot the high points. Does this sound practical? Or would the lapping plate and two sheets of glass work with the three plates technique? Thanks,
Alan
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The problem may be with the differing hardness, and embedability of the two materials. Cast iron to cast iron does work, but as I said, 3 are needed. each plate needs to be lapped against the others. you also need some way of checking so that edges do not get rounded. Best to read up on it first, as it has been many years since I have done any optical work.
Steve R.

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Do you know anyone with a surface grinder, which might get it near enough? T.W.

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Do you know anyone with a surface grinder, which might get it near enough? T.W.
Unfortunately not. Alan

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Wherabouts are you?
Mark Rand RTFM

On Sun, 31 Jan 2010 22:51:56 +0000, Mark Rand
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What Mark said. Plenty of us around with surface grinders that might be able to help.
Peter

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I'm in North East London.
Alan

Scrim wrote:
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Not without a reference flat.
If you have to buy one of those, you may as well just use the new one, of course!
If you can borrow a reference flat, you can scrap (or otherwise cut) your concave plate to the reference in the classic way.
BugBear

bugbear wrote:
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"scrape", obviously ;-)
BugBear

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I think you'd be there till Doomsday trying to lap or scrape that much out by hand, certainly on the side with 0.25mm wear. It needs either truing up on a lathe or mill first and then get it spot on by hand or surface grinding which should get it good enough to use straight away.
If it were mine I'd run a flycutter over it on the mill which should get it to within a fraction of a thou from true. Better than I'd ever need from a surface plate anyway not that I actually use them.

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So his out of true one is better than you need too ;-)
Regards, Tony

Dave Baker wrote:
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Nice mill!
BugBear

Thanks for all the replies. How flat would surface grinding get it?
Alan

Scrim wrote:
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I think surface grinders vary, but well under a thou is normal.
However, if you "just" want a surface plate, buy one!
(unless you can get free use of a surface grinder)
http://www.rutlands.co.uk/hand-tools/sharpening/lapping-compounds-&-plates/DKX31/granite-surface-plate
http://cgi.ebay.co.uk/Granite-Surface-Plate-12-x-9-x-3-Grade-0_W0QQitemZ290395708393QQcmdZViewItemQQptZUK_Measuring_Tools_Levels?hash=item439ceedbe9
That's INSPECTION grade for Pete's sake.
BugBear

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http://www.rutlands.co.uk/hand-tools/sharpening/lapping-compounds-&-plates/DKX31/granite-surface-plate
http://cgi.ebay.co.uk/Granite-Surface-Plate-12-x-9-x-3-Grade-0_W0QQitemZ290395708393QQcmdZViewItemQQptZUK_Measuring_Tools_Levels?hash=item439ceedbe9
That second one is very cheap! I was keen on the dual use aspect of a lapping plate, as I can also use it for occasional marking out.
Alan

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Depends on the grinder but a tenth of a thou would be easily possible on a good one.

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I was going to say that on my old nail of a J&S 1400, that's old enough to have the controls labeled in runes, I'd expect to achieve about 3 tenths on the size of that lapping plate. Say .0075mm. I'd need to do it in two halves, because the grinder's only 8"x24", but that's doable.
Downside is that at the moment, there's a couple of months work stopping me getting to the grinder (literally! I ran out of space to stack bits of lathe while I rebuild and paint the cabinet, so it's all piled in front of the grinder :), then I've got to repair a stripped thread on one of the grinder's hydraulic hose connections.
If you haven't had any luck by mid-late March, I should be close to a position to sort you out. I'm in Rugby, but could probably prevail upon a colleague to bring it up from Borehamwood on one of his twice a week commutes if you could get it that far.
Hopefully you'll get a better offer, but at least you can fall back to that.
What I would suggest is to lay out for one of Fordeight's (AKA Rotagrip) rocks off Ebay. Invest in the 18x12 rather than the 12x9. Save up the money if necessary, it'll last you a long time. Use that as your main layout and marking tool. Get some sucker (like me) to surface grind the lapping plate. Then either use it purely as a lapping plate or use it as a practice piece to learn scraping. From experience, it'll be close enough to make quick, satisfying, progress on when scraping for final finish.
Mark Rand RTFM

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Thanks Mark, much appreciated.
Alan