Flattening a Lapping Plate



I'm a bit put off getting one of those granite surface blocks for a couple of reasons apart from the money. First, my back, and second, it will take up yet more space. The whole idea of getting a lapping plate was to save space through its dual usability. Does anyone know how flat modern float glass is by comparison? I believe a '0' rated surface plate is flat to 5 microns.
Alan
Alan
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Scrim wrote:

Float glass is smooth ("locally flat") but not neccessarily flat ("globally flat").
It's also not rigid. As the founder of precision engineering (Sir Joseph Whitworth) pointed out, accuracy without rigidity (under normal working loads) is not very useful.
People sometimes (amusingly) support their hopefully-accurate float glass on a backing like MDF, thus getting the accuracy of MDF...
BugBear
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Thanks. Looks like I'm going nowhere with this...
Alan
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I put the lapping plate to one side for a couple of months but am back to it now. I've bought a scraping tool, but can anyone in the London area (near a tube station maybe) help me out by grinding the faces flat? It's iron/steel, 10 inches diam. by 1 1/2 inches thick. It has concave faces - I measure the dips in the middle of each face to be 0.25mm and 0.1 mm. Otherwise, can I still take you up on you offer Mark?
Alan
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You have mail :-)
regards Mark Rand RTFM
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Hello Alan, can't help with the grinding, but if you need an excellent dissertation on scraping, try Connelly's book: http://www.4shared.com/file/67579873/158c2190/Machine_Tool_Reconditioning_and_Applications_of_Hand_Scraping.html
I was so impressed I went and bought a hard-copy.
Richard
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Thanks for that. I tried a 1 inch wide scraping tool on the lapping plate the other day just to see how it went, but found that the grooves interfered with the process. It might be my poor technique, but my impression is I need a wider scraping blade to avoid 'falling' into the grooves - but could I apply enough pressure to scrape with a 2 or 3 inch blade.? I'm thinking perhaps I'll need to use a different technique to scraping, perhaps using abrasives.
Alan
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