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
Thanks. Looks like I'm going nowhere with this...
Alan
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
You have mail :-)
regards Mark Rand RTFM
Hello Alan, can't help with the grinding, but if you need an excellent dissertation on scraping, try Connelly's book:
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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