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Post by dickdastardly40 on Apr 18, 2007 14:21:10 GMT
Hiya Peeps, SWMBO whilst looking for something else has managed to drop my Granite surface plate, chipping a corner off! I thank god she didn't drop it on her foot or trap a finger under it. The damage looks fairly superficial, though she has managed to scratch both faces somehow possibly sliding it to get purchase picking it up. Obviously it was my fault putting it where it was but I do have clearance after some emotional blackmail to replace it. I see fleabay has some on BIN fairly cheap (£30 + post). The ones listed are 'grade 0' whereas J&L has some which are considerably more expensive which are 'grade 1' . The old one may then either get used for lapping, a chopping board or a pierrade! Does anyone know which is the better bet or can offer any guidance. Thanks in advance of any replies Al
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Alan
Part of the e-furniture
Posts: 287
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Post by Alan on Apr 18, 2007 17:49:20 GMT
i've seen some ropey looking granite plates in toolrooms. Some have small holes in them when thing/tools are dropped on them. As a model engineer, what are you worried about?
alan
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Post by chris vine on Apr 18, 2007 19:44:11 GMT
Hi Al,
I know it is a shame that it is damaged on a corner, but can you turn it round so that the bad corner is to the back and out of sight?
I suspect the damage offends you more than it will affect its usefulness??
Chris.
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Post by chameleonrob on Apr 19, 2007 7:53:29 GMT
one of the big advantages of granite surface plates is that when something is dropped on them they chip but don't throw up a ridge round the mark unlike cast iron, this means that even a surface plate thats looks really bad can still be used. if you want to replace it anyway then RDG and cheap. (http://www.rdgtools.co.uk/)
rob
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Lurkio
Seasoned Member
Posts: 101
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Post by Lurkio on Apr 19, 2007 21:54:49 GMT
Hi Al,
As Alan suggests, I wouldn't be too worried for our application. If you look at the requirements for a precision granite surface plate, they should be supported at certain points (indicated by the manufacturer) in order to maintain original accuracy, yet they are often just chucked on the bench in a toolroom, and generally abused - I haven't seen one 'properly supported' yet!. The odd scratch here and there shouldn't be too much of a problem for you.
Lurkio.
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Noddy
Statesman
Posts: 672
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Post by Noddy on Apr 20, 2007 14:21:33 GMT
As the guys have said, the advantage of using a stone like granite is that scratches and chips do not throw up positive burrs, so, as long as the feet of any measuring guages you use on it are wider than the scratches, they shouldn't be a problem.
Also, granite will chip rather than permenantly bend, so the bang it received will not have distorted it.
Interestingly, most rocks will have have locked in stresses, just like castings and cold worked metal do, but the ground surface should be stable unless fresh material is taken off the opposite side of the surface plate. even then, the distortion would probably only be in measurable in microns.
just as a digression, deep boreholes in areas like the Canadian and Finnish Shield areas, where deep glacial erosion took place a few thousand years ago and horizontal stresses in the rock are high, will often yeild core that spalls to discs that look like a pile of biscuits due to the release of stresses during drilling.
Keith
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