Granite Placemats as Surface Plates

Having recently aquired a 6" optical flat I thought it would be interesting to checkout the granite placemats and chopping boards
now stocked by the supermarkets. These are 15 to 20mm thick and small enough and light enough to be kept in a drawer when not in use.
Although the working surface is polished, it is only polished sufficiently to give a shine. There's severe "orange peel" surface disturbance - not good enough for optical interferometry checks. However the surface is ideal for the capillary/surface tension method.
http://xs434.xs.to/xs434/08011/granite_1a910.jpg
http://xs134.xs.to/xs134/08011/granite_1b902.jpg
http://xs134.xs.to/xs134/08011/granite_2a522.jpg
http://xs134.xs.to/xs134/08011/granite_2b880.jpg
Show the test results on two 15mm thick placemats. The 6" flat was supported at the top end with an 0.005" shim to give an airgap slope of about 5/6000. The equal thickness capillary edge displays a 6000/5 amplification of the flatness error - approx 1" per 0.001" Each plate was tested at 0deg and 90deg and showed errors of less than 0.0004".
The tests were repeated on a 300 x 400 x 15mm chopping board.
http://xs134.xs.to/xs134/08011/chop_board_0.005925.jpg
http://xs134.xs.to/xs134/08011/chop_board90_0.005344.jpg
http://xs134.xs.to/xs134/08011/chop_board_0.001674.jpg
The first two show the 0deg and 90deg results The third is a repeat of the 90deg result but with an 0.001" shim.
http://xs134.xs.to/xs134/08011/plglass_0.001_x_2in878.jpg
Is the result using an 0.001" slope on a 12" length of 2" x 1/2" plate glass (ex shop window display shelf)
http://xs134.xs.to/xs134/08011/splatemeths0.005256.jpg
Is an 0.005" slope test on a 12" x 18" surface plate. Although the lower faint capillary edge is fairly visible by eye, the contrast is too low for satisfactory photography. Both Meths and ispropanol (rubbing alcohol) gave similar results but a marking out fluid which appears to be heavily dyed alcohol gave excellent contrast. The snag is that the higher viscosity results in a long settling time.
http://xs134.xs.to/xs134/08011/splatemkgbu0.005571.jpg
http://xs134.xs.to/xs134/08011/splate0.001start328.jpg
http://xs134.xs.to/xs134/08011/splate0.001finish758.jpg
The fist JPG is an 0.005" slope test. The second shows the beginning of an 0.001" slope test. the third is the result 1/2hour later. The results show up the comparative roughness of a ground and scraped surface.
Jim
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<snipped for brevity>
Interesting post but, while I think I understand the basic idea, I'm not sure how you actually convert your observations to measurements.
I'm assuming that, for a given fluid, a sloping "gap" would give a straight line at the point were the "gap" gets too large to "hold" the fluid via capillary/surface tension. If the boundary is not straight it means the surfaces are not "flat". Is that correct, please?
As regards the measurements, do you do some trig to convert the deviations of the line to a local measurement?
TIA
Brian
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On Tue, 30 Dec 2008 07:14:03 -0000, "Brian Reay"

Surface tension of the liquid in the sloping gap ensures that the liquid/air boundary is of uniform thickness.
If both surfaces are truly flat, this boundary is a straight line at 90deg to the slope angle. If either surface has a flatness error the boundary moves to maintain its uniform thickness.
Since it is moving in a sloping airgap the change in thickness is multiplied by the inverse of the gradient of the slope angle - For a 1 in 1000 gradient the boundary movement is 1000 times the local change in gap thickness.
The deviation from a straight line is a direct readout of the flatness error adjacent to that line. Measurements can be made along lines higher up or lower down by choice of the amount of liquid.
Jim
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Thank you, sounds much as I had deduced.
Brian
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I'm not

straight
via
the
deviations
Jim,
How does surface finish and cleanness affect the result? I seem to remember an early chemistry lesson where the difference between fingered microscope slides and cleaned ones was demonstrated using a simillar wedge capillary gap, with the greasy one being ragged.
AWEM
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On Tue, 30 Dec 2008 10:24:34 -0000, "Andrew Mawson"
SNIP

For accurate results both surfaces need to be really clean AND free from dust particles. Both surfaces were first wiped down with commercial isopropanol and a fresh paper towel.
Airborne dust particles were avoided by first covering the clean test surface with a fresh paper towel and the placing the flat on top of the towel. When the paper towel is slowly withdrawn it carries away any stray particles that may have settled on either surface. After the towel is withdrawn the flat is slightly raised and the spacer shim inserted.
Dust particles generate easily identified sharp spikes on the boundary. Inadequately clean surfaces show up as improbable non wetted areas. This shows up well in the final measurements on the cast iron surface plate where complete cleaning is impossible.
Traditionally methylated spirits is used for these tests. I changed to isopropanol because my particular bottle of methylated spirit left a slight greasy film behind when it evaporated. This wasn't a problem during the test but it was nuisance to clean it off the flat before the next test.
Marking out fluid gives an exceptionaly visible result but is both messy and a nuisance to cleanup. For rough work, using a steeper gradient, a handful of identical small steel balls is less accurate but more convenient.
Jim
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On or around Tue, 30 Dec 2008 13:28:28 +0000, snipped-for-privacy@yahoo.com enlightened us thusly:

and more SNIP.
So, what was your conclusion? Can we go out and buy granite chopping boards as surface plates?
They'd be more use than the intended purpose. I see no point in a chopping surface which is only going to blunt my nice, sharp knife.
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