Is countertop granite flat enough for surface plate?

[gregl]

The header says it. We'll have some left over from the kitchen remodel and I hate to just toss it since I paid for it . I don't need inspection room accuracy, plus or minus .001 is more than good enough for what I do. THANKS!

[derekm]

measure it ... if its good its good

a piece of float glass and some blue will tell you

[SGW]

An old gent I knew who built absolutely beautiful model steam engines used a piece of busted-off granite facing veneer from an old store front for his surface plate. ("It drops off about three thou towards that corner, but I allow for it.") So I expect a piece of granite countertop will be at least that good, and at least that usable.

But the only way to tell, for sure, is to measure the piece of granite you have and see what it's like. Put a dial indicator on a surface gage and sweep it across the surface in several directions.

And let us know what you find. I'm curious.

[dp]

I bought a 12" square of black granite at Home Depot some time back and used it with great success for a lot of projects. I have a granite surface plate now but I still use this one for sanding and quick checks of non-critical parts.

You can easily check the flatness by placing the tiles face to face. Any two that resist spinning (surface tension) are good fits to each other. If a third is found that is also a good fit you probably have found as good a set as exists in the store. The store floor people will look at you oddly.

[lazlo]

The Enco 12x18, Grade B surface plate is $24.95 with free shipping:

http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMKD&PMPXNO=949402&PMAKA=640-0120

[Frank Ford]

Unfortunately, I believe they ship granite plates by truck now, so I don't think you can get the free shipping deal on them.

[Bones]

Ijust received a 12x18 surface plate from enco, free shipping, delivered to my door by ups.

bones

[BobC]

Unfortunately, I believe they ship granite plates by truck now, so I don't think you can get the free shipping deal on them.

Enco may have changed its policy but I purchased a 12x18 a few months back at the $24-$26 price and it came with free shipping via UPS or FedEX, forget which.
Bob

[Teenage_Machinist]

The two smallest sizes of surface plate are shippable by truck. Highly reccomend using the free UPS, there are still a few days left for a 25 dollar minimum.

If there is a Woodcraft store near you you can do that as well, They sell 9x12 grade B, as some woodworking tools need careful alignment aided by a reference..

Wish there was such a thing as "MetalCraft" sort of is in LMS but we could use some more variety.

It depends on what you consider a surface plate. Granite is good because it does not expand too much when heated, is abrasion resistant and cannot get burs, which mean that good counter top may be useable for less precise stuff such as marking.

Scraping and toolmaking == NO! NOT PRECISE ENOUGH!

[Just Bob Again]

You have to check the piece you get. I have a sink cutout from a granite counter. Nice and smooth but nowhere near flat. Out at least 20 thou from corner to corner. Over a small area, it's fairly flat. Too thin for a real surface plate. On the plus side, mine is 20 by 30 inches and I can lift it.

[John Stevenson]

Ironically this has just come up tonight on a UK forum.
I have taken the liberty of copy and pasting the post from a well know and respected UK member Jim Petagrid and all the following is Jim's work.


*****************

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

[dp]

Scraping and toolmaking == NO! NOT PRECISE ENOUGH!

They're perfectly suitable where 0.0002 is not necessary which at home is quite a bit. I used mine for scraping and improved the surface of the work immensely. I've done side by side checks with my granite surface plate and the granite tile and the differences for my environment are not significant.

Given the cost of true surface plates today the only justification for using a tile is convenience - they're easy to put away, and abuse. You can beat the hell out of them and not feel bad.

[Evan]

By a piece and test it. From my web site:

http://metalshopborealis.ca/laser/flattest.htm

If it doesn't make the grade it is still a handy work surface.

[Teenage_Machinist]

Not only that but they do not get burs from being beaten, in fact a good piece of tile may be better than the mill table.

[dp]

Not only that but they do not get burs from being beaten, in fact a good piece of tile may be better than the mill table.

They're the hot new thing on table saws and joiners, and probably router tables, soon enough. They're probably cheaper than casting iron for the same purpose.