Surface plate

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neanderman
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Location: Cincinnati, Ohio, USA

Re: Surface plate

Post by neanderman »

I bought mine from MSCDirect.com. They were having a sale on purchases over $99.
Ed

LeBlond Dual Drive, 15x30
US-Burke Millrite MVI
Atlas 618
Files, snips and cold chisels

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pete
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Re: Surface plate

Post by pete »

I've got one that size except it's only 2" thick. I hope to get a much bigger one at some point for checking and scraping machine tool parts. My small one lives when it's not being used in a tool box drawer in it's original and now well beat up cardboard box. Ideally surface plates should be always be permanently supported on there airy points. And that's very important with larger plates. Google has the details and how to locate those Airy Points if you use that as the search term. With a small plate like these I don't think it matters too much. Those Airy Points are a geometry trick to help balance out the effects of gravity and prevent it from bending the plate. And yes even large 12" thick granite plates will bend if there surface area is fairly large.These smaller plates are handy, but a little small when you start adding parts, a height gauge etc. But it's also helpful to fully understand how a truly flat part should "hinge" on a surface plate. In a nut shell it involves trying to gently and slightly rotate a rectangular shaped part 10-20 degrees left and right from each end while it's on the plate, the parts hinge or rotation point should be approximately 1/3rd in from the end your rotating, and both ends need to be checked using the same test and in the same way. It's an extremely sensitive but simple test just using the parts center of indicated rotation. 1/10,000" from flat can easily be detected. And if you learn how to read what's happening then it shows approximately where any issues might be. Checking for part rock with an dti tip on a corner and individually pressing down on each of the other 3 will give you a measurement for non flat or twisted surfaces. Obviously all 4 corners need to be checked against the indicator. A flat part shows no measurable rock, but that also doesn't mean it's not concave shaped. A very thin coat of Hi-Spot ink rolled out with a brayer will show that up instantly. In my opinion buying even a small plate is a smart move, it's a highly critical non optional shop reference surface. Many tests and checks simply can't be done without that known to be flat surface. For what these small plates cost there's not much reason to do without one.

There's special surface plate cleaners made, Starrett to name one makes there own version, by most accounts in industrial high accuracy conditions using simple Windex window cleaner or even drug store alcohol works just as well. And I've seen Marv's mention of using that non abrasive version of Fast Orange hand cleaner being used as well. Keeping the plate spotless is ultra important. And no matter how clean the plate is it should always be checked by wiping your palm and fingers across the surface to detect and wipe off any dust or dirt first. Normal room environments can drop dust, dirt, tiny paint chips off the ceiling that we wouldn't notice at any other time. 60 seconds after cleaning the plate I'll still wipe my hand across it to be sure it is in fact clean. Apparently your own hand and skin is a far better detector than any other known method. Clean enough to eat off of is still not quite good enough. And made from granite does not mean a plate doesn't wear. They can and will, so it's best not to always use the same area of the plates surface. Any rough rusty etc surfaces should never ever be put on the plate. The worst should at the minimum be a good machined surface finish. Scraped or surface ground surfaces would be better. Watching a few YT videos about how hand scraping is done helps show how important that cleanliness is. Having a surface plate is also disappointing due to how little in our shops is actually flat if you test them with that very thin coat of Hi Spot ink on the plate. So far it's yet to be proven to me, but some mention mineral based lubricating oil can be absorbed into the granite and it can affect it's flatness accuracy. If that's true then it might be something to at least be aware of.
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neanderman
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Re: Surface plate

Post by neanderman »

Thanks, Pete, for a very detailed answer! I considered a 2" plate, but the 3" was only a bit more money, so I went with it.

I wondered about "sagging," hence the real root of my question! I have a friend who welds, so I was curious if a dedicated, angle iron stand might be good.
Ed

LeBlond Dual Drive, 15x30
US-Burke Millrite MVI
Atlas 618
Files, snips and cold chisels

Proud denizen of the former "Machine Tool Capitol of the World"
pete
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Joined: Tue Feb 10, 2009 6:04 am

Re: Surface plate

Post by pete »

Your welcome. I would have bought the 3" as well. An angle iron stand might be helpful and with it raised a bit off the bench top it's then a bit easier to pick up and move around. And having a good grip on the angle iron also helps. Drop that plate on a concrete floor and they can easily break if it hits wrong, it will at least chip one or more edges for sure.

Normally a surface plate if it's not massive has 3 of those Airy Points where the supports are positioned. Two points at one end and a single at the other. Round numbers are close enough but Google gives the exact method of calculating the points for any sized plate. lets say the 12" dimension runs left / right or X axis as your facing the plate on the bench and you decide to put the two points on the left side. Flip the plate upside down and measure in from the left hand edge 1/3 of 12" or 4" that will be a line running towards and away from you or in the Y axis. You then measure in 1/3rd of the 9" dimension in each direction of the Y axis and that's 3". So the first two points will be 4" in from the left side X axis and each one 3" in from the front or back or Y on the plate. The third point is centered on the 9" Y axis width, but again 4" in X from the plates right hand edge. It's more complicated to explain it than to do it. :-)

But as I said, with a small plate like that and 3" thick plus an unknown level of true flatness I doubt having the correct positioning or even any support points will matter. If your unsure then an old folded bath towel under the plate spreads out the undulations of your bench top and supports the plate over a larger area a bit better. If I remember the size correctly Starrett sells a pretty expensive pink granite tool makers flat or granite plate a slight bit smaller and thinner than yours. The optional hinged wooden box it's meant to be stored in is just felt lined. Red of course. Ha!! found it, it's 8" x 12" x 2" https://www.starrett.com/metrology/prod ... il/G-81803 I don't believe there's any mention or use of support points at the calculated Airy Points. So good enough for Starrett then good enough for me. I think anything any larger than your plate then yes I would use the proper 3 pt support. And your plate weighs right around 27.5 lbs. depending on the granite type and density it was made from so that isn't an exact number.
LouStule
Posts: 86
Joined: Fri Sep 11, 2020 7:20 pm
Location: Washington State

Re: Surface plate

Post by LouStule »

Wow! That's a great price. Thanks for the link. A bank card would be nice too.
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