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Giant granite slab, What is used for?

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  • Giant granite slab, What is used for?

    I subscribe to a news letter from a recycle/salvage yard in Albany, OR. This morning they sent me a ad posting some BIG granite slabs with threaded inserts in them, they are big. One is 48"x 72"x 4" and several that are 31"X52"x8". Price is $500 for the smaller and $700 for the large one. My question is, What are these used for. Are they related to Machining some way? I thought they might be of some interest here.
    _____________________________________________

    I would rather have tools that I never use, than not have a tool I need.
    Oregon Coast

  • #2
    It's probably some form of purpose built optical table.

    Phil

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    • #3
      I bought 4-3'x3'x6" that were from an optics company & had machines bolted to the inserts.
      "Let me recommend the best medicine in the
      world: a long journey, at a mild season, through a pleasant
      country, in easy stages."
      ~ James Madison

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      • #4
        I don't know the original intent but it seems to me that would be a good start on a home made CMM if it's really flat. It might also be good as just a surface plate because sometimes it would be handy to be able to tie something down to keep it from scooting around for repeatable measurements. I have an 18 X 24 with ledges that I sometimes clamp to, but that is kind of restrictive on setups.

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        • #5
          I build robotic processing equipment and use granite surface plates for bases sometimes.

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          • #6
            Starrett at least will drill, slot, size, and basically custom build surface plates to whatever the customer specifies, and I'd guess it's fairly costly for them to do so. I'd also guess most other manufactuers would too. So it's origins could have easily started out as a surface plate. My guess is optics / lasers also. Depending on wear and overall condition, they might be at the high priced end. But could be pretty low priced if there exactly what's needed. They were probably pretty expensive to buy originally. There's a lot of work tied up in those. It's not likely that something was built like that and it wasn't flat to very high standards.

            Do it yourself and dirt cheap tombstone? Just screw the lettering on.

            Pete

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            • #7
              A previous company I worked for used surface plates like that for high precision laser processing tools. We ordered them with the threaded inserts pre-installed from our granite supplier. Ours were often around 48" X 72" X 15" (or larger). I say around because we made different size tools. The main tool was mounted on granite for rigidity, because the temp coefficient of granite is much less than steel (or aluminum), and because granite (so I was told) is homogenous. It doesn't have internal stresses that will cause the plate to twist and warp with change in temperature. The last reason we used granite was that the granite surface is flat and smooth so we could use air bearings to move the part being processed under the laser optical system. With the air bearings we could get fairly repeatable part position within 1 - 2 microns. Even in an air conditioned room, I was able to correlate the position error to the outside temperature. Fun stuff to work with.

              On a related subject, you can repair chipped and dinged granite plates with epoxy. The stuff from our granite supplier was their own 'secret sauce', in that they wouldn't tell us where they got it. I suspect that it had the same TC as granite. They showed us how to repair the surface far below the sub tenth level.

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              • #8
                Looks like the table off a CMM. So it is as good as a surface plate.
                Every Mans Work Is A Portrait of Him Self
                http://sites.google.com/site/machinistsite/TWO-BUDDIES
                http://s178.photobucket.com/user/lan...?sort=3&page=1

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                • #9
                  I just looked up what a CMM thing is and the U tube video showed something just like the one I posted. I guess I sure as heck don't need one. I do like the tombstone idea that Pete suggested,but I don't need one of those either, so I will have to pass on the stones. Thanks for the help guys.
                  _____________________________________________

                  I would rather have tools that I never use, than not have a tool I need.
                  Oregon Coast

                  Comment


                  • #10
                    I've seen several precision assembly machines (semi-conductor and solar cell assembly) that use large granite slabs as the machine structure instead of steel or aluminum. biggest one was about 6'x14' x 2' thick with 1'x2' vertical posts bolted on but I'm sure there are bigger out there. I think there are precision grinders that use granite as the machine structure as well. These applications require the thermal stability that granite provides over metal.

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                    • #11
                      Our Brown and Sharpe CMM at work has a table just like that.

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                      • #12
                        it's an optical layout table for very fast lasers (femto), can be used for gas chromatography, measuring stuff, testing pulse modulation. you usually see them in research labs.

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