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Lapping A Granite Surface Plate

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  • #31
    Thank you, Greg! I'll call Dapra in regards to Canode.

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    • #32
      I bought Prussian blue at the art store where I bought my brayer. Also bought yellow and red - all oil-based. They work great and clean up easily. Many of the methods, tools, and supplies used by factories can be duplicated more cheaply by the HSM with good results using off the shelf supplies from a good art supply store and fortunately, Seattle has several. Once you start marking your work it will become obvious what works and what doesn't.

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      • #33
        Dennis is probably right in that regard, but I am biased by high prices for everything here. At the local art supply a few tubes of paint cost about as much as two bottles of Canode shipped from the US. (I don't have a smiley to convey the frustration of that situation).

        Spotting medium is just an evenly milled pigment in a non-drying carrier. I am still working on the zen issues of scraping and didn't want to get into more variables quite yet.

        Speaking of variables, I bought a granite surface plate at an auction that included several obviously well cherished machines. I still wanted to get the plate certified rather than assume its flatness. The local calibration shop wants $600 for this service. How much are people paying for those Talyvels?

        Greg.

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        • #34
          Originally posted by Greg Q
          Dennis is probably right in that regard, but I am biased by high prices for everything here. At the local art supply a few tubes of paint cost about as much as two bottles of Canode shipped from the US.
          The big advantage of Canode is that it's water based, so it washes off. With Prussian blue, you have to wait for your skin to wear off

          By the way, red lead is widely available at chemical supply houses. I don't use it because I have little ones in the house that occasionally make a break for the shop.
          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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          • #35
            I am wondering about the use of expensive instruments to test these granite 'flats'.

            Amateur telescope makers have very precise testing procedure down to optical wavelengths using quite simple equipment. Of course the surface of the granite would have to be reflective, is it?

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            • #36
              No, granite plates are very dull-I guess from the lapping process. Look as flat as they are. I sure would love to discover a way to do this check with the tools that I already own (straightedges, 199 level, tenths indicator, surface gage.)

              Greg

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              • #37
                This method might be worth a try.

                http://www.cnccookbook.com/MTLaserMetrology.htm

                Steve.

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                • #38
                  Laser flatness check.

                  I tried that laser test on a woodworking machine and the results broadly agree with the straightedge/feeler gauge method. I couldn't get better than +/- 0.001 or so resolution. The other problem with the surface plates is that all of these are first order tools and should be better than 0.0001.

                  Since the accuracy rule of thumb is 10:1 for metrology steps, the result of scraping a spotting tool from the markings of a 0001" flat reference is a tool not quite that good. This will yield a scraping result on the machine of even less accuracy.

                  50 millionths (0.00005") is the number that you see on the certs for these tools, which is (I guess) the economical limit of accuracy/precision/repeatability for lapping these stones. Moore Special Tool Company (Jig Bores and grinders etc) built their reputation for unreal accuracy on their reference tools. They had hand scraped cast plates flat to better than 20 millionths

                  I was thinking of the results of scraping in my cross slide and saddle and gib strips. To get the celebrated results that are apparently possible I wanted not to assume that my plate was flat.

                  (I might however be making the new guy mistake of chasing crazy small numbers for no real world gain.-but I don't think so given what I've read and been shown)

                  Greg

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                  • #39
                    Originally posted by The Artful Bodger
                    I am wondering about the use of expensive instruments to test these granite 'flats'.

                    Amateur telescope makers have very precise testing procedure down to optical wavelengths using quite simple equipment. Of course the surface of the granite would have to be reflective, is it?
                    Yes, amateur telescope makers use a device called a knife edge tester. Trouble is, it does not work directly on flat surfaces as the light starts at a "point" source and is reflected back to a "point" image which the knife edge intercepts. I have a home made knife edge tester that cost all of about $25 and it can test spherical and parabolic mirrors to 1/4 wave or better. That's microns folks.

                    However, in order to test a flat with it, you would need a mirror, preferably spherical, that is already as accurate or better than the accuracy you want to test to AND that is as large as the flat itself. That's why the commercial, optical test instruments cost so much. I guess you could test in smaller sections and map them together, but you still need a good mirror. And the setup would be challenging. If the flat is horizontal, then the light beam would be vertical or perhaps at a 45 degree angle between horizontal and vertical. The known good mirror will need to be mounted so that it would not be distorted. Thermal effects are hell and it pays to set things up and then come back in 12 to 24 hours. Of course, you can not touch the mirror or the flat as the heat from your hands will distort things.

                    But it could be done for under $200, including grinding and aluminizing a 6" or 8" mirror, I would think. A 12" mirror would be better but more expensive. If you buy the mirror ready made, it could cost hundreds or even thousands.

                    An optical flat and monochromatic light source might be a better way.
                    Last edited by Paul Alciatore; 05-26-2009, 04:25 AM.
                    Paul A.
                    SE Texas

                    Make it fit.
                    You can't win and there IS a penalty for trying!

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                    • #40
                      Flatness Granite Surface Plate

                      Most high precision levels at 0.02mm/m can check flatness.

                      A 100mm long level will show 1 line bubble movement if it dips 0.002mm per 100mm.

                      http://www.leveldevelopments.com/eng...ock-levels.htm

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                      • #41
                        Originally posted by lazlo

                        I'm guessing you bought a big granite surface plate and it's not flat? You might be better off with a Talyvel -- an exquisitely sensitive electronic level (really a gauge head with a pendulum sensor). With a Talyvel and a lot of patience, you could record a 2D map of the flatness of the surface plate.



                        Mine has an analog display head, but the new ones have a digital interface with software that maps the 2D flatness of the surface (you can do this by hand with the analog head, of course):

                        We had a gent in recently, he had two Mahr units
                        http://www.mahr.de/index.php?NodeID=10257

                        He took readings across and diagonally using two sets of those feeding a computer that mapped the granites and then he executed a lapping schedule followed by cleaning and rechecking.

                        Clutch

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