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  • Scraping a surface plate

    If you guys recall my recent thread on the cast iron surface plate..
    I was going to leave it as-is, but after much reading, and quite a few
    you tube videos, I've decided to get myself a scraper and give it a go.

    Figure I couldn't make it much worse, right?

    The problem I face now is I have no reference surface. Buying a
    ref surface the same size as my plate sort of defeats the purpose of
    fixing my plate ... not to mention it'll cost big bucks probably.

    I'm wondering, is there any other way to at least get my plate better
    than it is?

    Glass?
    A giant piece of polished marble?
    a piece of granite?


    -Tony

  • #2
    No reference, no scraping.

    A granite flat costs about $90 for a 12 x 18, "OK" quality.

    or you can find 2 more same-sized plates, and scrape them all together. Properly done, that, while tedious, can get as good as you want.
    1601

    Keep eye on ball.
    Hashim Khan

    Comment


    • #3
      Thanks JT .. thats about what I expected but thought I'd ask.

      My plate is about 24x24"

      I'll start shopping around...

      Thanks.

      Comment


      • #4
        Hmmm, what does Connelly say in his book?

        I know there is a chapter in there about scaping surface plates and not just tiny old 2x2 footers. Some of the really big surface plates would be impossible to scape in with two other plates. Yeah, I now there are all kind of laser gadgets and what not, but there must be a simple, if incredibly tedious, mind-numbing, and physically exhausting way of doing it 'cause they didn't have usable lasers back in the 30's and 40's.

        Brian

        Comment


        • #5
          It would be possible to use optical methods, but you would need an optical collimator with a lens or mirror that is the diameter of the diagonal measurement of your plate. For a 24" X 24" plate that would be 34.95". This could be purchased for multi tens of thousands of dollars or of course, you could take up telescope mirror grinding. The good news is, you only need two pieces of glass and only one of them needs to be that size, the other one can be somewhat smaller, perhaps 25" of so. This is because you can make a perfect sphere with two surfaces while making a flat takes three. Of course, after making the spherical surface, you will need to convert it to a paraboloid of revolution. That will take more work and a knife edge tester which is fairly easy and inexpensive to make. Oh, and when testing the flat, you would have to at least partially polish it to provide an optically reflective surface for each test. A light coat of oil may work in the early stages, but polishing would be necessary at the final ones.

          Anyway, after a lot more work, you would wind up with a glass convex sphere, a glass concave paraboloid of revolution, a collimator that incorporates that paraboloid, and finally the desired flat. This may be the way to go in a factory that makes a lot of flats, but frankly, at home or in a small shop it would be a lot easier and a whole lot cheaper to just buy three flats and scrape them together. That is how they did it in the dark ages (40s or 50s).

          PS, they did actually have lasers in the 50s and they could and did use them for such work. They still needed the large mirror or lens to direct the laser light to all sections of the flat, just as we do today.
          Paul A.

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

          Comment


          • #6
            Originally posted by Tony
            .....

            Figure I couldn't make it much worse, right?

            .....

            I'm wondering, is there any other way to at least get my plate better
            than it is?

            Glass?
            A giant piece of polished marble?
            a piece of granite?


            -Tony
            Can't make it much worse? Wanna bet? Sure you can.

            Glass? Any 24" x 24" piece of glass that you can buy at your local glass supplier will sag very significantly over this kind of dimension. Perhaps several thousandths, hardly surface plate specs. Think of it as being made of rubber.

            Marble? Polished marble is fairly flat, but there is no guarantee. The edges can easily be rounded several thousandths. You would have to get it checked against a known good plate or by other means.

            Granite? Sure, just look in shop supply catalogs under surface plates. Prices are fairly decent for imports. But shipping can be a bitch. You can probably get away with an 18 X 24 and check it four ways with each edge aligned.
            Paul A.

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

            Comment


            • #7
              Didn't Evan have a thread about a laser and a surface plate?
              It did not seem all that complicated.

              Comment


              • #8
                Originally posted by H8Allegheny
                Hmmm, what does Connelly say in his book?

                I know there is a chapter in there about scaping surface plates and not just tiny old 2x2 footers. Some of the really big surface plates would be impossible to scape in with two other plates. Yeah, I now there are all kind of laser gadgets and what not, but there must be a simple, if incredibly tedious, mind-numbing, and physically exhausting way of doing it 'cause they didn't have usable lasers back in the 30's and 40's.

                Brian

                Large surface plates were scraped from a master, say about 18"-24" square, small enough to be lifted onto the big plate, upside down. The 'master' was, in turn, scraped fom another master - ad naseum. Original master(s) (3 off) can be scraped one to the other, until all three 'mark-up' essentially the same. The big plates - biggest I worked on was 10 ft x5ft were also checked with a precision level to ensure no 'roller coasters' in the surface. Using the master and overlapping the 'markings' enabled (eventually) the larger plates to be sraped flat. Big plate, as mentioned, could take 4 to 6 weeks, depending how good it came off the planer.

                NzOldun

                Comment


                • #9
                  I been down this road a dozen times. If a home shop guy wants to scrape in a cool 24 x 24 surface plate pick the simplest method of referenceing possible: buy a cheap granite surface plate from Grizzly, Enco, whatever. They're pretty good for flat and they have resale value once your cast iron plate is scraped.

                  Monkeying around with grazing light lasers, tight wires etc is not only dubious but checks only the path of the wire. laser etc. The area between check paths has to be referenced too.

                  An 18 x 24 granite surface plate will reference a 24 x 24 cast iron plate just fine. Simply rotate it 90 degrees and sideshift to pick up the gap.

                  If you find two more surface plates and decide to scrape them by the Whitworth "reduction of errors by averaging" technique you better figure on three to five times the scraping time. It can be done and if done correctly will result in lab master quality flats but it's a LOT of hard, very picky work. There is an excellent description of the technique in Charles M Porter's "Engineering Reminiscences" to be found in Google Books.
                  Last edited by Forrest Addy; 09-18-2011, 04:35 PM.

                  Comment


                  • #10
                    Originally posted by Paul Alciatore
                    It would be possible to use optical methods, but you would need an optical collimator with a lens or mirror that is the diameter of the diagonal measurement of your plate. For a 24" X 24" plate that would be 34.95". This could be purchased for multi tens of thousands of dollars or of course, you could take up telescope mirror grinding. The good news is, you only need two pieces of glass and only one of them needs to be that size, the other one can be somewhat smaller, perhaps 25" of so. This is because you can make a perfect sphere with two surfaces while making a flat takes three. Of course, after making the spherical surface, you will need to convert it to a paraboloid of revolution. That will take more work and a knife edge tester which is fairly easy and inexpensive to make. Oh, and when testing the flat, you would have to at least partially polish it to provide an optically reflective surface for each test. A light coat of oil may work in the early stages, but polishing would be necessary at the final ones.

                    Anyway, after a lot more work, you would wind up with a glass convex sphere, a glass concave paraboloid of revolution, a collimator that incorporates that paraboloid, and finally the desired flat. This may be the way to go in a factory that makes a lot of flats, but frankly, at home or in a small shop it would be a lot easier and a whole lot cheaper to just buy three flats and scrape them together. That is how they did it in the dark ages (40s or 50s).

                    PS, they did actually have lasers in the 50s and they could and did use them for such work. They still needed the large mirror or lens to direct the laser light to all sections of the flat, just as we do today.
                    Gould’s notes included possible applications for a laser, such as spectrometry, interferometry, radar, and nuclear fusion. He continued developing the idea, and filed a patent application in April 1959. The U.S. Patent Office denied his application, and awarded a patent to Bell Labs, in 1960. That provoked a twenty-eight-year lawsuit, featuring scientific prestige and money as the stakes. Gould won his first minor patent in 1977, yet it was not until 1987 that he won the first significant patent lawsuit victory, when a Federal judge ordered the US Patent Office to issue patents to Gould for the optically pumped and the gas discharge laser devices.
                    On May 16, 1960, Theodore H. Maiman operated the first functioning laser,[11][12] at Hughes Research Laboratories, Malibu, California,

                    Comment


                    • #11
                      I have a small 12" x 14" cast surface plate that I have always wanted to scrape, mostly for practice purposes but for about $50 I can have it Blanchard ground. I've been wondering if it's worth the effort.

                      JL.................

                      Comment


                      • #12
                        You can have it blanchard ground if you want but it won't give you a surface plate quality reference. If it's to be a good surface plate it will need to be scraped again after grinding.

                        There are probably a number of vagaries that can enter the grinding operation, heat being only one of them. The heat from grinding will cause expansion, and because the plate is not heated uniformly the ground surface will typically bow upwards into the wheel. As temperature normalizes the surface will be left with distortions. You're also depending on the bearings and geometry of the machine which, being less than perfect, will generate their own geometric artifacts in the ground surface. It's a good surface, no mistake, but just not a surface plate quality reference.

                        Originally posted by JoeLee
                        I have a small 12" x 14" cast surface plate that I have always wanted to scrape, mostly for practice purposes but for about $50 I can have it Blanchard ground. I've been wondering if it's worth the effort.

                        JL.................
                        .
                        "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

                        Comment


                        • #13
                          Laser methods of checking ARE used...... in industry.

                          You do not have the equipment to do that.*

                          it is best to forget about laser methods for home shop use at present.


                          If you have it, things like a Talyvel etc can help refine a good surface, but I suspect they are not much use to get a rough surface cleaned up smoother.

                          perhaps a countertop etc as reference can "get you close" and another method can take over from there. But "close" is still likely to be many thou off of flat. anywhere from 5 or 10 to 30 or more thou off maybe, depending on size.

                          That "only" 5 thou off is a mile when you are scraping a large surface.......!

                          * OK, there may be one or two of you with access at work, etc, but even there, it is unlikely to be useful in dozens of scraping passes as you flatten the plate. There will be better uses for the equipment.
                          1601

                          Keep eye on ball.
                          Hashim Khan

                          Comment


                          • #14
                            If you're gonna be a bear be a Grizzly bear. Not the brand of tools but the real bear with fangs and claws. If you're gonna scrape in a surface plate you need either a flat reference or a totel of three similar sized plates for the reduction of error techniqie.

                            If you have a granite surface plate available to you at work and you can get permission to blue it up and use it out of work hours or lunch time that will work too. It will also polish your reputation as a crafty machinist.

                            Here's a link to Grizzly's offering:

                            http://www.grizzly.com/products/18-x...No-Ledge/G9654

                            Shipping is more than the cost of the plate. OTH you can drive there and pick it up at the warehouse and save yourself shpping. Combine it with a family trip. Grizzly has warehouses in Bellingham WA, Springfield MO and Muncy PA
                            Last edited by Forrest Addy; 09-18-2011, 04:39 PM.

                            Comment


                            • #15
                              Originally posted by tdmidget
                              Gould’s notes included possible applications for a laser, such as spectrometry, interferometry, radar, and nuclear fusion. He continued developing the idea, and filed a patent application in April 1959. The U.S. Patent Office denied his application, and awarded a patent to Bell Labs, in 1960. That provoked a twenty-eight-year lawsuit, featuring scientific prestige and money as the stakes. Gould won his first minor patent in 1977, yet it was not until 1987 that he won the first significant patent lawsuit victory, when a Federal judge ordered the US Patent Office to issue patents to Gould for the optically pumped and the gas discharge laser devices.
                              On May 16, 1960, Theodore H. Maiman operated the first functioning laser,[11][12] at Hughes Research Laboratories, Malibu, California,
                              OK, I may have missed their USE by a year or two, but they did exist in the 50s, at least the late 50s.
                              Paul A.

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

                              Comment

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