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  • #31
    Originally posted by nickel-city-fab View Post
    I have seen versions of them made (among other things) where people would loctite a ball bearing into a hole (this is the foot), then skim the entire bottom of the part on the surface grinder. So that however many feet it has, they all end up in the same plane to within tenths or less on a decent machine.
    I dont know. What is wrong with that system. JR
    My old yahoo group. Bridgeport Mill Group

    https://groups.yahoo.com/neo/groups/...port_mill/info

    Comment


    • #32
      Originally posted by JRouche View Post

      I dont know. What is wrong with that system. JR
      Dunno, I think it would work just fine. I sure don't have that kind of equipment tho. Not sure I could hold those tolerances.

      Comment


      • #33
        Oxtoolco has a video to add to your watch list.
        https://www.youtube.com/watch?v=ZkCIVsP7-KY

        Steve

        Comment


        • #34
          Originally posted by Paul Alciatore View Post
          I have looked at the federal spec. It appears to show the feet as 3/6" diameter, which seems to be a strange way of saying 1/2". Is that an error? The draftsman did not know how to spell "indicating system" and the paper's author did not correct it so how much accuracy can we expect in that drawing. Perhaps I am not seeing the latest revision: the one I have called up is dated 9-12-73.

          It also says nothing about the shape of the feet. They are drawn as flat, but that is not a real specification. A sufficiently large radius, perhaps one that would only amount to a tenth or so of difference at the edges, would also look like a flat in that drawing. The text says nothing about it.

          I do not think a large radius would do any damage to the plate. Actually sharp edges on a completely flat foot could act as a scraper and be the cause of damage.

          On the practical side, a flat foot would be easier to lap in place. But then we can get into issues of how do we know/measure their flatness. Could we even detect the difference between a flat foot and one that is 0.0001" higher at it's edges? Use an optical flat, perhaps? But it would need to be a big one. Could this perhaps be one of the reasons why these repeat-o-meters are so expensive?

          I can completely agree that neither the plate nor the repeat-o-meter should wring. No sticking allowed.




          its 3/8. seems to be latest revision.

          Comment


          • #35
            I don't know either. If the surface plate is not perfect, with hills and valleys, then if the feet are perfectly flat and co-planer, they will NOT be making good contact. In fact, if they have sharp edges, they will scrape the plate when they run into a hill.

            I looked at the federal spec again and saw that the rear feet were drawn with a radius while the front foot and the foot on the arm had straight lines. I don't think that anything particular is being called for in that drawing and there certainly isn't anything specific about the feet in the text.

            But they do specify that 3/6" diameter for the feet. At least I think it is a diameter. I guess it could also refer to square or even rectangular feet. If anyone has a name brand one (Rahn or Starrett or ??), I would be curious to see a photo of the bottom and a measurement of the actual diameter (or dimensions) of the feet.



            Originally posted by JRouche View Post

            I dont know. What is wrong with that system. JR
            Paul A.
            SE Texas

            And if you look REAL close at an analog signal,
            You will find that it has discrete steps.

            Comment


            • #36
              Originally posted by Paul Alciatore View Post
              I don't know either. If the surface plate is not perfect, with hills and valleys, then if the feet are perfectly flat and co-planer, they will NOT be making good contact. In fact, if they have sharp edges, they will scrape the plate when they run into a hill.
              I've been thinking that it doesn't actually matter that much since the repeat-o-meter is making relative measurements, and not absolute dimensional measurements. So you can get away with being less-than-perfect on both the tool, and on the item being measured.

              Comment


              • #37
                Originally posted by Paul Alciatore View Post
                I don't know either. If the surface plate is not perfect, with hills and valleys, then if the feet are perfectly flat and co-planer, they will NOT be making good contact. In fact, if they have sharp edges, they will scrape the plate when they run into a hill.

                Originally posted by nickel-city-fab View Post

                I've been thinking that it doesn't actually matter that much since the repeat-o-meter is making relative measurements, and not absolute dimensional measurements. So you can get away with being less-than-perfect on both the tool, and on the item being measured.
                Yes!

                The point of the device is to find discrepancies. On a relative basis.

                If it reads an error when there is one, and reads the surface as perfect when it is perfect, then what do you want?

                If you want absolute measurements, then get either a "Planikator" or an Autocollimator.
                2801 3147 6749 8779 4900 4900 4900

                Keep eye on ball.
                Hashim Khan


                It's just a box of rain, I don't know who put it there.

                Comment


                • #38
                  Originally posted by J Tiers View Post

                  If you want absolute measurements, then get either a "Planikator" or an Autocollimator.
                  Nah, if I want absolute measurements I'll get a CMM
                  But even then, it's still one surface relative to another -- so the question becomes, "how much deviation is tolerable after accounting for our measurement setup?"

                  Hexagon Metrology is the parent company of Brown and Sharp IIRC: https://www.hexagonmi.com/products/metrology-hardware

                  Too bad I don't have a few million $$$ sitting around for those.......

                  Comment


                  • #39
                    Originally posted by nickel-city-fab View Post

                    Nah, if I want absolute measurements I'll get a CMM
                    .............
                    Too bad I don't have a few million $$$ sitting around for those.......
                    And there is the fly in the soup........ Betcha you can get an autocollimator for a few hundred, or maybe a grand, and it does more things..... align your 15' x 36" planer with it also!
                    2801 3147 6749 8779 4900 4900 4900

                    Keep eye on ball.
                    Hashim Khan


                    It's just a box of rain, I don't know who put it there.

                    Comment


                    • #40
                      I think you've figured it out. You really don't want the needle of that $1000+ indicator accurate to 20 millionths of an inch bouncing around while you're sliding it on the plate putting lateral stress it.
                      Mike
                      Central Ohio, USA

                      Comment


                      • #41
                        Originally posted by J Tiers View Post

                        And there is the fly in the soup........ Betcha you can get an autocollimator for a few hundred, or maybe a grand, and it does more things..... align your 15' x 36" planer with it also!
                        Very true -- I was shopping for older instruments on eBay a while back. Mostly Taylor & Hobson, and Hilger & Watts. anything decent seems to hover between $300 and $1k. But the reality of the matter is, I have no real need for this. My shop is just not that big. But yeah, it's cool to be able to measure slopes to a tiny fraction of a degree. I love the idea, but for myself I just couldn't justify it.

                        Comment


                        • #42
                          I bought a H&W AC from Ebay a few years ago for @ $200. It was a risk though because it was a TA-3. Works with an optional amplifier. I was lucky enough to find one for $75. Plugged them in and they worked - somewhat. Optics need to be gone over and I will be replacing the caps and the tube in it soon. Then re-assessing. Still I was pleased with the deal. I waited for some time to find the right price.

                          Best Regards,
                          Bob

                          Comment


                          • #43
                            actually there should be an inverted-repeatometer type contraption for the planekator, so you dont slide the instrument tip on the surface.

                            Comment


                            • #44
                              But.. You know you dont measure flat right?

                              I have what is considered the flat stuff 12" optical flat, so what...

                              Who is to say it is not dished? That is where a bridged, three point type unit is used. Checks and balances. JR

                              My old yahoo group. Bridgeport Mill Group

                              https://groups.yahoo.com/neo/groups/...port_mill/info

                              Comment


                              • #45
                                I tried following your link as well as just searching for other brands but none of them seem to say much about just how accurate their machines are. They all seem to throw around words like "accuracy" and "repeatability" and other terms that imply how great their machines are, but I could not fine a single figure on just how accurate they were. I guess it is one of those things where, "If you have to ask, ..."

                                So just for the pure heck of it, does anyone know what kind of accuracy they are capable of? And yes, I KNOW IT WILL VARY BETWEEN DIFFERENT OEMs AND MODELS AND STYLES AND OF COURSE, PRICE. But how about some ball park numbers from someone who has actually used one or more?

                                Oh, and just how do you verify that accuracy? Wouldn't you need some kind of standard like the ones that come with micrometers?



                                Originally posted by nickel-city-fab View Post

                                Nah, if I want absolute measurements I'll get a CMM
                                But even then, it's still one surface relative to another -- so the question becomes, "how much deviation is tolerable after accounting for our measurement setup?"

                                Hexagon Metrology is the parent company of Brown and Sharp IIRC: https://www.hexagonmi.com/products/metrology-hardware

                                Too bad I don't have a few million $$$ sitting around for those.......
                                Paul A.
                                SE Texas

                                And if you look REAL close at an analog signal,
                                You will find that it has discrete steps.

                                Comment

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