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Checking Flatness

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  • #16
    What purpose would that be?

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    • #17
      Originally posted by Neil Jones

      Yes, because I'm looking for just a small point of contact in three places.


      I need to be precise. Surprised others haven't commented as I would think this is a common situation for many machinists.
      I am puzzled. What is with the "small point of contact in three places"?

      I am rather familiar with getting things flat, and square......... I do hand scraping to do that.

      Since I saw the term "surface plate", why would you not blue it up and check for the contact "print" on the part? That is more-or-less what they are for..............

      Then as far as "square", you need a reference, but you set both on the surface plate, and offer up the reference square to the side of the plate, either using the "paper pull" method, or light transmission, to determine if your angle plate matches the square reference.

      If you have a granite 90 block, you can blue it, but I don't expect you do (I don't either, just some squares and CI parts)

      The 3 point support I have seen, but usually for setting a part on a grinder (or even a mill ) for an initial "flatting" when the surface is known to be uneven, so it won't set down well.
      CNC machines only go through the motions.

      Ideas expressed may be mine, or from anyone else in the universe.
      Not responsible for clerical errors. Or those made by lay people either.
      Number formats and units may be chosen at random depending on what day it is.
      I reserve the right to use a number system with any integer base without prior notice.
      Generalizations are understood to be "often" true, but not true in every case.

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      • #18
        "I am puzzled. What is with the "small point of contact in three places"?"

        I feel with a small point of contact I have a much better chance of getting a true reading with a dial indicator of the surface I'm checking for flatness.

        "I am rather familiar with getting things flat, and square......... I do hand scraping to do that."

        I asked some questions about scrapping in another thread that I put up here but didn't get much response. I'd like to learn to scrape and I asked some questions about the basic tools I'd need.
        Last edited by Neil Jones; 03-02-2009, 12:34 AM.

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        • #19
          There is a vast difference in checking flatness as opposed to a scraped surface. For one in aerospace applications , if a milled or ground surface is specified then best it be so. Flatness is checked by placing the part on 3 points exactly the same height ( done by grinding them at once) on a calibrated surface (plate). You the indicate the bottom of the part which gives you a comparisome to the reference plane, the surface plate. Indicating the upper surface alone gives you the total of flatness and parallelism. Bluing a plate and contacting it gives you not only flatness but the frequency of contact. Flatness will ALWAYS be checked on 3 points. The corners of 1-2-3 blocks do not qualify. They are planes,not points. Never indicate the top of the part for flatness.

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          • #20
            I should also point out that the B&S leveling plate is NOT used to check flatness. It has 2 screws, not 3. It is used to check parallelism. You adjust the plate to as near zero as possible. A part placed thereon is on a level surface (parallel to the surface plate). Running an indicator over the top of the part will give you the parallelism.

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            • #21
              "Flatness is checked by placing the part on 3 points exactly the same height ( done by grinding them at once) on a calibrated surface (plate)."

              Can't see why ball bearings wouldn't achieve the same result (or better). The rest of what you wrote I tend to agree with. I wish to move a way from using the corners of 123 blocks.
              Last edited by Neil Jones; 03-02-2009, 01:34 AM.

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              • #22
                Originally posted by Neil Jones
                "I am puzzled. What is with the "small point of contact in three places"?"

                I feel with a small point of contact I have a much better chance of getting a true reading with a dial indicator of the surface I'm checking for flatness.
                .
                Neil, JT is right, the simplest most direct method is to compare your work to the reference (plate).....after all with an indicator that's all you're really doing; its still the accuracy of the plate driving the bus.

                where the indicator usually comes into flatness is when you're getting a second surface parallel to one you've already got flat by scraping.

                maybe I'm misreading what you are trying to do - can you describe it more exactly? as td points out, setting something on three points just puts the plane formed by the three points parallel to the plate, and unless your indicating the underside doesn't check for flatness
                in Toronto Ontario - where are you?

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                • #23
                  "maybe I'm misreading what you are trying to do - can you describe it more exactly? as td points out, setting something on three points just puts the plane formed by the three points parallel to the plate, and unless your indicating the underside doesn't check for flatness."


                  I'm indicating the underside.

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                  • #24
                    Originally posted by Neil Jones
                    For the life of me I can't figure out how this device works or how it would be better than the 3 ball bearings that Les suggested for indicating flatness.
                    Hello Neil,

                    Flatness and parallelism are two different things and don't necessarily go hand in hand. There may be occasions where you need to check flatness of a surface that is not perfectly parallel to another surface.

                    You would need to support the part that needed to be inspected at three points and adjust those supports to where it's top surface is parallel to the surface plate. You would do so by placing three marks on the top surface of the part to be checked that are above the three support points. You would then check those three marked points with a test indicator or whatever and adjust the supports until the indicator read the same for all three ponts. Then you could sweep the entire surface of the part and determine flatness.

                    Hope my explanation makes sense!

                    Glenn

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                    • #25

                      "would 'parallels' be too easy an answer?"

                      Yes, because I'm looking for just a small point of contact in three places.

                      "It's not the most precise, but it works fine for things like car bellhousings..."

                      I need to be precise.
                      Funny. Neil, you taught me something about precision...

                      I made the assumption that you had a plane that you wanted to
                      parallel to your plate, and there was a protrusion that prevented you
                      from bluing it... probably because that was the problem I was most
                      recently trying to solve.

                      Rereading, you said 'grind' so I guess I could have inferred you needed some degree of precision.
                      The parallels got me to +-.001, which I thought was 'not very,' but in the
                      HS world, I guess 'not very' really needs numbers with it.

                      it's all chips...

                      t
                      rusting in Seattle

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