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Checking large plate flatness. HSM workarounds?

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  • Checking large plate flatness. HSM workarounds?

    12345678910
    Last edited by Arthur.Marks; 08-23-2010, 04:09 PM.

  • #2
    See this method I developed.

    http://ixian.ca/gallery/laser/flattest.htm
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      Master precision level moved around in a grid pattern.

      Originally posted by Arthur.Marks
      So I have been wanting to check the flatness of a 32"x45" ground steel plate. Budget runs around $300 max.
      How accurate? Optical interferometry type of technique like the home telescope mirror grinder guys use?

      How bout a master precision level moved around in a grid pattern combined with lots of surveyor mathematics techniques?

      Pretty darn cheap if you already have a precision level and many known thickness shims to align your lathe.

      I've seen new import master "precision" levels for like $150. I'm thinking $150 for the level, maybe $100 to contract out to someone like Mr Evan to explain how to do the surveying math and significant figures calculations (I'm not even going to try to explain it, although I could probably hack thru it if I had to) and $50 for headache pills and beer afterwards?

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      • #4
        12345678910
        Last edited by Arthur.Marks; 08-23-2010, 04:09 PM.

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        • #5
          Originally posted by vincemulhollon
          How accurate?
          12345678910
          Last edited by Arthur.Marks; 08-23-2010, 04:09 PM.

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          • #6
            Mpl?

            Originally posted by Arthur.Marks
            Ideally, I would like to know to .001" accuracy; however, in this case, budget may dictate accuracy moreso than my wishes.
            First thing when I google for master precision level is

            "Value 639-3618 8''/0.0005'' Per 10'' Master Precision Level" for $154

            Your budget and your wishes appear more or less compatible, assuming you'd want to do the "MPL surveying thing".

            You "need" a MPL anyway to align your lathe. Easiest way to see if you've twisted the ways is to level it "short way" at headstock and at the tailstock end. Also you can get the headstock and tailstock aligned dead center vertically by sticking in a test bar and getting it level "long way" on the ways and on the test piece.

            Folks whom ask why you'd want to level a lathe, forget that two things that are both perfectly level, also happen to be perfectly parallel......

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            • #7
              12345678910
              Last edited by Arthur.Marks; 08-23-2010, 04:15 PM.

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              • #8
                12345678910
                Last edited by Arthur.Marks; 08-23-2010, 04:10 PM.

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                • #9
                  12345678910
                  Last edited by Arthur.Marks; 08-23-2010, 04:10 PM.

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                  • #10
                    As shown the test doesn't identify the area that isn't flat. That can be checkd by masking the plate using a very thin fence of just about any material to shadow all but the area of interest. The salt test will inform you just what area is being tested.
                    Free software for calculating bolt circles and similar: Click Here

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                    • #11
                      Originally posted by Arthur.Marks
                      Ideally, I would like to know to .001" accuracy; however, in this case, budget may dictate accuracy moreso than my wishes.
                      Wow that's a challenge. sounding like a broken record here....getting something that large flat is not trivial...and that is a VERY large surface. Even the guy who blanchard ground it or however it was prepared would have to have been very good to achieve this flatness - without careful shimming the piece would been sucked down on the chuck and pulled flat, then ground, only to spring back after being released.

                      Going from a straight edge, which is very narrow to a large surface would be difficult; I'm not seeing how it would work. Scraping is in a way an inspection process and i've not seen a way to scrape (or inspect) a large rectangle for flatness using a narrow straightedge. there may be a way a way to map it if you used a parallel and a master precision level stepped it over, each time shimming to to level then measuring with feeler gauges, but even Starretts best is only good for .0005" over a foot so that won't work.

                      Just to maintain .001 flatness over something that size will require quite a support system...I'm imaging how much it could move just under its own weight. Can you give some context to its use?

                      Short of creative optical ideas like Evan's, the only way I know of to check for flatness is comparing to something flat (unless you want to generate three ). Fortunately I hear big assed old surface plates go cheaply. Even with a plate it would be a big job; I'd guess you'd have to lower the surface plate onto the work separated by three exact height spacers then indicate the distance between the two surfaces.
                      in Toronto Ontario - where are you?

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                      • #12
                        Surveying

                        Originally posted by Arthur.Marks
                        How exactly would you extend the test with, for example, a 12" level over the full 40+ inches?
                        That is that "surveying" bit that probably exceeds my ability to explain in English. If you find an understandable surveying book just use their explanation of "how to be a land surveyor" and disregard my attempt to explain.

                        Basically you'd break the 40+ inch thing into a grid using perhaps a 12 inch level. Then you draw out on the grid intersections how much shims you had to add at which end to make it level. You're kind of trying to make a topographic map.

                        So call the upper corner perfect zero. So one length out, 12 inches or whatever, to level it you had 0.020 shim on the edge and 0.021 shim 12 inches in. So, at the 12 inch mark its .001 low of "perfect"

                        Next measure 12 inches in to 24 inches in. This time it took a 0.025 shim on the 12 inch mark and a 0.020 shim on the 24 inch mark. Superficially it looks like 24 inchs in is 0.005 high. But wait, you've got to add the delta from the 12 inch mark, so its actually only 0.004 high. Err, I think. Remember, headache medicine before its done, beer afterwards.

                        Just do this, in 2d, across the whole thing. Then figure out the average slope across the width and the average slope across the length, and thats how much, on average, out of level the whole plate initially was. So, you proved that lengthwise your plate is 0.001 lower per foot because its resting on some swarf or beer can pull tabs or whatever under one edge. Now go thru and correct all your numbers again.

                        So 12 inches in you figured it was 0.001 low, but the whole plate on your bench is tipped 0.001 up so add 0.001 giving zero I guess over the first 12 inches its dead nuts perfect flat. You figured the 24 inches in point was 0.004 high but add that 0.001 per foot correction and its actually 0.005 high (oops).

                        Eventually you end up with a lovely topo map of peaks and valleys. Now you need a calculator or computer, to tell you the average and std deviation and Students-T test and things to tell if it all matters for your application. This is the fuzzy part that requires the headache medication and/or someone whom understands statistics, or at least can carefully explain it.

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                        • #13
                          How are you going to support the plate while measuring it? If your supporting surface isn't PDF (pretty dern flat) the plate is going to flex and give you erroneous measurements.
                          ----------
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                          • #14
                            How thick is this plate. You are going to have to figure a way to lay it flat so it does not bow or twist under it's own weight.


                            good luck

                            Dave

                            I type slower than SGW

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                            • #15
                              Flatness

                              I have used a Starret 36" straight edge to test something 34" long , just using light and shim stock down to 0.0005" - if the light shows through under the straightedge, see how big a shim you can get under there -it's rude and crude but when you ge to this kind of length, it will at least give you some idea. It takes patience, but is quite feasible.

                              You can reduce your expenditure by using a 36" across the plate, much cheaper than a 48" - and just slide the 36" across the 45" width and again check for light - I'm sure Evan will know for sure, but I read somewhere that the eye can see light coming through a gap of less than 0.0005".

                              You can buy a set of plastic shim stock from 0.0005" to 0.030" for about $25, and this is also priceless in the home shop anyway, you'll wonder how you ever did without them .

                              Although very old school, this works fine for 0.001" error, it gets a bit harder if you try going smaller than that. You will also have to take more seriously how this plate is mounted, as unless it is a few inches thick, it will conform slowly to whatever surface it is sitting on -just think of it as very hard rubber :-)

                              Of course, there is another problem you haven't yet figured -what do you do if there are more errors than you'd like -sometimes it's better not to know :-)

                              Richard in Los Angeles

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