I know... Concentricity! HELP ME

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  • I know... Concentricity! HELP ME

    OK, I know this is the 1000th post on this... but I think ours is a little different? Customer calls concentricity within .002" on opposite ends of a shaft 19" long. Each end has a 1" Diameter turned .354" back one is referenced as the Datum. How is this expected to be measured and can it really even be? I understand alignment will play a large roll in this but would think that any error in the cylinder projected across that length would make it impossible to check repeatedly?

    Could this even be checked as a positional dimension over that distance accurately with the wrist rotations involved? Currently setup on a star probe with a 2mm tip 50 mm long. Machine was just calibrated 2 days ago and I've recalibrated tips several time and depending how I do the alignment I can be anywhere from .004" to .025" out on concentricity. Tried Coaxiallity as well it reads the same.

    What would be the method if not on a cmm? the datum is such a small area to try to "chuck" onto and inspect using an indicator plus the part weighs about 10'lbs.

    I hate these callouts.

  • #2
    As you have already worked out the short length is not making a stable Datum to use for the calculation.

    You have a number of options all that will require agreement with the customer.

    First they accept the that the call out is unstable when trying to report it but are ok with that (very unlikely).

    If the main diameter of the shaft is turned at the same time as the Datum diameter request Datum is moved
    to this diameter and amend the tolerances as required.

    The Datum diameter must have a shoulder machined out to the main diameter and I guess that diameter is
    approx 40mm. This could be used as the Datum Level and the diameter for the centre point.

    Comment


    • #3
      Seems like you understand the issue perfectly.

      I agree with UKCMM - you need to communicate the issue to the customer (which is often the tricky part) and go from there.
      Automettech - Automated Metrology Technology

      Comment


      • YGOLORTEM
        YGOLORTEM commented
        Editing a comment
        When approaching the customer, what I've found works the best is to ask " We'd like to ensure that our and your data correlates, we'd like to ask in which method you are inspecting this feature or part?"

        Usually when you throw data correlation at them they are more apt to share info. Also, any good customer that wants to receive good parts will gladly work with you.

    • #4
      Originally posted by UKCMM View Post
      As you have already worked out the short length is not making a stable Datum to use for the calculation.

      You have a number of options all that will require agreement with the customer.

      First they accept the that the call out is unstable when trying to report it but are ok with that (very unlikely).

      If the main diameter of the shaft is turned at the same time as the Datum diameter request Datum is moved
      to this diameter and amend the tolerances as required.

      The Datum diameter must have a shoulder machined out to the main diameter and I guess that diameter is
      approx 40mm. This could be used as the Datum Level and the diameter for the centre point.
      Knowing this will be almost impossible to check accurately on the CMM what would be the best way to even give us a reasonable result on it? I'm measuring circles at each end then constructing a cylinder to create my "y" axis / X origin for the alignment. If you had to do it this way what would you do?

      As a note there is a shoulder turned @ 1.550" another 1" back at each end and then is turned down to 1.100" through the remainder of thepart. Current process turns the ends and shoulder (each side it's own operation) and then chucked on the datum while the other side is in a live center so we don't have the ability to tell our customer "it should be OK" since not all features or even the end features are turned in the same operation. We do however pre-turn the bar so that it is straight and indicate the part in at each stage to ensure it's running true.

      I think they use concentricity since their mating part is brackets at each end that have bores on them that will mount a roller of some sort, so they kinda just copied and pasted that to the shaft.

      Comment


      • #5
        2 matched V blocks at each end but leaving the datum ends exposed. two sets of circles at each end, 4 total. create a cylinder from all 4. level and origin to the cylinder. check everything else in between.

        im going to assume that the part is turned on a lathe and that each end is being held down whether it be the chuck or the tailstock.
        you may have to check the part while it is on the lathe with an indicator to give you a warm and fuzzy that each end is in line and that the lathe is turning true.

        the primary question will be how much does the part move when it's removed from the lathe.
        Che Guevara is a communist scumbag.

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        • #6
          OK so after meeting with our customer they are using a plane on the shoulder/face of the part for the primary datum in the alignment then a cylinder from one end of the part to the other. results are much closer to what I'd expect them to be but do vary each time I inspect the part around .001" just from loading and unloading.

          Comment


          • UKCMM
            UKCMM commented
            Editing a comment
            Good to know your customer listened to your concerns and acted on them.

          • BIGWIG7
            BIGWIG7 commented
            Editing a comment
            sounds pretty close to what I suggested. glad it worked out.

        • #7
          Help!!!!

          Comment


          • UKCMM
            UKCMM commented
            Editing a comment
            With what?

        • #8
          OK we've been back and forth on inspecting this for the last couple days, and run into too many odd issues when inspecting on the CMM namely If I flip the part over and inspect from the other side using it as the Datum the Concentricity is out 5X as much...

          Why can we not get a repeatable result? Even measuring as true position the numbers are the same and when the part is flipped it is out 5x as much this is crazy......

          Comment


          • #9
            PCDMIS does not calculate concentricity per ASME Y14.5. It reports it according to ISO standards. That is, it reports it exactly as True Position.

            B&S CHAMELEON/PCDMIS CAD++ V2011

            There are no bugs, only "UNDOCUMENTED ENHANCEMENTS!"

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            • #10
              Originally posted by Rocket88 View Post
              OK we've been back and forth on inspecting this for the last couple days, and run into too many odd issues when inspecting on the CMM namely If I flip the part over and inspect from the other side using it as the Datum the Concentricity is out 5X as much...

              Why can we not get a repeatable result? Even measuring as true position the numbers are the same and when the part is flipped it is out 5x as much this is crazy......
              Your two faces that you are using to level from are probably not parallel. I mocked it up in PC-Dmis and with only a 0.03° difference between the two faces I was able to get the concentricity to read good at one end but around 5x as bad when reversed. Take a look at the attached image for a visual indication of what could be going on. Levelled to A and origined to B the concentricity of D is good. Levelled to C and origined to B, the concnetricity of B is bad.
              CONCENTRICITY.png

              Comment


              • #11
                As Neil stated the error will change depending on what end is set as datum have attached my own sketch showing this
                Attached Files

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                • #12
                  Neil

                  Parallelism is within .0003" measuring across approximately a 1" area so that projected across the 19" would be .0057" off "positionally" if they were theoretically centered at each end. What's odd is that the parallelism doesn't change when flipping the part but the position changes. Am I missing something here... I should state I'm a machinist thinking how machinist do... I program and operate our CMM as well but by no means an expert.

                  Comment


                  • #13
                    As a side note the only way I can cheat the result here is creating a cylinder across the part, then constructing a plane from that cylinder and using that to level to and use as my primary datum in the alignment. I have an odd feeling this is what their inspection does since their QA measures concentricity about the same as parallelism. They are using concentricity on the shaft to help control a concentricity callout on the brackets mounted to this after assembly which is about 10" away from the shaft. With that being the case I'm sure parallel is much more critical then concentricity on this component yet they tolerance it opposite. Parallel is called out .001" but concentricity .002". The length of the part make it to where parallel would need to be within .0001" to have a chance to hold that.

                    They also allow up to .002" clearance from the diameter of the shaft to the diameter of the bore on the bracket which should allow for MMC to be used. Part is growing increasingly frustrating since their QA is not being very specific and will not share reports or allow us to view the part being inspected.

                    Comment


                    • BIGWIG7
                      BIGWIG7 commented
                      Editing a comment
                      you can level to the cylinder. you don't need to level to a plane.

                  • #14
                    did you get this to work for you?
                    Che Guevara is a communist scumbag.

                    Comment


                    • #15
                      I received results on a part we sent to them for inspection, They measured concentricity within .0003" .... Only way I can measure that is leveling to a constructed cylinder across those two bosses. Their CMM inspector insists they are using the shoulder perpendicular to those bosses to level their alignment... our results matched theirs within .0001" using the cylinder as the level so were just going to go with it :/

                      Our customers inspectors always seem very difficult to communicate with to the point it feels as though they are hiding something, I can't spend all my time emailing them at some point we've got to make parts so that's what we're doing using our method of inspection for the time being.

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