True Position , Concentricity Dim

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  • True Position , Concentricity Dim

    hi, Im wondering can anyone help with some advice . Im a begineer using v4.1.
    Im checking a part with 2 milled cylinders 1"dia x .5" long at each end they are 5" apart .The drawing calls for TP of .005" from one cyl to the other. Should i measure each cylinder as circles at each end? How does pcdmis work out TP ? Does it project one cyl to the other across the 5"distance? Does pcdmis do the same with Concentricity ?

  • #2
    You may want to upload a drawing or at least explain to us what the datums are so we may be of more assistance to you.
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    Xcel 15-20-10 - PFXcel 7-6-5 - Merlin 11-11-7 - Romer Absolute 7525SI
    PCDMIS 2012
    Windows Office XP

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    • #3
      Well, if it is cylinder-to-cylinder, then the datums will make no difference. You pick the cylinder you want to dimension FIRST, then the one that will be the 'datum' for the concentricity. It will tell you (I think) how far the center line of the second cylinder is away from the mid-point of the measured axis of the 'datum' cylinder.
      sigpic
      Originally posted by AndersI
      I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

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      • #4
        Can you post your EXACT print call? Like what are your datums????? A sketch would be great.

        What I would do: measure both features by using 3 to 4 autocircles at different heights (this tells you a lot about local circularity of the cylinder). I always use 8 points per circle, but the experts here disagree with that. Anything from 8 and up is probably not bad. If you have a small cylinder, stay with 8, if it gets bigger, use more points on the circle.

        Then construct a cylinder for each feature. Make sure you declare your datums properly (DATDEF under INSERT/DIMENSION/DATUM DEFINITION). Then use the XactMEasure GD&T (INSERT/DIMENSION/TRUE POSITION) to define the Feature Control Frame exactly as it appears on your print.

        PC-DMIS will do all the math for you. I believe it to be generally right.


        Jan.
        ***************************
        PC-DMIS/NC 2010MR3; 15 December 2010; running on 18 machine tools.
        Romer Infinite; PC-DMIS 2010 MR3; 15 December 2010.

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        • #5
          hi there, will try to get an image sorted. i think matt in on the rite track tho. yes it 2 cylinders on the running on the same axis (supposedly) 5.000"apart .Im getting results TP of .017" tol is .005 .I dont believe the results im getting I got the results as matt described .my problem is , what way is pcdmis working it out. Is it projecting one cyl across the 5"" distance to the other and therefore magnifing the error

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          • #6
            Originally posted by joes64 View Post
            hi there, will try to get an image sorted. i think matt in on the rite track tho. yes it 2 cylinders on the running on the same axis (supposedly) 5.000"apart .Im getting results TP of .017" tol is .005 .I dont believe the results im getting I got the results as matt described .my problem is , what way is pcdmis working it out. Is it projecting one cyl across the 5"" distance to the other and therefore magnifing the error
            First off, I'm not a 4.1 user so things may be different from 3.7MR3. Second, are you measuring both bores as cylinders. Are you measuring them as long as you can (distance between rows of hits)? Are you aligning to one of them? I think Matt is correct that it is irrelevent which one you use as the datum if no other datum structure is called out. You would most likely get the same results either way. I would pick the longer one to use as the datum because it would give you better rotation control in your alignment.

            If you are aligning to one of the cylinders it is "projecting" the cylinder across the distance and "magnifing the error" but if one of the cylinders is meant to be the datum then that is the correct way to do it.

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            • #7
              By definition, concentricity is 'measured along a projected' axial centerline. As you move further away from the datum feature any/all axial deviations are increased and increase the concentricity error. You should be able to see this effect on a small scale if you measure a circle near the end of the 'measured' cylinder (away from the datum cylinder) and a second circle on the same 'measured' cylinder (closer to the datum cylinder). Calculate the difference between the measured concentricy values and the length difference between the two circle locations (along the 'measured' cylinder length). At this point you can establish a simple proportionality to estimate the observed concentricity difference (between the two circles) if the concentricity were occuring over a 5" length. The equation would be something like this:

              (Concentricity difference between the 2 circles) divided by (locational difference between the 2 circles along the cylinder axis) are equal to (the estimated concentricity difference) divided by (the distance between the sylinders specified on the blueprint). I know this verbal description is wordy but I do not know a better way to try to explain it.

              If you have questions/concerns, let me know. My user information is available in the profile section.

              My Two Cents (Free and Worth Every Penny);

              Doug Wells

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              • #8
                Originally posted by Doug Wells View Post
                By definition, concentricity is 'measured along a projected' axial centerline. As you move further away from the datum feature any/all axial deviations are increased and increase the concentricity error. You should be able to see this effect on a small scale if you measure a circle near the end of the 'measured' cylinder (away from the datum cylinder) and a second circle on the same 'measured' cylinder (closer to the datum cylinder). Calculate the difference between the measured concentricy values and the length difference between the two circle locations (along the 'measured' cylinder length). At this point you can establish a simple proportionality to estimate the observed concentricity difference (between the two circles) if the concentricity were occuring over a 5" length. The equation would be something like this:

                (Concentricity difference between the 2 circles) divided by (locational difference between the 2 circles along the cylinder axis) are equal to (the estimated concentricity difference) divided by (the distance between the cylinders specified on the blueprint). I know this verbal description is wordy but I do not know a better way to try to explain it.

                If you have questions/concerns, let me know. My user information is available in the profile section.

                My Two Cents (Free and Worth Every Penny);

                Doug Wells

                Good explaination!

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                • #9
                  Thi smay be helpful too:

                  http://tech.groups.yahoo.com/group/Y...p/message/2102

                  Jan.
                  ***************************
                  PC-DMIS/NC 2010MR3; 15 December 2010; running on 18 machine tools.
                  Romer Infinite; PC-DMIS 2010 MR3; 15 December 2010.

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