Correct profile surface measurement

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  • Correct profile surface measurement

    Hi all
    It so happened that I had to rewrite my predecessor's program. But the measurement results of the part profile differ between our programs. I am taking datum B as a cylinder and my result is slightly out of drawing tolerance - 0.165. In my predecessor's program, datum B is a circle, and its profile dimensions are 0.006. Our engineers were happy with the old results, but now they are asking me questions. I have not come across this and I have no arguments.
    582.jpg

  • #2
    You are using the correct approach - datum B should be a cylinder. If you think of it in terms of an alignment, you could not use a circle for datum B because a circle is 2D and has no direction vector of it's own that you could accurately level to. The only time it is even remotely acceptable to level to a circle (or in this case to use it as a primary datum) would be on very thin, sheet metal parts, where measuring a cylinder is not possible. In that case you would have to take at least 3 sample hits on the top surface around the hole first and PC-Dmis would use the surface vector calculated from those sample hits as the direction vector for the circle. The reason for the difference in results is that the circle (even with sample hits) is only considering one specific section through the datum at the depth the circle is measured at. It is also not considering any orientation error of the hole - it will either inherit the current workplane orientation, or if it has sample hits, it will inherit the orientation of the surface the sample hits were taken on. The cylinder will properly define the full form (taper, circularity & straightness of the axis), orientation and size since you are considering the whole feature rather than a single section through it.

    So, in summary, wherever possible measure datum holes as a cylinders using enough points and levels to properly capture their size, orientation and form error. Only measure datum holes as circles when the material is too thin to allow a cylinder and always include at least 3 sample hits.
    Neil Challinor
    PC-DMIS Product Owner

    T: +44 870 446 2667 (Hexagon UK office)
    E: [email protected]

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    • Vladimir
      Vladimir commented
      Editing a comment
      Thanks ! This is quite understandable, it remains to explain this to others and remain in good health.

  • #3
    Vladimir

    This is all really great information from Neil.

    If you don't mind, I would like to jump into the GD&T.

    If one of my process engineers presented a blueprint like this to me, I would politely send it back and would require a secondary
    datum to be placed in the feature control frame. Surface profile is a form AND location tolerance. Therefore, you would need datums
    defined that control all three location axes. As it stands right now, you have X and Y are defined but we are missing Z.

    If the feature control frame said "surf prof" 0.010 |B|A|, then you would measured datum B as a cylinder, construct a generic line that
    has an X or Y vector, then level through -B-, rotate to the generic line, set -B- as your X and Y origin and your Z origin as -A-.
    Last edited by DAN_M; 07-17-2020, 08:43 AM.

    Comment


    • Schlag
      Schlag commented
      Editing a comment
      I see profile as form only all the time. There isnt a rule that says it has to be form and location but it is more common. I think of this part as an example of the thickness could be a linear tol with a Flatness and or PERP callout. That would really give you the same thing ?

    • Calvin.Korver
      Calvin.Korver commented
      Editing a comment
      I have to write a program next week that calls out a surface profile to itself. I don't even know that was an option I thought it had to have some kind of datum reference haha

    • DAN_M
      DAN_M commented
      Editing a comment
      I have seen & measured profiles with no datum callouts which just comparing the shape of the profile back to the CAD model but a full form and location needs the two datums and i assumed he'd that based off of the set of circumstanced he described

  • #4
    DAN_M
    I'm starting to understand what you are saying and it's good
    The original of my FCF for this plane looks like this:
    filedata/fetch?filedataid=19014https://www.pcdmisforum.com/image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw==‚Äč
    Last edited by Vladimir; 07-19-2020, 06:10 PM.

    Comment


    • VinniUSMC
      VinniUSMC commented
      Editing a comment
      Your picture didn't attach.

  • #5
    Why isn't this just perpendicularity? Was this the chapter the design engineer was on in their GD&T class?

    This is just perpendicularity, and it's going to suck as the ratio of the considered feature's length to datum length (based on the sketch) is way too high. Perpendicularity requires a datum with an axis, so it must be measured as a cylinder.

    Comment


    • VinniUSMC
      VinniUSMC commented
      Editing a comment
      That's my thought as well, but the OP came back and mentioned something about "the original FCF", so I am withholding until the OP comes back with the actual FCF, if it is different.

  • #6
    Strangely the previous post for me is displayed with an attached image. I hope it will be seen here for everyone.
    433254.jpg

    Comment


    • VinniUSMC
      VinniUSMC commented
      Editing a comment
      Ok, and what are datums A and D?

      Given the refinements, I would wager that the intent of the 0.15|B is definitely perpendicularity. Assuming D is the bottom face, or the lower top face, then the location of the surface is controlled to D, and assuming A is the surface that isn't D, then the runout controls the other relationship. And the final frame controls the perp.

      It seems like it might be over-toleranced, but I don't do enough work with turned/round/washers to opine any further.

  • #7
    VinniUSMC
    refinement by datums
    IMG_20200721_190145.jpg

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    • VinniUSMC
      VinniUSMC commented
      Editing a comment
      I can't speak very well to the overall validity of this particular set of feature controls, but my guess above seems to be correct. First, control the surface deviation from D. Then, the rotational deviation of the surface from A|B. And finally perp to B. Still, I'm not sure if it's over-constrained. How do A and D interact? A larger tolerance on that thickness? Is there any control from A to D, or D to A?

      I think that runout also controls orientation to B (simultaneous to location from A). It's been too long since I've done much with a rotationally controlled part like this.

    • Vladimir
      Vladimir commented
      Editing a comment
      Yes, there is a TP tolerance for this thickness from D to A 0.25, as well as parallelism to A|B and flatness control

  • #8
    Profile can imitate any other geometric control depending on how it is setup. I'd say it's legal but not the best choice. Purpose of any drawing is to convey design intent. The simpler the drawing is while still conveying intent the better it is. Perpendicularity is a more commonly used and understood control and should have been used.

    Comment


    • #9
      Gentlemen's, since the topic of perpendicularity was so touched upon, then it is also controlled in this drawing. This perp from hole B to plane A.
      3242.png
      Last edited by Vladimir; 07-21-2020, 05:31 PM.

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      • VinniUSMC
        VinniUSMC commented
        Editing a comment
        Yeah, that's not relevant to the original question though. The hole perp to A then the opposite surface perp to the hole. So, really, it could have been parallelism of the surface to A.

        I'm thinking more and more than this part is over-toleranced.

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