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Axis of a cylinder as Datum A - Datum B is, or is not, parallel to the Datum A plane

wish2bflying
I thought I'd revive this discussion. I've searched the forums and read the other threads on using the axis of a cylinder as Datum A. I don't think they answered the question I have here.

With reference to the print below, if I use the "Level Z+ to the cylinder" guideline, then in relation to the print, Z+ is pointing to the right (presuming vector directions). Then, it seems that the Datum B plane is PARALLEL to the [piece of paper] Datum A "plane", and therefore can't constrain rotation of the Z+ plane.

Now, the engineer insists that I'm wrong because the datum callout is to the AXIS of the cylinder, which is indeed perpendicular to Datum B.

My question is, how do I achieve this in PC-DMIS? If I construct a line in the Z+ workplane from A to C, and then use that to constrain the rotation, am I not effectively replacing Datum B with Datum C? Then Datum B simply becomes the place to put the Z origin.

How would you gurus approach this?

Another issue - the Datum C hole asks for A(M) - if Datum A is merely the axis line of the cylinder, can we even say it's a feature "of size"?

Also, would you probe Datum C as a cylinder, or as a circle with sample hits?




  • Datum "A" (shaft) controls 4 DOF, 2 rotational and 2 axial
    Datum "B" (surface?) controls 1 axial DOF
    Datum "C" (hole) controls 1 rotational DOF
    6 DOF, all done.

    I see no other way those datums can control all 6 DOF.

    CAN/MAY/MUST rule applies.

    "A" can control 2 rotational and 2 axial DOF, and it MAY since they are not previously controlled, so it MUST control those 4 DOF
    "B" can control 2 rotational and 1 axial DOF, but it MAY NOT control the rotational since they are already controlled, it MAY control 1 axial DOF, so it MUST control 1 axial DOF
    "C" can control 2 rotational and 2 axisl DOF, but it MAY NOT control 1 of those rotational and 2 of those axial since they are already controlled, it may control 1 rotational DOF so it MUST control 1 rotational DOF.

    This is for a callout of A-B-C. It all shuffles around if the callout isn't A-B-C

    Of course, this is all based on proper use of GD&T.
  • Okay, so it seems I was wrong, which isn't the first time. So does either or both of these look acceptable?



    Attached Files
  • both look wrong, you can't rotate the same axis as you already leveled to.

    Level to the cylinder, rotate to a line from the "A" to "C", I would do it by making pierce points of BOTH in the "B" plane.

    Level to cyl "A"
    Rotate to constructed line from A/B point and C/B point
    Set 2 axis to "A" and one to "B"
  • Your print gives fully defined instructions.



    New alignment that RECALLS STARTUP....
    Level to -A-
    Rotate -A- to -C- in the vector direction the features make ABOUT whatever you leveled to
    -A- is X&Y origin. -B- is Z origin.



    Regarding -C-...you can measure it as either a circle or cylinder. Looks like you'd be good with a circle in this case & sure why not? Use sample hits, it can't hurt...
    I use circles if I have a lot of tolerances and don't need to use that geometry for any fancy GD&T like concentricity or runout.

    Be careful! A2 is rotating twice. A2 is also an alignment that is fully defined with 6 DOF yet is recalling an alignment that isn't STARTUP (unnecessary). Finally, A2 's XYZ origins are out of order. When I train people, I always instruct them to LEVEL, ROTATE, XYZ IN THAT ORDER EVERY TIME so you're alignments are always "good"/you don't forget anything.
  • in reply to DAN_M
    "B" is a plane perp to "A", how can it origin 2 axis?
  • in reply to DAN_M
    Sorry I can't type today, is fixed now
  • in reply to Matthew D. Hoedeman
    Oh duhh. I was really tired and distracted last week. I know not to do that, so I'm not sure what I was thinking.

    Having said that, if we're essentially ignoring Datum B and constraining the rotation using a line from A to C, aren't we proving that Datum B is invalid?
  • in reply to Matthew D. Hoedeman
    nope, it just ends up being simply a single axis origin feature.

    Dan says the same thing as I said, just different words.


  • In response to #4.1.

    -B- isn't ignored. And it is valid! It just geometrically can't be used to constrain rotation because it is along the same vector as the feature you're leveling through.

    -A- has a Z vector, so you need to rotate to something that has an X or Y vector. Rotating from cir-cir gives you the Y vector.

    Level Z, rotate to X or Y about the Z, XYZ origin. Bingbangboom.

    CAN/MAY/MUST

    -A- CAN and MAY/therefore MUST be leveled to the Z.
    -B- has a Z vector so it CAN NOT be rotated about the Z for this alignment.

    I realize I sound repetitive but I am trying to break this down for your benefit. Hopefully its helping!
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