True Position - Drawing Issue?

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  • True Position - Drawing Issue?

    I have attached a stripped down drawing of a situation that I am asked to program. The part is symmetrical and they are asking for Position of each bore. Datum -A- is the shank, datum -B- is one of the faces (perpendicularity control). My concern is as follows:

    One bore is referenced to -A-, -B- and that makes sense. The other bore is referenced to -A-, -B-, and the first bore. This seems fine as to include the C/L distance. Here's the problem - If you get the right set of numbers, the part will be in tolerance one way but upon rotation of 180 degrees, the part will fail. In other words, the C/L deviation is only considered in one Position callout, so if the bore already deviates from the shank by a lot (but not OOT worthy), it may just fail if that bore is the one with the added C/L deviation.

    Management tells me: If you get a bad result, flip the part and it may be good.

    This seems absurd because the GD&T should allow it to be either good or bad. Not bood.

    B141786.jpg
    Attached Files

  • #2
    So, the bigger problem is the print is only classifying and controlling one face of the part as datum B (according to what you've shown), and if part is symmetrical, the print completely avoids communicating parallelism of the planes of the opposing side (which could be representing B when you flip).

    If you rotate the part 180° about this plan view, B datum is a new surface, without any control of parallelism or location between last measured B datum and the face that is now adjacent to your new D.
    If you flip the part over, the B plane is now a different surface and could possibly be projecting a plane origin that D datum bore is now perpendicular to...
    So, how do the 4 potential B datum faces get controlled to each other?
    --If you can make a passing part, with the other tolerances in spec, it's good.
    --If print doesn't control other side/face parallelism, I agree with your boss, try all 4 faces as B, to make sure it passes, before writing up! lol.
    Last edited by louisd; 05-08-2019, 01:51 PM.

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    • louisd
      louisd commented
      Editing a comment
      (following plan-view layout on CMM, with C & D cylinder axii projecting in Z axis)
      For C datum dimension: Level (X+) to A, Rotate (Z+) to B (about X), Origin XY to A, Origin Z to B.
      For D datum dimension: Level (X+) to A, Rotate (Z+) to B (about X), Origin X to C, Origin Y to A, Origin Z to B.
      Last edited by louisd; 05-09-2019, 11:22 AM. Reason: clarify with added parenthesis

    • JacobCheverie
      JacobCheverie commented
      Editing a comment
      I get how to align it, although I don't see how you can possibly origin XY to -A- as it is a cylinder.

      I just think a part should be good or bad, nothing in between. Is that not always the case in engineering? Is it because it is Wednesday today? Maybe tomorrow will be different

      PS Parallelism of other faces to -B- is controlled on print.

    • Kp61dude!
      Kp61dude! commented
      Editing a comment
      Leveling A will make the center line run up and down with Z therefore you could origin XY on -A- But Z could NOT be origin on -B- but rather on -C-, -B- would be used for rotation. That's one way to skin the cat.

  • #3
    It is important to note that we do not specify which bore is -C- and which is -D-. Upon rotation, they change places.

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    • #4
      Is design control internal or external?

      If internal, see if you can speak up, see if they can define B as a midplane of one side, like you suggest...

      If external, maybe have the machinist mark the defined B datum (setup plane) with a marker or something to help?

      If you can get parallelism to pass, and find the one B plane that makes the hole TP's pass the part IS good.
      You can make a routine to assess all 4 possible B datum locations...

      Comment


      • #5
        It is external, but internally approved. They do not want to go back and ask for clarification and are refusing to make any changes. -B- really has nothing to do with my question anyway. I'm more concerned with how both bores will be offset from -A- a certain amount. It is orientation dependent which one get's the effects from C/L deviation.

        And why would you say that if it is good in one orientation (out of 4), then it is a good part? The point shouldn't be to make it good in the right circumstance, it should be to make it good in all circumstances.

        Comment


        • Mike Ruff
          Mike Ruff commented
          Editing a comment
          The B-datum isn't defined very well for all of the reasons you listed. I agree with louisd though, since they do not specify which surface is datum B, as long as it is good to one of the four, it technically meets their print when that particular surface is used as datum B. If they want it conforming regardless of which one of the four surfaces is used as datum B, then the print needs to reflect that

        • JacobCheverie
          JacobCheverie commented
          Editing a comment
          Mike, what if I measure it one of four ways by choosing "the right" -B-, send the part to the customer, they choose a different -B-, and question our work/inspection methods? Do you see why I seek consistency? This issue came about before anybody even questioned -B-, which just adds another issue that needs to be addressed if we want reliability.

        • Kp61dude!
          Kp61dude! commented
          Editing a comment
          -B- needs to be shown and not explained. It could clearly be well defined as one entire side (2 surfaces like we're looking at it at the moment) and the other side (the other 2 sides again like we think) is controlled by perpendicularity.

      • #6
        Originally posted by JacobCheverie View Post
        It is external, but internally approved. They do not want to go back and ask for clarification and are refusing to make any changes. -B- really has nothing to do with my question anyway. I'm more concerned with how both bores will be offset from -A- a certain amount. It is orientation dependent which one get's the effects from C/L deviation.

        And why would you say that if it is good in one orientation (out of 4), then it is a good part? The point shouldn't be to make it good in the right circumstance, it should be to make it good in all circumstances.
        Well if one of the faces -B- that is chosen is not perpendicular to the bore in question (or from bore to bore) then no it should not work in all circumstances. But since you say don't worry about -B- for this issue since the high deviation is coming from -A- only then yes there's a problem. I wouldn't totally ignore those faces though because one of them could be introducing that little extra bit of deviation to cause it to be out yet still meet print on perpendicularity.

        Post some results if you could.
        Last edited by Kp61dude!; 05-08-2019, 02:32 PM.
        PcDmis 2015.1 SP10 CAD++
        Global 7-10-7 DC800S

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        • #7
          Here is an example, ignoring -B- from the DRF. Because yes, that is an issue on it's own regarding the definition of -B-. Inclusion of -B- will not correct THIS fundamental issue, only make it harder to find explicit cases.

          0B141786.jpg
          Attached Files

          Comment


          • JacobCheverie
            JacobCheverie commented
            Editing a comment
            The part is 100% symmetrical. Obviously CYL -C- will be off from -A- a set amount whether you call it -C- or -D- or -Q-, whether you spin it around, toss it in the air, etc. Relative to one another (as the FCF asks for) there is a set distance in a set direction that they are out. Whether you couple that with the C/L deviation is entirely arbitrary and depends on which unidentifiable cylinder you call -C- and which you call -D-. You and I can share a program. I can measure it good, ship to you, and you can measure it bad (unless you flip it). If I bought a toaster that didn't work, I wouldn't think to flip it.

          • Kp61dude!
            Kp61dude! commented
            Editing a comment
            I agree with you JacobCheverie I just don't have eyes on your print and program so I need you to tell me (or us) what exactly you are doing alignment wise. It's something I've learned to do no matter how good I or you or anyone thinks they are. The answer is probably sitting right in front of you if not I think you found a huge PCD bug that I am not aware of.

          • louisd
            louisd commented
            Editing a comment
            cool picture. solid explanation, BUT the print defines A as level axis and B as the rotation datum. IT IS IMPERATIVE to keep B in the conversation. taking B out is like comparing dragsters without thinking about the engines.

        • #8
          Walk us though your alignment if you could post the code. I have the feeling that there is nothing wrong with the datum definition BUT that's just me and really should have the entire print in my hand before making such claims... I'm a humble dude please don't kill me.

          Posting your code could say far more than one could attempt to articulate in a Forum while working.
          PcDmis 2015.1 SP10 CAD++
          Global 7-10-7 DC800S

          Comment


          • Kp61dude!
            Kp61dude! commented
            Editing a comment
            RULE #1 follow your datum scheme.
            RULE #2 and most importantly see rule #1

            or else have a good explanation on why you though better than the engineer unless you're the engineer (in this case please fix your datum scheme).
            or something in writing from whomever will potentially save your neck when **** hits the fan.
            or just gamble 50/50 win/lose.

            Level XPLUS/XMINUS to -A- (depending on your vector direction)
            Rotate ZPLUS to -B-
            Origin [email protected], [email protected], [email protected]

            Follow the order.

            I don't see anything wrong with the way the datums are defined (again nothing physically in front of me so could be wrong).
            Last edited by Kp61dude!; 05-08-2019, 03:37 PM.

          • JacobCheverie
            JacobCheverie commented
            Editing a comment
            I think that I am being misunderstood. I have gathered no data, I have not probed this part because it hasn't even been made yet. There is no result that is throwing me off. If you imagine this situation, theoretically, it is possible to have a part that is either good, or bad.

            Both bores will be located off of the axis of -A- by some amount, in the one direction that they can be, regardless of software alignment. The part is a real thing, and nothing is perfect. Now if you rotate the part any which way, they will still be off from -A-, by the same respective amount. Who disagrees with that? Rotating the part in physical space will not change the relative position of any of it's features. Now, if you measure the 2D distance from bore center to bore center, you will have a number that will deviate from nominal. Again, it doesn't matter how you flip it, those two bores will always be the same distance apart. Now, you have three numbers. Two deviations from -A-, one for each bore, and one 2D distance from bore to bore.

            If I ask you to calculate the true position, manually, as it is defined on the print, you will need to know which one to call -C- and which one to call -D-. The reason for this is that one calculation will take into account the C/L distance, the other will not. If the numbers just happen to be right, as in the image that I have posted, your determination of which is -C- and which is -D- is the determining factor of OK or NG.

            Does that make things more clear to anyone? This is purely theoretical and does not seem very logical or well-defined.

          • Kp61dude!
            Kp61dude! commented
            Editing a comment
            So your argument is from a design approach? If so wrong forum hahaha as much as we'd like to think we're smarter than the engineer... they're engineers (some listen to the CMM guy though...some). In my humble opinion it's 50/50 and in my experience some are smarter than others on both sides of the spectrum but we could argue this for a looooong time (or 'they' probably can). I'm no engineer btw but an Engineer Aide V (lol <--titles!)

            If I was a 'real' engineer I would've tied -D- bore with just -B- and -C- disregarding -A- so yes you could just flip the part 180 and be good... what's wrong with that?

            EDIT: nope I still don't get the 180° flip.
            EDIT EDIT: you're flipping about the bores centerline ah I get it!
            EDIT EDIT EDIT: nevermind on -D- to just -B- and -C- what if you wanted equally amount of meat around datum -D-? you must toss in -A-!

            The parts are not symmetrical according to GD&T! I've seen this before and it's way hard to explain to the machinists. Just machine the 2 bores really 'good' and problem solved, parts are geometrically and dimensionally (GD&T) symmetrical hahaha jk!!!
            Last edited by Kp61dude!; 05-08-2019, 04:39 PM.

        • #9
          I know I came late to the game but 1 question. CENTERLINE deviation was referred to more than 1x. Where would the centerline come into play ? , Which datum are you referring to ?

          Comment


          • JacobCheverie
            JacobCheverie commented
            Editing a comment
            Datum -D- is called out to -C- and there is a basic tying them together.

            More center to center than centerline.
            Last edited by JacobCheverie; 05-08-2019, 03:49 PM.

        • #10
          Sorry, I didn't know you hadn't made any parts yet, that changes things a bit. Maybe you could label the part with which features you used as the datums with something non-permanent like masking tape with writing so they can measure it the same way you did?

          Comment


          • #11
            Gents, as I mentioned before, "You can make a routine to assess all 4 possible B datum locations." it's not like its reinventing the wheel. Then if your customer gripes, share the routine, explain that parts have twist or are off center a bit, and explain that the print allows for it.
            B datum passes parallelism to identical faces.
            TP passes when you define -B-
            --as no, not that face,
            --no, not that one,
            --no not that third one...
            THIS FACE, RIGHT HERE. lol. (then facepalm).

            Comment


            • #12
              From my POV, if there is nothing on the part that can help identifying face [A] then the part shall be within tolerance regardless of 0° or 180°.

              Also, datum A seems to be a face, so why (M) for it in the FCF's?
              PC-DMIS CAD++ 2o19 R1 SP10

              Comment


              • louisd
                louisd commented
                Editing a comment
                Jacob, although we can communicate drawing shortfalls, the drawing is ultimately the voice of the customer. As flawed as it may be, give the customer exactly what they want. If you can prove product acceptance per the drawing, then the part is good. Your job is to inspect to accept, not the other way around.

              • JacobCheverie
                JacobCheverie commented
                Editing a comment
                louisd, that makes sense. That is how a lot of management sees it as well, as it was drawn by the customer. I am not trying to find fault in a part that we are making. I am trying to eliminate the possibility, for two reasons.

                This issue can be wiped away and resolved if nobody dimensions parts like this again, as it leads to confusion and can introduce sneakiness. The people that make these prints might need to think harder about what they are doing, but we all get smarter from that. Win.

                Inspection, with this current setup, may now have to spend 4x as long to inspect this part. We consider inspection "non value added", so why do we want this? Bulletproof program would lead to no flipping/rotating/trying again/throw away bad results/ethics issues. Win.

              • louisd
                louisd commented
                Editing a comment
                Fix the inspection method.
                Write a routine to measure the part once, including measuring all 4 potential B planes.
                Then, at end of routine, produce an alignment for each of the 4 possible orientations of the part.
                We can help you make code to either assess with a while loop, or even unlooped.
                That way, you can assess and output the dims that produce the best/lowest dimensional outcome of all dims evaluated.

                I'm picturing a fixture that holds the part with C and D cylinders projecting in X or Y axis, so B datums are easily accessible.

            • #13
              My take would be:

              If there are NO visual clues differentiating the two ends it is a drawing error to have a dependancy on only one (unidentifiable) end. Period.

              Your theoretical results show that one and the same part can be either good or bad when verified following the drawing information, ergo: the drawing is wrong.

              (Shall we call such drawing/dimensioning "Schroedinger prints"?)
              AndersI
              SW support - Hexagon Metrology Nordic AB

              Comment


              • JacobCheverie
                JacobCheverie commented
                Editing a comment
                Nice reference. This is how I was seeing the situation; admittedly it is a drawing/design issue, not PC-DMIS. I was just looking for other programmers to think about it / use GD&T to explain why we can/cannot dimension parts this way.

                Thanks @Andersl

            • #14
              Upon getting acceptable results, is it possible to part mark it, so that the customer knows what your primary datum is? As it's supposedly symmetrical, it should measure the same each way, but that's not always the case in the real world.

              Comment


              • JacobCheverie
                JacobCheverie commented
                Editing a comment
                It should measure the same way if it was designed that way or if the part was perfect, but it isn't. The arrangement of the GD&T allows for this error. We could mark it, but management is worried that we'll start marking parts and the customer will doubt all of the parts we've sent them. They do not want to go back to the customer with this. ASME Y14.5 indicates the datum features need to be readily discernible. In this case, they are not.

            • #15
              I think there's a lot more to this: material, method of manufacturing ??
              Talk to your manufacturing engineers, this may be a non-issue, or it could be a nightmare as your suspecting.

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