【转帖】proper gdt for cylindrical part with mounting pins

yang686526

proper gd&t for cylindrical part with mounting pins
i'm having to make changes to a cylindrical part used for calibration and have issues with the current gd&t methods used.  for one, datum a was palced on a centerline, which i disapprove of.  for another, the current schema doesn't address critical features for manufacturing of the gauge and its ultimate use.
the rod is 211mm long, 10 mm in diameter.  each end is machined back about 14mm in length, 5mm in diameter, and then grooved at 4mm from each end to provide a place for hanging a calibrated weight.  there are two 3mm dia pins (25mm long) mounted 40mm apart (basic) and centered on the rod.  these pins mount the rod to a dynomometer and imo should be datums (they are not.
i'm rusty on my gd&t and so my head is spinning at the prospect of fixing this.  i know what not to do (the way it's now done) but can't come up with a simple, comprehensive solution.  my thinking is that the only datums i need are the two mounting pins, and that all features should be wrt them, ie, using a-b in the datum space of the toelrance block.
the critical features are that if one pin is fixed (mounted) then the other is positioned in relation to it.  they should be parallel and "perpendicular" to the body of the rod (one thing i'm not sure how to describe using gd&t).  the grooves are critical in that as they deviate from the prescribed location, the calibration deviates as well (we are looking for torque).  straightness of the rod doesn't seem that important.
any ideas?
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what issue of y14.5 was applied on the original drawing?
that might serve as apart of the solution.
but, you are right about centerlines never being labelled as datum features.
i fought a similar 'battle' once but the drawing had been around so long that no would acknolege it as being labelled incorrectly.
you might want to consider making the od of the rod a datum feature and one of the pins and another datum feature and relating wrt those  features.
thanks for the reply ring.
there's no statement on the print for y14.5m, but i assume 1994 based on the types of datum callouts used.
i'd also considered using the rod body as a datum, but i kept running into circular references and it gave me a headache trying to resolve them.  my problem here is i'm not used to dealing with cylinders intersecting other cylinders, nor can i find any examples to reference.
in addition, i tend to go by the advice of my last instructor, ie, to begin applying datums that fixture the part and stop when you have enough to lock it down.  in this case, the two mounting pins do the trick and i can't see what making the rod body a datum would add.  i'm willing to be taught, though!
  first, as ringman says, get rid of that centerline datum.
you could then make datum a one machined end and datum b the opposite machined end.  this would allow you to orient one pin to the common axis (a-b).  label this pin as datum c.  from there, locate the other pin and the grooves relative to a-b and c.
  i'm not sure if this is exactly what you want, but it is the first approach that came to mind.
thanks for the response ewh.
the centerline datum was the first to go.  however, you're right in that a cylindrical axis will allow me to fix the first pin so i made the main body diameter (rather than the machined ends-- i'll get to why) datum a.  i then positioned the left pin wrt datum a and made it datum b.  the pin to the right of the centerline is part of a pattern so i used pattern positioning to ensure that pin1 and pin2 move together.  finally, since the pins together fix the calibration tool to the dynomometer, i toleranced the machined ends and grooves wrt to b-c.  this helps me avoid the circular reference (involving the datum a on the body) that made me nuts earlier.  since i am more concerned about where and in what condition the machined ends wind up, i want it to be with respect to the datums that affix the part.  i may also apply straightness to the overall body although that would only be to make it easier for manufacturing to keep the machined ends in spec.
i think this gives us what we want, but i'm still open to contrary input.  ; )
i feel that it is important to determine and specify the issue of y14.5 applicable, if any.  the issue definitely needs to be specified to avoid assumptions.
the datums are selected because they are the features that are the most important ones for the part to function properly. ones the mate or align are good. ones that both mate and align are the best. there is a logical chain of datums and directly toleranced features from the first to the last part. from this structure it should be implied exactly what the function of the mechanism is meant to be. if the gd&t is applied correctly then it's possible to derive the total stack tolerance of the mechanism. try doing that with the plus or minus system! since that is the understood official intent everyone in the process from fabrication, inspection and so forth is on the same page because they are fully gd&t literate. if the application is well done there is only one way to comprehend it and it will avoid the pitfalls of the old way.
if you want to see examples of cylindrical gd&t, try drawings for screws/bolts.  they routinely have references for cylindricity, concentricity, perpendicularity, and others.   
regards,
cory
more good points and i appreciate all the input.  i believe i am in sync with type26owner but have encountered resistance to that bit of common sense here-- one designer is of the opinion that the cmm should drive the datum applications!  i firmly believe the *usage* is paramount, most particularly the two pins since they fix the part.
the previous designer's contention was that he needed datum a on a centerline to keep the grooves symmetrical.  it took me some time to finally demonstrate to him that the grooves should be symmetrical about the two positioning pins, and that the centerline was essentially immaterial.  i say this because the machined ends could be at one or the other extreme end of tolerance and throw the centerline off, and my part doesn't care-- the grooves are the critical feature.  the light bulb finally went on for him yesterday.  ; )
as to ringman's point, i agree, but we lack true pdm at the moment.  that includes control of standards as well as robust checking.  i was pdm administrator in a former life and would love to be thus involved with my present employer, but they already have a guy supposedly doing it... the less said about that the better.  ; )
the asme y14.5m-1994 document is hard to extract the meaning from since it's written in legalese. however, the first few pages of chapter 4 explaining datum selection criteria are fairly comprehensible. the whole essence of the gd&t system is won or lost here by good or bad datum selection. have that person show you in the y14.5 were it states the cmm process matters diddily squat. btdt.
simple question:  does the drawing invoke y14.5?