Drawings with no datums

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  • Drawings with no datums

    Hello all,

    I was hoping to get a bit of advice and opinions regarding the use of drawings with no datums during cmm programming. I've been with my employer for three years and I'm the only pcdmis programmer. All our products are designed by our engineers and produced within our facility. The majority of machinists and engineers have only worked here and have no experience elsewhere. The problem is, the lack of any use of datums or GD&T. To be honest, many of the drawings are flawed, and this fact comes to light quite often during pcdmis programming. There are no datums structures, only simple dimensions, and a lot of the parts can be quite complex. This causes very cluttered and unorganized prints with centerlines and origins that are simply not explained to what features are used to establish these 'datums.'

    The practices here can be considered behind the times and the use of GD&T is often disregarded simply because the engineers and operators here do not understand it enough to use it as it is intended.This is not an effort to say that this practice is wrong, as this is a very successful company and has been for more than the last century. I just know that processes could be vastly improved.

    My biggest dilemma is during cmm inspection I simply must personally choose features to use as datums in hopes that I'm making the correct choice as the part was designed, since the drawing does not specify anything. This can be all well and good, but when parts are rejected, it's usually blamed on my programs using the wrong features to establish dimensions. It is in my opinion that with a properly constructed engineered drawing, there should be no uncertainties or ambiguities. I come from a background of cmm programming parts for the military, and this was never an issue. Has anyone else ever dealt with this before?

  • #2
    My company doesn't know GD&T very well either although thankfully for my sake they always define the datums, its the rest of the drawing that they have difficulty with here.

    It doesn't help you with installing best practices but I would just cover my own backside when you are programming a new part by dropping a quick email to everyone concerned outlining the alignment and datums you have used. At least then if they complain you can say that I informed you what I was doing, in the nicest way possible of course.

    Comment


    • DJLewis
      DJLewis commented
      Editing a comment
      BTD, Thank you, that is great advice. Problem for me, is I am on third shift until later this year, with no support from engineering. So, I usually get impatient and go with it. But great advice though!

  • #3
    I can imagine it can be quite frustrating for you. Only advice I can give you is to either set up off the features that have the leased tolerance or investigate the part to see which features are correct before programming. Another option is to find out what the part does and how it fits to its mating part and decide for yourself which features are critical.

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    • DJLewis
      DJLewis commented
      Editing a comment
      DS, thanks for the advice. Luckily, all of our parts are assembled here as well, so It's possible to investigate. Thanks for your reply!

  • #4
    I would focus on more of how the parts are manufactured. Stop edges and surfaces. How does it work being the only programmer and working on 3rd ? Not very well from the sound of it....

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    • DJLewis
      DJLewis commented
      Editing a comment
      Schlag, Thanks for your input! that's the method I've been using, but many of the machines and fixtures are being upgraded/replaced, so it's difficult to keep up, especially on 3rd shift, since support and communication is non-existent. No, it's not going well at all. Not only am I the only programmer, but I'm also the only process inspector on my shift for up to twenty operators. With a phone tied to my hip that rings every thirty seconds for inspections throughout the entire facility, it's next to impossible to have the focus and attention to be able to write programs successfully. It's more stressful that I ever imaged it could be, and 3rd shift is absolute H*LL on the body and mind.

  • #5
    My guess is since you are assembling everything at your facility, what is important is the relationship of the features to themselves. Not necessarily locations from any set Datum edge(s). I would align the part as you see fit, and then after everything is measured, do a "BEST FIT" of the critical features. This would probably correlate with how the part was machined.
    Marty Dobson
    Director of Quality
    http://www.linkedin.com/in/mjdobson

    PC-DMIS CAD V4.2 MR1 & V2011
    PH10MQ
    TP200

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    • #6
      Are a bunch of linear measurements taken from one surface or feature or maybe two features in different directions? If they are treat those as your alignment features and datums as they need to be rock solid for all of the measurements referenced to them.

      Mike
      You can never tell exactly how deep a mud puddle is without getting your feet wet -- Dennis the Menice

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      • #7
        Funny to see we are all concerned by the same problems
        Same thing here, our engineers sometimes use some GD&T, but i see they don't really understand it...
        I think there is really a lack of training concerning the engineers, i have a 3d design diploma, and i don't remember having practiced much the GD&T, or just in a simple manner. Even if i'm not, i could be today a mechanical engineer and create drawings....
        But i think that with the development of CMM, which are more and more used nowdays, this should be better in the future.
        Now, maybe i'm a bit oldschool, but in my opinion GD&T is a bit complex, and not necessary, or only in some cases/jobs. As you well said, the company can totally produce functional products with "simple" drawings.

        I know, i'm not helping you but concerning me, i'm doing like others said: analyse how the parts are mounted / machined, but i admit we sometimes had problems to be credible when problems occurs....

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        • Schlag
          Schlag commented
          Editing a comment
          I have seen the whole range on GD&T. Done correctly its a godsend and can give you a huge tolerance bonus. Done incorrectly on the print or when the operators don't have much background in it? Well... that's why most people hate it and think its complicated. We have a "group" here to hash through customer prints when needed. I have had my ASME certification for over a year now. Nice little diploma with dust on it cus the engineers are never wrong !

      • #8
        Back in the day I was doing some supplier audits. The set up person at the supplier came up to me and asked me about a part he was setting up for us. He stated that if he uses the outside of the part to establish the center lines all of the dimensions are out of spec. If he uses the inside edges of the part all the dimensions are good. What should I use to establish the centerline ? I looked at the drawing and sure enough no datum structure. I looked at him and said.... Well I would use the one that gives you the correct dimensions. He asked what if the part does not work ? My response.........That will teach the FN engineer to start using GD&T and datum points........ Just saying .

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