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Thread: Drawing Standard Practices

  1. #1
    Associate Engineer
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    Drawing Standard Practices

    Hello, just found the forum, I've been looking for a community like this recently so I'm hoping that I can get some advice I need while also offering assistance when I can based on my own experience.

    A little background about myself, I have been a CAD Designer/Engineer for close to 15 years, working on a multitude of different types of products and tooling from automotive, aerospace, furniture industries. Before getting into design I worked under my father's tutilage from a young age in his fabrication shop, building conveyors and other material handling systems, cutting, welding, painting, we did it all as it was a small shop. I also worked in maintenance and as a machinist for a few years. So I have a pretty diverse background from both sides.

    I recently started a new position as a design reviewer & drawing checker for a large material handling company in their R&D department. They have a long history and outdated standards. They also are very set in their ways and while the managers are fairly young and wish update the standards at some point we aren't there yet. With all that said, I have some issues with how they like to do things because I've never seen it done in the ways they are doing it. Now I'm not saying I'm right, I'm trying to find clarification because they have tasked me with bringing a fresh perspective and trying to steer them towards better manufacturability of their parts and yet I'm meeting incredible resistance, which I expected.

    First issue, they like to do what I call "chaining" their dimensions together when it comes to holes. Being tasked with trying to find and eliminate tolerance stack-ups this strikes a nerve with me because I have never seen it done this way. I've worked in many tool and die shops as well as other types of places and this is the first place I have ran into where they dimension to the first hole, then dimension all the other holes in a pattern, and sometimes even other holes that are different all off of that first hole location. Most of them claim that they are controlling the pattern by doing that, which I contend is not true because you are stacking the tolerance of the dimension to the first hole with the tolerance of the dimension from the first to the second hole. Is this a common practice that I have just not ran into somehow over the years?

    Something else, there is not one control point on the part, they will dimension from 2-3, even sometimes more edges to dimension something, which also does not make sense to me.

    Another issue that came up today again is dimensioning to a centerline, then dimensioning from the centerline to a hole or slot or to locate a part in an assembly. The problem I have with this is, when you are building it you have no centerlines. Today it was a rectangular part, with 2 angled slots, the centerline was dimensioned then the slot end centerpoints were dimensioned from the centerline. If I were going to make that part I would want to know the location of the slots from one of the edges. Yet the response I get is "I've been dimensioning to centerlines for 40 years". To top it off its a charted drawing as it has multiple lengths and the slot locations change. So the reason I was given was he dimensioned it according to the "intent of the design". Which when I had him show me what he meant it was what the sketch in the model was drawn like, so he dimensioned it the way the sketch was made. While I understand it had to be made that way from a design perspective, to show it correctly on the drawing and make it "manufacturable" I need dimensions I can use. Because the length varies his centerline dimension was "L/2", then dim B goes back from the centerline to the slot location. While I realize that yes, you can make it from the information given, having to do the math to figure out what L/2 is introduces the possibility of human error in manufacturing which I am not comfortable with. There is no reason it cannot be dimensioned from the edge of the part, then that dimension tabled instead. Thus giving me a dimension I can build from and check to.

    Anyway, sorry for the long post, but I could really use a gut check if anyone would like to chime in and see if I'm off my rocker or on the right track because this is getting a bit irritating.

    Thanks,
    Last edited by Macguyver; 03-06-2012 at 04:04 PM.

  2. #2
    Technical Fellow jboggs's Avatar
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    Welcome to the forum Mr. Macguyver. After skimming your post, and looking back over more experience than I care to mention, all I can really say is... I feel your pain. You have hit one of my hot buttons, but its late and I will forego starting yet another opus at this time. Suffice it to say that you have two choices: you can "fight the good fight" and risk ending up bloody and disliked by all the old farts, or you can simply do your best to try to learn what their system really is (not what they say it is) and just try to catch glaring drawing errors, within that system. Standards are a wonderful thing, but I think too many young folks place way too much importance on them. If its ugly but works, it works. You will get much better results in the long term after the folks there see you as part of their team, rather than the new guy that's there to "fix" them. Then when some problems do start to come up repeatedly, you can get them involved in finding root causes, like maybe dimensioning to fictional features, or from multiple datums. Let them suggest the solutions. If they can feel like part of the solution, rather than the source of the problem, they will be supporters of the changes. Hang in there, and pick your battles very carefully. One day you will be the old fart.

    One other bit of advice that has served me very well over the years: a marketing consultant I once knew told me that the purpose of good communication isn't to make sure you can be understood. It is to make sure you will not be misunderstood. The same applies to drawings. Just because someone can make a part that works from the drawing, doesn't mean the drawing is right. The drawing is right only when the part cannot be made incorrectly.

  3. #3
    Administrator Kelly Bramble's Avatar
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    I've been training dimensioning and tolerancing to ISO, ANSI and ASME standards for many years and have visited hundreds of different organizations. I have encountered a lot of creative drafting practices – some of the creative dimensioning and tolerancing practices are intuitive and others I need an explanation.

    When I encounter organizations that produce engineering drawings with dimensions and tolerances that DO NOT reflect end-item fit, form and functionality, I politely assume that manufacturing is doing an awesome job and deserve credit as such.

    To fair, not all parts and assemblies need precision and specifically applied dimensions and tolerances. For example, fabrics, thin sheet metal, plastics and polymers that are non-rigid and the manufacturing processes employed to create parts tend to more capable than the end-item geometric requirements. The parts will flex and stretch and world goes around.

    However, when one has rigid components with very specific fit, form and function requirements to do the job they need to do – well, this is where an improperly defined part causes problems post manufacturing.

    Another interesting point I want to make is that when an organization builds all components internal – they have their own manufacturing facility and folks, the manufacturing folks tend to understand what the products needs to be – they have learned via. experience. Ultimately, in these environments very good parts get built regardless of the effective quality of the engineering drawing dimensions and tolerances. In other words – manufacturing is doing the real design (yup, I said it).

    I have had many customers over the years where they closed down their internal manufacturing facility and began outsourcing. In those organizations that had weak engineering drawings the contracted manufacturing folks had a hard time creating parts that met the fit, form and functionality requirements. It is an interesting moment when engineering, design and drafting folks claim that they have incompetent manufacturing folks for the challenges they are having. Anyway, they end up hiring a guy like me to train the staff on how to properly dimension and tolerance a drawing so that a job-shop can build a useable part. The difference here is that the internal experienced manufacturing folks knew what was required and did it, but the job-shop folks only had a drawing to work with.

    Ultimately, organizations should have adequate documentation on their products – design through manufacturing as eventually the knowledge people are going to die, retire or just leave the organization.

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