For context: I work designing one-off science exhibits. Generally we design around pretty sloppy tolerances when we can, so things can be cheaper to make, with less precise tools, and at more facilities. We sometimes make everything in house.

Question: Occasionally, we'll call out a GD&T feature for a vendor, on a welded frame for instance. Parallelism, lets say. I generally don't expect the vendor to actually check this parallelism directly, but use it as a way to indicate design intent: what's important, and what's not. Should the one-off part come back significantly off, we will escalate to actually measuring the feature. We can then use this feature spec deviation to talk the vendor about a replacement.

Is this an appropriate use of GD+T? Is this CYA (Cover Your Ass) type usage relatively common? How often are GD+T features called out but not measured by the fabricator, or our QA until a problem arises?

Certainly our usage case is not the most common one, and of course for aerospace and etc, the answers will (potentially) be different.

Just wanting some outside voices of what the correct approach is.

  • $\begingroup$ Doesn't sound that crazy to me $\endgroup$ Dec 29 '20 at 7:45

To my experience yes, this Cover Your Ass usage is a relatively common, especially when you are outsourcing the machining and assembly.

Actually, I've had an bad experience, at a company where I worked for. There was a particular order for solar trackers, that the CAD designer did not put in a welding specification, and somehow it was not spotted during approval. The order and dwgs were sent to a low cost machine shop. A few months into (a heavy admittedly) winter, the machines from that particular order started to fail due to snow. Even though, it was poor workmanship, since no standard specifications were on the designs of the order, the company could not claim replacement. Even though I was not involved in the fallout, that experience has made me realise that the time spend to properly put all specifications can save a lot of time.

And to be honest once you pick that good habit, its not as time consuming as most people think.

  • $\begingroup$ Especially considering that any gain you may get is lost when problems occour. $\endgroup$
    – joojaa
    Dec 29 '20 at 11:04
  • $\begingroup$ You mean gain from not including the specs? $\endgroup$
    – NMech
    Dec 29 '20 at 11:27
  • 1
    $\begingroup$ Yes any possible time saved for not including G&T is paid multiple times in any event that you have a problem with the manufacturer $\endgroup$
    – joojaa
    Dec 29 '20 at 12:21

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