Design Intent 1

[ptruitt]

Consider simultaneous requirements Vs SEP REQT in the case of a part that a machine shop could make in it's sleep. (An ultra-process-capable part.) If simultaneous requirements are presented, maybe just one checking fixture or one CMM setup would result. But what if the SEPT REQT callout caught the eye of a savvy estimator and it result in a more costly quote because they would be thinking that the inspection would be more involved. The other thought I have is: What if the SEP REQT (instilling design intent in the document) allowed part-to-part variations visible to the naked eye? Perhaps there would be endless questions with everyone from the fabrication department to the shipping department worried about the variations. Conveying design intent is a powerful benefit of GD&T, but I am wondering if there are situations where I may run into some unintentional outcomes. I'm just worrying too much. Right?

Peter Truitt

 

[pmarc]

Peter,
Let me reference to some of your statements:

But what if the SEPT REQT callout caught the eye of a savvy estimator and it result in a more costly quote because they would be thinking that the inspection would be more involved

I actually face with such way of thinking quite often. I would compare it to a situation when one thinks that every GD&T symbol on a drawing adds extra costs to a part's inspection. I hope we both agree that it is not true, but a lot of guys think otherwise. SEPT REQT means that there is no geometrical relationship between toleranced features so logically the requirement is less severe than in case of simultaneuous requirements. Unfortunately in many cases designers set their priorities to satisfy inspector by having one gage or one CMM setup and not to grasp the most important factor - part's function.

What if the SEP REQT (instilling design intent in the document) allowed part-to-part variations visible to the naked eye? Perhaps there would be endless questions with everyone from the fabrication department to the shipping department worried about the variations. Conveying design intent is a powerful benefit of GD&T, but I am wondering if there are situations where I may run into some unintentional outcomes

Assuming that designer knows what he is doing, his intent allows such variation, so I see no reason why somebody should be worried about it. Endless questions appear every time when other people have no trust to a designer or do not understand part's functional requirements. SEPT REQT is not so common practice (as it maybe should be), but IMO, if correctly used and interpreted, can be very practical and helpful.

 

[Belanger]

I'm brining back an older thread here, but it's because I recently encountered a similar situation. Peter, I think you can look at it either way, but I'm essentially going to agree with pmcarc.

Some folks might see SEP REQ as more expensive because it involves separate gage fixtures. But I would say that it's less expensive, because it allows more variation (if design intent allows this, of course). Here's what I would suggest if the gage folks give you a hard time: I would tell them to use a single gage...one treating the patterns as if they were a simultaneous requirement. Only if they DON'T pass this single gage, then the separate gages could be pulled off the shelf. This secondary check (of the SEP REQ) will then filter out the good vs. bad parts.

So it depends on the viewpoint. Sure, it sounds more expensive to have those gages on the shelf. But the alternative is that we might be rejecting some parts that are actually functional. I think tossing out good parts is the more expensive way to think!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems