Position callout on a radius

There is no definition that covers this case. Explain what you are suggesting to the engineering team and require written acceptance or have them send their solution and use it.

The 120° arc internal radius is not a feature of size, however if they insist on specifying position the MMC modifier helps somewhat to try to do something that makes sense with it anyway. I would go for the surface interpretation. Consider an R50 fixed "size" and fixed in place (relative to the DRF) VC boundary. The actual radius value can grow from that to R60, and it cannot violate the VC.

Edit: The problem is, you would still need some other limit from the other side (within the material).

Do you manage to derive an axis repeatably from a 120° arc?

Thanks all. I was able to repeat the axis method. We ran a gage r&r on all the features and this one did fine. It a ct scan so you can capture a lot of surface area even though it is a small feature. We also can get these callouts quite repeatable on the CMM, when we can reach the feature, by being meticulous with the datums and probing the feature itself. The parts are also made to tight tolerances, so inherently it helps, but it's all relative so our repeatability must match the tight tolerance. Once it is below 90 deg all bets are off from what I see so far.

All our parts are from our customers so the conformance of callouts is really out of my hands, and I'd say 75% of parts have one or more of these locations without opposing sides, ie not a feature of size.

I don't fully understand the axis vs surface method. My understanding is the standard says the form or orientation deviation(within the limits of size) can be so bad that the surface method takes precedence....but what does that mean? Does that mean that you MUST use hard gage to make sure the feature passes? You can't really hard gage many internal features so then would I just make it a surface profile measurement? Or perhaps cut the parts on an EDM and hard gage the parts?

Secondly, does anyone have some type of example where the form or orientation of the part can be so bad, yet w/in the limits of size, that you would need to or it would be better to use the surface method vs axis? I've never really absorbed this concept past just knowing it exists and understanging hard gaging the surface is always the gold standard due to it being a real life fit check.

"hard gaging the surface is always the gold standard due to it being a real life fit check" - That's true.
Coming from design and not inspection, I never quite understood why the surface method is considered more difficult without a hard gage, compared to the axis method. Is the math required to virtualize the VC boundaries and calculate surface method actual values that difficult for modern metrology software?

External customers - submit the inspection process to them for approval.

Axis vs. surface is trivial. Axis isn't a real thing. It is derived from the surface by fitting another mathematical idealization to it. What it "means" is if the approximation is suitable, use the approximation but be aware that the approximation isn't the actual surface.

If a hole is allowed more location tolerance than the size of the feature then its virtual condition is negative, so no fixed gauge can be made and either it requires a complex gauge or comparing the location of the derived axis to the requirement.

Dave,

Thanks I'm gonna explain my thoughts to engineering and have them proceed.

B,

I don't think its harder I was just taking the standard at its clinical hard core definition, I would think if for some,reason you couldn't obtain a repeatable axis you can just make it profile in one direction away from the VC with a tolerance the size that that feature can grow radially.

I will say these small arcs can shift, grow, shrink drastically when using a max circumscribed or min inscribed math computation on the feature. I'm pretty much always using least squares which is much more stable, but make a hole slightly bigger or a boss slightly smaller as it is an average and not the highs.

sendithard,
The problem with less than 180° arcs and position tolerances is that less than 180° arcs have no defined axes, so whatever axis the software calculates is not per definition.

Now, measurements do not always have to strictly follow the Y14.5 definitions (and measurement errors that result from such lack of strict correlation may be reasonable risks that are part of any measurement). However, the product definition you are dealing with is nevertheless flawed because it doesn't describe geometrical limits unambiguously. Since you are GDTP-Senior, do not give up on pointing that out to whoever is concerned.

Burunduk,

I agree with you on these ideas. I've been theoretically beaten up a bit, due to the amount of these I encounter and have to deal with. Some are customer driven, and others are self imposed tolerances for our own production verification. We have some good, and somewhat proud engineers that won't agree with the legalese of the standard so I can only bark so loud on these issues. You all would get a kick out of some of these position callouts. Some are theoretical point widths with bonus positon where another part will and cannot touch said theoretical point.

What to you limits a 120 deg arc from having an axis? Is it the idea there is no 'envelope' ie...there are not opposing sides to stop the expansion or contraction around a feature....there is no referee per se. Is it this real life envelope that causes you the most concern or the idea that in the literature of the standard it reads as if you must have an envelope and hence opposing points at least?

The term feature of size and opposing points is like beating a dead horse around here a little bit.

I assume your go to for the 120 arc would be just go to surface profile, no? I've substituted some callouts for profile...just did one today. GOM doesn't allow for runout on a sphere, so I just used profile as the size is controlled tighter in a separate callout so profile is doing the job of the runout with out worrying the profile is overbearingly checking size.

I just took a re-read of Dave's comment on the axis..."Axis vs. surface is trivial. Axis isn't a real thing. It is derived from the surface by fitting another mathematical idealization to it."

That really hit home finally...axis is the dream world we are making up with real parts that have to mate together with real surface contacts...I'm gonna use that example that the axis isn't a 'real thing' in a meeting and see if I get fired...kidding.

I have a better grasp of the standard invoking the surface method takes precedence after this discussion...it does, b/c it is the only real thing that exists.

sendithard said:

What to you limits a 120 deg arc from having an axis? Is it the idea there is no 'envelope' ie...there are not opposing sides to stop the expansion or contraction around a feature....there is no referee per se.

Click to expand...

Yes, it is that. If there was some other supported method for deriving the axis like a best fit cylinder, there would be "no problem" (just with axis measurement purposes, that is), but ASME is based on the functional approach of mating parts, high points they touch, and envelopes that correspond to that. For an internal radius the unrelated mating envelope can expand on and on without getting limited properly or at all by the actual surface. Now, with a contracting envelope about an external partial arc - it's different, but we want to be consistent, hence the "180° rule" which as you know is not spelled out in the standard, but it's concluded from the definitions of FOS and AME (without a feasible AME, you can't treat/use an FOS as one).

sendithard said:

I assume your go to for the 120 arc would be just go to surface profile, no?

Click to expand...

Yes.

sendithard said:

GOM doesn't allow for runout on a sphere, so I just used profile as the size is controlled tighter in a separate callout so profile is doing the job of the runout with out worrying the profile is overbearingly checking size.

Click to expand...

It could also be handled by the dynamic profile modifier of Y14.5-2018, eliminating the effect of size from the profile control. however I think there is currently no released GOM Inspect version that supports that, just yet. Am I correct?

Burunduk,

Correct, we run 2021, but I have 2022 installed to make sure a lot of my python coding runs properly if/when we update.

2023 will be released in the next month or two and I heard the dynamic modifier will be in there, not sure though.

sendithard said:

2023 will be released in the next month or two and I heard the dynamic modifier will be in there, not sure though.

Click to expand...

Yeah I heard a while back about the planned update that is supposed to include it, too. Nice to know it's that soon though, thanks for the update.