Apply GD&T on Threaded Feature?

[clkwad]

Can I use GD&T position tolerance on an internal or external v-thread with MMC?

[Kelly_Bramble]

Yes, you can for ASME Y14.5-2009 and previous USA standards. By default the indicated MMC tolerance applies to the axis derived from the pitch cylinder. One can specify the position tolerance and associated material modifiers applied to the major or minor diameter. MMC specifications will require manufacturing/quality to actually do more than a simple go, no-go thread gage inspection, thus complicating the inspection effort and for that matter the design.

In general, an MMC tolerance applied to the derived pitch diameter is rarely necessary or practical. For example, take a 250-20 UNC - 2B threaded feature. The pitch diameter according to ANSI/ASME B1.1 is specified minimum dia. 0 .2175 and maximum dia. 0.2248. This gives a total variability of .0073 or bonus tolerance. Since the v-thread will center the mating screw this variability is practically meaningless as the feature that needs to be clear is the Major diameter of the mating screw and the mating hole feature.

Hopefully, the designer has specified a tolerance structure for the threaded feature and mating hole feature that is easy to manufacture and functionally robust.

In general, I recommend v-threads specified at RFS, however I do recognize that there are potential designs that may benefit from a MMC position, orientation or datum reference tolerance at MMC or LMC for that matter.

[Belanger]

Kelly, how about the whole idea where MMC is the "ticket" to functional gaging?
I'm not advocating either way on this; I agree that there is negligible bonus tolerance in terms of real numbers. But as you know, GD&T experts over the years have dug into two camps on this one, and I myself have waffled back and forth. But since a pitch diameter does have some tolerance, could the notion of a single-size thread gage have any merit?

[Kelly_Bramble]

Tough question!

Kelly, how about the whole idea where MMC is the "ticket" to functional gaging?

Well, utilizing the MMC modifier in many applications and ********** is the “ticket” to functional gaging, however not ALL applications or ********** can benefit from functional gaging.

I think the MMC concept has been over-sold as well as under-sold in selected applications and manufacturing environments. It is not always practical or cost effective to design and manufacture a functional gage nor does increasing available tolerance with MMC always a good idea.

In short one does not throw an MMC modifier on everything just because of the potential for functional gaging or the bonus tolerance.

Tolerance design should be matched to application, target manufacturing process capabilities and end-item quality and costing requirements.

But since a pitch diameter does have some tolerance, could the notion of a single-size thread gage have any merit

Sure, so how does one do this correctly? What does the fixed boundary functional check gage look like?

I think the question that should be posed first is why? Since v-threads are self-centering how does the MMC tolerance structure benefit the end-item?

Implicit methods to verify a thread location or orientation are:

• A fixed size hole gage that is oriented and located relative to the reference datum structure sized so that it verifies the MMC major diameter outer-boundary for that external thread.
• A fixed size gage pin that is oriented and located relative to the reference datum structure sized so that it verifies that MMC minor diameter inner-boundary for an internal v-thread.

One would still need to verify the threads with a go & no-go thread gage. This method is technically wrong but if the thread features are manufactured in the same setup chances are the major, pitch and minor diameters are manufactured almost perfectly co-axial and to the same size variability.

In other words .. “good enough”.

This measurement method could become correct if the tolerance MMC is specified to apply to “Major Diameter” or “Minor Diameter” appropriately.

What do you think about all of this?

[Belanger]

I agree, maybe with minor qualifications. One thing about using a fixed gage for a pitch diameter... although threads will self-center, it could be argued that a threaded hole that departs from MMC will require more turns of the bolt (or torque or whatever) before it engages properly. So the departure from MMC (bonus, if you will) is there, although it's not felt as looseness in the sense of a clearance pin that jiggles around.

And of course, the question revolves around pitch diameter. If we were talking about GD&T applied to a major or minor diameter then the MMC creates no problem at all.

I wholeheartedly agree that we should not throw an MMC modifier on everything just because of the potential for functional gaging. But I hesitate to make blanket statements, so I am not of the strict camp that says you can always throw an M in there for the thread, but I also wouldn't say that we should never use an M on a thread. (The standard shows M on a threaded hole in Fig. 7-21).