Virtual condition of threaded holes (modified MMC)

[Burunduk]

One of the things I am yet to wrap my head around is threaded holes modified at MMC and their gaging.
Attached is a link to a tip by Tec-Ease tip of the month June 2003
The threaded holes are positioned at MMC and with a projected tolerance zone.

The virtual condition considered at the design of the gage is not the virtual condition of the threads themselves (the pitch diameter of the internal thread of the component being toleranced), but of the fasteners, after they are assembled into the holes (their external threads' major diameter or their unthreaded shank portion). I perfectly understand the logic of this and how it represents the design intent, but what part of the Y14.5 standard suggests that the tolerance can be inspected this way?

I also understand how it sort of makes sense with the projected tolerance zone modifier, but If there was no MMC modifier, just the circled P, then the axis of the pitch diameter(unless otherwise specified), extruded above the part, would need to fit in the projected tolerance zone. It would still be the pitch diameter of the internal thread that is being controlled. The normal situation for features verified for conformance to position at MMC is that the virtual condition of the controlled feature itself is being simulated. But in this case, the 10.3 diameter is not related to the MMC size of the internal thread's pitch diameter. The figure indicates it as "Virtual condition of the mating part". However, the mating feature of the internal pitch diameter being controlled is the pitch diameter of the fastener, not the clearance hole of the part installed above the toleranced component. And the thread go-gages representing the fasteners are not at a fixed distance relative to each other according to how this gage is designed, contrary to how virtual condition gages for patterns of holes are expected to be per the requirements of Y14.5. Is there some kind of convention that justifies this gaging method (other than it makes sense functionally)? Something from Y14.43 perhaps? Does the usual meaning of virtual condition change when dealing with threaded features? This doesn't seem to be mentioned in Y14.5.

And another question - how would the gage look like if there was no projected tolerance modifier, but only MMC in the position feature control frame of the two M10 holes?

 

[drawoh]

Burunduk,

I do not understand threads at MMC, or LMC for that matter. The implication is that there is a bonus tolerance for me somewhere. Threads are fairly accurate. There is not much bonus on stuff I have designed.

Projected tolerances are another matter. I have assembled thick, machined housings onto sloppily tapped holes. If you are clamping sheet metal, projection is not an issue. If your clamped part is thick, you had better understand and manage perpendicularity error in your tapped holes.

--
JHG

 

[Belanger]

Burunduk -- I recall this being discussed several times over the years. Perhaps you've already done a search, but here's one thread that might be helpful:

[URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=381644[/url]



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

 

[Burunduk]

Thank you both.
Belanger, the thread you linked to is one that I somehow missed in my searches. That was an interesting read.
I'm still struggling with finding answers to the questions I asked though. Whenever this subject comes up there are arguments for and against using MMC on threaded holes. The main "against" argument is self-centering and negligible "bonus", and the main "for" argument is the possibility to apply hard gaging, as in the Tec-Ease tip I linked to above. I am not so interested in "for" or "against" arguments at the moment. I am specifically interested in understanding the hard gaging aspect and its relationship with the concepts described in the standard. I will try to explain the issues I see a bit better:

The hard gage as described in the Tec-Ease tip seems to simulate the virtual condition of the screws at the part+screws assembly level, not the virtual condition of the threaded holes as derived from the part detail drawing of the considered part. One couldn't design such a gage without having some data related to the screws available (in the tip even the "virtual condition of the mating part" is considered but I think they could do without that). I find it unusual. I think that if all was "as usual", knowing the MMC (Min. limit) size of the pitch diameter of the threaded hole and the positional tolerance for that size would suffice. And as I mentioned, fixed spacing between the gage elements that engage with the holes would be required, which is not the case in the tip. I understand that this gaging method is based on the fixed fastener formula (with projected tolerance zone) from Appendix B of ASME Y14.5 and perfectly captures the design intent, but it anyway seems fundamentally different from how virtual condition gages are applied to non-threaded features, with non-projected tolerance zones, while it seems that no separate definitions for the virtual condition concept exist within Y14.5 for this application.

Here is a related question that I think addresses the same:
Is there a Surface Interpretation (as opposed to Axis Interpretation) of threaded holes with a projected tolerance zone and an MMC modifier in the position/orientation tolerance, and if yes, does it deal with the surface of the threaded hole itself (the internal feature) or with the surface of the mating fastener (the external feature)? If the latter (as suggested by the tip and makes sense), how is this invoked by the Y14.5 standard?

Another question is the question I asked in my first post: "how would the gage look like if there was no projected tolerance modifier, but only MMC in the position feature control frame of the two M10 holes?"

To summarize the issue, I am trying to figure out how the relevant standards support this gaging method. Y14.5 is not a gaging standard but it defines the virtual condition concept on which the gaging is based. Maybe some support can be found in Y14.43? (unfortunately, I no longer have access to a copy of it).

 

[drawoh]

Burunduk,

Here is my diagram of a screw and its clearance hole.

Both the screw and clearance hole are shifted opposite the maximum distance that allows assembly. I assumed that this was a clearance hole and the major diameter of a screw. Tolerances, whether they be [±] or positional need to be based on this model. If the "screw" and the "hole" are matching threads, I do not see this model doing anything useful. A screw in a functional tapped hole has a bit of clearance. You can force the screw off the centre of the hole by a very small amount such that the two axes do not line up. I do not see a functional difference between surfaces and axes. The threads are approximately conical and will force the screw into the centre. If your design relies on this not happening, your screw will be difficult to tighten down.

I am looking at a 1/4[‑]20UNC- class[ ]2 thread here. Based on pitch diameters from my Machinery's Handbook, I see clearances of between...

.2175-.2164=0.0011 to .2224-.2127=.0097

My standard clearance hole would be 9/32", which provides of clearance of at least 1/32=0.0312". My positional tolerance on the tapped and clearance hole would be [⌀].016". There is a bonus there, but not a particularly big, useful one. I am trying to visualise the geometry of the screw shifting in the tapped hole to fit through the clearance hole. Your screw torques on the production line will be interesting.

The maximum OD of the 2A thread is .2489". There is my bonus tolerance!



--
JHG

 

[Burunduk]

drawoh,
Those using MMC on threaded holes, do it not to provide "bonus" tolerance (which is negligible), but to make use of thread gages legitimate during the inspection. They argue that specifying position RFS on threaded holes means that if physical gaging is the preferred procedure then the thread gages would need to be of adjustable size (like expandable pins with a thread)* which doesn't make much sense. Then CMM or some sort of scanning is arguably the only option for a standard-compliant inspection and not everybody wants that.

* putting this another way (and even without the use of physical gages), with RFS it is theoretically required to simulate the unrelated actual mating envelope of the pitch diameter of the internal thread. Which is an awful thing to attempt.