Interpretation of projected position tolerances at MMC

[pylfrm]

Imagine a drawing that defines a part with two holes, each having a length of 8 mm, specified as follows:

hole 1:

diameter 6 +/- 0.1 THRU
[box]position[/box][box]diameter 0.4 (M) (P) 32[/box][box]A[/box][box]B[/box][box]C[/box]​

hole 2:

M6 X 1 - 6H THRU (pitch diameter 5.350 - 5.500)
[box]position[/box][box]diameter 0.4 (M) (P) 32[/box][box]A[/box][box]B[/box][box]C[/box]​

Drawing interpretation is per ASME Y14.5-2009. Units are millimeters. The datum reference frame is fully constrained with respect to the part. The true position of each hole is fully defined with respect to the datum reference frame. The projection direction for the position tolerances is defined.

What is the meaning of each position tolerance?


pylfrm

 

[3DDave]

This looks like a trick question. The main obvious difference is the pitch diameter tolerance on the screw thread is likely to be smaller than on the hole. I could look up that thread tolerance to be sure, but that would involve standing or using Google search.

 

[CheckerHater]

Projection tolerance zone is used with so-called "fixed fasteners", that's it, bolts, studs and press-fit pins.

Diameter 6 +/- 0.1 THRU looks like clearance hole and therefore will not benefit from using projected tolerance zone.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

The tolerance zone in not throughout the length of the considered feature PLUS the projection. it is simply the length of the projection. That is, it is NOT an "Extended" tolerance zone, it is a "projected" one.
copy-paste from one of the training materials

In my opinion, there are three different characteristis:
- size of the hole or size of the thread (PD)
- location of the feature (hole -thread)-specified in the tol zone diameter
- how far out the projected tol zone will go (distance-also shown with a chain line)

Diameter 6 +/- 0.1 THRU looks like clearance hole and therefore will not benefit from using projected tolerance zone.

I am not sure I understand this. Why THRU means clearance? Why cannot be a press fit? "fixed fastener"

 

[CheckerHater]

Because 0.2 tolerance on size 6.0 is too rough for press fit.
For appropriate press fit see ISO 286 / ANSI B4.2

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[3DDave]

It would not be a press fit with an MMC modifier.

 

[greenimi]

It would not be a press fit with an MMC modifier.

Why not?

The definition (of the projected tolerance zone) itself in the standard has (by default) MMC modifier. So, are you saying that the projected tolerance zone cannot be used for press fit applications?

I am not inclined to buyoff on CH explanation about the "roughness" of the press fit........0.2 tolerance on 6.0 size. Not convinced, but he has more experience than I do with this stuff.......

 

[3DDave]

That's the most convoluted conclusion of the week. I suggested that MMC is not usually used with press fits and you conclude that I meant something about projected zones?

 

[greenimi]

3DDave,
So how would you specify a projected tolerance zone where a plain hole for studs or pins are located on a detail part drawing?
(plain holes for studs or pins used in a press fit applications)

MMC or not MMC?

 

[3DDave]

Tell me what you would do.

 

[greenimi]

3DDave,

Tell me what you would do.

I am not an expert in this language (GD&T)....I barely know a little bit of English, let alone dimensioning and tolerancing....I have some books to guide myslef / stir me in one direction or the other.
I am here to learn from the experts/ more experienced folks willing to share their knowledge.

 

[drawoh]

pylfrm,

I can interpret your specifications by GD&T. If these are mating parts, the design does not make much sense to me. Generally, MMC is not a useful concept for tapped holes. When you tighten the screw, it centres. Does MMC apply to the major diameter, the pitch diameter or the minor diameter? Projected tolerances are a useful concept for tapped holes, since the hole orients the projected screw. I would not bother with the projected tolerance for the mating clearance hole. The clearance hole must clear the screw plus the positional tolerance. Note how applying a projected tolerance makes your positional tolerance more accurate.

In the case of a projected tolerance on a tapped hole, you could specify that the tolerance applies to the pitch diameter. You then make a male threaded fixture at MMC that screws into your hole. It would then wiggle around to test the projected size. Are you willing to do this? The minor diameter is easier to inspect, but much less useful.

--
JHG

 

[3DDave]

MMC on tapped holes is used for the examples of projection in the Y14.5 standard, and the default feature for tapped holes is the pitch diameter, so there is no need to specify it otherwise. In general MMC is fine for tapped holes as it allows the use of fixed size gages. The amount of available shift is usually small as the clearance in threads is usually small and, as noted, the fastener will tend to relocate and reorient itself after assembly during the tightening process. This latter effect results in (usually) elastic deformation, an area that is outside the typical dimensioning and tolerancing consideration.

 

[pylfrm]

Perhaps a bit more explanation is in order. The drawing I described is completely imaginary, dreamed up just for this question. There are no functional requirements and no mating parts, so don't read too much into the specific dimensions and tolerance values I chose. My goal was simply to provide a concrete example for discussion.

I did include the pitch diameter tolerance of the thread in the original post, but I will repeat it here along with the minor diameter tolerance for reference:

Pitch diameter: 5.350 - 5.500
Minor diameter: 4.917 - 5.153


I'm hoping someone will provide an answer describing the meaning of the position tolerances in geometric terms, complete with numbers where appropriate.

Additionally, if anyone thinks either tolerance is invalid, I would certainly be interested to hear about that.


pylfrm

 

[3DDave]

Ha - sorry. Too lazy to read past the thread callout. I was expecting that info to be separate, but there it is.

Basically either one controls the location and orientation of the projected axis of the feature cylinder from within the body of the part, just as shown in the standard, such as Figure 7-21, in the 2009 version. The MMC modifier allows the feature simulator to be moved to be within this cylinder.

 

[pmarc]

pylfrm,

I am intrigued by this question, especially by "Additionally, if anyone thinks either tolerance is invalid, I would certainly be interested to hear about that.", and that is why I will take a risk and try to give you an asnwer .

Hole 1:
When the hole is produced at MMC, the position tolerance zone is a cylinder of dia. 0.4, 32 mm long (at minimum) that fully lies outside of the part in specified direction. The tolerance zone is perfectly oriented and located with respect to the specified datums.
When the hole is produced at LMC, the position tolerance zone diameter increases to 0.6.

Hole 2:
Here I would simply refer to fig. 6-11 in Y14.5-2009, which is pretty similar to your made up scenario. Does your question have anything to do with what feature the MMC modifier applies to?

 

[pylfrm]

Basically either one controls the location and orientation of the projected axis of the feature cylinder from within the body of the part, just as shown in the standard, such as Figure 7-21, in the 2009 version. The MMC modifier allows the feature simulator to be moved to be within this cylinder.

Could you expand on that last sentence a bit? It seems closely related to the aspect of this I'm most interested in.

Hole 1:
When the hole is produced at MMC, the position tolerance zone is a cylinder of dia. 0.4, 32 mm long (at minimum) that fully lies outside of the part in specified direction. The tolerance zone is perfectly oriented and located with respect to the specified datums.
When the hole is produced at LMC, the position tolerance zone diameter increases to 0.6.

What must fall within the tolerance zone cylinder you describe?

Hole 2:
Here I would simply refer to fig. 6-11 in Y14.5-2009, which is pretty similar to your made up scenario. Does your question have anything to do with what feature the MMC modifier applies to?

I'd say the MMC modifier applies to the tolerance, not to any feature on the part. I probably haven't understood your question properly though.

Note: A threaded hole is located and gaged from its thread profile at MMC.

What do you suppose this means?


pylfrm

 

[pmarc]

What must fall within the tolerance zone cylinder you describe?

The extended portion of the axis of the hole.

I'd say the MMC modifier applies to the tolerance, not to any feature on the part. I probably haven't understood your question properly though.

What do you suppose this means?

I admit the note in fig. 6-11 is muddy. The default rule in Y14.5 is that in absence of extra notation, such as MAJOR DIA or MINOR DIA, the position callout applies to thread pitch cylinder. So technically I would think that the amount of available position tolerance should be determined based on the actual size of pitch diameter. Dia. 0.4 for pitch diameter 5.350, dia. 0.55 for pitch diameter 5.500.

 

[pylfrm]

The default rule in Y14.5 is that in absence of extra notation, such as MAJOR DIA or MINOR DIA, the position callout applies to thread pitch cylinder.

More specifically, it applies to "the axis of the thread derived from the pitch cylinder" (para. 2.9, emphasis mine). The axis detail is something that never really sank in for me until now, and I guess it rules out a surface interpretation for MMC and LMC orientation and position tolerances on threads, projected or not.

Para. 2.9 also applies to datum feature references though. If you only have an axis to work with, it seems MMB and LMB would not be meaningful.

I find this all a bit strange, especially considering the usual theory that MMC and MMB allow the use of hard gages with fixed-size elements.

pylfrm

 

[3DDave]

LMB and MMB refer to the simulators, not to the result. The reason for axis interpretation is that there are cases where the MMB, in particular, can degenerate - one could produce a dia .125 nominal hole with a dia .500 location tolerance so the virtual condition is dia-.375, making a fixed location hard gage simulator problematic.

Since there is no projected surface, the axis interpretation is the only meaningful one, but it doesn't rule out hard gages.