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Interpretation of projected position tolerances at MMC

Interpretation of projected position tolerances at MMC

Interpretation of projected position tolerances at MMC

(OP)
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
positiondiameter 0.4 (M) (P) 32ABC

hole 2:
M6 X 1 - 6H THRU (pitch diameter 5.350 - 5.500)
positiondiameter 0.4 (M) (P) 32ABC

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

RE: Interpretation of projected position tolerances at MMC

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.

RE: Interpretation of projected position tolerances at MMC

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

RE: Interpretation of projected position tolerances at MMC

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)

Quote (CheckerHater)

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"

RE: Interpretation of projected position tolerances at MMC

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

RE: Interpretation of projected position tolerances at MMC

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

RE: Interpretation of projected position tolerances at MMC

Quote (3DDave)

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.......

RE: Interpretation of projected position tolerances at MMC

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?

RE: Interpretation of projected position tolerances at MMC

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?

RE: Interpretation of projected position tolerances at MMC

Tell me what you would do.

RE: Interpretation of projected position tolerances at MMC

3DDave,

Quote (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.

RE: Interpretation of projected position tolerances at MMC

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

RE: Interpretation of projected position tolerances at MMC

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.

RE: Interpretation of projected position tolerances at MMC

(OP)
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

RE: Interpretation of projected position tolerances at MMC

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.

RE: Interpretation of projected position tolerances at MMC

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 bigsmile.

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?

RE: Interpretation of projected position tolerances at MMC

(OP)

Quote (3DDave)

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.


Quote (pmarc)

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?

Quote (pmarc)

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.

Quote (ASME Y14.5-2009 Fig. 6-11)

Note: A threaded hole is located and gaged from its thread profile at MMC.
What do you suppose this means?

pylfrm

RE: Interpretation of projected position tolerances at MMC

Quote (pylfrm)

What must fall within the tolerance zone cylinder you describe?
The extended portion of the axis of the hole.

Quote (pylfrm)

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.

Quote (pylfrm)

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.

RE: Interpretation of projected position tolerances at MMC

(OP)

Quote (pmarc)

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

RE: Interpretation of projected position tolerances at MMC

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.

RE: Interpretation of projected position tolerances at MMC

That brings up a good point... The standard mentions two interpretations for a position tolerance: Axis interpretation (particularly for RFS) and then a boundary interpretation (particularly for MMC).
So we often resort to "boundary" for MMC, but with the projected tolerance zone it's best to stick with the axis idea.

RE: Interpretation of projected position tolerances at MMC

Quote (J-P)

So we often resort to "boundary" for MMC, but with the projected tolerance zone it's best to stick with the axis idea.
But this does not mean that hard gages could not be used.

RE: Interpretation of projected position tolerances at MMC

Right -- MMC is the ticket to using fixed functional gaging.
I was just pointing out that we often speak of boundary with MMC, yet with a projected tolerance we still want to speak of the axis (in terms of the theory).

John-Paul

RE: Interpretation of projected position tolerances at MMC

Well, this is how the committee chose to explain the concept in the standard, but technically they could use surface interpretation too.

Maybe the reason why they did not decide to explain it in terms of surface is that in some/many(?) cases the boundary generated by position @MMC with projected tolerance zone requirement would not really mimic true functional worst-case boundary.

RE: Interpretation of projected position tolerances at MMC

pmarc,
How ISO is using the projected tolerance concept?
Since in ISO the axis is defined as "derived median line" (more or less closely related with it) then how this concept works in ISO? Just curious.


RE: Interpretation of projected position tolerances at MMC

greenimi,

It is true that in ISO the axis of a cylindrical feature is more like the Derived Median Line in ASME (they call it the Extracted Median Line), but when it comes to the projected tolerance zone concept the feature that has to fall within the projected tolerance zone is actually the extended portion of a perfectly straight axis derived from an associated cylinder. That associated cylinder is pretty much the same entity as the Unrelated Actual Mating Envelope in ASME.

RE: Interpretation of projected position tolerances at MMC

(OP)
Just for completeness, let's add another feature to the imaginary part described in the original post. I will also repeat the existing feature specifications for reference:

hole 1:
diameter 6 +/- 0.1 THRU
positiondiameter 0.4 (M) (P) 32ABC

hole 2:
M6 X 1 - 6H THRU (pitch diameter 5.350 - 5.500)
positiondiameter 0.4 (M) (P) 32ABC

hole 3:
M6 X 1 - 6H THRU (pitch diameter 5.350 - 5.500)
positiondiameter 0.4 (M)ABC

What would the design look like for a hard gage intended to check each of these position tolerances?


Quote (pmarc)

Well, this is how the committee chose to explain the concept in the standard, but technically they could use surface interpretation too.

How do you imagine a surface interpretation for projected tolerances might be defined?


3DDave,

Good point about the degenerate cases. An axis interpretation for MMB or LMB datum feature references is something I haven't really considered before, so I'll have to give that some more thought.

pylfrm

RE: Interpretation of projected position tolerances at MMC

Quote (pylfrm)

How do you imagine a surface interpretation for projected tolerances might be defined?

Similar to regular position at MMC callouts with the exception that the VC boundary is located outside of the toleranced feature, not inside, meaning that it is not the surface of the feature that can never violate the VC boundary, but the surface of a mating part's feature, if you will. Verification of this requirement creates some practical difficulties as far as use of hard gages is considered, but it is doable. If you have access to ASME Y14.43-2011, take a look at fig. B-20. It shows how to use hard gages to verify projected position tolerance (at MMC basis) applied to threaded holes.

RE: Interpretation of projected position tolerances at MMC

(OP)

Quote (pmarc)

Similar to regular position at MMC callouts with the exception that the VC boundary is located outside of the toleranced feature, not inside, meaning that it is not the surface of the feature that can never violate the VC boundary, but the surface of a mating part's feature, if you will.

I have a guess at the general concept you're describing here, but I don't really understand in detail.

I probably won't have access to ASME Y14.43-2011 in the near future, but I'll see what I can do to change that. Thank you for the suggestion.

pylfrm

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