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:
hole 2:
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?
hole 1:
diameter 6 +/- 0.1 THRU
positiondiameter 0.4 (M) (P) 32ABC
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
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
RE: Interpretation of projected position tolerances at MMC
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
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)
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
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
RE: Interpretation of projected position tolerances at MMC
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
RE: Interpretation of projected position tolerances at MMC
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
RE: Interpretation of projected position tolerances at MMC
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
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
RE: Interpretation of projected position tolerances at MMC
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
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
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?
RE: Interpretation of projected position tolerances at MMC
What must fall within the tolerance zone cylinder you describe?
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?
pylfrm
RE: Interpretation of projected position tolerances at MMC
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
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
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
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
RE: Interpretation of projected position tolerances at MMC
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
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
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
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
hole 1:
positiondiameter 0.4 (M) (P) 32ABC
hole 2:
positiondiameter 0.4 (M) (P) 32ABC
hole 3:
positiondiameter 0.4 (M)ABC
What would the design look like for a hard gage intended to check each of these position tolerances?
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
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
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