Refinement to positional tolerance
Refinement to positional tolerance
(OP)
I have a differing of opinions on a tolerance scheme best described in the attached file. Basically it is over some nuances regarding what a colleague calls a "composite" feature control frame (I think incorrectly), and what I would simply call 2 single segment control frames.
The part has a post that I want to control location and orientation in general, and then refine the location & orientation in one direction.
We are using ASTM Y14.5M-1994, but I'm curious if it breaks any rules of the 2009 version.
The part has a post that I want to control location and orientation in general, and then refine the location & orientation in one direction.
We are using ASTM Y14.5M-1994, but I'm curious if it breaks any rules of the 2009 version.





RE: Refinement to positional tolerance
And your intent says position of .010 at MMC, but the feature control frame shows .10. I'm sure that's just a typo, but better to catch it now...
Neither sketch is what we would call a "composite" feature control frame. For composite, one position symbol would be shown, centered. And any datum references on the bottom part of a composite callout would have to be repeating a datum referenced in the upper part, and in the same order of precedence.
I suppose it's OK to say "compound" position tolerance when verbally discussing GD&T; I don't think that term is used in the standard but it gets the idea across that there's more than one position tolerance invoked. (Both the 1994 and 2009 standards would refer to all this as "single-segment" feature control frames.)
Your colleague's explanation #1 isn't strictly true. It sure makes sense to have the MMC modifier but it's not required (FYI -- in the 2009 standard that would be called the MMB modifier). It's the function of the part that dictates "M" or no "M" ... not the gauging.
Explanation #2 isn't really true. Since it is two single-segment position tolerances, you can do whatever you want regarding the datum references. It's not common to have a tertiary datum suddenly become primary, but it's legal.
And explanation #3 is not true at all. Without a diameter symbol in front of the number, a position tolerance reverts back to being two parallel planes. There are times when that's needed. However, I think that the position tolerance of .05 should be in the right-hand view, so that it is clear what direction those parallel planes lie in. Datum C sort of conveys the idea, but it's best to show it in the view where the position actually operates, unless it's multidirectional.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
1 - Wrong - MMMC may or may not be used on feature of size. It depends upon the situation.
2 - Compound Feature Control Frame?? What is that? I think that you mean single segments feature control frames.
3 - A diametrical tolerance zone only applies to feature of size that are round except when bi-direction tolerances are applied. One can also delete the diametrical tolerance zone on round feature of size and it will mean that tolerance is applied along the X and also the Y axis. We would end up with a square tolerance zone similar to co-ordinate tolerancing.
Dave D.
www.qmsi.ca
RE: Refinement to positional tolerance
On both drawings, I do not understand your datums A and B. I think they do the same thing. Perhaps Datum_A should be the long bottom face, and datum_B the Ø.25" hole!
The upper toleranceing scheme makes sense to me, as long as you want your feature located accurately to the Ø.25" hole and located less accurately to the width.
On the bottom tolerancing scheme, the first tolerance block requires a Ø.01" position with respect to datum_C, rendering the second block redundant.
On the upper tolerancing scheme, I agree with Belanger that the diameter symbols are not appropriate.
RE: Refinement to positional tolerance
Here's the tolerance zone I envisioned where the parallel lines would be parallel to the plane simulated by Datum C with a width of .05" and the diameter of the "(" & ")" entities would be .
___
(___)
drawoh: I think you have it backwards or I'm not understand your point. In both schemes the first tolerance block has a looser tolerance (.10") than in the second tolerance (.05") where I intended to refine the positional tolerance. Regarding the order of precedence, in use the part is slid over a post until the bottom surface (datum "B") makes contact and at that point the datum "C" centers the part between 2 posts. My order of precedence reflects that. Does my order still not make sense?
RE: Refinement to positional tolerance
The MMB modifier is essentially saying that in the function of the part, there may be some looseness or slop around the datum feature, and not only can we live with that, but we'll use that slop to make the feature being positioned appear to have more location tolerance. Notice how I'm phrasing it in terms of the function. Part are designed to function, right? Not to be gaged. (Gaging is a secondary process which must be performed but it is usually not the "driver" of the design process!)
So when you mention suppliers who say that "the boundary condition doesn't come into play when inspecting parts on a CMM for example unlike the material conditions in order to apply bonus tolerances," I can't say they're wrong. But that statement about the CMM process not a cause; it's an effect. If you deem that the function of the part will allow a little bit of slop on the datum feature, then their CMM program should be able to mathematically factor that in! Otherwise they are going to be checking parts with an "RFS" mentality -- which means exact mathematical center, not a boundary condition -- and perhaps rejecting parts that will function just fine.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
Thanks for the reply. I definitely need to unlearn my (mis)conceptions about material boundaries.
You had mentioned "since it is two single-segment position tolerances, you can do whatever you want regarding the datum references. It's not common to have a tertiary datum suddenly become primary, but it's legal", but I'm still unclear whether the way I specified the tolerance (bottom sketch) or the way my colleague suggested it (upper sketch) says 2 different things if my intent is to have the axis lie within the tolerance zone at MMC that I sketched. Do you think an inspection house would interpret ours the same way? I've attached a new file so we have the same boundary modifiers.
Thanks,
Dave
RE: Refinement to positional tolerance
For your colleague's sketch, the upper frame would require: a fixed-size gage pin of .15 inserted into hole A. Then stabilize the part by holding a flat plate against surface B. Now we have to measure the position of the pin's axis with respect to those degrees of freedom that A and B have constrained. (I might have put B ahead of A, but that's a different discussion!)
Now, the upper frame in your sketch is almost the same but you also have the tertiary datum C. (It sounds like you want C to constrain the last degree of freedom, which is the rotation around the gage pin's axis.) This is simulated in physical gaging by first contacting A and B as done for your colleague's sketch, but then having the part slide in between two plates spaced exactly 1.10 apart.
But basically, there would be no difference in the end result, because you are going to have the position to datum C be "trumped" by your lower frame. Thus, having C as a tertiary datum in the upper frame is not wrong, but it doesn't really add any value. The only value would be if there are other feature control frames on the drawing that reference A(M) B C(M) -- then I would leave C in order to imply simultaneous gaging.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
Thanks for addressing my question. I think your explanation including "simultaneous gaging" helped a lot. I do find it interesting that you say the datums in the lower frame trump the upper. I was under the impression that the lower frame only refines the tolerance zone but does not further constrain any degrees of freedom. Could the upper frame if it were |A|B| stand on its own if there was no lower frame?
As far as the the order and selection of datum features, I think I'm falling into agreement with you and Drawoh. In the actual smaller part (that I based this simplified model), the thru hole (Datum "A") is an hour glass shaped revolved cut and not cylindrical. In use, the part is slid over a pin and then moved down until it contacts the surface represented by "B". Then another part engages with the extruded boss and the surfaces that comprise datum "C" help to orient that added part. I based my datum selection and precedence on how the product is used, and not necessarily how best to constrain the part. Does that make sense?
RE: Refinement to positional tolerance
Your datum sequence makes sense. The only reason I was thinking twice has to do with the MMB modifier on A. If there is any looseness around datum A, which is permissible by MMB, then datum B will stabilize the part flush on that bottom surface (3 points of contact), sort of making B the main datum. But it's not wrong, and the new standard actually shows this practice in Fig. 4-21(d). It means that if datum feature A is at .15, then surface B may only make contact at one point. So I guess you can leave it as is...
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
Datum A removes 4 d.o.f. [x,y,u,v]; Datum B takes away 1 d.o.f. [z]; which leaves 1 d.o.f., the rotation about Z [w]. If Datum C isn't referenced in the top fcf, then there is no rotational constraint on the inspection setup, and you can be at a substantial angle to the inclined mating face (i.e. rotated about Datum-A) which would produce a significantly different part though acceptable to the tolerance.
Material modifers on the two datum fos's (A and C) are subject to the functionality of the part; if those datum features are going to mate to something with permissible slop/mismatch, then MMB modifers on the datum references make sense. On the flip side, if those datum features are going to mate intimately then RMB is more appropriate.
It's true that many inspectors don't consider MMB or LMB when they're CMM inspecting ... but they should. As I recall pretty much any software under 10 years old at this point should be able to handle MMB or LMB datum modifiers without a hiccup ... as long as it's not on a surface profile control. The value of using MMB or LMB is still there regardless of whether or not a CMM is used as it provides a better means to fit the part features.
"Compound" was commonly used in the bad old days by instructors and laymen who didn't know the standard particularly well, and it still percolates through the system. "Composite", "Single Segment" and "Multiple Single Segment" are the terms used correctly. Calling a multiple single segment fcf a "compound" fcf may lead to misunderstanding it as a composite control.
As for the shape of the tolerance zone in the second single-segment fcf, it makes sense for the functionality you seek; a cylindrical tolerance zone would not add anything beneficial.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Refinement to positional tolerance
I get how it uses probed points to determine a feature's size in order to determine and apply a bonus tolerance, but I still can't get my head around how a CMM factors in the material boundary of the datums. Like I said in one of my easier postings, I've always associated applying material boundaries in the design of functional (attribute?) gauging. Maybe the disconnect for me is that a gauge is tangible counterpart to the part to be inspected while construing surfaces based off of a cluster/pattern of points is on the abstract side of things. Would you be able to humor me with an example of how a CMM algorithm applies MMB, LMB, etc.?
RE: Refinement to positional tolerance
For comparison, do some visualization exercises based on the design you submitted. If you engaged the workpiece with a hard gauge, you could shift the workpiece or the gauge around within the limits established by the allowable datum shift. It's usually a fairly small shift, but it can be substantial if the tolerances so permit. As you move the piece or gauge wrt the other, if you achieve a mating condition wherein all constraints are satisfied, the part is good. Fortunately the human brain is still the most versatile computer we have, so spatial reasoning is a doable for most engineers. For the CMM, it's all processing power and algorithms. I would suspect that the algorithm tries to float the first modified datum while achieving the best fit of the workpiece within the tolerance zone, then progressing from there ... which is essentially what we do cognitively in hard-gauging.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Refinement to positional tolerance
There is a composite callout with the upper segment having a diametrical tolerance zone of 0.72 mm and a refining perpendiclarity to datum A of diameter 0.36 mm tolerance zone.
The "issue" is that there is "only one weld stud" and the argument is that the composite or multi segment callout (1994 std) refers to the top segment as the "PLZTF" and thus the argument as in the drawing example here and in my case..."one feature". The PLTZF is for a "pattern". The argument is that "one feature" is NOT a pattern.
I changed the callout for a location tolerance FCF and used a refinement of perpendicularity FCF under.
Comments regarding "Pattern Locating Tolerance Zone Framework"?
I also have a question regarding finding the "center of a pattern of holes" on the actual produced part; for setup on
CMM. Can show specific concerns on a sketch to clarify my question if neccessary. This thread or a new thread?
RE: Refinement to positional tolerance
new thread
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Refinement to positional tolerance
Any comment on the "pattern" question? Would like your point of view.
New thread, good enough... tomorrow... gone for today.
RE: Refinement to positional tolerance
Specifying a composite position tolerance for a single feature isn't a good idea, but not because of the word "pattern" in PLTZF. This acronym is just a general guideline, it doesn't really define anything. The issue is more that a composite position tolerance doesn't add any value when applied to a single feature, and will likely just make things confusing.
The only thing that distinguishes a composite FCF from two single segment FCF's is that the lower segment of the composite FCF controls orientation only. This allows you to refine the orientation of patterns (orientation tolerances can only be applied to individual features). But if you only have a single feature, you might as well just use an orientation tolerance (in this case, perpendicularity). So changing it to a position tolerance and a refining perpendicularity tolerance was a good call.
Finding the "center" of a pattern of holes using a CMM is a whole other can of worms. A new thread would be best for that.
BTW, Jim nailed the description of CMM datum shift pretty well, considering he's a design guy ;^)
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Refinement to positional tolerance
One item from Evan's post; second and subsequent levels of a composite positional FCF refine inter-feature location as well as orientation to the datums referenced. So, if you used a composite control on a single feature, then you'd get orientation alone from the second level. Unfortunately too few people understand that position includes orientation and therefore can (technically) be used to control orientation. I've seen it on many prints, and while it is "technically" correct, it's not user-friendly and will not be understood ... hence ignored by most.
Two single-segment FCFs (both positional or one positional & refined with perpendicularity {option #2 is easiest for most people to understand}) is appropriate for what you seek.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Refinement to positional tolerance
I'm not trying to nitpick -- and I know my statements here are in disagreement with a Tec-Ease tip -- but this is something that I've seen quite frequently. Granted, it's a minor topic, but I cordially suggest that paragraph 7.2 of the standard means that the position symbol must by its very nature always have an element of "location" involved.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
If you want to take 7.2 (2009) at face value alone, then position wouldn't control orientation at all because it doesn't mention it. But, as the orientation of the feature's axis or center plane is constrained within the positional tolerance it is indirectly controlling the orientation as well, even on a single fcf. It's this "hierarchy" as Tec-Ease calls it which tells you that for some applied control you get some other lower-order controls for free.
I haven't studied the '09 standard as I have the '94, but I haven't found anything that precludes position's use in this way. Unfortunately that arguably makes it "valid" by extension of principles and thus technically legal. It doesn't mean that I like it or would ever advocate the use of it, but it is, strictly speaking, legal unless it violates some other aspect of the standard. I have had a number of my students ask me about using it and I carefully tell them that it is technically legal, but really bad practice. It is not widely understood to be used in that way, and therefore introduces interpretation error into their system ... with no upside.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Refinement to positional tolerance
I believe I understand all points of view that you have presented. They are affirmation of my understanding.
My point of view regarding a single hole for example, with a positional FCF to only control orientation, is
that a positional FCF should have locating dimensions to be a positional tolerance, therefore I would use an orientation FCF
in that case. My opinion.
Even though as Jim presents that it would be "technically" legal to use the composite FCF; I changed the FCF
for the sake of clarity. My point of view these days is that the less explaining required, the better the callout.
Clarity with agreement, while not compromising integrity, is better for me than being technically legal.
One thing that does "bug me", is when Evan responds to the PLTZF as being a general guideline. It is a technical standard
and in my opinion should have weight to the terms is uses. If one considers certain terms as guidelines and not meaning specifically
the term used, and add to that the "extension of principles"; the communication at that point can become way too messy for me.
I understand the need for extension of principles, however if a term is intended as guideline, then say so.
I value all of your responses and appreciate your time to respond.
RE: Refinement to positional tolerance
Please keep in mind that my opinions on Y14.5 terminology are just that, my own opinions.
I sometimes question the usefulness of the terms PLTZF and FRTZF to describe the upper and lower segments of a composite FCF. The PLTZF (Pattern Locating Tolerance Zone Framework) does more than locate, it also orients. The FRTZF (Feature Relating Tolerance Zone Framework) does more than relate the features, it also orients them and in some cases can also locate them. So to me the terms FRTZF and PLTZF are oversimplifications that gloss over some of the intricacies of composite FCF's.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Refinement to positional tolerance
Position by definition is used to control location. Oh by the way, orientation often comes along for the ride. Therein lies the "hierarchy."
To use the position symbol when the only quality to be obtained is perpendicularity is akin to using parallelism when all you want is flatness; you're putting the hierarchy out of order. Think about it: Parallelism by definition is meant to control orientation, but you're saying that we aren't really controlling orientation. Non sequitur!
Maybe I'm being too literal with the standard. But from a logic point of view, it doesn't make sense to point to a single hole and "position" it to a plane that intersects that hole.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Refinement to positional tolerance
So I don't put too much weight in classifications or hierarchies for geometric tolerances in terms of controlling size, form, orientation, and location. What gets controlled depends heavily on the geometry of the considered feature and datum feature(s), and the nominal relationship between the two. It's very case-specific and defies simple categorization, as evidenced by the awkward groupings in Y14.5. Surface Profile is the only geometric tolerance that can locate irregular surfaces, but it is not included as a tolerance of location. Size tolerances also control form. Position is lumped in with Concentricity and Symmetry as location controls. The runout tolerances are "composite controls" even though they control location to a datum axis in very much the same way as Concentricity. Position can sometimes control orientation only. It's a mess.
Here's an example that bent my mind a bit when I first thought of it. Picture a cylindrical shaft with a diameter tolerance of 1.000 +/- 0.005 and a Straightness tolerance of 0.020 at MMC. The cylindrical surface would have to conform to a virtual condition boundary of diameter 1.025. Now let's add an o-ring groove halfway along the shaft, splitting the cylindrical surface into two sections. We can't use the same Straightness tolerance, because form tolerances can only be applied to individual features. To get the same virtual condition boundary as we had with the Straightness tolerance on the single cylindrical surface, we would have to specify a Position tolerance of 0.020 at MMC (with no datums) on what is now a pattern of 2 cylindrical surfaces. So in this case Position (a tolerance of location) applies very much the same control as Straightness (a tolerance of form). This flies in the face of the "hierarchy" in a big way.
Here's a question. Would it be legal to apply a Surface Profile control, with no datum features, to a single planar surface? Or are we obligated to use Flatness because it exists?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Refinement to positional tolerance
The new standard has a "CF" modifier, so I'd say we are now allowed to use straightness of 0.020 MMC on those two surfaces interrupted by the groove. But you're right that in 1994, we would be forced to use position.
However, I would still say that using position in that case qualifies as a "location" control, because we are locating one cylinder to the other (take your pick as to which one becomes the "datum"). So it doesn't really mess with the hierarchy.
Position doesn't "sometimes control orientation only." It often controls orientation along with location. But I agree with your other statements about runout, profile, and the other jumble of categories.
To your last question: yes, it is legal to apply profile of a surface to a single planar surface. Profile's main job in life is to control form (usually it has datum references to do other things as well). So this case would be equivalent to flatness. (And I think flatness would have been better, but profile is legal.)
Yet this is still different from position's "orientation only" usage because in this case profile is fulfilling its main duty; in the other case position does not even fulfill its main duty of location.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Refinement to positional tolerance
See paragraph 8.2 of the standard: "Profile tolerances are used to define a tolerance zone to control form or combinations of size, form, orientation, and location..."
Notice that "form" is always an integral element of profile.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Refinement to positional tolerance
I'm not sure that I agree with you about the role of orientation in position tolerances. For features of size whose orientation can be controlled (cylinders, slots), a position tolerance will always control orientation (perhaps relative orientation). The lower tier of a composite position FCF allows the datum features to constrain rotational degrees of freedom only - this is the example I was thinking of when I said that position sometimes controls orientation only. I will admit that the relative location of the features in the pattern would be controlled by the lower tier as well. So you are right that a position tolerance always has an aspect of location. Except in the case of a single cylinder ;^). But I would say that a position tolerance always has an aspect of orientation as well. Except in the case of a single sphere ;^).
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Refinement to positional tolerance
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Refinement to positional tolerance
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com