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Combined Controls Extension
4

Combined Controls Extension

Combined Controls Extension

(OP)
Here is Figure 8-24 from Y14.5-2009, with an additional position tolerance added:



The additional callout refines the relationship of the feature to Datum A.

Is this a valid application? ;^)

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Combined Controls Extension

Evan,
This question boils down to the perenial dillemma: is it okay to use position callout when the only geometrical characterstic to control is orientation?

If we say it is okay, then there is nothing wrong with the callout.

If we say it is not okay, then the callout should be changed to perpendicularity.

There would be no dillemma if there were two or more features like this. In that case the position to A would undoubtedly be valid calout as it would also control spacing between the features.

RE: Combined Controls Extension

(OP)
pmarc,

I agree with all that. The meaning of the callout is clear, but whether or not it is "legal" by Y14.5 is debatable. Using position to orient a feature is questionable, but applying perpendicularity to an irregular feature of size would be going out on a limb as well - there is no mention of this in Section 6.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Combined Controls Extension

I don't like position being used when it doesn't control, well... position. I interpret the definition of position as involving location, and location involves a distance. Many GD&T symbols can be control multiple qualities, but never at the price of losing their own, main definition.
It's true that the standard doesn't offer a solution to this dilemma. But rather than allowing position to do only orientation, they should simply mention perpendicularity as a sidebar to the explanation about the boundary concept (or add another paragraph to the perp section to allow this). With the boundary idea in place, the picture given in the OP would be perfectly fine.

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

RE: Combined Controls Extension

Interesting...
If you read carefully thru 6.4.2, orientation tolerance zone is clearly NOT defined for complex shapes.
The work around could be to use Profile the way similar to 8-23, or Perpendicularity of "each element" the way similar to 8-25.
I would be leaning towards Profile.

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

RE: Combined Controls Extension

CH -- profile's main job in life is a form control. Yet the OP graphic clearly wants the form liberated to 1.2. So I wouldn't go that route.
I do kind of like your idea of perp with each element. The only hitch is that each element is a surface control and here they want to use MMC. Shrug.

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

RE: Combined Controls Extension

Well, it all starts with half-baked attempt to combine profile with MMC, right?

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

RE: Combined Controls Extension

You can always write a note to describe what you want. I would be very interested to see how that note would be worded.

RE: Combined Controls Extension

CH,

I assume by your statement that you're not a big fan of combined controls using profile and position (or other controls) at MMC? Even though its supported in the standard have you seen a lot of issues when applying it in real world applications/gauging etc..? I am genuinely asking because everything here except the question of using position to only control orientation seems kosher.

RE: Combined Controls Extension

@chez:
It's not a matter of "fandom".
It is said in every textbook that MMC/LMC modifiers do not apply to profile, there is no virtual condition and functional gauge cannot be used.
Then, for those who still want to have it their way, Para 8.8 and fig. 8-24 offer work-around even with the picture of functional gauge.
So, when Belanger expressed his discomfort with the idea of surface control being used with MMC, I couldn't help but notice that the rules were already stretched.
Not to mention that perpendicularity of each element would refine profile, not the position; and position at MMC is still boundary control, not the axis control to begin with.

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

RE: Combined Controls Extension

I'm sorry if this is not exactly on the subject the OP wanted to discuss, but i'd like to ask something about the original fig. 8-24 (without Evan's addition of the last segment in the FCF).

If the same complex feature was defined only with profile of a surface tolerance, referencing the datums and disposed unequally: PROFILE[1.45 (U) 0.85][A][B][C] , single segment FCF without the position. Would the meaning be the same? Could the same functional gage be used for the boundary?

RE: Combined Controls Extension

CH,

When you say "the rules were already stretched" you mean the as written rules in the Y14.5 standard right? As in you are saying the MMC boundary concept with profile is sort of a sanctioned bending of the rules (sanctioned because its shown clearly in an example in the Y14.5 text) and I guess should be avoided if possible? Because the only difference between Evan's example and the Y14.5 standard is the addition of another position tolerance with a single datum reference A.

I ask because this topic seems to come up a lot - people want to control irregular features of size the same way as regular features of size, with MMC.

RE: Combined Controls Extension

I am saying, if the standard is suggesting Fig. 8-24 as the "way to go", then there is nothing wrong about adding perpendicularity requirement.
It does not contradict the other ones:

But semiond is also asking legitimate question: do we always have to specify several controls? If position and perpendicularity can be satisfied, then "too loose" profile spec probably has no benefits and the whole thing can be simplified.
(Although, we still cannot check profile with functional gauge smile)

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

RE: Combined Controls Extension

CH,
I see where you're coming from - as long as that satisfies the functional requirements for the part/feature then thats fine as it accomplishes something similar, but not the same as MSS position with MMC.

semiond,
I'm a little confused with your numbers but I think you mean [0.6 (U) 0.85]Sorry you're numbers were fine! This would result in the same virtual condition but to your question (1) it would NOT be the same as the entire profile could vary between these bounds - essentially the size of the tolerance zone is 1.45 now instead of 1.2, the original control only let that 1.2 boundary float with some additional profile tolerance and (2) as CH stated, it cannot be checked with a functional gauge anymore because MMC cannot be applied to a profile tolerance.

That being said - the more I look at this the stranger it looks and the less clear it seems to me. I assume that from the way its shown in the standard that the feature is limited at its LMC size by a fixed profile boundary basically dimensioned to the datums and at MMC size it can float around the "positional boundary" shown in 8-24. However I don't think its clear what happens when all or part of the feature is between MMC and LMC - it seems to me the profile could float around the positional boundary in such a way that local deviations outside the original intended 1.2 width profile zone are allowed without actually violating the outer boundary set by the LMC size. The only way this could be fixed is if the LMC boundary follows the MMC boundary when moving around this positional boundary, but thats not the way its shown in the standard. I might be thinking too much into this but it seems unclear to me.

Edit: Sorry semiond - had a brain fart, you're numbers were fine!

RE: Combined Controls Extension

Chez311, i understand what you are saying about the 1.2 total tolerance zone vs. the 1.45 tolerance zones in my scenario.

And now i think i even may be able to clear up some of the confusion you expressed:
The profile tolerance within 1.2 is only related to the basic dimensions of 30, 35 R10,R3 - not to the dimensions given from the datums. It builds a 1.2 wide "frame" all around that true profile where the entire surface must fit, but that frame can move in all directions because the profile segment of the composite FCF doesn't call out any datums. To constrain the location of that 1.2 wide tolerance zone, the second segment of position comes into play. It creates an internal positional boundary that takes into account:
1. the locating basic dimensions (25 & 25). 2. the profile tolerance zone width (half of it actually).
3. It's own tolerance zone (0.5, also one half - 0.25).
The above 1, 2 and 3 are being translated into a fully defined "size" and location of that boundary in relation to the datum reference frame.

Hope it might help.

RE: Combined Controls Extension

Here is one way how a gage might be used to inspect profile:
Consider a simple case - a hole of basic size 5 is defined with 0.2 profile tolerance. An expanding pin gage is used. Let's say it was able to expand till 4.95 until it was blocked by the hole's surface. That means we have another 0.15 tolerance zone to be used (edit - for the diameter, which means 0.075 radially). Now we put the part for inspection on the optical comperator with the pin in the hole, and check that the portions of the hole surface that depart from the pin are at a distance less than 0.15-edit-correction:0.075. from the pin.
That was just a simple example offhand.
Where does the convention that gages can't be used for profile come from? Textbooks?

RE: Combined Controls Extension

You can use gauges to control profile.
It's just since LMC/MMC modifiers do not apply to profile, you cannot rely on "functional" gauges taking advantage of "bonus tolerance" when checking fit between two parts.
Para 8.8 and Fig. 8-24 are exactly dedicated to create workaround, the way to check profile same (or rather similar) way you check position.

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

RE: Combined Controls Extension

semiond,
CH got to the answer before me - note in my response I did include the phrase "functional" gauge.

In relation to my confusion on the profile/position interrelationship, thanks for your response after reading it and then looking back at the figure today it became immediately clear to me - I can be real thick headed sometimes. I didn't take notice of the fact that the profile tolerance did not have any datums applied so of course its allowed to float and shift freely around the boundary created by the position tolerance. I apologize for such a simplistic question, I should have thought about it a little harder before posing it.

RE: Combined Controls Extension

Chez311, you sound like your being too hard on yourself. We're dealing with a field where it's very easy to get confused/lost. Especially since most of us probably do this for enrichment purposes, and we don't have the time to concentrate on details like we do with the projects at work.

CH/Chez311 this is probably my misunderstanding. I guess that i'm not dealing with GD&T "by the rules" for long enough to be familier enough with the terms and knowing the difference between a "functional gage" and simply a "gage" (i suppese that an RFS gage is not referred to as "functional"?)

RE: Combined Controls Extension

(OP)
semiond,

If you wanted to convert the combination in Fig. 8-24 to a single profile tolerance to A|B|C, I think that the tolerance value would have to be 2.85. So the FCF would be [PRF|2.85(U)0.85|A|B|C].

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Combined Controls Extension

Evan, i'm left floundering about 2.85.
Here is my explanation to the numbers:

If we consider the Virtual Condition (max. material) boundary and the Resultant Condition (min. material) boundary, i think the distance between them is 1.45; the RC is at distance of 0.6 from the true profile. The VC is at distance of 0.6 (half profile) + 0.25 (half position) from the true profile. Total width of the profile tolerance zone (per my proposed alternative) is 1.45. The portion allowed to be outside the material relative to the true profile is 0.85.

Here is the graphical depiction of the tolerance zones from the bottom of the figure:

RE: Combined Controls Extension

semiond,
Thanks for the understanding - it just seemed like an obvious thing I should have caught and I try to put a decent amount of thought into my questions, but you are right about this being enrichment.

I think typically when we refer to "functional gauges" we mean a fixed gauge. An RFS gauge would have to be adjustable.

As far as your numbers I see where Evan is coming from but I think it actually falls between both of your proposals - I would say the total tolerance zone should actually be 1.7 as the additional position tolerance would allow the entire 1.2 original zone to shift 0.25 up to the "positional boundary" which actually would increase the size of the tolerance zone on both sides - so the total zone increases by the full position tolerance. So in this case it would still be an equally disposed profile tolerance of 1.7 - at least I think.

RE: Combined Controls Extension

I think i'm also starring starting to understand where Evan is getting at and per my last calculation it is actually 2.9!

Consider the widest feature allowed per the 1.2 profile FCF segment at the direction of 30 basic dimension, and put it in a location where it contacts the positional boundary on the right side. Now check where the left side of the as-produced profile reached. That scenario is widening what i previoulsy was considering the RC boundary.

This is getting interesting...

RE: Combined Controls Extension

semiond,

I think you may be correct, the combined control is really throwing me off as I'm not considering the bonus tolerances allowed by the MMC position callout for a feature of LMC size. Its safe to say though that this would not be "equivalent" because as opposed to the profile/position combined tolerance a single profile control of this size would allow a much larger (and smaller) feature. This would have to be changed to a MSS or composite profile tolerance to get a similar control - albeit without the ability to apply MMC/LMC and without available bonus tolerance.

RE: Combined Controls Extension

Chez311:
Here is what i would currently consider a tolerancing scheme exactly equivalent to the one shown in fig. 8-24:

[profile][2.9(U)0.85][A][B][C]
[Profile][1.2]

The second segment is of course a refinement of the first one.

I even think it could be gaged/inspected in the same way despite of the fact that technically the term MMC can't be used in regards to the composite profile.

Objections?

RE: Combined Controls Extension

semiond,

I'm trying to think of a way to disprove that as it doesn't seem like we should be able to get the same result without position at MMC, however it looks like the tolerance zone would behave in almost exactly the same way - down to replicating the behavior of bonus tolerance afforded by the position control in the example 8-24, without technically providing bonus tolerance obviously as MMC/LMC cannot be applied.

Perhaps it comes down to the fact that the tolerances in the FCF's must be exactly matched to provide the proper behavior - any mismatch (ie: one or both too large/too small vs. the other or improperly distributed) and the behavior will be different vs. the boundary concept provided by combined position at MMC/profile tolerance. This will work out to a pseudo-"bonus tolerance" that will not work out correctly or intuitively what we're used to with standard MMC/LMC bonus tolerance with position.

Anybody else care to comment on this - maybe I'm missing something? Evan/CH/Belanger/pmarc?

RE: Combined Controls Extension

chez311,

Here is how I see it:
https://files.engineering.com/getfile.aspx?folder=...

I used slightly more regular contour than the one in fig. 8-24, but I believe that this should not change anything when it comes to drawing a general conclusion.

There is one more interesting observation to make, in my opinion. In the profile/position combo scheme, if one or both tolerance values are big enough to allow the actual contour to rotate more or less freely about the MMB boundary (in a similar way to what I have shown in the attached document), this may lead to some unexpected and undesired consequences.

RE: Combined Controls Extension

pmarc,

Thank you as always for your detailed figures - it definitely clearly shows the interrelationship of the combined callouts. I do wish that the standard had more detail in this area to more clearly explain the behavior that results from using the boundary concept but alas that seems to be a pattern with some of these topics.

That is definitely an interesting note about the unexpected behavior of the combined profile/position control vs. pure profile. My first thought is that this is only really significant when the tolerance is very near the size of the feature (in your figure the tolerance zone width of 2.9 is very nearly half the true profile of 6.0) and becomes proportionally less of an issue as the size of the feature approaches several multiples of the tolerance zone size - for example if your true profile had a size of 30 then this deviation allowed by the position/profile combination vs. pure profile is still present (depending on corner radii as well - a sharp corner limits this significantly), but to a much smaller degree. To this end I think that you have shown that in many use cases the MSS/composite profile control that semiond suggested can very nearly replicate the behavior of a profile/position at MMC control - but not precisely which precludes the ability to treat them the same (ie: with gauging).

HOWEVER - as I think about it more, since profile can be applied to just about any shape one can imagine, I would think that there is some shapes with generous radii or nearly circular shapes/sections that some strange and unexpected behavior could be present with either (a) profile/position combined control or (b) an overly generous profile tolerance (with either profile/position or MSS/composite profile) which means that the designer must carefully analyze all the factors when applying such tolerance schemes. As far as I can tell the MSS/composite profile tolerance is the more conservative in most cases.

RE: Combined Controls Extension

pmarc - isn't the 10.1 value on sheet 2 the size of the Resultant Condition, not the LMB?

RE: Combined Controls Extension

chez311,
I agree that the bigger the "ratio" of size of the feature to the values of the position and/or profile tolerances, the smaller the effect of the unexpected behavior. I also agree with your HOWEVER part of the reply. Could you just help me understand what the MSS stands for?

EDIT: Nevermind about the meaning of MSS. I figured it stands for Multiple Single Segment.


3DDave,
I decided to use term LMB instead of Resultant Condition just like I used MMB instead of Virtual Condition. I guess I could also call them Inner Boundary and Outer Boundary.*

The key thing here is that the 10.1 dimension is not really the true size of that extreme boundary, regardless of how we will call it. The figure on sheet 3 shows that the extremities of the toleranced feature can reach farther than just 5.05 (10.1/2) from the true position. That is why I think the idea of conversion of profile+position combo to a composite profile by calculating size of the extreme outer boundary based just on a possible translation of the feature is not a good idea - at least not for features other than circles/cylinders.

---------
* In my opinion life would be so much easier if the number of terms used to describe the extreme boundaries of a feature was minimized in the standard. Inner Boundary, Outer Boundary, Virtual Condition, Resultant Condition, Maximum Material Boundary, Least Material Boundary - this is one of the areas in the standard where people trying to understand it could get a severe headache.

RE: Combined Controls Extension

pmarc, your analysis discovered a very interesting thing (the unexpected behaviour allowed by position).
I must add from the practical aspect, that it is hard for me to imagine a real application that could be intentionally allowed to behave that way.
In my workplace, if the produced product differs visually from what is depicted on the drawing, even if all dimensions are produced within their specified tolerances, the people at the production plant are obligated to ask the designer's approval for the visual discrepancy. I'm not sure if similar procedure is practiced everywhere else, or if this is dictated by some quality assurance standard, but nevertheless...

In direct continuation to the said above i must add, that i maybe biased here, but i think that the combined profile describes the most probable design intent in this case, better than the datumless profile combined with MMC position. It addresses the allowed tolerance zones and the resultant & virtual conditions boundaries directly, whereas in the standard figure the same zones and boundaries are the stack up result of the specified tolerance values and require some work to be discovered. Obviously it also prevents that unexpected behaviour, and i think that it is fair to say that in the manufacturing buisness "unexpected" is almost a synonim to "unwanted".

RE: Combined Controls Extension

pmarc - seconded; since Virtual Condition was established a while back and Resultant Condition is the opposite, I would have ditched the rest outside of the mathematical definitions standard. They are useful in describing descriptions, like naming the parts of sentences and parts of speech, but less so in the application of these concepts. It just triples the number of terms with no additional clarity to the end user.

Edit because [Submit Post] is just a bit too close to where I wanted the cursor.

RE: Combined Controls Extension

pmarc,

Yes you are correct - MSS stands for Multiple Single Segment, it was abbreviated in much of the lessons I took on GDnT so its stuck with me that way, apologies for the confusion.

Quote (pmarc)

The figure on sheet 3 shows that the extremities of the toleranced feature can reach farther than just 5.05 (10.1/2) from the true position. That is why I think the idea of conversion of profile+position combo to a composite profile by calculating size of the extreme outer boundary based just on a possible translation of the feature is not a good idea - at least not for features other than circles/cylinders.

Wouldn't that be an example of why the composite profile is the preferable choice? As semiond noted typically unexpected = unwanted and it looks like while maybe not entirely preventing this, composite profile seems to limit some of this behavior if the zone and limits are set up correctly - of course the same result could be had with composite profile if the tolerance zone was made excessively large.

RE: Combined Controls Extension

I hope it is not too late to get back to Evan's original question about the orientation refinement. What is not so clear to me is - since the 0.2(M)|A position uses the boundary interpretation to refine orientation, how will the boundary look like in this case? If it should be at uniform distance of 0.1 from the MMB of the complex feature, will it not make the positional boundary of the second segment (at uniform distance of 0.25) redundant?


Edit:
I made a check by myself and discovered that the position tolerance WRT datums will not be redundant, because the refinement position boundary floats together with the profile tolerance. However, I also discovered that the resulting tolerance zone of the orientation is not obvious from the specified value and it actually equals the profile tolerance+ half of the position tolerance refinement. Attached is a sketch displaying it. I modified the tolerance values to make the zones more visible:

*profile is within 2.

*position WRT datums A,B,C is within 1.4.

*position as orientation refinement is 1.

Displayed is one possible location of the tolerance zones. The dashed thick lines are the profile tolerance zone, the dashed orange thin profile is the refinement boundary, the continuous internal profile is the virtual condition boundary established by the position tolerance WRT to the datums. Maximum orientation deviation is 2.5. I find the fact that it's not directly accepted from the refinement FCF disturbing.



Edit: if my analysis is correct, the "refinement" tolerance zone ia actually larger than the Profile specification used to define form. Then it is not really a refinement. I would currently answer Evan that it's not a good application since it fails.

RE: Combined Controls Extension

semiond,

Two things:
1. As you correctly noticed, the MMB boundary created by the position callout wrt A is not redundant because it can translate toghether with the profile tolerance zone. The only relationship to datum plane A that is actually controlled by that callout is perpendicularity. So if the only relationship is perpendicularity, the question is, does it make sense to use position symbol at all?

2. As for your sketch, look at it again and ask yourself what the maximum possible orientation error could be if there was no "position of 1 orientation refinement" callout. And keep in mind that no one said that this extra callout was form refinement. Orientation requirement is what we have been talking about.

* Hopefully you did not edited your post for the third time while I am writing this comment winky smile

RE: Combined Controls Extension

pmarc,

1. I would say the answer is no. For orienation refinement, my choice would perpendicularity of "each line element", I'm with CH on this matter.

2. I would say the maximum error will be 2, as dictated by the form control (Profile).

* The addition in the bottom of my post under the picture is from a few hours ago. So I hope that your assumption is correct, I didn't edit the post while you were writing the comment smile

RE: Combined Controls Extension

Quote (chez311)

Wouldn't that be an example of why the composite profile is the preferable choice? As semiond noted typically unexpected = unwanted and it looks like while maybe not entirely preventing this, composite profile seems to limit some of this behavior if the zone and limits are set up correctly - of course the same result could be had with composite profile if the tolerance zone was made excessively large.

I think that there is a place for both. Composite profile is more stringent requirement, but that does not mean that profile + position @MMC combo should not be the preferable choice in some cases. The combo in case of irregular features of size is a similar technique to applying position @MMC to directly toleranced regular features of size (pins, tabs, holes, slots), except that in case of the combo the size and form is controlled with profile tolerance and not with Rule #1. There are certain risks behind using it (the unexpected behavior under certain specific conditions that we talked about), but that does not always disqualify it.

RE: Combined Controls Extension

semiond,

1. Perpendicularity of "each line element" does not result in the same geometric requirement as the position/perpendicularity tolerance @MMC wrt A, so I am not sure I understand why you add it to the conversation. In case of perpendicularity of "each line element", regardless of the feature size the amount of available perpendicularity error would have to be the constant. In case of position/perpendicularity tolerance @MMC wrt A the possible amount of perpendicularity error changes (increases) with the increase of the size of the feature.

2. Incorrect. The maximum possible orientation error (for feature at LMC) would be 2.5. I guess you said 2 because you assumed that the profile tolerance zone is perpendicular to datum plane A. That is not what the profile tolerance is saying.

RE: Combined Controls Extension

I made a typo in my last reply. There should be 2.7 instead of 2.5.

RE: Combined Controls Extension

pmarc,
You are right, I didn't take into account the profile tolerance zone is not constrained in orientation. Thank you for pointing that out.
So eventually there is a refinement of 0.2 to orientation, resulting from the specified value of 1.
I don't like the indirectness of it.
I guess that's why I brought up "each line element" - more straightforward = less trouble in practice and more useful. Much like in the case of composite profile vs. profile + position boundary dilemma.

RE: Combined Controls Extension

pmarc,
Regarding your answer to chez311 - we've seen that the composite profile tolerance, in terms of tolerance zones, provided the same flexibility for production as the profile+ MMC position with it's bonus. That is with the additional advantage of design intent clarity and less "unexpected" scenarios.

RE: Combined Controls Extension

semiond,

It is your right to like or dislike whatever you want - just like I have a right not to like position wrt A applied to a single feature of size that is nominally perpendicular to datum A. My opinion is that as long as a geometric control or a combination of geometric controls does not clearly violate any rule from the standard, it is at least worth considering.

Another thing is that geometric tolerancing is not only to define the end product in full and unambiguous manner. It is also to provide as much tolerance as possible within "boundaries" determined by functional reuquirements. So if using more generous tolerancing scheme (position + profile combo vs. composite profile OR perpendicularity @MMC vs. each line element perpendicularity) does not interfere with the design intent, I do not think the scheme should be replaced by something else just because this something else is more straightforward.

RE: Combined Controls Extension

pmarc, I agree with almost everything said, And by the way, I too don't like "position wrt A applied to a single feature of size that is nominally perpendicular to datum A." Why not use perpendicularity?

However you mentioned that MMC position + profile combo vs. composite profile that we discussed earlier, is a comparison by which MMC position + profile comes out as the more generous tolerancing scheme. I'm not sure why you still stick to this viewpoint, after we've seen with the help of your excellent diagrams that the tolerance zones in these 2 cases are equally generous and behave almost the same, with the exception of the unwanted behaviour of the profile + MMC position that widens the RC boundary just a little bit (In my opinion, such "generousity" won't do any good).

I agree that all legal tolerancing schemes are legitimate and should be available to choose from, but sometimes there is a clear advantage to one over the other, that should be pointed out.

RE: Combined Controls Extension

semiond,
The point of my excellent diagrams, as you called them, was exactly to show that the 2 cases are not equally generous. If they were, we would not be able say that certain condition of the as-produced feature meets one scheme and does not meet the other.

RE: Combined Controls Extension

pmarc,
I looked at your diagrams once again.
We calculated the same MMB (VC) and LMB (RC) boundaries that appear to be equivalent for both controls. Yes, I know the rotation phenomenon for the profile+position combo makes a difference and widens the LMB boundary in a peculiar way, but it didn't even make you update calculated 10.1 LMB width/height to a larger value, and for a good reason.

The 2 schemes not equally generous?
If you choose to call the phenomenon that caused the feature pass one control and be rejected by the other generous, so be it. In this particular case - I'd rather call it dangerous smile

RE: Combined Controls Extension

3
semiond,

It is the second time in last few weeks when I have a feeling that no matter what I will say I will not convince you. So with almost no hope of success I will leave you with following example:

In Y14.5 world, if a cylinder of nominal diameter 10 is controlled with tolerance +/-0.1, this requires that the cylinder must be produced within 9.9-10.1 size limits and must have perfect form when produced at MMC.
If the same cylinder is controlled with basic size 10 and profile tolerance of 0.1, this also requires that the cylinder must be produced within 9.9-10.1 size limits and must have perfect form when produced at MMC, but in addition it also means that the cylinder must have perfect form when produced and LMC.

Rethorical question #1: Are you able to say which of the two schemes is more generous?

Rethorical question #2: If from functional point of view it does not really matter if the cylinder, when produced at LMC, has perfect form or not, does the first scheme deserve to be called dangerous? Or should we rather say that the second scheme is too restrictive?

RE: Combined Controls Extension

pmarc,
I do understand the point that you convey with the rethorical questions.
But please notice that I emphasized the words "in this particular case" in my last post.
I learned to never make generalizations, unless those generalizations are narrow enough to describe a very unique type of cases. A pin produced in a way that it meets local size limits in every cross section but somewhere it violates the perfect form boundary at LMC is not necessarily a bad product, in many cases it's good enough for it's function.
That leads to absolutely no conclusions about a feature that is toleranced in way that, in your own words:"may lead to some unexpected and undesired consequences."

Edited for typo correction.

RE: Combined Controls Extension

pmarc,

I think your last post summed it up nicely, I see the point you are trying to make and can appreciate it. Whereas the second scheme may be more restrictive/conservative in most cases this is not necessarily desired, especially if its not required for the function of the part - just as we wouldn't apply the most restrictive tolerances to a part unless required. Of course this means that in every case, especially with more generous tolerances or shapes with larger radii/circular sections, we must analyze whether the chosen scheme might result in unwanted behavior, but that goes without saying.

RE: Combined Controls Extension

chez311,
You summed up nicely what I tried to sum up winky smile

I read somewhere some time ago that applying a GD&T to fully and unambigously define product is like being at Level 1 of the game. As long as an individual or organization does not apply the GD&T (so that as much tolerance as possible from functional point of view is available), Level 2 will remain unreachable.

RE: Combined Controls Extension

I just want to add to pmarc's last remark my observation, that Level 2 is only available to "design players" working in organizations where the people "playing" at the production floor have enough spare time at their work day to interpret sophisticated feature control frames using colourful tolerance zone diagrams and making fun calculations, in order to get just a little bit of extra tolerance "bonus" that they are probably not going to utilize anyway.
That is instead of the boring practice of just reading the limits directly from a simple and unambiguous specification, which finds a way to both make sure part functionality AND enough flexibility for the technologist to conduct an efficient and manufacturable process.

If you work in the kind of organization I described, adopt Level 2 strategy and have fun. If the latter boring option is more suitable to your situation, you better stay at level 1. smile

RE: Combined Controls Extension

Quote (semiond)

.....working in organizations where the people "playing" at the production floor have enough spare time at their work day to interpret sophisticated feature control frames using colourful tolerance zone diagrams and making fun calculations, in order to get just a little bit of extra tolerance "bonus" that they are probably not going to utilize anyway.

That is instead of the boring practice of just reading the limits directly from a simple and unambiguous specification

Someone, on this forum, said:

..... But that is the price of simplicity - the requirements become tighter. With GD&T there is usually a trade-off between tolerance and complexity - to allow more tolerance, the specification (or inspection method) must be more complex.

RE: Combined Controls Extension

semiond,
In order to get at level 2 organizations do not have to use what you called "sophisticated feature control frames". In many cases proper, that is functional, datum feature selection is well enough to be able to open up tolerances to a point where everyone benefits by that. This is just one example and there is much more that can be done without even stepping into the unsafe/uncertain teritory of extensions of principles or sophisticated callouts.

Side question: Could you please stop making allusions to my "colourful graphics full of fun calculations" if you do not understand what they are for here? Over and out.

RE: Combined Controls Extension

And one more for the other side of the argumnet argument:
“The law of diminishing returns means that even the most beneficial principle will become harmful if carried far enough.”

RE: Combined Controls Extension

greenimi,
I appreciate your second quote very much - that was said much better than I could ever say to express my message.

But please allow me to use your first quote with a little modification - I will replace just one word by a one that is closely related to it and more relevant to this discussion and it's specific context, and let's see what comes out. This is also for the benefit of others that may have misunderstood me:

"..... But that is the price of simplicity directness - the requirements become tighter. With GD&T there is usually a trade-off between tolerance and complexity - to allow more tolerance, the specification (or inspection method) must be more complex."

Does that statement still make sense? No? I don't think so too.
Well, by simplicity I did mean clarity, and directness. Sometimes you can have more direct specifications than others, that supply the same useful flexibility for production, and an advantage of better product definition, even if they don't include the M in a circle symbol or require calculations to determine the total size of the usable tolerance zones.
I'm not against MMC position in general. It is often very useful, direct, efficient and unambiguous - particulary with the axis interpretation. But here we saw a specific example where it may be less preferable.

I have no clue why pointing it out causes such a fierce resistance.

RE: Combined Controls Extension

pmarc,
Regarding the level 1/2:
I can justify aspiration to get to level 2, as long as what it takes to be at level 2 doesn't come in expense of the main goals of the strategy of level 1 which you described: "fully and unambigously define product".
With modern production equipment, many manufacturers do not get to the extreme boundaries of the allowed tolerance zones, and certainly don't utilize the whole range. If giving the that little extra tolerance comes at the expense of clarity of design intent, and with the price of allowing "unexpected" things like we've seen - which means more ambiguous product definition, this may lead to damaging consequences.

All I'm saying is - there are 2 different aspects that should be considered. There is no reason to emphasize one and present it as a more advanced level, and underestimate the other. Each one should make his choice, but bear in mind the two sides of the coin.

Edit: By the way, I'm sorry if I made it look like I was sarcastic towards the effort you've done with the figures. I appreciate them, they were useful for me and contributed a lot to this discussion. I just wanted to convey the point that manufacturers don't always have the luxury to make the best value out of such specifications through tolerance analyzis. And often a more straightforward approach is more appreciated. I'm sorry again if you found the way it was conveyed offending.

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