Two Pattern of Holes

[mkcski]

I have a somewhat unique application for GDT that I can find no examples of in the Standard or in any of the reference materials I have access to. So I’m requesting some assistance with some symbology and its interpretation. See attached partial sheet 1 and sheet 2 drawings

We have a component with a circular pattern of (20) “features”. A section through one “feature” is shown in Detail C – upper left on sheet 1. They that are BASICally located to datums in the FCF at the top of the drawing; Datum plane A - 3 motions (tol. zone perpendicularity to primary datum), Datum axis B - 2 motions (center of BASIC circle), and Datum axis C - 1 motion (rotation–clocking with other features). At each of the (20) Detail C “features” there are (2) 3.625 diameter coaxial holes.
As I see it, there are two patterns operating here: a circular pattern of (20) upper “features” with each of the (20) “features” being a coaxial pattern of (2) holes. The position tolerance for both the 20X circular pattern and between the (2) coaxial holes is the same value because: 1) an oscillating shaft must simultaneously passes through the two bushings in Detail C and a single bushing in the mating part, and 2) when the components are assembled the (20) shafts do not interface with each other. Material condition modifiers have been analyzed and are RFS/RMB - (Rule #2).

Y14.5-2009 only illustrates examples of a single location for position of a coaxial pattern - figures 7-43, 7-44, so we have “concocted” the dimensioning schema in Detail C to functionally dimension the part.

The intended interpretation of the GDT in detail C: The FCF for the upper holes controls the position of the entire 20X circular pattern of upper holes to Datums A, B, and C. Because the oscillating shafts that pass through the bushings do not interface with each other, there is an independent relationship between the (20) individual pairs coaxial holes. Given these parameters, and no examples to refer to, we specified the upper hole as Datum D - 20X INDIVIDUALLY and positioned the lower hole to Datum A to maintain the required functional perpendicularity, and to Datum D to maintain coaxiality with the upper hole. The tolerance zone for the lower hole would be oriented perpendicular to Datum A and be aligned with the RMB related actual mating envelope established from the upper hole.

Composite position was considered. But the fit-up analysis determined the position tolerance for the circular pattern and the coaxial holes is the same, so the PLTZF and FRTZF tolerances are the same value – not a smaller refinement – a requirement for composite position.

Questions for Forum members:
1) Does your interpretation match the intended?
2) Is there a “better” approach? Could we eliminate Datum D, the FCF and size dimension for the lower hole by applying the by adding 2X to the upper hole diameter so that a single tolerance zone projects “down” from the upper holes.
3) Would a continuous feature “CF” concept apply? The 3.38 “neck down” between the holes causes me concerns with this.

 

[Nescius]

It may or may not help, but your situation immediately reminded me of Fig. 7-26 in the 2009 standard.

 

[Belanger]

CF would make them equivalent to a zero position tolerance at MMC. Since you don't have M's then CF probably not work. (Was that the intent, or are the oscillating shafts fitting in a clearance/loose fashion?)


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

 

[pylfrm]

mkcski,

I am not following you reasoning for the two position tolerance values being the same. The datum feature references are different, so the tolerances control completely different things.

Your current scheme effectively allows diameter .004 position error of the lower holes with respect to the A|B|C datum reference frame. If you apply this tolerance to both the top and bottom holes as you mention in question 2, then you are effectively allowing diameter .008 position error of each lower hole with respect to the individual A|D datum reference frames. Something doesn't jive.

More information about the assembly and fits with the shafts and mating part(s?) would be very helpful. As it is, I'm not clear enough on the functionality to offer much in the way of specific suggestions.

 

[patdh1028]

Here are my thoughts:

1. I understand what your requirements are on the drawing. I think you're right that in the standard there is no "shorthand" notation for assigning unique datums to multiple single features and then referencing each one of those datums to a respective local feature instance. So your drawing notation as-presented isn't explicitly supported by the standard; I understand what you mean, but it seems others here don't and that isn't a good sign...

2. Your best approach here in my opinion, is to take the "messy" route and assign unique datums to each of the upper holes, and have unique FCFs for each of the lower holes calling out the tolerance with reference to the respective DRF. This method means a lot of notation on the drawing, but captures your design intent and is unambiguous by the standard. It's a weakness of the standard that there isn't a shorthand for this, but what can you do besides wait for the next revision or risk a shop misinterpreting and making non-functional parts when they mis-interpret your invented shorthand?

3. You do not want to use CF. Belanger is almost right, but it's actually slightly worse than Zero @ MMC because the CF feature is physically longer than each individual one so in practice it is a tighter perpendicularity and location tolerance. That would be a major decrease in total tolerance versus what you have, and it sounds like that is way overkill for your design intent. CF would be a clean option if you could relax your size tolerance in accordance to the location tolerance you are restricting, but since you are pressing bearings into those bores that probably is not an option.

 

[pmarc]

I too have some difficulties to understand why this particular dimensioning scheme has been chosen based on the description provided, so will I just focus on providing a general answer to this: "Y14.5-2009 only illustrates examples of a single location for position of a coaxial pattern - figures 7-43, 7-44, so we have “concocted” the dimensioning schema in Detail C to functionally dimension the part.", as I believe it will give some amumnition to solve mkcski's dillemma.

It is true that the 2009 standard does not explicitly show how to deal with a pattern of coaxial features that is subsequently patterned, but I think it (the standard) provides some/enough tools to grasp this requirement - correct me if I am wrong, but I would say the main concern here boils down to find a way to allow the lower hole in each group to locationally follow the corresponding upper hole without being directly locationally tied to the other lower holes, which would probably be the case if some kind of composite positional callout was used.

So please take a look at the attached picture. It shows modified version of fig. 7-44 from Y14.5-2009. We have 2 patterns of 4 coaxial holes. Size of all 8 holes is controlled by separate callout. Location of two holes (one in each pattern), which correspond to 20 upper holes on mkcski's drawing, is controlled by a position callout to the global datums. And then there is this second position callout with no datum references that only controls spacing within each group of 4 holes (this is what '4 COAXIAL HOLES' is for), but does not control spacing between 4 holes in one group and 4 holes in the other group (this is what '2X INDIVIDUALLY' is for). Does it hold water?

If so, I think it can be easily adopted to mkcski's example, provided it matches with functional needs. I would just add that to me, based on what I see, it looks like in this very example both holes - the upper and the lower - in each group will probably still have to be assigned as individual datum features, because there are some patterns of holes on both sides of the flange that most likely should be located from the axis of each individual hole.

Side note 1: Per the standard (figs. 2-8 through 2-10), when the continuous feature modifier is used, 'nX' prefix is not put in front of dimensional callout, so the size callout for datum feature B on the drawing deviates from that rule.

Side note 2: Have you ever considered applying all the stuff we are talking about to the IDs of the bushings, instead of to holes diameters?

http://files.engineering.com/getfil...b4bcca0&file=position_pattern_of_patterns.png

 

[3DDave]

As in all tolerancing methods it depends on what the desired outcome is. If the maximum deviation for each pair of bushings relative to the shaft is important and the shaft is not an interference fit, then make each pair a continuous feature with a perpendicularity callout relative to A, the CF modifier, and use MMC to allow for more tolerance when the bores are bigger. Typically there will be more clearance with the mating shaft when the bores are bigger, so this reference can reflect reality. Then the pairs of holes can be located with a larger location tolerance relative to A|B|C using MMC/RFS/RMB/whatever terminology is in vogue. If the bores need to be more continuous/well formed, use a cylindricity callout and the CF identifier.

What is described in the sample drawing is more in line with manufacturing steps - start with bores on one side and then telling the machinist how much room he has to match the locations when machining the other bores from the other side. It's convenient to do this, but it makes understanding the way this part works with the mating parts more difficult. It's not uncommon to do so. but it makes tolerance analysis a more difficult task.

 

[mkcski]

To all who responded: Not ignoring your comments. I am super busy today and will review comments ASAP.

 

[mkcski]

pmarc:
You caught the exact requirement in your paragraph: "It is true that the 2009 standard does not explicitly show how to deal with a pattern of coaxial features that is subsequently patterned, but I think it (the standard) provides some/enough tools to grasp this requirement - correct me if I am wrong, but I would say the main concern here boils down to find a way to allow the lower hole in each group to locationally follow the corresponding upper hole without being directly locationally tied to the other lower holes, which would probably be the case if some kind of composite positional callout was used."

I like your suggested approach in the sketch too. But I have a few clarifying questions to ask:

1) I cannot “find” what Y14.5 “statement” would lead me to understand that the 2X FCF only applies to the two holes on the left in your drawing (the upper holes in my design) and not all (4) coaxial holes as a group (like in the composite position examples).

2) Your example appears to have the .08 coaxiality tolerance as a refinement of the .25 position of the upper holes. Functionally in my design the position of the upper holes as a group and the coaxiality of the holes require the same tolerance - .002”. Does this change your thinking?

3) I assume your FCF with no datums is an application of 7.6.2.3 and fig 7-59, yes?
Your statement about the holes surrounding the bushing “holes” is true and it will approach Design Engr to add them to the template drawing.

Side Note 1 is appreciated. This is a subtle one that I missed in my review.

Side Note 2 is understood. The bushings are a composite material that is difficult to machine and control size. They are final machined in a lathe before "shrink" installation for more control. The relationship between the ID to OD is tightly controlled to maintain wall thickness. The position error between the ID and OD is part of the fit-up analysis for the assembly (with the bushings installed).

In addition: Although the Draft Comments period is closed for the new release of Y14.5, I contacted ASME and they sent me a free paper copy of the Draft – sweeeeet!

 

[pmarc]

mkcski,

Here are my answers to your questions:

1. I do not think such a statement exists. The logic in my example would be following: since 4 COAXIAL HOLES note placed under the FCF on the right means that the FCF applies to all 4 holes in a group, then lack of 4 COAXIAL HOLES note under the FCF on the left means that the positional callout applies only to leftmost hole in each group. I agree, it is not perfect, but in your example you can clarify the requirement by adding something like UPPER HOLES ONLY under the FCF.

2. No, it does not change my thinking, because I would not say that the 0.08 coaxiality callout in my example is a refinement of the 0.25 position callout. The 0.25 callout is only to control the leftmost holes, so there is actually no dependency between both values. As a matter of fact, from geometric definition point of view, it would be perfectly legal to even have the 0.25 position callout and for example 0.8 or 1.6 coaxiality callout.

In your case, both values can be .002 and there will be no conflict between them.

3. Yes, the position FCF with no datum feature references is somewhat similar to fig. 7-59. We are just not talking about establishing a single datum axis from multiple coaxial holes.

In addition: That is great to hear you were able to obtain a copy of the draft. As far as I have been told, there will be another round of public review, and I think one of the outputs of this discussion should be a comment submitted to the Y14.5 committee with a request to add a figure or two clarifying how to deal with pattern-of-pattern position controls. I have written that down, and will definitely send something as soon as the second round of review starts.

 

[mkcski]

pamrc:

Thanks much for your insight. Clarifying notes are always a possibility and that I tend to under utilize. I lead a weekly GDT Case Study with the Design Engr Dept. I will present your approach and see what discussion ensues. I will keep you posted.

This "pattern-on-pattern" is the first time I could not find a close example or apply other concepts to communicated design intent. I too will add this to my list of Comments. The other is Continuous Feature <CF> like the two coaxial holes in my post that function as a single feature and that have a "interrupting" feature between them. There needs to be more clarification for this geometry and <CF>.

Could you please start a new thread when you hear that another Draft is released and a new comment period has started?

 

[pmarc]

Sure, mkcski.

Perhaps it makes sense to start a new thread where thoughts/opinions about current version of the draft could be collected?

 

[CheckerHater]

I think it was already started

http://www.eng-tips.com/viewthread.cfm?qid=408171

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

 

[mkcski]

I can start a thread but before I do... I assume posts to this thread would be few and infrequent so...being new to the Forum: Are there any "sunset" laws related to threads and how long they stay active if there is no activity?

 

[pmarc]

I think it was already started

http://www.eng-tips.com/viewthread.cfm?qid=408171

Yes, this one can be used too.

 

[mkcski]

It may be beneficial to start a new thread with a more descriptive heading like "Collection of Public Review Comments for next Draft of Y14.5" Yes/No?

 

[CheckerHater]

Just do it (TM)

You may also report first thread for deletion to reduce confusion

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

 

[pylfrm]

mkcski,

If you keep the original position tolerance on the top hole, remove the original position tolerance on the bottom hole, and add a tolerance of |position|diameter .002| on both holes of each pair (20X individually), then I believe you will allow a condition that would measure as a position error of diameter .016 to the |A|D| datum reference frame. I can't say whether this is a good or a bad thing. My point is that the various tolerance schemes discussed are vastly different (a factor of over 8 by one measure) despite using the same diameter .002 tolerance value in the FCFs.

Regarding CONTINUOUS FEATURE, I wouldn't object to using it on the pairs of diameter 3.625-3.627 holes. I do agree that clarification in the standard might be helpful though, but for other reasons. The text only mentions using it with features of size, but Fig. 7-45 shows something very different. Recently I was wondering about using it with an angle tolerance on an interrupted conical surface.


- pylfrm

 

[mkcski]

pylfm:

I am confused as to what the FCF would be placed or "attached" to in your suggestion. Could you attach a hand sketch of your approach.

Yes figure 7-45 does not follow the FOS examples elsewhere in Y14.5. It is on my clarifications list for the next public comment to the Draft

 

[mkcski]

pylfm:

Of note: I received a copy of the DRAFT of Y14.5 this past Mnday. I just now looked up Fig 7-45 (now 10-46) and have attached it. Very interesting to say the least. I don't have time right now to review the definition of Continuous Feature