Datum shift + Stack-Up
Datum shift + Stack-Up
(OP)
I just finished looking at the latest Tec-Ease tip (http://www.tec-ease.com/gdt-tips-view.php?q=253) regarding datum shift and its message that the modifier applied to a datum feature does not change the tolerance on the feature or features being toleranced, but should (or can) the modifier be included in the tolerance stack up?
In the attachment I provided an incomplete drawing of a part with coaxiality controls. The primary datum is an external thread, although a cylindrical boss could have been just as pertinent. I also included a tolerance stack up to look at wall thickness using both worse-case and statistical models. The handling of bonus tolerances and datum shift was based on a series of papers written by Ngoi et al (affiliated with the Nanyang Technological University) in lieu of having access to any stack up analysis textbooks, etc. 2-D CAD drawings were used to check my work based on the guidance from the aforementioned papers.
Should I have excluded the datum shifts? Can anyone refer me to a good text that discusses inclusion of geometric tolerances in a stack up analysis?
In the attachment I provided an incomplete drawing of a part with coaxiality controls. The primary datum is an external thread, although a cylindrical boss could have been just as pertinent. I also included a tolerance stack up to look at wall thickness using both worse-case and statistical models. The handling of bonus tolerances and datum shift was based on a series of papers written by Ngoi et al (affiliated with the Nanyang Technological University) in lieu of having access to any stack up analysis textbooks, etc. 2-D CAD drawings were used to check my work based on the guidance from the aforementioned papers.
Should I have excluded the datum shifts? Can anyone refer me to a good text that discusses inclusion of geometric tolerances in a stack up analysis?





RE: Datum shift + Stack-Up
Think of it this way: any shift available from datum A is simply looseness which would allow the part to jiggle a little bit around the threads. But that jiggling doesn't change the wall thickness between the ID and OD on the left end.
By my calculations, the wall thickness you are looking for would be .2075 max and .1575 min (worst case).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum shift + Stack-Up
Here's a link to such a paper < pib.sagepub.com/content/214/3/235.full.pdf > that you probably have to cut and paste in your browser's address bar.
Any good resources for stack up analysis (besides course work, training courses, etc.)?
Looks I'll be going back to the tolerance analyses I recently performed for my actual parts!
RE: Datum shift + Stack-Up
If you are trying to calculate wall thickness (amount of material between OD dia .750 and ID dia .375) datum shift should not be included in the tolerance stack.
Why? There is no datum shift between features that are gaged simultaneously (i.e. that are contolled by a geometrical tolerance using the same datum references, with the same material modifiers and in the same sequence). In your example both position controls references to the same A(M) datum.
I could recommend you 2 books on Tolerance Stacks with GD&T usage:
- Fisher B. - "Mechanical Tolerance Stackup And Analysis"
- Krulikowski A. - "Tolerance Stacks Using GD&T"
I especially like the second one. It is really comprehensive and offers very logical (at least for me) step-by-step explanation of whole process of calculating tolerance stacks for different geometrical characteristics.
RE: Datum shift + Stack-Up
min. wall thickness -- .1575
max. wall thickness -- .2075
RE: Datum shift + Stack-Up
But what is purpose to specify M modifier with datum A for pos. requirements if it do not allow to float datum from ideal axis of RFS thread.
Contrary if it is allow to float datum then it will affect in variation with worst case.
RE: Datum shift + Stack-Up
pmarc: Does Krulikowski's publication address the datum shift quandary for stack up within a component? As I noted to J-P, the papers (see attached) I've been leveraging claim to use examples from Krulikowski's text to validate their method and the examples include the datum shift in the stack.
RE: Datum shift + Stack-Up
Yes, Krulikowski's book offers datum shift calculations within single component as well as within larger assembly.
And your interpretation of my previous statement is right - for your example datum shift matters only when considering position of one feature (ID or OD) relative to datum feature A referenced at MMB (Maximum Material Boundary).
RE: Datum shift + Stack-Up
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Peter Truitt
Minnesota
RE: Datum shift + Stack-Up
The "nominal/plus/minus" method you're using can also factor in bonus and shift, but they must be rolled into the nominal and plus/minus numbers. More info if you need it.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum shift + Stack-Up
I guess I need to get the Krulikowski text to understand when and where datum shift should be included (I've found it's not always intuitive). However I was under the impression, and maybe a false one, that Krulikowski's method always factored in datum shift into the stack up. I just need more education (and time in a day).
RE: Datum shift + Stack-Up
When to include datum shift in a stack depends on how it's used and what the stack problem is. In general, there is no effect on a stack from datum shift if:
• the datum is referenced RMB
• the features are gaged simultaneously
• the shift acts in a direction different from that of the stack path (may be a partial trigonometric effect from shift, depending on the angle)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum shift + Stack-Up
Peter Truitt
Minnesota
RE: Datum shift + Stack-Up
RE: Datum shift + Stack-Up
However, regardless of the thread call out, the axis of the thread should be derived from the pitch cylinder unless MAJOR or MINOR DIA is specified by the datum flag per ASME Y14.5-1994 (& 2009). The one piece of info I left off is that all GD&T is per ASME Y14.5M-1994. My datum stack used the pitch diameter tolerance from ASME B1.1-2003 for the 5/8-11 UNC-2A thread.
I too considered measurement bias and thought maybe this is why one would add the datum shift into the stack. The only gauging scenario I could think of is attached, but I can see huge measurement bias creeping up given that the OD of the gauge would simulate the axis of the threads.
RE: Datum shift + Stack-Up
Peter Truitt
Minnesota
RE: Datum shift + Stack-Up
I also noticed that the mean of the inner and outer boundaries for the O.D. do not coincide with the nominal size of the O.D. which means that the fabricator will need to decide whether to aim for the size or the position. I'll guess that they would aim for the size nominal, but there is no law that says that they must.
So I moved datum 'A' to the .375 diameter. Then I determined the outer and inner boundaries of the O.D. based on the .1575 (min) and .2075 (max) walls, calculated by several folks. The mean of these boundaries is .74 Dia.
Then I went out on a limb a guessed that, since the original drawing allowed the O.D. to float within larger boundaries, the design might allow the basic O.D. nominal to be the same as the mean of the boundaries.
Then, I set the second segment of the feature control frame at .015 (to cover all of the .750+/-.005 sizes) thinking that more size tolerance could probably be allowed if the nominal size was at the mean of the boundaries. (Another assumption, I know.) But the stack-up didn't look good, so I backed off on the .015 and ended up at .009.
I would be pleased to hear comments on all my assumptions and concerns about making the size nominal the same as the mean of the boundaries. Also, does my stack-up add-up? Do you find the stack-up easier or harder?
Peter Truitt
Minnesota
RE: Datum shift + Stack-Up
The mean of the inner and outer boundaries for the OD doesn't coincide with the nominal size of the OD because of the MMC modifier. Reason: the bonus tolerance allows more position error when the OD is smaller, so it's not equally distributive.
This is why you gotta be extra careful when using a stack spreadsheet that tries to lump everything into a "nominal/plus/minus" format. Here's how I would do a stack for your original drawing using that Excel template: First, we all agree that there is no shift tolerance But to account for the bonus tolerance, you can simply roll it into the size and use the "boundary" dimensions in the stack. Take line 1 for example: instead of having .375 followed by a position tolerance line and then applying the bonus line separately, we could just have one line: a positive NOM of .3725 (half of the OD's LMC) and the ± column as .015 (half of the total possible position tolerance). I think this is really what you were saying: the midpoint of the boundaries (my .3725) is not the same as the midpoint of the size alone (.375, radially speaking).
Line 2 on that spreadsheet would then simply reverse things for the ID: the negative NOM would be .19 (half of the ID's LMC) and the ± column would be .010 (half of the total tolerance happening to the ID). That's it! Only two lines in that stack, and the resulting answer will still be .2075 max wall thickness and .1575 min wall thickness.
The sketch from you last post is a bit different so I'll have to digest that one a bit... But I would say that while moving the datum around can make the stack numbers easier, in a real design the datum should be selected based on functional interface. For that reason, maybe keep the datum on the threads?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum shift + Stack-Up
Would the part be any different if someone were to remove datum 'A' and all reference to it (from the hand sketch)? Would the stack-up change? Would it violate ASME Y14.5? Is this datum implicit or explicit in what it does for the design?
Peter Truitt
Minnesota
RE: Datum shift + Stack-Up
I understand (and appreciate) J-P's comment about simplifying everything to 2 lines, but the contributions to the stack are "hidden" requiring further explanation to someone trying to make heads or tails of the analysis. Having a line item for each contributor is better in that regard. What column each bonus (or shift) tolerance goes into is another matter (i.e. less intuitive) and I had to rely on 2-D sketches to make sure I was putting everything into the right column.
RE: Datum shift + Stack-Up
Peter, sorry that I was combining my reply to Dave and you together. I like your idea of using profile, because it tackles size, form, and location all together. But it doesn't allow bonus tolerance. Not sure what to do...
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datum shift + Stack-Up