Tolerance stackup calculation (ref pmarc example Alex Kurlikovski Fundamentals GD and T book) 1

[greenimi]

I am learning tolerance stackups and I am using Alex Kurlikovski book (Fundamentals of Geometric dimensioning and Tolerancing, 2nd edition). I have a question about the stackup tolerance calculation --fig 9-14 page 265—attached—
Minimum distance X min calculation shown is 4.5mm. Someone around here, who has way more experience than me in those kind of calculations, is claiming that the “real” X min calculation should be 4.1 (not 4.5 shown) because the form error was not included.
In the book: X min is :
69.6 (min length) – 50 (basic) - 10(basic) – 8.6/2 (max size for the hole) - (0.6+1)/2 (hole at the LMC, hole position is at MMC) = 4.5mm
Our expert is claiming the calculation should have started from 69.6 - 0.4 = 69.2 and not from 69.6. Therefore, the result would be 4.1mm and not 4.5mm.
Justification: the size of the feature (length) still has to be ±0.4mm (70.4/69.6 = 70±0.4), if the opposing points meet the size specification and the envelope meet rule#1, then the length meet the requirements. The form error was not taking in consideration for the calculation in the book. (a gage can use all 0.4mm in form error and still be making contact with the datum feature simulator)

I know pmarc had some issues with X min calculation in fig 9-12 page 263 (x min should be 2.7 and not 2.9) and here is that specific thread.

http://www.eng-tips.com/viewthread.cfm?qid=335464

And it’s exactly as pmarc stated: “It is weird to disagree with such authority”
Now, going back to our issue (page 265): Ii is our “expert” points us in a right direction or “the unclaimed form error” is not applicable here?

Thank you

 

[CheckerHater]

It just doesn't add-up to me.

 

[pmarc]

CH,
Right face on both pictures on the second page of my graphic is not the right face of the part (notice thickness to length proportion). I should have probably used thin wavy line in order not to mislead anyone.

That said, on the lower picture on page 2 for instance, the right face of the part will be most likely inclined to datum plane A at the similar angle as flatness tolerance zone to satisfy the size requirement.

 

[greenimi]

Pmarc,

I am still digesting…… reading, re-reading, thinking of your last 2-3 posts (I am not very sure I am still capable of comprehending today, but the weekend is coming so…I have time to do it)
But, anyway…. Hmmm: are you saying that the sketch attached still not violating the AK book (sketch constructed to get the X max: 70.4 max size, 60 basic becomes 59.5, hole at MMC: 8.00) with 10° angle as you suggested in your earlier post. As you can see I got 26.9 max, but is also thickness dependent.

 

[CheckerHater]

Wavy line or not wavy line it doesn’t matter.

Orientation is always refinement of the size.

See ASME Y14.5-2009 Fig. 6-2 for example. “The surface must be within the specified limits of size”

 

[pmarc]

greeimi,
In general the part could look like this, with one remark however -- 70.4 dimension should not be measured in a way you are showing it. It is a distance between two inclined parallel lines, not the distance measured in a direction normal to datum plane B.

CH,
I fully agree with fig. 6-2, but it is not what we are talking about here. We are not debating on orientational relationship between left and right face (that is two nominally parallel surfaces) of the part in my and Alex's example.

BTW, could you point me to a place in the standard where it is really stated that ORIENTATION tolerances must be a refinement of the size. From what I see such statement exists only for PARALLELISM tolerance.

 

[CheckerHater]

First of all, there is no difference between parallelism and perpendicularity. Both are angularity with different value of an angle.
I know “math” is 4-letter word on this forum, but I still have to say they are mathematically equivalent.
(It isn’t only me who says it, Y14-5.1M does too)
Now, will somebody please explain the difference between two cases shown on the picture?
In both cases tolerance zone is trapped between 2 parallel planes oriented in space the same way.
Why in one case this zone is allowed to exceed size limits and in another is not?

 

[CheckerHater]

If you need exact legalese saying there is no difference between parallelism and perpendicularity, see Para.6.6 ALTERNATIVE PRACTICE.

 

[pmarc]

CH,
Another graphic. Maybe this will help.

http://files.engineering.com/getfil...66d4a259550&file=perpendicularity_vs_size.JPG

Para 6.6? Where exactly? I must be missing something.

 

[CheckerHater]

What makes you think you can measure size in random arbitrary direction?

 

[pmarc]

Well, I can't find proper text '09 edition of the standard, but in '94 you can find a note at the end of para. 4.4 saying that in order to relate linear or angular dimensions to a datum reference frame an explicit note must be placed on a drawing. To me this means that as long as there is no such note, the dimensions are not related to datums and shall be measured independently of the datum reference frame.

I am pretty sure that in A. Krulikowski's GD&T Fundamentals there is a figure showing this, but in the light of what I have been trying to do throughout the whole thread, using his book as a reference may sound at least funny.

 

[CheckerHater]

It’s funny, but now I start agreeing with you.
2009 doesn’t add clarity, especially if you look, say, at Para.1.9.1 Rectangular Coordinate Dimensioning referencing Fig. 1-49.
Dimensions on the left picture do not only “locate features”, but definitely create size (same with Para.1.9.2/Fig. 1-50).
Then does the rule “linear dimensions specify distances in coordinate directions from two or three mutually perpendicular planes” apply to the sizes (like 45 and 90 on Fig. 1-50) as well?
One may only guess :-(

 

[pmarc]

I am glad that we start agreeing on something.

I do not want to sidetrack the discussion, but my general comment about chapter 1 to any reader would be - please be careful, the chapter just shows some general techniques of dimensioning, not how to make proper tolerancing.

And coming back to our main topic on perpendicularity tolerance value vs. size tolerance value, does it mean that you have been convinced that perpendicularity tolerance can be greater than the size tolerance? (I am not asking you just to make me feel better. I think greenimi has not been convinced yet and needs additional votes on this). And going further, does it also mean that in the absence of perpendicularity control between A and B in Alex's example, you would agree that the MAX value in the stack-up can't be properly calculated?

 

[SeasonLee]

pmarc / CH

Attached is the figure mentioned by pmarc.

Season

 

[CheckerHater]

I mostly agree that unlike edges and axis standard isn’t clear about interpretation of dimensions being parallel and perpendicular to each other.
I am quite sure that in example from AK’s book ALL and not only basic dimensions are aligned with DRF and the whole problem is simpler than we think.

 

[pmarc]

SeasonLee,
Since the first picture in this thread was taken from one of previous editions of AK's GD&T Fundamentals (based on Y14.5M-1994), I am curious whether similar exercise is shown in the newer edition of the book (based on Y14.5-2009). Could you please check?

 

[CheckerHater]

I really don’t think it’s from Fundamentals.
It’s from Advanced, or separate book on Stack-up.
It just cannot be from the same book OP example is taken from.
Dimension C is clearly aligned with the DRF, so form error is definitely not considered.

 

[pmarc]

It is from Fundamentals - I have this book in front of me.
While the stackup calculations indeed show "Max width dimension from datum B", as you underlined, the drawing at the top does not specify this is the way the 69.6-70.4 dimension should be interpreted.

Imagine one has to do this stackup (or similar) on his own without looking at this example. What makes you think that the interpretation of 69.6-70.4 dimension will be in line with AK's interpretation?

 

[CheckerHater]

What year is your “Fundamentals”? My only has Fig. 9-14, but not Fig. 13-17.

While the stackup calculations indeed show "Max width dimension from datum B", as you underlined, the drawing at the top does not specify this is the way the 69.6-70.4 dimension should be interpreted

You cannot be serious. If max dim is measured from datum and equals 70.4 and min dimension is measured from datum and equals 69.6, then how else they are interpreted?

When one has to do stackup on his own without looking at the example one is free to cut the corners as one pleases.

The only thing I am certain – I am even bigger fan of Independence – just to know for sure NOTHING is related unless said otherwise.

 

[pmarc]

What year is your “Fundamentals”? My only has Fig. 9-14, but not Fig. 13-17.

I was talking about fig. 9-14 (the very first picture in this thread) and not 13-17, which is probably taken from a newer edition. Since SeasonLee posted 13-17, I thought he could search in this edition for an example similar, or better yet, identical with 9-14.

You cannot be serious. If max dim is measured from datum and equals 70.4 and min dimension is measured from datum and equals 69.6, then how else they are interpreted?

You know min and max are measured from datum, because the answer says so. The print is not.

 

[CheckerHater]

You know min and max are measured from datum, because the answer says so. The print is not.

The question is WHY the answer says so? Because "one" can simplify his/her work the way one sees fit.
This is what I said in my very first post on this thread: if you don't agree on your assumptions you cannot agree on your results.

I hope you agree that it's really sad state of affairs, if Fig.9-14 is the only example of stackup available to the users of Fundamentals.