Tolerance Stack-ups with Bonus and Datum Shift. 1

[Agaurav]

thread1103-43357<sub>8

Hi</sub>

I was doing Stack-up for the question in thread referenced.
Then I compared my answer to PDF provided by pmarc, I could only get Part C right, solution for A and B are wrong,
I'm at a very initial level of stack ups, can anybody detail out what im doing wrong in my solutions a and b.

Thanks for all the help.
Anant

 

[Belanger]

Your spreadsheet only shows one row (the ID of 29 to 30). Please show the rest of your data/dimensions.
It's most likely that you didn't get all the solutions because you didn't include what we call "bonus tolerance" and "datum shift" somewhere?

For everyone's reference, the original thread link is:
[URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=433578[/url]

 

[Agaurav]

Hi J P

My bad, i updated the link here.
Please take a Look
My answers are
Case a: Min7.5 /Max12.5
Case b:Min8 /Max12.5
Case c:Min7.5 /Max12.5

Since i'm sure im missing something but not too sure what, im beginning to overthink this and imagining what not.
This is a cry for help.

Thanks
Anant

 

[Belanger]

I believe it's because your calculations don't include the form error on the two diameters. Recall that a position symbol is positioning the "unrelated actual mating envelope." But if they are not perfect circles, then the envelope only feels the high points; the actual answers could be further increased/decreased by any bubble or dent in the circles.
In the thread that I linked to above, look for the last graphic that was uploaded (by pmarc). By doing the stack based on MMB/LMB you can account for the true limits of the two edges. Again, the key is the form error (out of roundness).

 

[Agaurav]

I'm definitely missing a point.
Few other user were missing that too, so that feels good. Haha
How LMB is 48 and not 49.

Are LMBs tricky. Cause in Part C MMB is straight 50.

What am i missing.

 

[Agaurav]

How datum shift actually influences the stack-ups, if it's not a assembly tolerance stack up.

 

[pmarc]

How LMB is 48 and not 49.

Agaurav,
Maybe this will help you understand why.

https://files.engineering.com/getfi...6-9a3a-254af603d7fd&file=LMB_external_FOS.JPG

Briefly speaking, it's because there is no requirement for the feature to have perfect form at LMC. However, if the datum feature A in the stack up was controlled with a zero straightness at LMC tolerance, the requirement for perfect form at LMC would show up and then the LMB size would be 49.

 

[Agaurav]

Thank you pmarc. I was really struggling with this.
Happy thanksgiving guys.