Using spreadsheet for Tolerance Stackup 5

[irfan08]

We manufacture pressure vessels, DFMA reports following critical parts,

1. Neck
2. Top & Bottom Head
3. Shell
4. Stud

I would like to have a good understanding on how to implement tolerance stackup using data from spreadsheet. For starter we are only concerned in 1-D (Height Only) analysis for POC.

I have seen the excel file from KENAT, its just I am new to statistical tolerancing or even tolerancing, I need a basic work around which is simple and easy.

Any help will be appreciated.

Thanks

 

[KENAT]

irfan, simple tolerance stacks are explained in ASME Y14.5M-1994 section 2.6.

In your case lets say:

1. Neck 1+-.01
2. Top & Bottom Head 2+-.01 (is this one part or 2, I've assumed 1?)
3. Shell 5+-.01
4. Stud .25+-.01

So 1 + 2 + 5 + .25 =8.25 nominal

You can add the tolerances in the same way.

(+-.01) + (+-.01) + (+-.01) + (+-.01) = +-.04

In cases where it's not equal bilateral +- tolerance you can have two tolerance columns in your spreadsheet. One for plus one for negative.

Also, you need to consider if your part is geometrically positve or negative, a simple stack is always positive but when you go up to one 'face' and then back away from it, the 'backing away' dimensions are negative.

Any help at all?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[irfan08]

I am following "Traditional Approaches to Analyzing Mechanical Tolerance Stacks" by Paul Drake. I do understand loop diagram (somewhat) and its just I don't want to get tangle in Statistical lingo too much.

 

[KENAT]

irfan, there is no "statistical lingo" in my above post. It's a simple 'worst case' analysis.

In fact, as regards the excel spreadsheet of mine you reference, there is no statistical lingo there (assuming you're refering to my holes tolerance calculator).

Is this link any help.

http://books.google.com/books?id=4Z...=X&oi=book_result&resnum=3&ct=result#PPA61,M1

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[irfan08]

Oh I didn't mean in your post, its just my own read and research I am doing. My case is for statistical and not worst case.

I can now see how loop diagram is formed. Thank you very much.

Let me give you a brief idea what I am trying to do here is, to achieve a scenario, where I can mock up different tolerances on Variable parts(not fixed) to get a desired stack up height of my Vessel.

 

[KENAT]

Take a look at this.

I didn't create it but have tidied it up and used it a few times.

It's error prone when you insert rows etc so be careful.

I wouldn't necessarily use as it but it may give you some ideas.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[Brodie79]

Good spreadsheet Kenat. Stack analysis is probably me least favorite task but one of the most essential.

 

[irfan08]

That's great. Is "Geom factor" same as sensitivity?

What is the importance of process capability indices, like Cpk on statistical tolerancing for stackup? Is it too far stretched of analysis if I include Cpk. Any easy reading reference?

Thanks

 

[KENAT]

Geom Factor is whether it's positive or negative contributer to the stack.

Hopefully the attatched sketch helps clarify (sorry the terminology isn't quite right).

Have you searched on this site about cpk, there was a thread fairly recently as I recall.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[irfan08]

thanks for clearing it up, now I fully understand.

What is the importance of tol^2 (normal) in the wokrsheet you have uploaded previously. How will this feed back to my tolerance.

 

[KENAT]

Have you heard of the term "root sum squared" or "root mean squared"?

If not, then frankly, you may be in over your head - or at least have some serious reading to do.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[irfan08]

Phew, that is a lo blow to a learner. I have and continuosly doing my reading. I apologize If my questions seem redundant or lacked experience.

Thanks for your help though, I really appreciate it and I do have a better understanding now.

 

[KENAT]

You took it the wrong way.

I asked if you knew the terms which the tol^2 refer to.

I then said if you didn't know the term that you may be in over your head or have a lot of reading to do.

It wasn't a dig.

However, RMS/RSS forms the basis for a lot of statistical analysis of tolerances. So it's a pretty fundamental principle.

I've been accused of being blunt before, if it causes offence sorry, but seeing as this is free advice you're getting...

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[irfan08]

No offense taken whatsoever. For a free advice in this day and age I will take any hit.

My first goal was to create a spreadsheet for assy, where I can plug n play for clearance gap required at different point of interests. then Use statistical tolerancing to resize where applicable to minimize errors in our stack up height.

I have read about RSS and MRSS and I guess time will tell if I succeeded in my project.

thanks for the insight.

 

[DanStro]

irfan08,

You may want to look at the website below. I use the spreadsheet a lot (modified somewhat) and I learned a great deal about stack up calculations from the resources on it, both normal and statistical.

About the only things I usually worry about from a statistical end are the RSS and I do Monte Carlo simulations so I don't know much about Cpk and other things that relate to 6 sigma tolerancing.

http://adcats.et.byu.edu/home.php

I hope this helps.
Dan

 

[irfan08]

That was great. Thank you DanStro