Determining Position Tolerance Value

[Dikuza]

Hello,

I understand most GD&T and how to interpret a position callout, however, I'm still a little confused on how to actually determine the correct value for diametrical position callouts.

Please let me know if my below method is correct:

1. Two parts are to be bolted together via a pattern of 4 .25-28 UNF-2B bolts.
2. From online tables, I have determined the correct close-fit clearance hole to be .266 +.006/-.001 (.265 to .272).
3. From this, subtract the bolt major diameter from the smallest hole: .265 - .250 = .015
4. Therefore, I have .015 tolerance to split between the two 4-bolt patterns (.010 and .005 or .007 and .008 for example)

I feel like this is too simple. Does the threaded hole not come into play at all?
What book and or website do you guys use/recommend as a good resource for GD&T?

Any help is much appreciated.
Thanks!

 

[KENAT]

Retsied, ASME Y14.5M-1994 has the equations in appendix B4 and I've posted an excel sheet* before that has the most basic 'fixed' fastener version that you invoke above in it.

It also gives guidance on projected tolerance or over-sizing the hole in lieu.

I suggest you take a look at the proverbial 'horses mouth'.

BTW, I'd think twice putting much less than a .010 tolerance on a threaded hole - it's often better to split the tolerance allowance with a bit more going to the threaded hole than the clearance hole due to typical process capabilities for threaded V plain holes.

The typical 'close fit' holes may be more applicable for floating fastener than fixed fastener so take them with a pinch of salt as they can lead to unnecessarily tight position tolerances.

(*Be careful there's a bit of an error on a tab relating to converting +- dims to pos tol.)

Posting guidelines faq731-376

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

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[SeasonLee]

If you have Y14.5 2009 std, pls look at page 191~193 Appendix B.

Season

 

[Dikuza]

Thanks for the replies and reference to Y14.5-2009. After looking over this, it appears I have done it right. Just seemed too simple to me. Thanks guys!

 

[pmarc]

Not sure if this discussion is still active or not, but I think it is worth to mention that what OP said in initial post (that there is 0.15 to split between the clearance holes and threaded holes) is true only if projected tolerance zone concept is used in conjuction with position tolerance applied to the pattern of threaed holes.

If the projected tolerance zone concept is not used, possible orientation error of the threaded holes relative to mating face needs to be taken into account - and this is where additional threaded hole parameter, that is ,the minimum depth of engagement of threaded member comes into play. The formula in Appendix B.5 of Y14.5-2009 shows how it is done.