Positional tolerance of helical threaded inserts 2

[BriPat]

Can anyone tell me what positional tolerance can be reliably achieved on helical threaded inserts--especially the smaller sizes?

 

[3DDave]

Tap drill size sets the minor diameter of the tapped hole. If it isn't still there, the tap was being used as a drill, which doesn't work well. Not that it can't work, just not well.

 

[KENAT]

3DDave, depending on material and thread forming method I don't believe that's true. Tap drill size will be close to thread minor diameter but may not be within the required tolerances.

Explicitly applying the FCF to the minor diameter is sometimes done to simplify inspection, but again strays slightly from the true functional requirement.

Posting guidelines faq731-376

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

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

 

[ewh]

I agree with KENAT. I prefer to specify either MINOR Ø or MAJOR Ø (or let it default to pitch diameter, which while more difficult to verify is most accurate).
I don't care what size drill the machinist uses as long as the resultant thread meets spec.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[tbuelna]

...Well, it makes in process inspection easier but not finished part/receiving inspection as the feature no longer exists - or am I missing something?

That's a valid point. But if the deliverable end item includes installed inserts, then none of the tapped hole thread features would be accessible for verification by receiving inspection, right? It is common practice to rely on in-process inspections as a means of quality control of a finished component. The in-process inspection records are delivered with the finished component, and they are reviewed by your receiving inspection as part of your QA acceptance process.

If your primary concern is truly the PD positional location of the installed insert threads, then you need to develop some method to inspect these features after installation. But this will not be an easy task.

 

[3DDave]

I was referring to the suggestion that no trace of the tap drilled hole remains: "Well, it makes in process inspection easier but not finished part/receiving inspection as the feature no longer exists"

The minor diameter generated by the tap drill should still be present even if much of the original hole surface is carved out by the tap. It's not as accurate a representation as the theoretical pitch diameter, however that diameter is established with any certainty, and should not be held as a definitive of which way a screw will be located or oriented when tightened.

Screw threads are of some difficulty for position callout - because screws are installed loosely, the threads they go into are really subject to movement in accordance with a Maximum Material Condition reference, but then they have conical alignment when tightened, suggesting a Regardless of Feature Size treatment.

This search turns up an amount of unsatisfying answers:

https://www.google.com/?gws_rd=ssl#q=position+tolerance+inspection+of+threads

such as

https://www.linkedin.com/groups/Is-any-one-using-Bonus-1781397.S.59478259

and

http://www.eng-tips.com/viewthread.cfm?qid=374050

 

[tbuelna]

The reason thread PDs are mainly used as the positional locating feature is because when the mating component threads are fully engaged, it tends to produce a self-centering effect about the relative PDs. The mating threads contact at their flank surfaces, and not at the major/minor diameters or roots/tips. The number of engaged thread pitches also has an effect on the positional location between the mating parts. With just a single engaged thread pitch there will usually be significant clearance. But with, say, six engaged thread pitches there will be more of an accumulated interference fit that will result in a greater self-centering effect about the thread PDs.