Thread Class/Fit question

[kwal89]

Does any one know if threads that are the same size and pitch but have a different class designation still fit into each other? Say I have a 10-32 2A thread would that mate with a 10-32 1B or 3B hole?

I would assume that since 1B is the "loosest" designation that a 2A or 3A might fit into it, but not the other way around.

I am asking this question because I am trying to determine the thread of two components and I am uncertain as to what they are. I measured the major diameter to be 11.00mm and the pitch to be around 0.75. So I thought great its a M11 x 0.75 and bought a tap to help me confirm. The tap I purchased is an M11 X 0.75 - 6H. The tap fits just a little loose when I put it in the female portion. But when I taped a new hole in some scrap I had, the male piece did not fit great. It threaded in fine for two turns but after that there was so much interference that I could no longer turn the male piece.

Either my thread class is an issue or the pitch is custom and not a standard size.

Any one have any experience with mixing and matching thread classes?


Thanks
Kyle

 

[KENAT]

If memory serves, simplistically class 3 nut on class 3 screw results in line fit at worse case tolerance stack.

Any other combination will have clearance.

This may not apply regardless of length though - for long engagements you may face an issue but it's been a long time since I flicked through ASME B1.1.

Of course if you're talking metric that's not class 2A-2B type fit per B1.1 and I'm not familiar with the metric spec.

Posting guidelines faq731-376

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

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

 

[DerbyLoco]

Does the screw lie comfortably in contact with the tap. Even a small pitch variation will show up and I find that screws are very sensative to pitch variation so the fact that you only get two pitches before binding would make me question the pitch first.
11mm is an unusual diameter so could it be 7/16" x 32 tpi although I cannot find such a thread in my ref book so it may be a special. Another thought is that your sample has got streched and so elongated the pitch.

 

[hydromech]

If the thread tolerances are correct, the male part should go all the way in with a marginal increase in torque caused by friction.

If the torque increases as the male thread is engaged, then the pitch is wrong.

This is assuming that the male thread is straight.

 

[Cockroach]

Sounds more like a thread pitch issue than class concern. Commercially available threading is class 2, you can cut or perhaps buy specialty tooling otherwise.

I would get a pitch gauge and measure pitch. Just because the tap fits loose, you may have a 0.8 mode thread for example. So look up the pitch possibilities given your thread size and measure pitch with a gauge, not another tap.

So I think it is your method that gives you the problem at hand.

Regards,
Cockroach

 

[kwal89]

Thanks for the input guys.

 

[Tmoose]

http://www.docstoc.com/docs/93865888/STANDARD-INCH-PITCH-DIAMETERS

According to the image on page 2 here the largest 2A external thread pitch diameter would always have some clearance with any internal thread. 1B, 2B, or 3B.

http://www.fastenal.com/content/feds/pdf/Article - Screw Threads Design.pdf

 

[tbuelna]

The published dimensions for a given thread class are limits. Whether a given pair of threads will engage depends upon their specific dimensions. With small diameter thread forms like your number 10's, any tolerance accumulation can be critical, such as pitch diameter runout, axial pitch variation, etc. Larger thread forms are less sensitive to these tolerance variations. The most problematic issue is the axial pitch tolerance accumulation with long thread engagements.

 

[desertfox]

hi kwal89

In the UK M11 is not a preferred first choice thread size, it jumps from M10 to M12 although both these threads can both have a 0.75mm pitch, are you sure its a metric thread?

http://www.roymech.co.uk/Useful_Tables/Screws/Screw_Threads_1.html