stumped by large threads...

[WRMorrison]

A friend of mine called this evening looking for help, but I can't seem to find an answer anywhere. He needs to cut some internal threads that he says are 6.5" - 13TPI. I've never worked with anything that large, and he needs to know the what the minor diameter should be. Isn't there a formula somewhere that determines the minor? I can't seem to find any information, and any help would be great.

Thanks,
WRM

[annoying]

Well, though it may not be the absolute correct way, on "normal" size threads, I always subtract the pitch from the major diameter which, of course, always coincides with predrill sizes listed on charts.

6.5 - (1/13) = 6.423077

So, I'd bore it to 6.423"

But there may be a more proper way to calculate

[txcncman]

Did you try looking it up in Machinery's Handbook?

[WRMorrison]

I found this formula last night:

Minor = Major - (5*sqrt(3)/8)*Pitch

So in this instance:

6.5 - (5*sqrt(3)/8)*.0769 = 6.416"

So, according to this calculation, the minor should be 6.416"

Sound about right?

-WRM

[txcncman]

Yes. I came up with:

Internal -

Minor Diameter Minimum = 6.410"
Minor Diameter Maximum = 6.428"

[basalt]

The minor diameter of a UN thread at maximum material condition is--- UN BASIC MINOR DIA. = basic O.D. - (2 X .54127P) You can find all you need at my web site "UN-UNR-UNJ-THREADS" Hope this helps.