Anyone w/UNJ threading experience?

We are getting ready to thread mill a part and I am looking to confirm (or deny) something our threadmill supplier told us...

The actual thread spec says,

"3.875-12 UNJ Class 3B Other Thread x .460 deep per MIL-S-008879"

Our threadmill supplier says we should just use a standard UNF thread mill and that opening up the minor dia is what turns it into a UNJ thread. None of us here has had any experience making UNJ threads, but from looking at the MIL spec, we are worried that we wouldnt be meeting the correct radius requirement with a UNF thread mill.

Can anyone out there with experience at this confirm or deny what our supplier has told us?

Thanks,

Tim

Tim,

Your thread mill supplier is correct. The external (bolt) 12UNJ thread has a very specific root radius of 0.0125 to 0.0150 inch. The internal 12UNJ thread needs to clear that radius, hence the enlarged minor dia. See ASME B1.15 in addition to MIL-S-8879. I just happen to be starting a job with a 1.0625-12UNJ thread so I have all this in front of me.

Cliff

Can anyone out there with experience at this confirm or deny what our supplier has told us?

Click to expand...

I'll confirm the confirmation.


http://closetolerancesoftware.com/index.html

I can verify the confirmation. Just use the J specs for minor dia.
Old Bill

Everyone is correct. The female J thread only has a modified minor dia. The male J thread has a larger root radius than standard UN thd.s for strength. The female thd. needs the larger minor dia. to clear the larger root radius on the male thd.

(The pitch dia. is standard on both male & female.)

Hope this didn't confuse you but It's about the simplest way I could explain the J threads.

Thanks, Everyone

Thanks for all your replies.

I feel better hearing it from people with experience who AREN'T trying to sell me anything.


Tim

Does that MIL spec require a thread comparator to inspect? (for function AND PD)

Something I wrote a while back:

The UNJ threadform is a modification of the UN (Unified . . . NOT "Unified National", despite a fair number of textbooks and other references using that erroneous term) requiring 1) a radius of between 0.15011 x Pitch and 0.18042 x Pitch, tangent to the flanks of the screwthread, at the Minor Diameter of the external thread, and 2) a Minor Diameter of the internal thread large enough to clear the radius of the corresponding external thread.

The basic profile of the internal UNJ thread has flats at both the Major Diameter and the Minor Diameter. The basic flat at the Major Diameter of the UNJ internal thread is 1/8 x Pitch (exactly as with the UN threadform), while the Minor Diameter flat of the UNJ internal thread is 5/16 x Pitch, which is larger than the 1/4 x Pitch Minor Diameter flat of the UN threadform.

With the flank-to-flank angle of the UNJ screwthread being 60 degrees when measured in a plane containing the axis of the screwthread, the basic length of the UNJ internal thread flank is (1 - 1/8 - 5/16) x Pitch = 9/16 x Pitch, which makes the basic Single Depth of the UNJ internal thread 9/16 x Pitch x Cosine 30 degree.

The basic Minor Diameter of the UNJ internal thread is the basic Major Diameter - 2 x basic Single Depth = Major Diameter - 1 1/8 x Pitch x Cosine 30 degree.

The Basic Minor Diameter of an internal thread essentially defines the maximum material condition, or in practical shop terms the tap drill size for a 100% thread. Any allowance (none for Class 3 threads) and tolerance is added to the Basic Minor Diameter of an internal thread.

John