The UNJC thread form specifies a controlled root fillet radius on external threads. You can use any process you like, including rolling, grinding, milling, single-point turning, etc as long as the finished threads meet the tolerances defined in relevant specs like MIL-S-8879.
As Terry mentioned, there is no limitation on manufacturing method. Many suppliers of carbide threading inserts offer UNJ products. Here is one such offering:
kevlar49... to expand on what Tbuelna just stated...
UNJC [coarse threads, -1A or -2A quality, external] are relatively uncommon for Aerospace 'spec hardware'. In that case, good machining techniques and non-aero applications [or lightly loaded aero applications] may be fully acceptable. When application demands quality, such as -3A [highest quality, external], then I would expect nothing less than roll-forming.
However, high quality UNJF [fine threads, -2A, -3A quality, external] are virtually impossible to attain without being roll-formed. For this reason, and the obvious grain-flow benefits derived from cold-rlling, for fatigue life/strength improvements, etc, virtually all aero applications for UNJF threads are specified as critical and shall be roll-formed [whay pay so much for poorer performing cut threads?].
NOTE. All UNJF internal threads [-2B and -3B, internal] will be cut threads in forged parts such as for high quality/durability/re-usable high strength NUTS. UNJC-*B threads [internal] will be cut in all forms of material... forgings, castings, wrought-stock [MF from bar/wire, sheet or plate, etc) only when UNJC quality is required [otherwise UNC would be "good enough"].
Regards, Wil Taylor
Trust Me! I'm an engineer!
Trust - But Verify!
We believe to be true what we prefer to be true.
For those who believe, no proof is required; for those who cannot believe, no proof is possible.
Unfortunately, in science what You 'believe' is irrelevant – "Orion"
To elaborate a bit on wktaylor's excellent post, if you read MIL-S-8879 the stated purpose of the J threadform is for fatigue sensitive applications. So obviously the ideal method of producing an external thread would be roll forming. MIL-S-8879 also describes J threadforms based on 16-pitch, 12-pitch, 8-pitch, etc that go up to 6 inch diameter. At large diameters a rolled thread may not have the accuracy required to provide a fit with its mating part without excessive interference. Large diameter cl.3 threads often must be finish ground to obtain a suitable fit.
MIL-S-8879 also only describes 3A/3B class for any threads, including UNJC (table II).
tublena... Dang... You are so right... Thanks for the reality check.
That'll teach me to re-review the specs [MIL-S-8879, AS8879] before engaging mouth.
Been too wrapped-up too tightly with UNF/UNC specs [H28] lately [on the old lady I work on]. Lots of detail parts [shafts, large pins, etc] use cut threads for the retaining nuts.
Regards, Wil Taylor
Trust Me! I'm an engineer!
Trust - But Verify!
We believe to be true what we prefer to be true.
For those who believe, no proof is required; for those who cannot believe, no proof is possible.
Unfortunately, in science what You 'believe' is irrelevant – "Orion"
One other thing came to mind about methods of producing internal J threads. There are types of flute-less taps called "roll taps", which actually cold roll/displace the drilled hole surface inward as the tap is turned, and this forms the thread flanks without producing any chips. So it is possible to roll-form threads in a tapped hole, and these roll-formed internal threads would have similar benefits as roll-formed external threads in terms of fatigue.
However, there are a couple issues with roll tapping that must be carefully considered when used for producing J threads. One issue with roll-tapping is that the process tends to create burrs/flash of displaced flank material that protrude beyond the tip minor diameter, and these tip burrs/flash can interfere with the mating external thread's root fillets. The burrs/flash can also break loose when the mating part is threaded into the hole, producing metallic debris. Because of these issues, roll tapping is sometimes not permitted for aerospace applications.
When I worked as a machinist many years ago we made bot rolled and cut J threads going all the way down to 10-32 for aerospace applications. As stated above, rolled threads will provide a stronger thread, and the J profile was developed to account for the fact that a rolled thread cannot have a sharp V at the root. An important thing to remember is that you cannot inpspect J threads with typical thread rings. You have to use "Johnson" gages to properly inspect J threads. The gage is a pair of dial indicators set up with rollers. One checks the thread form, and the other checks the thread pitch lead.
I had not heard of this type of thread gage before so I did a bit of research. The Johnson Gage Co. has a nice website discussing how these gages work. Thanks to your post I learned something new today!
I agree that the Johnson website has some very good information. While this is not specific to measuring threads with Johnson-type gages, I found this document (sorry, German language only) to be of very high quality for documenting inspection procedures for various fastener features according to requirements in ISO 4759-1:
