Form tapping a no-no in aerospace?

[Metalcutter]

Electrical Conductivity

I had a "run in" with an engineer on rolled threads. After a "meeting of the minds" he informed me that for microwave work when trying to tune their filters the rolled threads made it very difficult to tune because of the rolled thread form.

The main problem was they liked to tap the holes as deep as they were drilled. *Ugh

0-80 taps make better studs than holes in this case.

So Electrical Conductivity is another feature of cut threads. I found this interesting.

Regards,

Stanley Dornfeld

 

[Tank]

I asked a friend at the Johnson Gage Company. Here's his reply:

"To our knowledge, we have never heard of any restrictions on form tapping in aerospace. It does create a stronger thread, but also you run into the "fish lips" condition on ID thread at the crest if not machined properly. It may create a void & reduce the structural integrity of the thread. This void could have chips that break off and reduce the amount of flank contact. Plus pieces flowing around in the assembly is not a good thing, especially in a rotating part."

MRAINEY, I find this interesting. In my previous post my statistic about 70% of aircraft failures... was from the CEO of Johnson Gage Co. I attended a very good semenar that he put on. I would classify Mr. Johnson as a 3rd gereration thread expert, as the legal system does. (As a legal witness thread expert).

Now that I think of his position there was no time spent on how the thread is produced. It was totally on on inspection methods and thread form, ect. If you could meet spec.s and chisel in the correct form it would not matter how it was produced. As long as it met all the dimentional requirements.

My companys testing shows that internal formed threads in aluminum shear at the root raduis before cut threads do. This is due to the cold working or work hardening of that area. My guess is most of the larger aircraft companys have done the same type of testing with similar results.

For most land based threads that are not marginal in design strength (due to weight restrictions) this is not a problem.

I want to make this post clear. I consider any info. coming from the Johnson Gage Co. as expert data on dimentional form of threads and the inspection there of. I do believe that their expertice stops there and manufaturing of and material choices/joint strength requirments of threaded designs are up to the engineer of any given application. Myself I also have seen many posts form MRAINEY and always consider it very usefull to the topic at hand, as was this. I just needed to put mt 2 cents worth in.

 

[mrainey]

It looks like there are several important considerations with form tapping:

Geometry at the crest ("fish lips").

Integrity at the crest (breaking off).

Metallurgical characteristics at the root (possible shearing).

Will the customer accept the formed threads?


Why take a chance, especially with critical parts?

 

[viper]

I think where I was trying to go with this thread was to ID certain standards that might prohibit forming and what the language looks like. I realize it can be controversial but usually in aerospace, everything is spelled out in the relating control documents. I realize many will have a differing opinion on forming so I was more or less looking for factual info like standard referrences or first hand experiences in aerospace.

 

[strokersix]

This discussion reminds me of when I used to work for a company that provides aircraft flight hardware. There was a design where a component was held in place by the edge of a pan head screw. Of course the screw head bend when the screw was torqued. I proposed a design change to provide full diameter bearing under the screw head and produced prototypes to illustrate.

The rules are that the hardware must be requalified meaning a series of expensive tests for any design change. Result is that even when the proposed design change is clearly superior, it may not be implemented due to the high cost of verification.

Unintended consequence of the regulations.

 

[Hansdie]

Thats why there is a process of cut-form threading that gives stronger threads without being too brittle and no fishlips...not in widespread use...but some are doing it

 

[machine1medic]

The broader question might be:

What's more important? What is likely to be better, infact allmost certainly.
Or what is absolutely concretetly beyond any shadow of any dought
certifiably and documented to be OK (addequit as tested and pidgionholed)
to be OK , so that lawers have a basis to ply their trade on,
in the very unlikely event of failure.

 

[Tank]

I think where I was trying to go with this thread was to ID certain standards that might prohibit forming and what the language looks like. I realize it can be controversial but usually in aerospace, everything is spelled out in the relating control documents. I realize many will have a differing opinion on forming so I was more or less looking for factual info like standard referrences or first hand experiences in aerospace.

Like has been stated, areospace companys all maintain their own standards or documents that cover just about everything - O-ring grooves, snap ring grooves, threads, ect.

Our thread standard is clear on the threads. "Form threads are NOT allowed on ID threads on any product unless explicity allowed by a note on the part drawing. If allowed formed ID threads must conform to MIL-S-70335".

I guess maybe I'm missing the delema here. If you are not sure if form threads are allowed contact the company.

If you know they are not allowed and think they should be, well the company is paying you to follow their spec.s, know matter how stupid you or anyone thinks the spec. may be. Myself, I would follow the customures spec.s.

If you did produce a formed thread that was not allowed by your customer there is a chance that your parts will get bounced. My guess is that everyone who has responded to this post could glance at a thread and tell you that it was formed. Aircraft assemblers are trained to question anything that looks abnormal. Our assemblers always think formed threads have burrs in them. They are not used to seeing them and they look differnt, plain and simple. If formed threads show up on a part where its allowed we have to explain that the "burr" on the minor dia. isn't a burr but is acceptable and what you get with a formed thread. If the part doesn't allow formed threads all parts go back to the supplier as scrap.

Our prints where cut threads are required call out the minor dia. of the thread. It would be the wrong size for a formed thread. If the minors are called out on the drawing it's a good indication of an intended tapped hole.

Where formed threads are allowed only the thread is called out and a lead drill depth (no dia.) with a min. thread depth.

If you were to try to form a thread when the minor dia. is called out your hole size is oversize below where your form thread closed the minor dia., thus you have a non-conforming part. It would effect nothing, but the part would not meet print requirements.

Do the prints you are working with call out the minor dia. of the thread to a depth below the min. thread depth?

 

[Pat Murray]

Many if not all of the bolt and stud specifications used in the petrochemical industry do actually specify in exact terms that there are to be no cut threads... This may be because the petrochemical industry is more concerned with preventing leaks then bolt or thread failures... Also in many cases both pressure and temperature are significant factors... Most of the bolting in these industries are concerned with pressure containment rather than with assembly... In my many years as an inspector I have only seen a small few stud failures using form threads, and in every case it was related to high operating temperatures where the bolts had relaxed at operating temps... Buit to answer your question Yes there are codes and specifications that do in fact spell out the thread method...

 

[bbiggs]

Formed vs cut threads

On small Piper airplanes with wings above the "body" they had a problem with the cut threaded "Jesus" fork bolts that hold the wings on breaking. The FAA requirement now requires rolled threads, and all the cut threaded forks in the field were mandatorily replaced.

Aircraft Mechanic-Inspector

 

[Pat Murray]

I've seen a lot of "aerospace" external threads that specify rolling. All the NAS and MS bolts, nuts and nutplates are all rolled/formed threads.

If the print and the specs say "don't do it" don't. I wouldn't rely
the tiniest little bit on what somebody says.

Read the applicable specs, adhere to those and screw everybody else.

For some folks that may not know... When the specification says "Use this" or "Do it this way" they use words that mandate the use of certain materials in a certain way... Like "Shall be used" or "Will comply with..." This then becomes a legal mandate... and violation is cause for prosecution... It has happened where craftsmen have been held accountable... And some are actually facing criminal charges for "shooting from the hip" Don't do it... when the spec says it they really do mean it...

 

[SND]

I think any design that fails on it being a cut vs form thread, is cutting it pretty f-king close and I wouldn't wanna be anywhere near it either way...

 

[chipload007]

- two small crests rolled together into an upside-down W shape.

Wouldn't that be an "M"?

 

[Noah Katz]

"Aero" and "space" can be quite different.

I work at Lockheed in the latter, and the design goal as far as loads is fatigue life sufficient to get through a total of about 10 min of vibration (testing and launch).

 

[ipmmiller]

I can tell you why areospace companys are fussy about the're threads. 70% of all aircraft accidents that involve a mechanical cause are due to thread errors. Now this could include design, assembly, or manufacture but none the less, the miniscule amount that a company could save in the total cost of a large aircraft probably makes changing this rule impossible.

Kind of makes a "simple" thread seem alot more important.

this topic is way beyond my technical experience but this percentage of failure due to thread errors would seem to be a great reason for changing something about the rules or the process?

 

[badvette]

On small Piper airplanes with wings above the "body" they had a problem with the cut threaded "Jesus" fork bolts that hold the wings on breaking. The FAA requirement now requires rolled threads, and all the cut threaded forks in the field were mandatorily replaced.

Aircraft Mechanic-Inspector

That is a bolt, outside thread which I think is a whole different animal. Is this a AN bolt, is there STS for this, I presume this is an AD? Just curious. Those old rag wings!

 

[ietech]

Bolt body size?

That is a bolt, outside thread which I think is a whole different animal. Is this a AN bolt, is there STS for this, I presume this is an AD? Just curious. Those old rag wings!

It is also true that OD threads for roll forming are made from reduced size stock. This makes the body diameter of the fastener smaller than a bolt with cut threads.

A 1" thread is rolled from .912" round stock giviing a full 1" thread with a reduced body size.

I am not an expert on this just read it somewhere --- how critical the body size is could also be a factor in some manufacturing requirements.

 

[ietech]

Found where I read about the stock size

Isn't the body diameter left larger? It's only reduced in the area of the rolled threads isn't it?

I would think that the full size stock could be cut down to the correct dismeter to roll form but this would eliminate the labor cost saving by adding lathe time to the cost. I would also eliminate the weight reduction.

I don't know if this information will help but I found the link where I read about this topic.

http://www.portlandbolt.com/faqs/rolled-vs-cut-threads-bolts

Hope this is helpful to someone here.

 

[USMCPOP]

Isn't the body diameter left larger? It's only reduced in the area of the rolled threads isn't it?

On better bolts, yes.

You can see the "step" at 2:48 in this video about making bolts.

YouTube - ‪Making Nuts & Bolts‬‏

 

[ietech]

Thanks

On better bolts, yes.

You can see the "step" at 2:48 in this video about making bolts.

YouTube - ‪Making Nuts & Bolts‬‏

Great video - very interesting.