Form tapping a no-no in aerospace?

[viper]

Our customers do not have a problem with it BUT I talked to 2 people today that indicate aerospace and form tapping do not mix... Gunna have to call some engineers to confirm this one. Anyone else have some info on this? I know designers (my brother included) do not like to see anything that distorts or otherwise changes the grain structure of the material but I think we would all agree that forming DOES make a better, stronger thread.

 

[crij]

It does make a stronger thread, but it also makes a brittler thread, since the threads are now workhardened. Depending on the alloy being used and the courseness of the thread, the threads could be close to failure or already cracked.

Knowing how sensitive the aviation industry is to anything that might fail catastrophically, just the theory of something having the risk of internal cracking will send them into fits.

Just my observation,

Rich C.

 

[Conrad Hoffman]

I'm the biggest fan of form taps, but the resulting thread does have a small groove at the peak and with certain fasteners, and especially with power tools, there's a slightly higher chance of cross threading.

 

[Ferrous Antiquos]

Compared to a cut thread where the crest comes to a 60-deg point with a flat on top, the rolled thread is actually a double crest - two small crests rolled together into an upside-down W shape. The mini-crests are prone to breaking off, leaving little chips floating around, not good in any kind of closely fitted moving parts. What remains of the thread form may still be strong enough though.

 

[viper]

I guess I did not want to get off on a tangent of discussing the pros and cons of form tapping since there are obviously a few cons. I would like to determine if someone has been prohibited from forming, and what the situation was. As well, we are looking through the standards to see if there is actual specific language that would prohibit it. At this point, I am thinking this is a case by case basis.

 

[Bobw]

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.

 

[Limy Sami]

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.

X2 . what he said.

40 + years ago I was taught ;- ''If in doubt, ask.'' and it's served me very well.

 

[Ox]

I have seen this on OD threads on a SS part once. Said right on the print:

"Doan doo it Lucy!"

The part would have been cake-walk to roll, but cutting it was a bleeding nightmare!

I don't know 100% for sure the app. It could have been aero, but I was told that it was for a sub.


---------

Think Snow Eh!
Ox

 

[SeymourDumore]

Viper

If this is an aerospace part, the the original design will almost definiately be controlled by their own manufacturing processes. Let that be P&W, Sikorsky, Airbus, Rolls, GE, Honeywell or whoever.
When you look at the print, the only thing you normally see is toleranced dims, untoleranced dims and explicitly specified features.
All othe UOS dimensions, manufacturing control processes, surface characteristics, heat treatment processes etc etc are called out by a spec number, usually in the notes section.
If you don't see a note about the thread manufacturing method on the print, then you have to look in the referenced manufacturing process spec. If that doesn't explicitly disallow thread forming taps, then you can use thread formers.
Similar issues arise with EDM operations. If there is no statement controlling it's use ( or forbidding it's use as it may ) then you can EDM the part.

As for formers, I have only heard of it mentioned by someone with regards to STI threads, but whenever I look at any MS, AS or direct Helicoil specs I cannot find any reference or indication that they cannot be used.

Bottom line is, with Aerospace parts you need to find the specifications to determine what can or cannot be used. If it's not mentioned, then typically you're in the clear to use it.

 

[viper]

For a reference, we selected the parts we are running now. The threads are controlled by SAE-AS8879. I cannot seem to find it online nor do I have it in my stash. If anyone does and would be kind enough to share, please PM me or simply look through the standard for formers. I am pretty curious about this. Enough that I will probably call some people with Boeing to get the skinny on this deal.

 

[Tank]

I work for an areospace company that does not allow form tapping on any of our parts unless explicity allowed by a note on the print. I think the total parts of ours that would allow this would be around .05% of our parts. The only reason these are allowed is because it was a product of a company that we bought and there is pedigry of this process working in the field (sky).

I know there are other threads where this has been debated to death, might be able to find them on a search. I have tried to get this changed myself at my company to no availe. Much of the testing is quite dated on this but finding an open mind is often difficalt.

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.

 

[SeymourDumore]

For a reference, we selected the parts we are running now. The threads are controlled by SAE-AS8879. I cannot seem to find it online nor do I have it in my stash. If anyone does and would be kind enough to share, please PM me or simply look through the standard for formers. I am pretty curious about this. Enough that I will probably call some people with Boeing to get the skinny on this deal.

Viper

I believe the SAE spec will have no problem with it, but the Boeing spec does override it anyway so do have to check with them.

As far as Tank's comment, they better have that explicitly declared in their own specificiation.
IOW if it's not in the spec and not on the print, you're OK to do whatever you want.

 

[dicer]

A rolled thread is less likely to have the stress risers that a cut thread will have, I would think that would make them mandatory in aviation. Though it seems form threads do not have the same percentage of threads that a cut thread has, yeah I know the charts.

 

[viper]

The spec we were looking at was the 8879 mill spec that has been taken over my the sae spec so I am not sure how the Boeing spec would be involved. If it is not in the sae spec, we should be fine on the part we were thinking about but certainly something to consider for Boeing spec parts which are common here.

 

[SeymourDumore]

Viper

My point was that the Boeing drawing might reference a feature to be an AS or MS component - say a boss having one end to be AS53614-12.
You will have to get the dimensions from the AS spec, but you will also have to observe the Boeing specifications for the part. If they collide, the Boeing spec wins.

 

[cncprogman]

+1 to Seymour's replies. I worked for a defense contractor for 10+ yrs, and unless the MS spec said no , we rolled it 4-40 and up.

 

[MikeMc]

We have done both rolled (formed) and cut for aerospace work. Most of the time the spec will be called out on the print, sometimes it will list either the rolled (or formed) or cut spec, sometimes both. It never hurts to ask. We just finished a job doing a quad-lead formed thread, what a hoot.

 

[machinistrrt]

At the aircraft OEM I used to work for, female threads were spec'd by size/pitch, location, depth of thread, max depth of hole, thread form (UNJ, for example) and class of fit (almost universally 3B). Whether or not it was formed or cut was up to mfg engineering, who generally used form taps where possible.

Note that this applied to fastener holes only; threaded holes that were a part of motion control or other highly loaded elements were different.

 

[mrainey]

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."

 

[Mike Ortega]

The toolcrib here doesn't even stock them, but we do use them on fixtures and some non critical/flight parts.