Rolled threads or cut threads

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BJC

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Rolled, or really cold forged threads are ALWAYS preferred unless it is intended to break.
Yet thousands of airplanes have flown with cut threads on tie rods in the wings and streamlined wires holding the wing in place.


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BJC

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IIRC ALL of the McWhite flying wires were cut threads?
I think that that is true, but I can’t verify it. Lots of McWhyte wires flying on Pitts and other airplanes.

BTW, just saw a photo of a Bruton’s wire (rolled threads) that failed through the base thread on a flying wire.


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TFF

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McWhyte was a semi licensed version of the Bruntons. I inquired a couple of years ago to McWhyte if they had the tooling maybe curious if it could be obtained. I got the response that it was in storage and they were keeping it.
 

Hot Wings

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Tidbit of history I found while searching for the proper spelling - which I just guessed at:

 
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imacfii

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Those are cool tools, but they only seem to be available in metric threads. Kind of pricey too. I'll have to look around and see if any come with standard sizes at a reasonable price.
we've been producing Streamline Flying Wires (AN671-676) and Tie Rods (AN701-706) for almost 15 years, all but the earliest ones with rolled threads. The advantage of rolled threads goes way beyond the strength issue. Cut threads leave a stress riser at the shank end of the thread which on a tail brace wire, unsupported in the middle will vibrate and eventually break We've replaced many tail brace wire which started out with cut threads. The rolled thread does not leave a stress riser, as the heads open at the shank end, leaving a nice smooth ramp.

The other thing about rolled threads is they are microscopically smooth, unlike cut threads which are microscopically rough and cam induce galling. When cutting a thread on stainless it is done very slowly, to avoid chatter, whereas we are rolling threads at 3500rpm. The resultant thread is smooth as you are not removing material, just repositioning it, with the metal grain flowing around the rollers.

A piece of steel is analogous to a piece of wood, both have grain running along the major axis. When cutting a thread you have unsupported grain, with a rolled thread as the grain is formed around the rollers, there is no unsupported grain.

Most commercial bolts have rolled threads however the process is slightly different. The bolt blank comes down a track and the thread is 'wiped' on by a pair of plates. The other difference is that in rolling a thread, the major diameter must be reduced to the effective (half way between the major and minor diameters) for a 1/4-28 thread the major diameter is .250", the effective diameter is .2262". This allows half the material to be rolled up from the bottom of the thread. Some manufacturers grind the shank down to the effective diameter, we use a rotary swager to reduce the shank down to the effective diameter, which in turn increases the tensile strength of the shank. Rolling the thread also increases the strength of the shank, so the resultant thread is much stronger that the untouched shank.

We have been concerned with possible galling on stainless threads. While the wire or tie rod is a 3 series stainless, (an austenitic, non magnetic steel which can have its strength increased by cold working) the clevis is a 4 series stainless, which can be heat treated and tempered back. Generally the clevis fitting (AN665 series), has its female thread cut, but we have had good results roll forming the female thread. The main contributors to galling are heat and friction. You can reduce the friction on the wire by thread rolling, but there is still a possibility of friction from the female thread. as even though it is hard, is it still microscopically rough. Roll forming the female thread makes the female thread microscopically smooth. We've also had good results 'chasing' the female thread with a rolled formed chaser to microscopically smooth the female threads before the clevis' are heat treated.
Russ Ward
www.vintageaero.com
 
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