Rolled threads or cut threads

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Dana

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I can't give a quantitative answer, but the rolled threads will be stronger, both in initial strength as well as from a fatigue standpoint.

-Dana

PADDLE FASTER!! I hear banjo music!!
 

Jay Kempf

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Which is better and why? How much strength difference is there?
Application dependant. Rolled threads are WAAAAAY stronger, if done cold rolled, than a single pointed, stamped, or cast thread. The reason is that with rolled threads instead of cutting through longitudinal grains in the metal you end up rearranging them so that they make nice reinforcements for the actual threads sticking up from the minor diameter so that the shear strength is better. Instead of shearing along intergrain boundaries the shear failure has to cut across if that makes sense?

jfk
 

Autodidact

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I think that rolling threads instead of cutting them does something similar to what happens when you forge something in steel, like a crankshaft or connecting rod; it makes it tougher. Cutting the threads interupts the grain structure while rolling them keeps the grain intact, though a bit tortured.

Oop! Beat to the punch!:gig:
 

PTAirco

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Bruntons only ever used rolled threads on their flying wires and tie rods. The American company McWhite used cut threads and I doubt it caused a vast number of sudden failures, but I am sure they had to be a little heavier than than an equivalent wire with rolled threads. In cut threads, you remove material so the root diameter of the inner part of the thread is what determines your tensile strength. The rest of the wire is reamins the original thickness and adds nothing to the strength, only weight. With rolled threads, no material is removed and the root diameter is the same as the rest of the wire.

Curiously, all female threads are tapped such as the barrels of turnbuckles and spoke nipples. In talking to a manufacturer of custom spokes, in all their tests (with rolled threads) the spokes broke outside the threaded area and never pulled out of the nipples.

if you can afford it and you can find somebody to make them for you, rolled threads are the way to go, but with some consideration you can use carefully cut threads where appropriate.
 

Dan Thomas

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Most bolts are made with rolled threads. Nuts are usually cut with a tap. The bolt is often the first to strip, given a nut made of the same material, because the nut's thread root area is larger than the bolt's thread roots. Diameter, see?

Dan
 

skeeter_ca

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I'm thinking male threads here. I'm considering the different approaches to drag/anti-drag wires inside a wing. Some use a turnbuckle system while others use a threaded rod going through the spar. I like the threaded rod going through the spar approach for simplicity but am unsure of the strength differences to adjust for. How exactly are rolled threads made?
 

PTAirco

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I'm thinking male threads here. I'm considering the different approaches to drag/anti-drag wires inside a wing. Some use a turnbuckle system while others use a threaded rod going through the spar. I like the threaded rod going through the spar approach for simplicity but am unsure of the strength differences to adjust for. How exactly are rolled threads made?
I'm going to use the same system for my biplane wings, like many other designs; threaded rods passing throuigh the spars, secured and tensioned by nuts. Turnbuckle prices have reached insane levels and rods tend to require far less periodic adjustment, if any. I'll cut my own threads unless I can find a shop that will roll threads cheaply for me. Strength is dependent on whatever root diameter you're left with. I'll add a little diameter for peace of mind.
 

skeeter_ca

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PTAirco

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Those devices will last about three threads if used on any high strength steel. Take a look at how massive the rollers are in Dan's video; it takes a lot of power; even for something like a 3/16th thread. Find a custom spoked wheel maker, such a a custom motorcycle shop; you can get spokes up to 3/16th in diameter or close - good for roughly a couple of thousand pounds in tension. Or use thinner and double up where necessary. End fitting may be a little fiddly, but ought to be cheap to make.
 
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Keyepitts

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Just a side note on the female threads, there are "form" taps available that that perform an action similar to external thread rolling for female threads. The tap drill diameter is usually slightly larger for a form tap than it is for a cutting tap. Be careful when ordering taps these days, because form tapping is becoming more popular, and its difficult to tell the difference between form and cut taps. And another note- not all materials can be roll-formed.
 

Chilton

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Not wires, but Tie rods. About 30 years ago an AD in Europe mandated changing all Tie rods in the Stampe SV4 series to remove a large number which had been made with cut threads, the trigger being a fatal accident when the wings came off one in flight. About 10 years back we got a similar AD on the DH Tiger Moths after an Australian example shed the wings in flight. Both accidents were Tie rods with cut threads failing through the minor diameter of the thread.
 

Thomas Balmer

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Bicycle spokes are a great example here. Tiny, yet under great tension. Rolled threads are preferred.
 
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