Galling

Galling occurs mainly because of friction and heat. A 300 series with a 300 series fastener will gall under pressure – ask the yacht guys. Hardening the stainless will help, as will a clean surface. A 300 series wire with a 400 series fitting is better.

A300 series is not very cooperative under normal machining operations. Try drilling a hole in 300 series, go very slow and get rid of the heat or the drill bit will turn blue with heat and fail. The same thing happens when you try and cut a thread in 300 stainless – the cut thread is microscopically dirty and rough.

A piece of steel has a grain, like a piece of wood, running length wise down the bar. When you cut a thread you have pieces of unsupported grain hanging out in the breeze. Plus with a cut thread, you have that big stress riser where the cut thread ends at the shank rolling, and then the thread is rolled back up to the specified diameter.

With a rolled thread, the grain runs around the rollers, so the grain is not interrupted, but continuous and the thread is microscopically clean and smooth. With a rolled thread, you have to reduce the shank of the wire to the effective diameter, which is called the pitch line. and all the force goes down through the pitch line. With a 1/4-28 thread, you need to reduce the diameter of the shank down from .250" to .226". You can grind the shank, which interrupts the grain flow, or swage it. By using a rotary swager, we can reduce the shank diameter down to the effective diameter, preserve the grain structure of the shank and work harden it. Then we roll the thread up to the nominal diameter, or just slightly below it.

Watch on YouTube: »Threading on the Old School Aero channel.

This gives us a microscopically clean and smooth thread surface, reducing friction. Swaging the shank increases the hardness of the shank, rolling the thread increases it further, without making it brittle.

The 300 series wire is the part that galls, just as an aluminum bolt will gall on an aluminum nut. The 400 series fitting does not tend to gall, it is heat treated so is hard.

The AN spec calls for a medium fit thread, certainly not a close tolerance fit, presumably to prevent galling. You need a few thousanths of an inch clearance to get a bit of lubrication in there. The force all goes down the pitch line, so with a 1/4-28 thread any thread diameter above .226" is a bonus. I installed a 9/32" (BSF) fitting on a 1/4-26 (BSF) wire to see where it would let go. It took over 3,000lbs to make it give up.

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