Re: Ugghhh cross threaded CW shaft into CW shaft adapter with metal threaded sleeve


Christopher Erickson
 

Many moons ago when I was learning machining under the tutelage of a master gunsmith, I learned that all stainless steel and aluminum formulas, at the microscopic level, actually "tear" instead of smoothly cut when machined. This tearing leaves a microscopic feathered surface that can catch and "cold-weld" with other machined aluminum or stainless steel surfaces when they are pulled across each other under pressure. For many years, this characteristic of stainless and aluminum prevented them from being used in many applications and especially in firearms. Sturm Ruger eventually developed a proprietary process of casting stainless in such a way that the cast parts didn't have a feathered surface and could be cast within .001" accuracy. This allowed stainless parts to be put in mechanisms where they would be sliding back and forth or around other stainless parts, without galling or cold-welding. 

Aluminum is a bit different. Because it is softer, springier and has a lower melting point, it can gall in more situations than stainless steel.

I also learned that careful polishing of machined stainless or aluminum parts that knocked down the feathered surface considerably decrease galling. But not prevent it 100%.

Also that using two different stainless formulas with different feathering characteristics on mating parts could help a bit.

Also that machining at angles that didn't result in feathers squarely facing each other helped. But that's just about impossible to do in thread cutting.

About the best thing I found was coating/impregnating the polished or unpolished stainless/aluminum surfaces with Teflon. And if you can't do that, use a Teflon-based lubricant between the two searing surfaces. My favorite Teflon lubricant is Break-Free CLP. One or more drops at the point where the two threads first meet does an amazing job of minimizing galling.

One usually-effective way of preventing stainless steel and/or aluminum threads from galling is to machine them with a tiny bit of extra clearance so they don't ever have enough pressure between them to allow the feathered surfaces to cold-weld to each other. However this approach fails in threaded parts if there ends up being any contamination in the threads, such as aluminum slivers from the last threaded aluminum hole. And I think this is exactly what happened to this particular counterweight shaft and adapter.

How to prevent galling (cold-welding) of AP stainless counterweight shafts and stainless threaded collars? My suggestion would be to carefully inspect and clean the threads of your counterweight shaft with stainless steel brush (fiber or brass brush would be too soft) to make sure there is no contamination or slivers of aluminum left over from the damaged aluminum threaded hole of the old counterweight shaft adapter. Then I would add a couple drops of Break-Free CLP or some other Teflon- based lubricant to the threads before putting them together.

And it is possible to have a sliver of aluminum cold-welded into the thread grooves on your counterweight shaft. If you see that with your 2.5+ reading glasses or magnifying loupe, I would use an Xacto-knife to cut and work it out of the stainless threads before screwing your counterweight shaft into the new stainless adapter. A threading die (1.125x14?) would be the best way to quickly and easily remove the aluminum contamination from the threads, if you had one.

I hope this helps.

"My advice is always free and worth every penny!"

-Christopher Erickson
Observatory Engineer
Summit Kinetics
Waikoloa, Hawaii


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