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I often wonder if the locking is caused by poor quality machining.
I bought some cheap eBay adapters to suit some DSLR camera where the thread is clearly torn up rather than cleanly cut, (bearing in mind that the thread cutting process is in fact a tearing process !)
This is usually caused by using incorrect cutting speeds, feeds and depths of cut, poor lubrication, inadequate swarf clearnce and also cutting tools that arent sharp.
Threads cut where the swarf has got caught up in the thread that has just been cut, actually causes the hairlike fine strands of swarf to be friction welded back onto the thread thats just been cut and this can lead to additional galling and seizure.
A good thread for our sort of optical threads - eg extra fine pitch, ought to have a high quality surface finish, it should look almost shiny.
So while this is hard to fix once you have it - its possible to use a small amount of fine cutting compound and repeatedly tighten and loosen the thread, so as to slightly polish the surfaces but this only works if you can clean the thread afterwards - so not practical for most situations
It also will not necessarily prevent galling if the two metals on either side are either the same material or are soft material - aluminium being a prime culprit.
An ideal thread combination is a hard and soft material (eg Stainless and phosphor bronze) on each side - this becomes self lubricating, but we rarely find this.
The other problem which I unfortunately had first hand experience with on some expensive equipment is that not all threads are correctly made
That is the diameter, pitch and the form may not have been cut exactly the same
One thread may have been cut on one type of machine eg lathe - using one type of tooling and the other thread cut on a milling machine using helical thread cutting
So its no wonder that sometimes two threads that ought to be mating are not properly mating.
Trouble is you may not even know it.
In my case a $1700 focusser.
The thread machined into the body was cut incorrectly (too small and incorrect tooth form) and I had to recut a thread on the adapter that was made correctly to fit it !
I couldnt recut the thread in the focusser because it wasnt possible to dismantle it or hold it in a lathe to recut it.
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