Questions on thread profiles and cross-threading

[kosPap]

hi all!

I would like to ask something about tailcap thread compatibility between the Surefire and a chinese copy...

Initially they do not match as described in this thread

So I managed to fit the latter to the former, till I tried to screw a TNC one-cell extension and the original tailcap back in. They wouldn't start!
Being persistand with strong half turns they now screw alright without being cross-threaded.

Now I am afraid the chinese tailcap has a different thread profile and I just form-fited one into another.

Can this be? What is there to know about thread profiles other than the diameter and revolutions per inch specs?

TNX kostas

 

[PhotonFanatic]

hi all!

I would like to ask something about tailcap thread compatibility between the Surefire and a chinese copy...

Initially they do not match as described in this thread

So I managed to fit the latter to the former, till I tried to screw a TNC one-cell extension and the original tailcap back in. They wouldn't start!
Being persistand with strong half turns they now screw alright without being cross-threaded.

Now I am afraid the chinese tailcap has a different thread profile and I just form-fited one into another.

Can this be? What is there to know about thread profiles other than the diameter and revolutions per inch specs?

TNX kostas

Hopefully you know that there are Imperial and Metric threads, some of which are very similar, but not identical. To determine what any thread actually is, you need some thread gages, one Imperial and one Metric, and a good magnifying glass. :devil:

If you think you've managed to weld them together, I would first apply some NanoLube to the threads, let it sit for a day, and then try unthreading them.

 

[gadget_lover]

There are several parameters that make up the thread. There are standards that set forth formulas that determine exact profiles and depths. There are different thread shapes, as in the acme thread or the UNC thread. The simplest shape is the sharp V thread, where the male and female are mirror images, with sharp peaks on the male sides.

I'm not sure where your problem is, but it sounds like you have a case where the threads of one part are properly cut UNC, with flats on the bottom of the groove and the top of the threads. The other part could be sharp V, in which case the peaks of one side are hitting the bottoms of the grooves on the other side.

But.....

If the first turn or 2 are pretty normal, then it gets real tight... you most likely have mismatched threads. Some metric threads are almost exactly the same as imperial threads. If they are close enough you can force them together, bending the metal and deforming the threads. The oiled, anodized surfaces will help the deformation.

Forcing threads can have several outcomes. You can deform the threads of one or both parts. You can pressure weld them together. The threads themselves can fracture and break off. The parts can gall and become permanently stuck.

If you try to force the old parts back together, you run the risk of stripping the threads off.

Daniel

 

[Mark A.]

Wikipedia has a good primer on screw threads, at this link: http://en.wikipedia.org/wiki/Screw

Clicking on the link for Metric will produce great detail.

I believe they list some rare threads as well.

Also, any copy of Machinery's Handbook will yield a lot of good info; for instance, the "class of thread" noting the tolerance to which they are held. Sometimes, just differences in the quality of threading in the same size & pitch will introduce poor results.

 

[gadget_lover]

I forgot to mention.... The threads of my surefire L4 and L1 are different. The L1 uses what looks like an acme thread (wide and squared off) and the L4 looks like a sloppy V thread (no flats on the crests and the bottoms of the gooves are curved).

 

[kosPap]

ouch! of all your comments the case of metric vs imperial threads somehow struck a nerve....

So with the potential of harmimg my surefire I will reconsider.

Thnak you for the eye-opening info, Kostas