Things that thread together are broken down into "classes of fit" (or "tolerance class"), which determine the dimensions for inside and outside threads for compatibility and proper fit.
https://www.fastenal.com/content/feds/pdf/Article - Screw Threads Design.pdf
Unified Thread Standard - Wikipedia, the free encyclopedia
We cut and then gauge the threads on our cans to the Class 3B (internal) specifications, and therefor the muzzle threads need to fit
within the Class 3A (external) thread specs. The thread specs on our web-site,
Thunder Beast Arms Corporation [TBAC] | Tech | THREAD SPECIFICATIONS
are not special or unique to us at all. We simply use the pitch diameter from the Class 3A specs which is from the Machinery's Handbook but you can find it in any online reference,
Unified Screw Threads, Standard Series
Unified inch screw threads and tolerances - Ames
There is no class "tighter" than Class 3, so if a 5/8-24 thread has a thread pitch diamter larger than 0.5979, then it is not cut to any
specification. IE, "Class 4" if it existed would be an interference fit-- and that's why cans get stuck if the can's internal pitch
diameter is smaller than Class 3B specs or the muzzle's pitch diameter is larger than Class 3A specs.
Threading on a CNC lathe involves using a single tool that repeatedly inscribes the same path on the thread surface over and over until the desired cut depth is achieved. This is called "single point threading". I found a video on youtube which shows how this works, although it is not a barrel threading video.
THIS IS NOT OUR VIDEO - I JUST FOUND IT ON YOUTUBE
https://www.youtube.com/watch?v=0UcurjixoGA
At 0:50 you can see the thread cutting tool approach the thread portion. Each repeated run it makes from that point until about 1:06 is taking off additional material from the threads. Right at the end you can see the threads. Like I said that is not a muzzle threading video, but it's the same general process that a CNC operator would follow to cut muzzle threads.
If he were threading a muzzle, at the end of the video when the machine has made the initial cuts, he would use a thread micrometer, thread wires, or a GO gauge to determine if the threads have been cut small enough to be in spec (ie pitch diameter smaller than 0.5979 for 5/8-24), or if he needs to do additional passes. The can should
never be used as a primary method to determine if the threads have been cut enough. (This is why I wrote the note in the other post about not needing the can to do a muzzle threading job.)
Most muzzle brakes, flash hiders, and thread protectors are cut on the loose side, some extremely loose. For muzzle devices that are not meant for suppressor mounting, there is literally no reason they have to have a precise thread fit. With a muzzle device used for suppressor mounting, there is a desire to have it fit tighter, but even then many of them are a little large.
With regard to having it on and off a couple times before it got stuck. There are a couple possibilities here. If all remaining
clearance has been sucked up by the muzzle being 0.002" over, then all it would take would be a miniscule particle or piece of carbon to create an interference fit. Another possibility is that minor galling (damage to the surface of the metal) had been occurring the first couple times it went on and off due to insufficient clearance and eventually that accumulated to such an extent that it seized.