I am getting into cue repair and love it. But how do you determine which ferrule to put on which shaft, as a replacement.
Not exactly sure how to answer this question, but I'll try.
Some cues you won't know what kind of ferrule/tenon you'll be dealing with (threaded or not, capped or not, what size of tenon) until after you've taken off the old one. You can cure some of the odd-sized stuff by putting in a new tenon of the typical 5/16th size tenon.
As far as the material of the ferrule, I'm not an expert yet but I'm learning as well to be able to tell what material the ferrule was made out of that I've taken off by the way it comes off with my lathe. There are several different types of materials used in ferrules and if the customer wants to replace it with what was on it previously then it would be nice to have a few of each one of theses types of ferrules on hand. There are a lot of different ones out there though. PVC, ABS, Delrin, Fiber, Phenolic, Aegis, Saber-T, Porper, Elforyn, Juma, Titan, Ivorine, Ivory, etc.
I personally carry both 13MM and 14MM ferrules on hand of a few different types of ferrules so if you run into a shaft that is at or below 13MM you don't have to turn down the ferrule as much. I also carry 3/4", 1", and 1-1/4" ferrules of a few of the different types because you will encounter different lengths of tenons/ferrules as well. Some of the ferrules I mentioned above come in just one or two lengths, some you can get in all of the lengths.
Do a little research of the different suppliers out there like Mueller's, Prather, Atlas to name a few and you'll see what i mean by all of the different ones out there.
Hope this helps answer your question.
Trevor