Anyone who has the ability to hold the cue in a lathe with the pin exposed can do it. I've modified many in a lathe with a hand file. You can do it by hand with a file, but it is harder to get professional looking results. Not impossible, just harder. A steady hand and a benchtop belt sander...
If the insert isn't an open cylinder, then the bottoming tap may work. The inserts I used to use were open cylinders. Regardless, a machinist could likely drill the end of the insert without harming the existing threads.
If you aren't concerned with originality, the Adam screw could be...
Any UNC 5/16-18 bottoming tap *should* be able to cut a few more threads if my theory is correct, but I didn't consider that it had an insert, so it probably would not work. The ball nose might be compressing the wood beyond the insert, in which case it would need to be bored a bit. Could...
If it is only the last couple of turns, I would expect that the Valhalla pin is longer and you are screwing into the partial threads caused from a starting tap.
Why should I make it better? There is too much information given freely to hacks which undermines custom cue making as a whole.
Read his posts, I only responded in kind.
I also gave him the advice he is following now with the wood lathe and chuck.
Finally, n the tooling is something that...
Since @Sheldon is saying what the rest of us are saying, it is how it is cut before the splice is assembled that determines the width of the bottom flat.
I don't know if you're joking or not, but I assure you that ferrule is correct. It's an odd progression from the original definition, which was an iron band (derived from ferrous), but then came to mean any thin metal sleeve to reinforce parts, which then made sense when they used brass, on...