I remember doing something similar on splicing of a super long leadscrew. It was like 40-something feet long. It consisted of two pieces of leadscrew around 21-22 feet in length. These were 3" in diameter with a 1" pitch special square thread cut on them. My job was splicing them together! I pretty much did an exercise like Joe Pie did on figuring out how much to take off to get the two threads to line up. That was almost 45 years ago!
