chasing dial

I assume you are talking about the threading dial, a gear that rides the threading leadscrew and has usually 4 numbered lines? First, it indicates when the leadscrew threads are roughly aligned with the half nut threads, so you can make a clean engagement. Also, for threads coarser than 8 TPI, or not evenly divisible by 8, it allows to to align the half nuts ou multiples of 8 TPI, so that you get in sync with the thread.

For threads that are multiples of 8, you can engage at any thread on the leadscrew. The dial just gives an indication where the good spots to engage are, so you don't mash the threads. What about 11 TPI, 27 TPI, 4 TPI? For 4 TPI, you can only align to any even thread on the screw, if you go to the one between them, you will be half a thread out of sync with the thread you are trying to cut. Any mark on the dial will be OK, even if it has 8 marks (assuming 8 TPI screw and 16 teeth on the dial's gear.) For 11 and 27 TPI you need to engage only every inch, or 8 teeth. Mark 1 and 3 will be OK. For 4.5 TPI, you can only sync every TWO inches, so only use the same mark every time.

Jon