Improvised tachometer

It's seldom that one needs to know the rotational speed of the lathe accurately but, if you do and you don't have a tachometer about, here's one way to do it.

Get a length of all-thread and a nut to match it. Grip the all-thread in the chuck and make two marks on it a convenient distance apart. Thread the nut on up to one of the marks. Now, with the lathe running, grasp the nut (use needle-nose pliers) and time how long it takes to move to the next mark.

Given:

d = distance between marks (in)
p = pitch of all-thread (tpi)
t = time for nut to move between marks (sec)

then:

speed (rpm) = 60 * p * d / t

I would never do that because I have other ways to measure RPM, but this is a practical idea for those who have no easier way.