Make your own Lathe from other peoples rubbish

Step 18 Speed measurement (optional little interlude)

Ok so now we have a working, spinning, dancing (almost) headstock, we can measure the speed and press that motor tachometer into useful service.

There are a number of different ways to do this. I used the 'threaded rod and nut' technique.

Simply attach the multimeter to the tacho output, set to read voltage (20V scale).

Setup a threaded rod so it spins with the drive shaft. You need to know the pitch of the thread (i used some 1mm pitch stuff) Mark two places on the rod (A and B), a given distance apart (say 60cm).

Now set the lathe running at a slowish speed and record the multimeter reading. Hold a nut stationary on the rotating rod - it will travel lengthways along the rod, because the rod is spinning.

Start a stopwatch when the nut passes point A, stop the stopwatch when it passes point B. Record the time it took.

You can now work out the rpm (revs per minuet).. If A and B are 60 cm apart that requires the shaft to turn a full 600 rotations for the nut to travel the distance (because the pitch is 1mm - that's one rev per mm).

So what is the shaft speed if: it took 60 seconds to travel between A and B? Answer: 600rpm because it revolved 600 times in one min...

If we make a little table or rpm with corresponding voltage readings we only need a few reading and can draw a graph (which should be a straight line) to give us the rpm, at any given voltage.