Cheap tach - was r.p.m. kit??? / 4 digit under $20

Hi, I just tried one of the suggestions on another group regarding
this subject. I was intrigued by the simplicity of the idea and the
fellow mentioned that this is an old trick thats been around a while.
You just need any small DC motor, a potentiometer, and a volt meter.
The first potentiometer that I found in the junk drawer was a 50K
although other values will work. I hooked it up as in this picture.

http://www.members.shaw.ca/axxus4/cheaptach.jpg

The motor is connected across the pot and the meter is connected from
the center of the pot to one side. To calibrate, spin the motor in the
chuck, and adjust the pot to read your known rpm (in mV or whatever).
I was spinning it at 1460rpm in my mill drill. I adjusted the pot to
read about 1.460V on the 20V scale. I walked over to the drill press,
and spun the motor. I read 0.354V. I was amazed to see that the drill
press was set to 360rpm. Very neat trick. I will use this method to
measure servo and stepper rpm.