After switching my lathe to a variable speed DC motor I had no way of knowing it’s range so rather than buying a digital tachometer for a one off measurement I used some old tech… a homemade strobe tachometer. A google image search yielded a variety of discs designed for measuring speeds from 60 rpm up to 7200 rpm. After printing out 4 papers discs I found my lathe is capable of 240 rpm up to an estimated 2,800 rpm (estimated because this method jumps from 2,400 to 3,600 rpm). I did hit 3,600 rpm by loosening the bearing retaining nut but I don’t want to run it that way, 2,800 is fast enough for 60 year old bearings. In the video you’ll hear a knocking sound, I don’t know what that was but after filming I removed/replaced the arbor and fixed it.
The stationary band tells me the lathe is running at 300 rpm
Strobe tach disc (from internet or homemade)
glue (spray glue or glue stick)
Wood to mount the paper disc
To make your own start with an image search for “strobe tachometer disc”. If you live in the U.S., lights operate at 60hz, overseas they are 50hz, so make sure you are using the right disc or your results will be off. It is possible to make your own and I’ll cover that further down. Print the disc to fit inside the swing of your lathe, mine are 5 inches diameter, and carefully cut it out. Cut a wood circle, drill a hole in the center and true it using a lathe, disc sander, drill press or whatever you have. Actually I guess it doesn’t have to be a circle, a square would work fine. Center the paper disc on your wood and glue it down. Mount the disc to your lathe or drill press (I used a screw chuck on my lathe) and turn off (or point away) all light sources except for one fluorescent above the machine. Incandescent bulbs will not work, neither will energy saver fluorescent bulbs (the kind that replace normal bulbs). Turn on the lathe and slowly adjust the speed until one band appears to stand still.
If you want to make your own disc from scratch, here is the math. Neither the length of the bars nor the distance between them matter, all that matters is they are equally spaced. Rather than make a circle, you can make them in a line and glue it around the outside edge of a wood circle.
Formula: 120 x hz/rpm=equally spaced bars or 120 x hz/bars=rpm
So to make a strobe disc for 480 rpm in the US (60Hz)
120×60/480= 15 bars equally spaced
120×50/480= 12.5 bars (doesn’t work out evenly so we’ll adjust the formula to work with whole bars)
120×50/13 (bars)= 461.5 RPMs (fraction)
120×50/12 (bars)= 500 RPMs (nice even #)
Quick Reference Chart