Lathe Tachometer

Lathe Tach - no moving parts!

Ever have an idea that is just AWESOME, only to find out that someone else beat you to it? Well, this is one of those ideas.

I noticed one day that at certain speeds, the teeth on my lathe chuck would change color - red, then blue, then red, etc. This lead me to the conclusion that the fluorescent light above my workbench was flashing, and that each flash wasn't the same color throughout it's duration, but started out red and turned blue - or maybe the other way round.

But it got me thinking. Could the fluorescent light be used as a strobe light? Could it be used to measure the RPM of the lathe?

My idea was to wrap a piece of paper, bearing evenly spaced markings (specifically, numbers), around the chuck. When the chuck was turning at just the right speed, the numbers would appear still. Using different numbers at different spacing, I could directly read various speeds.

First Try

It didn't work. The flash from the fluorescent bulb just wasn't short enough to properly freeze the numbers in place, resulting in an unreadable blur. The red and blue effect, however, was quite prominent. I decided to use the red and blue effect instead of the numbers, and developed a maximum-contrast pattern, shown at the top of this document.

Second Try, showing rainbow on 150 rpm band

This worked pretty well, though I had the spacing wrong. Turns out that fluorescent bulbs flash twice per cycle - once on the positive half, and once on the negative half - or 120 times per second. As a result, the speed of the lathe in the picture above is actually 300 rpm, not 150 as shown.

Third Try

The corrected version works great. To confirm that it was indicating the correct speed, I arranged a mechanical counter so the jaw of the chuck would trigger it as the chuck turned. Adjusting the speed to 100 rpm according to the tachometer, I timed exactly one minute - during which the mechanical counter was triggered 303 times. Dividing by 3 jaws gives 101 rpm. I double checked at 200 rpm, with similar results.

During the process of trying to find out how many times per second a florescent bulb flashes, I stumbled across the tips section of the Sherline web site, which offered a free rpm gauge (pdf). This is a disk, rather than a band, with a very similar pattern to the one I developed. The shape is different, but the principle remains the same. Subsequent searches found lots of references to this device, so I guess it's an old idea.

Ah, well. I guess I should skip the patent application.

You can download my version of the lathe tachometer here. Printed at 300 DPI, it is sized to fit a stock 3" mini-lathe 3-jaw chuck, which has a diameter of 3.15 inches and a circumference of 9.9 inches. It can be resized for other chucks just by scaling it up or down. The seven bands correspond to 100, 200, 300, 400, 600, 800, and 1200 RPM.



© 2003 W. E. Johns