Tuesday, August 26, 2014

Shop made strobe tachometer

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 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. I tested all the discs against each, including the one I made from scratch, and found them all reliable. Thanks to this method 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 that left a small amount of slop in the spindle and I don’t want to run it that way, 2,800 is fast enough for 60 year old bearings.

The stroboscope wheel I made covers most lathe speed ranges. To use, save the file to your computer and print to fit a letter size sheet of paper. The disc will work at any size, just make sure it fits the swing of your lathe. I recommend cutting a wood disc on your lathe, cutting out the paper disc and then gluing paper to wood, center best you can.

Materials needed:
Strobe tach disc (provided below)
Wood to mount the paper disc
60hz fluorescent light or stroboscope

2017 Update, this new color version makes it easier to distinguish bands.

Video of wheel in action and explanation of how it works.

Still of disc in action:

The goal was to accomplish this: each # on the tape represents a band on the tach wheel. 

So I could make this

If you want to make your own strobe tach 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

In Europe
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 #)