What's the RPM

[churchjw]

OK, imagine the circumference of the shaft flattened out. Now at 1725 RPM, how many feet per second (or inches, or MM or CM, as you wish) does it travel? Calculate how far apart the marks need to be to represent 1/60th of a second. Now convert it (mentally) back to a circumference, and you know how far apart to mark the shaft. Of course, there are limits to consider, a 1/4" shaft may not be "long" enough to mark, as the circumference may be shorter than the distance traveled in 1/60th of a second. I haven't done this kind of tach work in ages, but it does work. The 60Hz (or 50 in places) is generally very accurate. It's what keeps out plug-in clocks running

Here is a page that kind of goes with what Tony is saying. I used this method years ago for a project and it worked well.

http://www.sherline.com/rpmgage.pdf

Jeff

 

[brucer]

I've been thinking about making a tachometer for my step pulley bridgeport... I was thinking of milling a pocket in the upper bearing locknut and epoxy a magnet in the pocket, then mount a housing on the bearing sleeve locknut and mount a hall effect sensor in the housing, then just use one those cheap digital tachometer breadboard kits.. then mount the readout in my vfd remote.

I dont really need it, but would be cool to have an actual tach readout on my vfd remote, instead of the normal frequency readouts that are available.

might get around to it when I get with the cnc plasma finished.

 

[Tony Wells]

Just so you guys don't think that idea was mine. Many of you may remember the days of the turntable, the ones that played LP's. Some of them had a neon lamp, which flashes at 60 times/second near the edge of the platter, which in turn had marks properly spaced for each speed the tt was able to run, usully 33 1/3 and 45 RPM. There was an adjustment to "freeze' the timing marks, just like the old days of a timing light on a harmonic damper.