I just got a Pro membership a couple of days ago, thanks to Random_Canadian's generosity. (Thanks for the hookup!) I figured it was time to quit lurking and actually contribute, so here goes my first Instructable. 

As a CNC technician, there have been a few occasions where I've wondered if a spindle or motor shaft was spinning as fast as the motor controller claimed. An old trick I learned a long time ago involved a digital guitar tuner. The trick was to hold a piece of stiff metal shim stock, around .020" or so thick, against a protrusion on the shaft. As the motor strikes the shim it should cause it to vibrate at a frequency that the tuner would turn into a note reading. If you were to look up the frequency for that particular note, some quick math would give you a reasonably accurate RPM. 

The problem with this method is that most industrial machinery is not located in a concert hall. The ambient noises in a typical factory reduce even the most careful measurement right back into guesswork. So, if we generate an electrical signal instead of relying on sound, we can isolate and refine our measurement into something useful, and gain a tolerance of around 5 percent.

You'll need: 
A small permanent magnet
Old headphones
Enameled magnet wire (pretty fine; mine feels around 0.010")
A digital guitar tuner with an input jack (you can use a good one - we won't be modifying it in any way)
A 1/4"  male to 3.5mm female plug adapter 
The patience to splice very thin, very fiddly wire

Step 1: The sensor

The sensor is as cheap and easy as it gets. I picked up a package of 1/2" ceramic magnets from Radio Shack for around three bucks. (I buy these five-packs every time I need one magnet, and then the rest vanish. Apparently, they're biodegradable, or I have mice that are far more sophisticated than I would expect.) By wrapping some fine enamel-coated wire around the magnet (see photo) we have a magnetic pickup, like in an electric guitar. It's tough to see in the picture, but there's probably fifteen or twenty wraps of wire on there. I didn't worry about north or south poles, and I don't think that would really have an impact on function.
Very helpful and the links were the best. I can't wait to build my own. Can't wait and see what you got coming next. <br>Thanks for DIY project. Would have never thought to make it out of a tuner. Till next time.
In #3 it should read <br>Many CNC and other machine tools have a RPM output FROM THE MOTOR (built in opto or magnetic sensor). Connect that to a tach or frequency meter like in #2.
A black magic marker and a photo tach (eBay $13) would be easier and SAFER. It isn't safe for you or the machine to hold something against a rotation shaft. If its circular as most rotating shafs usually are then unlesaa they have a grove or protrusion this wont work well. <br> <br>My method: #1 Best one <br>Use a black Permanent marker to paint about 1/2 to 1/4 for 1 section per rotation and hold up a laser or photo tach. Or attach an opto sensor in series with a 10K resistor powered by 5v for input to any tach. This way you know the speed without affecting it. For a CNC head that moves just attach the opto sensor and run the wires on the machine axis with the other wiring. <br> <br>#2 <br>Many digital meters also have a frequency measurement setting. Use the opto sensor like in the last paragraph but measuring the frequency output and multiply the result x60. <br> <br>#3 <br>Many CNC and other machine tools have a RPM output (built in opto or magnetic sensor). Connect that to a tach or frequency meter like in #2. <br>

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Bio: I'd rather be efficient than professional.
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