5. Milling Machine Tachometer Movie

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To summarize, a portion of the top of the otherwise black spindle has been painted white. The circuit board has reflective-pair optical sensor (IR LED bouncing infrared off the spindle and into a phototransistor). The microcontroller measures the time between the start of each bright reflection or counts the number of bright reflections over a given period. The code calculates how many rotations this would equal in one minute (RPM) and displays the result on blue numeric LEDs.

Click to see a movie of a homemade electronic tachometer display the speed of a hobbyist milling machine.

The MicroLux labels read up to 1100 RPM in low gear and up to 2500 in high gear.

My older mill measured 10 RPM at start (without stalling) to 755 (center) to 1113 (maximum) in low gear. It measured 10 RPM at start (without stalling) to 1737 (center) to 2560 (maximum) in high gear.

My newer mill is the same model as the older mill. It measured 15 RPM at start (without stalling) to 804 (center) to 1204 (maximum) in low gear. It measured 41 RPM at start (without stalling) to 1870 (center) to 2763 (maximum) in high gear.

These results tell me two things:

1. The mills perform better than specified -- even after years of use. (Note: Your maximum motor speed may be different, as speed is dependent on the AC voltage at particular location.)
2. The speed doesn’t match a common-sense ratio with the dial position. That is, the center position is approximately 66% maximum speed, not 50% speed as might be expected. That’s not a big deal. But, just be aware of it if you’re trying to get the cutting tool to match a particular speed.