I was inspired by the “Lathe & Mill Tachometer” posted by Catfish (see his 4-2-2014 post by Randy Richard and his YouTube video), and decided to install the same type of product on my old 12”X37” geared-head metal lathe. I previously used the same type of display and sensor for my 7” swing mini-lathe (see Homemade Mini Lathe Digital Tachometer).
Randy advised searching for “Hall Effect Tachometer” parts on eBay. I found many offered in the range of $13 to $20 and I purchased one with a blue LED display. The eBay purchase included the LED display with bezel and its attached circuit board that snaps into a rectangular cutout, the cabled Hall Effect sensor NPN with nuts and lock washers, and a small rare-earth magnet. The simple wiring diagrams are posted with the products photos on eBay. I also purchased for $3 a small 110VAC to 9VDC power supply circuit board. I used eBay “Buy It Now” and the parts arrived within a few days.
I mounted the digital display into an aluminum case (1.5”x2”x4”) after cutting a rectangular opening the old fashion way with a nibbler (could have milled it but I wanted to show another way to do this). I machined, from ½” aluminum rod, a support column threaded 3/8-24 and ½” long on each end and drilled a ¼” dia. thru hole for the wires. 3/8-24 jam nuts are used to attach the support column to the aluminum case and to the lathe electrical box. On the side of the case is a SPST miniature toggle switch for powering on/off the tachometer.
I created a simple aluminum bracket for mounting the Hall Effect sensor. The bracket positions the sensor less than a 2 mm gap from sensor end to the spinning magnet mounted near the rear of the spindle (sensor specs require a 1 to 10 mm gap). The rare earth magnet is epoxied to the spindle after verifying the north-pole end is pointing up (the sensor only works this way). An existing threaded rod bracket for attaching the lathe rear cover was used to attach the bracket to avoid drilling into the headstock. The sensor wires are held safely in place with tie-wrap mounts and guided around the back of the lathe and into the lathe electrical box.
In the lathe electrical box I installed the 110VAC to 9VDC power supply circuit board (secured in place with tie wraps) and used solid core wires with screw lug connectors to connect to the line power. The 9VDC wires and the sensor wires both go through the hollow support column and connect to the circuit board. I choose a circuit board with surface mount electronics that draw less than 40mA current and the small 500mA 9VDC power supply circuit is more than sufficient.