I was inspired by the “Lathe & Mill Tachometer” post by Catfish (see the 4-2-2014 post by Randy Richard) and decided to install the same type of product on my 7" swing mini lathe I bought new in 1993. 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 used eBay “Buy It Now” and the parts arrived within two days.
I mounted the digital display into a rectangular hole I cut into the cowling covering for the change gears at the rear of the lathe. The cowling space had sufficient room for this thin electronic part and did not require mounting a separate enclosure. I cut the opening using a Dremel tool with a reinforced abrasive wheel. However, the enclosure near the top of the cowling has very little room inside for the cabled sensor. This restriction required installing the sensor from the outside at the back of the cowling covering. This mounting works well and allows 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 north end of magnet must be facing the sensor for it to work correctly. You have a 50:50 chance doing this correctly so I delayed epoxying the magnet in place until this could be tested. I decided to use 5-minute type epoxy to make sure the magnet stayed in place when the spindle spins at over 3000 RPM.
The sensor and power supply wires were secured to the inside walls of the cowling with silicone caulk and this keeps the wires safely away from any spinning gears. Also, inside the cover there are several strengthening ribs that I drilled through for passageways to secure the wires very close to the side walls. The tachometer electronics requires 9V – 15V DC and I measured the current at less than 40 mA. I purchased a small 120 VAC to 9 VDC power supply listed on eBay. The power supply was mounted inside the lathe speed control box (see photo) and connected after the power switch into the 120 V AC screw terminal connector. Later I may consider adding a separate on/off switch for the display. Finally, I used an optical tachometer verified the digital tachometer output was correct.