“Single-point threading” on a lathe is the process of cutting threads, such as for a bolt, into the material through turning. The spindle/workpiece spin and the carriage moves linearly at a precise amount per turn of the spindle. That linear movement is the thread pitch. But this process usually requires several passes to reach the final depth, which presents a problem: how do you start each thread at the exact same point each time? Daniel Engel’s Arduino-based mod solves that problem on mini lathes.

To facilitate single-point threading, most lathes have a lead screw that can engage with the carriage in order to move the tool at a constant rate. That lead screw connects to the spindle through “change gears” or a transmission system, letting the machinist adjust the linear travel of the carriage relative to each spindle turn in order to achieve a desired pitch. 

But what happens when you finish the first pass and need to go back for a second? How do you keep the leadscrew (and therefore carriage) in sync with the spindle? One method is to run the lathe in reverse, but that has problems of its own. Some lathes have half-nuts designed to sync engagement, but many mini lathes lack that feature.

Engel’s modification does two things to help: first, it provides a gear reduction to the spindle. That lets him perform single-point threading a much lower RPM, increasing torque and making it easier to stop at the right time (such as meeting a shoulder). Second, it lets an Arduino UNO Rev3 board monitor the spindle position via Hall effect sensors that detect four permanent magnets mounted on the spindle from the factory.

The Arduino records the stop and start positions, displaying them on an LCD screen along with the spindle RPM and current position. After finishing a pass, Engel can use that information to return to the exact same starting position in order to perform the next pass.

This is a clever solution to a common problem and Engel’s tutorial has all the information you need to perform the same modification on your own mini lathe. 

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