"Projects" at georgesbasement.com

Oufitting a Sebastian treadle lathe, continued
Part B. Making the threading dial that is essential to permit moving the carriage back to take another threading cut without losing the registry between the gears and the lead screw, so as to avoid having to reverse the lathe while keeping the half nuts engaged.
Roughing out the frame of the threading dial

   A chunk of scrap steel was hacksawed and then drilled to enable breaking out the waste between the the upper and lower fork.
Partially completed threading dial
   Here the threading dial is partially assembled to test its fit and clearance next to the half nuts.

   The axis of the dial's spindle is tilted three degrees to match the helix angle of the lead screw.
Bottom view showing attachment of threading dial
   The threading dial is clamped in place with a turned-down Allen-head shoulder screw.

   The plain spur gear rotates on a bronze bushing to which it is held by a cotter key.
The following pair of images shows the completed threading dial.
The lead screw has a prime number of threads per inch (i.e., 5 tpi) and the spur gear of the threading dial has twenty teeth, so the gear train comes into alignment with an even number of threads being cut four times in each revolution of the dial, i.e., every inch along the lead screw, as marked at right below.
Partially completed threading dial in place
Completed threading dial in context
Next: Making another necessary accessory, the crossfeed threading stop.