Nothing crazy exciting here. We needed a new lathe chuck key for our Sharp 1340 lathe. Our spindle nose uses a D-4 camlock system which shares a 3/8″ square drive with 3-jaw and 4-jaw chucks. Our 5C collet chuck uses a smaller, metric square drive and has its own key.

One of the biggest things we needed was a shaft height that cleared the headstock as the hold-over chuck key we had would frustrating wrap your knuckles into the head stock. As a frequent 4-jaw user I also wanted enough leverage to be able to crank down a 4-jaw chuck.

So I set into Fusion360 to design a new chuck (drawing below). I decided on using 1144 (aka stress-proof) for the handle as it’s been partially heat-treated and tempered to create a stable material with some durability. The chuck key we had previously was made of CRS and eventually work-hardened and then rolled over at the edges to the point that it spun in the square drive slot.

The new skill I shot for here was to work on shrink fits. I had designed this to work as an FN2 fit. Unfortunately that didn’t go as well as I was hoping. Even though I had worked out the thermal expansion numbers I was able to get the parts to slip fit. I need to read more as to whether FN2 implies a press fit assembly. Based on my numbers though by heading the chuck body to 400° F and and cooling the handle shaft to about -30° F, I should have had a slip fit.

Lacking any machine tool grinding capabilities, I ended up bumbling around on the lathe with some abrasives to knock down the diameter to get to the point where I was able to get a light interference fit that required hammering with a brass hammer to drive the handle into place. So, FN1?

Time will tell how this holds up.

Finished product