I wrote last time about the very bad run out of the 3-Jaw chuck. Did some more research and found that there isn't too much can be done about the chuck itself, other than buying a high quality one with high precision. But that is just one of the factors that affect the precise centering of the work piece. A 4-Jaw chuck is still a little better, but it takes much longer to get everything true. I have a dial gauge with a magnetic base on order and should be here soon. In the mean time, I learned that using a ball bearing can make the work piece on a 3-Jaw chuck run much more true than without.
Got a broken bicycle bottom bracket that has a ball bearing still on, and it broke at the perfect place. I really don't need to modify it, except to thread a screw on it, and then clamp it on a tool post, like this:
Work piece centering tool using a ball bearing from a broken bicycle bottom bracket.
To use it, mount the work piece just tight enough that it won't come off when the lathe is turned on. Run the lathe in slow speed, and gradually a advance the ball bearing until the bearing start to turn. The work piece should now run true enough to start turning. I don't have the dial gauge yet to check the run out, but to my eye, it was light years better than when I was turning the Angenieux 70mm f1.5 projection lens.
All these changing of tool posts and change the belt position for different speed of the lathe is quite tedious. All the more motivation to convert this lathe to use a variable speed DC motor, and buy a quick change tool post.
Water Quality Sampler Post - Zeiss Opton Tessar 75mm f3.5 & Sony A7