I have an alignment problem with my 1980s Frejoth 12x37 lathe (very like a Grizzly 4003, and various ENCO etc) and 6” D1-4 3jaw chucks. I know 3 jaw chucks are not that accurate but I think it should be better than what I’m seeing. D1-4 backing plate is running acceptably true - within 1 thou radial and axial. The back edge of chuck (where I can get a gauge) is also running within 2 thou of true. A rod set in the jaws describes a cone. Close to the jaws, the rod is 10-15 thou out, but 4” down the rod the error increases to 30 thou or so. Tracing back along the rod into the chuck, this would suggest the minimum error on the rod - the point of the cone - would be back inside the chuck - if you could measure it. Thinking is was a jaw issue, I got a new-to-me 6” 3 jaw d1-4 chuck, chinese, relatively new and in good condition. It shows almost the same error! I then tried slipping a shim inbetween backing plate and chuck. Oddly this did not change the error much. I’m quite confused and would be grateful for any insight or advice.