Set up square bar in a lathe
You need a four jaw chuck to hold a square bar, but the jaws are not self-centring…
The jaws all move independently – concentric lines are provided as a guide to help set each jaw to approximately the same depth. Use these to get the bar roughly central. 2
Round bar can be set with a clock gauge, but using the cutting tool works well here. Move the tool forward until it touches and then move the lathe saddle up the bed, out of the way. 3
Rotate the chuck half a turn and bring the saddle back across, adjust jaws until the gap is equal either side. Once done, rotate 90° to set the other plane – it’ll take a few goes... 4
... and don’t fully tighten the first jaw pair till the end, as the bar needs to slide in them to adjust the other plane. If you can get an even score from the tool on each face, it’s about right.