I don't know if this is an old trick or not (it probably is – few things are new in this game). Anyway, the job is to mount some bar ends in the chuck for facing, and you want them to run true, but neither end is machined so no reference point.

For this I use the 'bearing on a stick' method shown below. Simply, it is a ball bearing (extracted from a broken computer hard drive) mounted on a suitable length of square section steel, which itself is mounted into a tool holder.

To use: mount the workpiece in the chuck as accurately as you can by eye, do not tighten the chuck firmly – it just needs to be tight enough to hold the piece so it won't move on its own, yet able to be repositioned when the bearing is pressed against it. The photos are self-explanatory (I hope). Spin the workpiece slowly and bring the bearing into contact (which will be intermittant), then continue pressing until it runs true. The sound it makes is as good an indicator as any. Back off the bearing and tighten the chuck more firmly and re-check, if OK tighten fully.

By similar means the bearing trick can also be used to make thin-ish disks run true by pushing it up against it's front face, it just needs mounting along the line of the bed instead of across it.