Sunday’s turning between centers class had a variety of challenges and responses. Ultimately the flywheel was not turned due to a bent mandrel, a result of too much press force. I have since had a chance to think about the un expected result.
If a bushing is pressed on to a bent mandrel and then turned sufficiently between centers, the surface of the bushing must end up concentric to the axis of rotation. It won’t matter that the surface of the bushing is not concentric with the surface of the mandrel; what ever is pressed on to that bushing can still be turned true and balanced. Is that not so?
If there is still interest I am game to reconvene the turning between centers class on Sunday, starting with the flywheel already mounted on to the mandrel with a bushing that restores the required concentricity.
Even so, the re-drilled center hole of the fly wheel seems to have taken out some of the un balance; as now I can get the wheel go to about 500 rpm, max by hand, and it doesn’t vibrate the machine much at all. The side to side wiggle is still embarrassingly large, but doesn’t seem to vibrate the work much.
Agree the bushing will end up concentric with the spindle after turning. If the mandrel is severely bent, then the bushing may have to be turned down very far to reach a spot that will be concentric. As I recall, the mandrel wasn’t too far away from straight, and the end of the bushing was already oversized at the tailstock end so I think you can do that.
I’m guessing you will want to leave a little bit of a shoulder on the tailstock end of the bushing so the flywheel doesn’t spin off? You could use a LH tool and turn the headstock end of the excess bushing down to the desired diameter. As I recall, the hole in the flywheel is 0.749 per pin gauges.
If the mandrel were severely bent I would be concerned that it would be too unstable to turn that heavy flywheel at high speed, but again, as I recall, it wasn’t super bad.