I recently purchased a Sheldon 13" lathe that had been sitting unused for a while. One of the issues was a 3 jaw chuck the refused to unscrew from the spindle. I tried several things over a period of 4 or 5 days. Kept shooting PB Blaster on it. I started by chucking up a piece of hex bar and locking the back gear. I wrenched on the hex bar as much as I felt I could without breaking teeth. No luck. I left the wrench with weight hanging from it overnight. Nothing. Tried an impact wrench. Still nothing. Tonight I removed the chuck. (I forgot to mention that someone had mounted a 1 1/2-8 chuck to a 2 1/4-8 plate that was screwed onto the spindle.) I drilled a piece of bar and attached it to two bolts on the mounting plate. I turned a piece of bar so it was a tight fit in the rear of the spindle bore and then clamped a dog on the end of the spindle. Attached a chain from the pedestal to the dog. I used about 5' of cheater bar on the the mounting plate and it finally broke loose. I would have expected to see some of the penetrating oil on the threads after spraying it as much as I did, but it didn't appear to have wicked in at all. The threads were completely dry. Not real pleased with the PB Blaster.
Chuck
Chuck