REMOVING A SCREWED-ON LATHE CHUCK - some hints and tips...

Stage 1
Arrange the chuck so that its key is at the 12 o'clock position, sticking up vertically. Select the lowest spindle speed (with the lathe switched off, of course) or put the lathe into backgear. Grasp the chuck key and pull it towards you - if  the chuck has not been over-tightened, this should remove it. Do not hit the chuck key with a hammer - you may damage the headstock gearing. If this fails, try Stage 2

Stage 2
Arrange a block of neatly-cut hardwood on the bed so that, when the chuck revolves backwards the face of one jaw will strike it. Put the lathe in backgear and run, at bottom speed, in reverse. As the jaw hits the wood it should undo the chuck. This may have to be repeated several times before it works. One trick, which has been known to work on smaller chucks, is to heat the threaded section of the backplate using a small heat gun - a jewellers' torch works well - then flood the inside of the spindle with plumbers "Frost Bite" - the stuff used to freeze pipes so work can be done without having to drain a system. Obviously you need to block the open end of the spindle with rags pressed in tightly. If still no success, move on to Stage 3.

Stage 3
The problem now is to lock the spindle without damaging the backgears gears - which must NOT be used to lock the spindle, so many have had a tooth broken off when using that method. If the lathe is belt-driven , twist a steel bar though the belting to form a tourniquet. This may damage the belt, but that's cheaper than having new gears made. With the spindle locked (and using the heat/freeze technique described above) use a heavy hammer on the chuck key - do NOT put a bar across the jaws, you'll either stain them or even strip the teeth off.
An alternative is to drive a thin wooden wedge between the largest spindle gear and the inside face of the headstock - experience shows that this will need to be repeatedly knocked further into place as work progresses.
Still no luck? Time for Stage 4.

Stage 4
You might like to attempt the manufacture of the very effective "chuck removal tool" shown on this page" http://www.practicalmachinist.com/vb/south-bend-lathes/how-remove-stuck-chuck-tutorial-165028/

In case the instructions and illustrations on practicalmachinest.com are lost, the pictures are repeated below.

Stage 5
If none of the above methods work I'm afraid to say that the only option left is to remove the chuck from the backplate (they normally bolt on) and then machine the backplate off the spindle. The chuck will be secured to its backplate by three or more bolts; remove these and prise the chuck off - if the chuck is a close fit on the backplate (as it should be) it will be necessary to get something into the interface between them). With the chuck removed, the backplate can now be turned off, though extreme care is necessary when doing this  - especially, of course, when you get close to the  spindle thread. Having just exposed the thread crests on the spindle, it will be necessary to carefully "pick" the material out with a sharp, preferably hardened pointer.

To prevent this happening again make sure that:
- chucks, backplates and faceplates have clean, lightly oil threads and screw onto the spindle easily
- tighten using light hand pressure only - work will tend to tighten the fitting sufficiently
- make sure that, when the a fitting is removed from the spindle it is:
1) thoroughly cleaned of swarf
2) oiled before being replaced.

Use either an item as shown, or a block of hardwood that spans the full length of the slightly opened chuck jaws and then, using backgear and electrical reverse, cause the chuck to rotate backwards against it

If none of the above methods work, I'm afraid to say that the only option left is to remove the chuck from the backplate (they normally bolt on) and then machine the backplate off the spindle. The chuck will be secured to its backplate by three or more bolts; remove these and prise the chuck off - if the chuck is a close fit on the backplate (as it should be) it will be necessary to get something into the interface between them). With the chuck removed, the backplate can now be turned off, though extreme care is necessary when doing this  - especially, of course, when you get close to the  spindle thread. Having just exposed the thread crests on the spindle, it will be necessary to "pick" the material out with a sharp, preferably hardened pointer.