REMOVING A SCREWED-ON LATHE CHUCK - some hints and tips...
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
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. If not, try stage 3.
The problem now is to lock the spindle without damaging the backgears gears - which must NOT be used to lock the spindle. If the lathe is a belt-driven type, 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, use a heavy hammer on the chuck key - do not put a bar across the jaws, you'll 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.
If the chuck still won't come off a "removing bar and clamp assembly" will need to be made - an item far simpler and cheaper than a set of backgears that so many thousands of people have wrecked trying to get a chuck off. This is the link
If none of the above methods work I'm afraid to say the only option left is to remove the chuck from the backplate (they normally bolt on) and then machine the backplate off. The chuck will be secured to a removable 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 it will now necessary to remove the backplate by machining it off. Extreme care is necessary when doing this - especially of course when you get down to the spindle thread. Having just exposed the thread crests it will be necessary to "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
- make sure that when the fitting is removed it is:
1) cleaned of swarf
2) oiled before being replaced.
Fitting a new chuck--see the advice here