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.
• 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.
• The problem now is to lock the spindle without damaging the gears. 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 you are now faced with a machining operation. 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.