Some lathes ( especially larger ones) often have chucks with integral threads or other mounting mechanisms – Long-nose Taper, Camlock, ISO, etc. – but most small lathes (and older larger ones) use a simple “backplate” where a suitably threaded disc – preferably made from drawn cast iron – is screwed on (or otherwise attached) to the spindle nose and then turned very carefully so that a spigot, raised in its centre, will fit closely into a recess in the back of the chuck.
- At all costs avoid steel backplates; they can bruise or otherwise damage the spindle nose and, if they become stuck, will be much more difficult to remove.
- Contrary to popular belief, the bolts that pass through the backplate and screw into the body of the chuck do not provide a location – they simply clamp the two components together; the alignment of the chuck on the backplate (and hence its position relative to the centre line of the headstock spindle) depends upon the spigot, (machined on the backplate), being made a very close fit within the chuck body.
- A further important consideration concerns the surfaces of the backplate and chuck that come into hard contact with each other.
- This is determined (of course) by which surfaces the mounting bolts pass through – and can be either on the raised outer ring (annulus) of the chuck, or the circle formed inside it.
- Whichever surfaces come into contact make sure that the other two (non-contact surfaces) have a little clearance between them – about 0.025″ (0.5 mm) is sufficient – in other words, the depth of the spigot must not be too deep, nor too shallow.
- Needless to say, if you have more than one chuck each will require fitting to its own backplate. Even when chucks have identical backs removing and refitting them (on a shared backplate) would not only waste time but introduce inaccuracies..