J for Jaws and jam chucks Trevor Branton
Woodturning chucks and jam chucks offer the woodturner additional means of holding work pieces, but they can also present the risk of serious injury to the woodturner from several different hazards.
Sharp edges of revolving jaws and mounting jaws
These are capable of inflicting considerable damage to any part of the turner’s body which comes into contact with them. The risk can be reduced by rounding off the sharp edges which protrude outside the chuck body. Care should be taken to remove similar amounts of metal from each of the jaws in order to keep the chuck balanced. A slightly simpler way to protect from sharp metal edges is to place a sleeve made from an old rubber inner tube around the chuck.
If jaws become unsecured from the chuck whilst at speed there is not only the danger of being hit by the loose jaw(s) but also by the rotating work piece which may be ejected from the chuck. In order to minimise this risk scroll chucks should always be engaged in several teeth of the mounting jaws – it is not sufficient to have the mounting jaws just engaged on the first tooth.
Incorrect work holding
When using a woodturning chuck the optimum grip is obtained when the jaws are at their design diameter, the work piece spigot is appropriately sized to optimise the jaw grip and is also shaped to match the jaw profile.
Chuck Capacity and Spigot Size
Do not try to hold a work piece on an unsuitable work holding e.g. too large a work piece, too small a diameter spigot or if the spigot material is cracked or unsound.
Jaws should only be used in appropriate chucks. Whilst jaws may appear to fit in another make of chuck there may be minimal engagement between the scroll and the jaw teeth.
Jam chucks rely on the ‘interference grip’ between work pieces and jam chuck. There is obviously a limit to the force which can be applied to the work piece without it coming out of the jam chuck. Wherever possible the tailstock should be brought up to locate the work piece in the jam chuck. It is best if smaller sized tools are used and they should be freshly sharpened to minimise the force on the work piece and jam chuck.
LEARNING POINTS -
Keep your hands and fingers away from the revolving chuck jaws.
Round off or shroud sharp edges which protrude outside the chuck body.