Thursday 16 September 2010
Gabor Lacko shows you how to make your own indexing ring and plunger which allows you to make holes for spokes, chair legs, a clock face, or for various decorative features
Indexing, which is a built in feature on some lathes, allows the spindle to be locked in predetermined positions. Indexing is useful, when holes are required for spokes, chair legs, a clock face, or for various decorative features.
To achieve 2, 3, 4, 6, 8 or 12 indexing divisions within one turn of the shaft is easy. All these numbers go into 24, so, with a 24 division index ring all the divisions mentioned are available. The device is called an index ring because the indexing holes are located on the circumference.
Metal turning and ornamental lathes have a wider variety of divisions, for example, 360, 192 and 144 are common. To accommodate this many combinations the face of the plate is used, and the device is called an index plate.
More about the indexing ring
Indexing is a feature of most 'upmarket' woodturning lathes. Quite often the indexing plunger also serves as a spindle lock. To prevent the lathe being switched on while the plunger is engaged, lathe manufacturers sometimes incorporate an interlocking system which disables the starting circuit. Lathes with no indexing usually rely on indexing on the back plate of the chuck.
Back plates for most chucks are available with 24 indexing positions. Some manufacturers make add-on index rings to fit over the chuck. With either of the two solutions, all the turner has to do is to make some sort of plunger arrangement which will lock the lathe spindle in the position selected on the index ring.
First, make the index ring - 150mm (6in) diameter, approximately 25mm (1in) wide with 24 indexing holes equally placed, all 6mm (1/4in) diameter, about 10mm (3/8in) deep - made from good quality hardwood, such as beech, oak etc. On one side of the disc bore a 15mm (5/8in) deep recess to fit snugly over the back plate
Replace the three back plate screws with studs cut from threaded rod. Screw the studs into the chuck and use nuts and spring washers to refit the back plate
With the back plate as a template, drill three holes for the studs. On the chuck side, counter bore the index ring for the nuts and washers. To fit it over the back plate, use thin half nuts
To select any of the index ring holes you need to make a plunger stand. This right-angle assembly will serve as a drilling jig initially. On the vertical part, exactly centre height, drill a 6mm (1/4in) hole to guide the drill when drilling 24 index holes
Cut a plywood disc, about 12mm (1/2in) thick on the bandsaw, and divide into 24 equal parts. Make a 2-3mm deep saw cut on the rim of the disc in all 24 positions. Bore a hole in the centre to mount it on the chuck which has the index ring attached
Replace the toolrest with a piece of round timber, set it about 3mm (1/8) below centre and mount a 20 x 30mm (3/4 x 1 1/8in), 3mm thick piece of wood on it with a chamfered edge to fit into the saw cuts on the rim
With the help of the little wedge, index the assembly round in all 24 positions, and drill a 6mm (1/4in) hole, 10mm (3/8in) deep
On top of the plunger stand, mount a small 'pointer' to show you where you are. If desired, you can also number the holes. Sacrifice the bit you used to drill the indexing holes, and with a small handle, turn it into a plunger - it's bound to fit