Many turners like to re-turn the bottom of a bowl to get rid of chuck marks and leave it with a shape that complements the rest of the piece. There are a number of ways to do this, including:
Some of these options are expensive. I seldom make natural edged bowls, so my preferred option is homemade stepped jaws. You need an ordinary four jaw chuck with steel accessory plates that bolt onto the jaw carriers. If you have the jaw carriers with their scroll thread, you could quite easily make the accessory plates too.
- proprietary chuck jaws ('Cole' jaws) with adjustable buttons
- stepped jaws that fit the bowl rim
- friction drive against a wooden faceplate, with tailstock support
- friction drive from rim of a wooden faceplate inside the bowl, with tailstock support
- friction drive from padded post against inside of the bowl, with tailstock support
- vacuum chuck
- jam-fit wooden chuck
- homemade scroll chuck
- homemade clamping ring that holds the bowl against a faceplate.
The jaws are made of plywood. I used MDF, but precautions are needed to stop it delaminating and losing its grip. MDF is not strong and can split easily. Good quality ply is stronger, but also can delaminate under stress. Neither is as strong as metal, and allowances must be made for this when using the chuck. This is how to make the jaws.
First glue up several squares of good quality ply, free of voids, to make a thick slab. There is no critical thickness, but about 30 mm would be good. Make the laminated square big enough to contain the outer diameter of the finished chuck. Ensure that you glue thoroughly, clamp it up well, and allow time for the glue to set. You don't want it to come apart.
Then cut it into quarters. A table saw is good for this as it will give nice straight edges.
Lay them together, back in their square shape, and mark a circle centred on the intersection. Bandsaw out the curved edges of the quadrants. Screw each chuck carrier plate onto the inner corner of its quadrant, leaving no clearance between them. Use short screws so they don't project into the recesses you will make for the bowl rims, but use plenty of them.
Assemble the jaw blanks on the chuck and turn the rim true. Sand away the sharp edges. Now turn recesses in the face to suit the sizes of bowls that you make. The rim and the recesses will only form true circles at one diameter, but will still grip. The middle recess can be deep enough to almost touch the screw tips, or you can leave it thick in the middle. Undercut the sides of each recess slightly (not too much as it will make delamination more likely). Now it is ready to use.
You will probably find that you occasionally have to widen a recess to fit a particular bowl rim, and eventually the jaws will need replacing.
Two of the four jaws
Chuck jaws made of wood are not as strong as steel or aluminium. They cannot withstand the same stresses and their grip on a bowl may not be as secure. Segmented ply or MDF jaws like this can flex and don't grip as securely as a jam fit chuck made of solid wood. So this kind of chuck must be run at low speed, with light cuts, and with tailstock support where possible, particularly if the bowl is deep. They aren't suitable for shaping a bowl, just for refining the foot. Don't make the outer recess too wide, in case the material splits away at the edge. You could insert screws to reinforce the edge and make sure the top layer of ply doesn't lift.