Getting a Grip on Four-Jaw Lathe Chucks
Shape the bottom
When shaping the outside of a bowl, form its bottom to fit your chuck jaws so you can reverse the blank, grip it with the chuck, and form the inside. For contracting dovetail jaws, form a dovetail spigot as shown right. Cut the inside corner where the spigot meets the bowl body crisp and clean to provide a positive gripping corner for the chuck jaws. Leave a flat bearing surface for the jaw ends. Because the jaws pull in on the spigot as well as squeezing it, make the spigot slightly shorter than the jaw depth, so it does not bottom out. Sand and finish as much of the outside of the bowl as possible.
The same jaws that contract onto a spigot also will expand into a recess hollowed out in the bottom of a bowl, as shown below right. When planning the shape of your bowl, leave extra thickness in its bottom to accommodate this recess. Make it about 1/8-1/4" deep, cleanly cut, and with its edge following the shape of the jaws. Leave sufficient wood around the recess to support the outward force exerted by the expanding jaws. Gripping the bowl in this manner allows you to completely form the outside, including its bottom. With the outside of the bowl formed, sand it and apply a finish.
Generally speaking, contracting onto a spigot provides the greatest holding power. However, for turnings of large diameter but shallow depth, such as a large platter, expanding into a recess gives better results. Regardless of how the chuck holds the workpiece, be sure to periodically check its tightness.
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