Getting a Grip on Four-Jaw Lathe Chucks
How a chuck grips
Chuck jaws either contract around a turned spigot (round tenon) or expand into a hollowed-out recess. In contracting mode, the jaws commonly supplied with chucks grip spigots from 1 1/2" to 3" in diameter. These same jaws expand into turned recesses from 2" to 3 1/2" in diameter.
Because chuck jaws form a circle when completely closed, they make continuous contact when contracting around a spigot or expanding into a recess that is just slightly larger than the diameter of the circle, as shown right. This provides the greatest holding power and safest operation. As the jaws approach their maximum open position they make only point contact with the workpiece, as shown right. Gripping only at these points (eight in contracting mode, or four in expanding mode) is less secure and risks marring the wood. Manufacturers offer accessory jaws to cover a wide range of gripping diameters.
When turning, be careful of chuck jaws adjusted near their maximum diameter. The farther open the jaws, the more they extend beyond the chuck body, and if touched, the more likely they are to cause injury.
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