Pie Jaws with Bottom Master Jaws
First developed by watch and clock makers, chucks with pie-shaped jaws offer two main advantages. Boring the unhardened jaws to hold a particular diameter part means that instead of gripping the part at only three or four tiny contact points, they grip the part all the way around its diameter. This increased gripping area allows for much less clamping pressure and less chance of deforming a gear or other delicate part held in the chuck. In addition, because the face is bored to an exact and consistent depth, placing additional parts in the chuck for turning or milling means each part surface is held at exactly the same depth as the previous parts, saving setup time.