The term workholders includes all devices that hold, grip , or chuck a workpiece to perform a manufacturing operation. The holding force may be applied mechanically, electrically,hydraulically, or pneumatically. This section consider worholders used in material-removing operations. workholding is one of the most important elements of machining processes.
Figure 4-1 illustrates almost all the basic elements that are present in material-removing operation intended to shape a workpiece. The right hand is the toolholder, the left hand is the workholder, the knife is the cutting tool, and the piece of wood is the workpiece. Both hands combine their motion to shape the piece of wood by removing material in the form of chips. The body of the person whose hands are shown may be considered a machine that imparts power,motion, position, and control to the elements shown..except for the element of force multiplication, these basic elements be found in all of the forms of manufactuiin setups where toolholders and workholders are used.
Figure 4-2 shows a pair of pliers or tongs used to hold a rod on which a point has to be ground or filed. This simple workholder illustrates the element of force muliplication(增加) by a lever action, and also shows serrations on the parts contacting the rod to increase resistance against slippage.
Another large group of workholders are the chucks. They are attached to a variety of machine tools and are used to hold a workpiece during turning, boring, drilling, gringing, and other rotary operations. Many types of chucks are available. Some are tightened maunally with a wrench, other are power operated by air or hydraulic means or by electric motors. On some chuck, each jaw is individually advanced and tightened, while others have all jaws advance in unison. Figure 4-5 shows a workpiece clamped in a four-jaw independent chuck. The drill, which is removing material form the workpicec is clamped in a universal chuck