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The term workholder 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 considers workholders used in material-removing operations. Workholding is one of the most important elements of machining processes.
Figure 4-1 illu7strates almost all the basic elements that are present in a 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 may be found in all of the forms of manufacturing serups where toolholders and workholders are used.

Figure 4-2 shows a pair of pliers of tongs used to hold a rod on which a point has to be ground or filed. This simple workholder illustrates the element of force multiplication by a lever action, and also shows serrations on the parts contacting the rod to increase resistance against slippage.
Figure 4-3 shows a widely used workholder, the screw-operated vise. The screw pushes the movable jaw and multiplies the applied force. The vise remains locked by the self-locking characteristic of the screw, provides means of attachment to a machine, and permits precise placement of the work①.
A vise with a number of refinements often used in workholders is depicted in Figure 4-4. The main holding force is supplied by hydraulic power, the screw being used only to bring the jaws in contact with a workpiece. The jaws may be replaceable inserts profiled to locate and fit a specific workpiece as shown. Other, more complicated jaw forms are used to match more complicated
workpieces.
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 turming, boring, grinding, and other rotary operations. Many types of chucks are available. Some are tightened manually with a wrench, others are power operated by air or hydraulic means or by electric motors. On some chucks, 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 from the workpiece is clamped in a universal chuck.
Purpose and Function of Workholders. A workholder must position or locate a workpiece in a definite relation to the cutting tool and must withstand holding and cutting forces while maintaining that precise location. A workholder is made up of several elements, each performing a certain function. The locating elements position the workpiece to the machine; and clamps, screws and jaws apply holding forces. Elements may have manual or power activation. All functions must be performed with the required firmness of holding, accuracy of positioning, and with a high degree of safety for the operator and equipment.
The design or selection of a workholder is governed by many factors, the first being the physical characteristics of the workpiece. The workholder material must be acrefully selected with the workpiece in mind so that neither will be damaged by abrupt contact, e.g, damage to a soft copper workpiece by hard steel jaws②.
Cutting forces imposed by machining operations vary in magnitude and direction. A drilling operation induces torque, while a shaping operation causes straight-line thrust. The workholder must support the workpiece in opposition to the cutting forces and will generally be designed for a specific machining operation.
     
Many workholders are used in industry that are not used on material removing operations. Workholders may be used for the inspection of workpiece, assembly, welding, and so on. There may by very little difference in their basic design and their appearance. Quite often a standard commercial design may be used in one application for a turning operation and for the same or another workpiece in an inspection operation.