Which device is used to grip and hold a workpiece during machining?

The device that is typically used to grip and hold a workpiece during machining is a clamp. Clamps are essential tools in machining operations as they secure the workpiece in place, preventing any movement that could lead to inaccurate cuts or surface finishes. By firmly holding the workpiece, clamps ensure both safety and precision, which are critical factors in machining processes.

In the context of the other options: a chuck key is a tool specifically designed to tighten or loosen a chuck, which is another device used to hold workpieces; it does not grip the workpiece itself. A dolly is generally used for transporting items rather than securing them during machining. A conveyor belt is utilized for moving materials from one location to another and does not have the function of holding a workpiece in place during machining operations. Thus, clamps are the appropriate choice for gripping and holding workpieces securely.