Jig or template to hold a workpiece

Technical English for a foreigner - please correct and rephrase if you can come up with better alternatives.

A machine in manufacturing usually is fed material or a workpiece to be processed. Sometimes, the orientation of the workpiece [synonyms?] is defined by a kind of jig into which the workpiece is inserted.

Is there a better word for jig?

No -- jig is the correct technical term for any bit of tooling designed to hold a workpiece for the purpose of facilitating a specific operation. A vise or clamp is a general-purpose tool; it lacks the constraints that make a jig a jig.

A jig may incorporate a template, as in the case of a jig designed to hold a workpiece for routing a particular shape against a bearing or bushing guide, but the template is not a jig until the clamps, stops and handles (as may be appropriate) are added.

Within the sentence "Orientation of the workpiece is defined by a kind of jig into which the workpiece is inserted", it's likely the word fixture will serve better than jig. As noted in the wikipedia Fixture (tool) article,

The main purpose of a fixture is to locate and in some cases hold a workpiece during either a machining operation or some other industrial process. A jig differs from a fixture in that it guides the tool to its correct position in addition to locating and supporting the workpiece.

The question refers to defining orientation of a workpiece; it does not refer to guiding the tool that operates upon the workpiece or guiding the workpiece against the tool. Thus, the device in question is more precisely a fixture than a jig.

It is untrue that "jig is the correct technical term for any bit of tooling designed to hold a workpiece for the purpose of facilitating a specific operation". Rightly, many machinists say fixture for devices that hold a stationary workpiece but don't guide a tool, and jig for devices that guide a moving tool or workpiece; others may use one term or the other for all such devices.