A jig guides the cutting tool as well as holding the work. A fixture holds and locates the work but does not guide the tool — the machine does that. On modern CNC equipment most of what people call jigs are fixtures, and treating one as the other leads to designs that cost more than they need to.
6 min read · Last reviewed 21 September 2026 · All resources
On a manual drilling machine, a drill bush is doing real work: it is the only thing holding the hole position. On a machining centre, the axes already know where the hole goes. Adding bushes there usually adds cost and a wear item without improving position.
The practical consequence is that a fixture for a CNC machine should concentrate its effort on locating repeatably, clamping without distortion and loading quickly — not on guiding the tool.
A fixture that clamps hard but locates poorly produces parts that are consistently in the wrong place. Locating comes first: establish the datums, constrain the degrees of freedom deliberately, then clamp in a direction that pushes the part onto those locators rather than away from them.
Over-constraining is the common failure. Six points define position; additional locators fight each other and the part ends up sitting differently depending on the order the clamps are tightened.
Thin-walled and cast parts deform under clamping and spring back after release, which shows up as a part that measured correctly in the fixture and incorrectly on the bench. Spreading the load, clamping over supported material and reducing force are all cheaper than discovering this in production.
Where distortion is a genuine risk, it is worth measuring the part both clamped and free during the first-off, so the effect is known rather than suspected.
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Occasionally — for a secondary hand operation, or where a feature is added off the machine. But if the machining centre is positioning the tool, guiding it again is usually redundant.
Batch size and cycle time decide it. Manual is cheaper and simpler for low volume; powered clamping pays back where loading time is a meaningful share of a short cycle, or where consistent clamping force matters.
With real parts, not just the first one. Loading and unloading repeatedly, by more than one operator, surfaces the problems that a single careful first-off will not.
Often, particularly where the change is to clamping or a locator rather than to the fundamental datum scheme. Whether it is worthwhile depends on how much of the original design still suits the part.
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