In a lot of shops the words jig and fixture are used for whatever happens to be holding the part, and the ambiguity costs money at the design stage. The two devices answer different questions. A jig holds the work and also guides the tool that cuts or drills it. A fixture holds the work in a defined position and guides nothing; the operator brings the tool to the part.
Both locate, both clamp, both control geometry. The difference is whether the device tells the tool where to go, and that single difference changes what has to be built, how rigid it must be, and who is allowed to use it.
A jig carries the reference for the cut. Because the tool is guided by the device, the accuracy of the feature depends on the device rather than on the operator's eye and hand. That makes a jig worth building when the same feature has to appear in the same place many times, and when the feature is awkward to mark out on the part itself.
The trade is rigidity against access. A jig that guides a drill has to be stiff enough that the drill cannot walk, and open enough that the operator can see the cut and clear the chips. Those requirements pull in opposite directions, which is why jigs for tube work tend to be built around one operation rather than around the whole part.
A fixture answers a different question: where does this part sit, and where does the next part sit relative to it. It is a locating system with clamps, and its job is repeatability of position, not guidance of a tool. The accuracy of the finished work still depends on marking out and on the tool, but the relationship between components is fixed.
In welded assemblies that relationship is the whole game. A fixture that puts two tubes in the correct relative position and holds them while the joint cools does more for the finished geometry than any amount of care at the bench. It is also the device most often built too lightly, because its job looks passive.
Build a jig as if it were a fixture and the guided feature will drift, because the guide is not stiff enough to resist the tool. Build a fixture as if it were a jig and it comes out heavier and more closed than it needs to be, which usually means the welder cannot reach the joint and the part has to come out to be welded, defeating the purpose.
Naming the job first also settles the tolerance question. A jig has to hold the tool path to whatever the feature needs. A fixture has to hold two parts to each other, which is often a looser requirement on absolute position and a tighter one on relative position. Those are different drawings and different inspection plans.
In either device, locating and clamping should not be confused. Locators define position; clamps hold the part against the locators. A clamp that also positions the part is doing two jobs and doing both badly, because the force needed to hold is rarely the force that should be used to place.
The practical test is simple. With the clamps just touching, the part should already be in the right place. If it moves when the clamp is tightened, the clamp is positioning it, and in production that shows up as scatter between parts that all look identical on the bench.
This is worth writing into the build notes for any custom fabrication project, because the locating scheme is what a second operator has to reproduce on a different shift.
Tube is awkward to hold because it has no flat faces. A jig for tube work usually locates on the outside diameter in a pair of V blocks, with the guide positioned relative to those blocks rather than to the bench. Everything depends on the tube being straight in the region the blocks touch, which is why a jig built for one bend radius is not automatically good for another.
The guide itself wears. A bushing that has drilled a few hundred holes is no longer the size it was, and the drift is invisible because the jig still looks right. Any jig for bent tube work should be designed so the guide can be changed without rebuilding the body.
A welding fixture has to survive heat, spatter and the occasional hammer. Its locators need to be out of the weld path or shielded, and its clamps need to hold the part against shrinkage rather than merely hold it up. A clamp sized for handling will lose the argument with a cooling weld, which is why the stainless TIG welding procedure and the fixture are best designed together.
It also has to allow the assembly to come out without springing. If the part has to be prised off the locators, the fixture is over-constrained or the part has already moved, and forcing it out only hides which. A fixture that releases cleanly is a fixture whose geometry is honest.
Small assemblies often use a single plate with pins, blocks and a drill guide, and that is fine. The distinction matters when the device is being designed and when it is being justified. A combined device is heavier, needs more clearance, and is harder to modify when the part changes by a small amount.
A useful habit is to build the fixture first and add guidance later if the volume justifies it. Most first articles need position, not speed, and a fixture that can be adjusted between parts is more valuable at that stage than a jig that locks a geometry nobody has confirmed yet.
Neither device is worth building for one part unless that part is large or the consequence of getting it wrong is expensive. The case for building one is volume, repeatability or access: the same joint in the same place many times, or a joint that cannot be reached once the assembly is closed.
Where a device is justified, the money usually belongs in the locators rather than the frame. A stiff frame with soft locators produces parts that vary, while a light frame with hard, well-placed locators often produces acceptable parts. That proportion is worth remembering when a tooling budget gets trimmed from the wrong end.
A fabrication quote depends on how the parts will be held, so the holding method belongs in the conversation rather than in the shop's private notes.
Deciding which of the two jobs a device is doing, before it is drawn, keeps the build honest. The assemblies described under other parts and the tube work in the exhaust pipe range both depend on that decision being made early, and the fabricated components pages show finished parts where the holding method is visible in the geometry.
