A jig holds a part in a known position while it is measured, cut, drilled or welded. A fixture does the same job for assembly, keeping several pieces in the right relationship until they are joined. Together they replace the operator's judgement with a fixed reference.
That reference is the point. Without it, every part is positioned by eye and corrected by hand, and the result depends on who built it and how tired they were. With it, the tenth part starts from the same geometry as the first.
In custom work an exhaust or intake assembly is a chain of bends, joints and brackets that has to fit a specific vehicle. If a bend sits slightly out of rotation, or a hanger is welded to the wrong side, the whole assembly stops fitting. Jigs freeze those relationships before welding starts.
On a titanium fab component the case is even stronger, because correcting a mistake is expensive. Titanium is harder to rework, and a part that has to be cut apart and re-welded loses both time and material. Getting it right the first time is the cheaper path.
A good jig is rigid and simple. It has fixed stops for the tube ends, blocks that set the rotation of each bend, and enough clearance that the part can be welded without the fixture fighting the torch. Anything that flexes under clamping load will release a distorted part.
It also has to be correct to begin with. A jig is usually built from a known-good part or from measured geometry, and it is checked before it is trusted. A jig that was made badly simply produces the same error more reliably.
Holding a part in position while the welds cool does more than keep dimensions. It resists the pull that shrinkage applies as each joint cools, so the finished part stays where it was set instead of drifting. That is especially valuable on thin-wall assemblies with many joints.
Positioning also affects access. A jig that can be rotated lets the welder work in a comfortable position for each joint, which produces a more consistent bead than welding overhead or reaching around a fixture. Consistency of access leads to consistency of result.
A jig removes variability, but it does not remove skill. Deciding where to place the tack welds, how much heat to use and how to sequence the passes are still judgement calls, and on a one-off part the fixture may be little more than a few blocks and clamps on a flat table.
This is the balance in custom work. Heavy tooling only pays for itself across repeats, so for a single bespoke part the fabricator builds a light temporary fixture and accepts a little more hand-fitting. The method follows the quantity.
The check is straightforward: build several parts from the same jig and compare them. If they stack together, bolt to the same pattern, or sit in the same place on a reference surface, the fixture is doing its job. If they vary piece to piece, the jig is not holding what it is supposed to hold.
Recording the checks matters as much as making them. A note of what was measured and how it compared keeps the process stable across batches and makes it obvious when something in the fixture has moved or worn.
