A workpiece in space has six ways to move: three translations and three rotations. Locating it properly means removing exactly the ones you do not want and leaving the rest free. That is the whole theory of fixturing, and almost every over-constrained jig on a shop floor is a jig where somebody removed a freedom twice.
The problem with removing a freedom twice is that the two restraints rarely agree. The part cannot satisfy both, so it deforms elastically to fit the fixture, and it stays deformed until the clamps come off. What the welder sees is a part that fits beautifully in the jig and comes out twisted.
Understanding this changes how a fixture is judged. The question is not whether the part is held firmly. It is whether the part is held by the minimum set of contacts that fixes the geometry you care about.
For most fabricated assemblies, three points on a primary face, two on a secondary face and one on a third are enough to fix position and orientation. Everything else should be a clamp that presses the part against one of those contacts, not a contact of its own.
A tube assembly is a special case, because a cylinder does not sit on three points the way a plate does. It usually needs two V-blocks along its length plus an end stop, with the V-blocks doing the centring and the stop doing the axial position. Adding a third V-block in the middle feels safer and is often the beginning of trouble, because three V-blocks on a slightly bent tube will fight each other.
The same logic applies to a flange. A flange face located against a machined pad is a good datum. The same flange pinched by four separate clamps, each with its own setting, is four competing datums.
Clamps are force, and force applied at two points on a flexible part produces a shape that depends on which clamp was tightened first. Tighten the left one, the part bows right. Tighten the right one, the bow is pushed back through the middle. The final shape depends on the order of operations, which means it depends on who was working that day.
This is the practical signature of over-constraint: the part measures differently depending on how it was clamped. If two operators produce two different pre-weld geometries from the same components and the same fixture, the fixture is asking the part to do something it cannot do consistently.
The cure is usually to remove clamps rather than add them. A part that is located properly needs clamps only to keep it in contact. Two well-placed clamps on a located part hold better than six clamps arguing with each other.
When a part is forced into a shape and then welded, the weld freezes the part in that forced shape. The elastic strain that the clamps imposed becomes locked into the assembly. Release the clamps and the part springs toward its natural geometry, dragging the weld with it.
That spring is not always visible on the bench. It can appear as a flange that will not sit flat, a hole that no longer lines up, or a crack that opens at the toe of a stiff joint after a few heat cycles on the vehicle. The stress was there from the moment the clamps came off; nothing announced it.
Thin sections are the most sensitive, because they store strain with very little force. A thin titanium panel clamped flat over a bowed frame will hold the bow out of sight until release, and then return most of it. The weld did not cause that movement. The fixture did.
It helps to separate two jobs that look identical. A fit-up clamp brings two edges into contact so they can be tacked. A shape clamp forces the assembly to a geometry. The first is temporary and can be removed after tacking. The second is a statement about the finished part and should be used sparingly.
Most good fixtures are all fit-up clamps. They hold edges together, hold a part against a stop, and then get out of the way. The moment a clamp is used to pull a part that does not want to be there, it has become a shape clamp and it is bending metal rather than holding it.
Where a shape clamp is genuinely needed, the part should be redesigned so it does not need one. A bracket that has to be pulled into place is a bracket with the wrong hole position or the wrong bend allowance, and clamping it only hides that until the next operation.
A fixture should be designed around what happens when it opens. The useful question is not how the part is held but where it will move when the last clamp is released, and whether that movement matters.
If the movement is predictable, it can be pre-set. If it is not predictable, the fixture is over-constrained and the part will find a shape nobody chose. Repeatable fixtures produce repeatable errors, and a repeatable error can be compensated at the drawing stage.
Quick-release clamps and slotted locating pins help here, because they let the part settle before final tightening. A pin that has to be tapped in is a pin that is forcing the part to a position it did not reach on its own.
Stainless and titanium do not behave the same way against tooling. Titanium galls against steel contacts under load, particularly when the clamp is tightened and the part shifts slightly. That galling marks the surface and, on a visible part, becomes the reason for a reject that had nothing to do with the weld.
The remedy is soft contact faces, clean tooling and clamping that does not allow sliding. Brass or polymer pads on the contact points cost little and remove most of the problem. Keeping the same tooling away from stainless afterwards avoids transferring contamination.
Titanium also springs back more than stainless after forming, so a fixture that assumes the part will relax into contact will leave a gap. The clamps have to close that gap without bowing the part, which brings the design back to locating properly and clamping lightly.
It is usually simpler than the one it replaced. Fewer clamps, more machined contacts, clear access to the joint, and a shape that can be checked with a straight edge while the part is still in it. Complex fixtures tend to be compensating for a part that is difficult to fit.
It also carries its own datum. A fixture with a marked reference face lets the operator check the part before welding rather than after, and that check is what turns distortion planning into a routine instead of a post-mortem.
Our own four factories run repeat fixtures for the exhaust and fabrication ranges, and the pattern is consistent: the fixtures that survive years of use are the ones with the fewest moving parts.
Whether a fixture is part of the price depends on volume and repeatability. What matters to the buyer is the geometry that comes out, so the drawing should say what is critical and what is free.
How a custom part is quoted, drawn and sampled is set out on the customization page, and the plant and equipment behind it on about us. Welded fabrications and brackets appear under titanium fabrication components and other parts, tube work under exhaust pipe, and material options on shop by material.
