A machined part is a chain of dimensions, and the tolerance on the finished part is the sum of the tolerances on each link. A weldment is the same chain with one extra term that is not on the drawing: the assembly moved after it was made, and it will keep moving slightly for a while after that.
That term is why weldments acquire a reputation for being difficult to hold. The cut lengths are right, the bends are right, the fixture is right, and the finished part is still out of position at one corner. Nothing was made wrong. The assembly simply took up a shape that nobody had calculated.
Weld metal occupies less volume after it solidifies than it did as a liquid, and the metal around the joint contracts as it cools from welding temperature to room temperature. Both effects pull the joint together. On a single weld in a restrained assembly the result is residual stress; on a weldment that is free to move, the result is movement.
The amount of movement depends on how much weld there is, how much restraint the fixture provides, and how much material surrounds the joint to resist it. A long continuous bead pulls more than a series of short ones. A joint welded both sides pulls more than one welded on one side. A thin panel pulls more than a thick one, because there is less material to hold the shrinkage in check.
None of that is unpredictable in direction. A weld pulls its two sides toward each other, and an assembly with several welds pulls in several directions at once, with the final shape being whatever balance those pulls reach.
A fillet along one edge of a plate curls the plate toward the weld. A butt joint between two plates closes the root gap and can lift the ends. A circumferential weld around a tube pulls the tube in and can ovalise the end. A weld that joins a plate to a tube pulls the plate toward the tube, which is usually visible as a dish rather than a bow.
Once a fabricator can predict the direction, the correction becomes a design decision rather than a rescue. Pre-setting the parts slightly away from the weld before it is made, or leaving an allowance that a later machining cut removes, both work because they anticipate a pull whose direction is known.
Two fabricators can weld the same parts with the same fixture and get different results, because the order of the welds changes where the accumulated error lands. Welding a frame from one corner around the outside concentrates the pull at the closing joint, which is why the last corner often closes tight or opens wide.
The usual disciplines apply: balance the welds so that opposite sides pull against each other, work from the middle outward so the assembly is free to move in both directions, and leave the joint that matters most until the assembly is already close to shape. Backstep welding and staggered sequences do the same job on long seams.
These are not refinements. On a part with several joints, sequence is often worth more than any tolerance on the individual pieces, and it costs nothing except planning.
A tolerance on a weldment is only meaningful relative to a datum, and a weldment gives the designer a choice of datums that a machined part does not. Reference the assembly to the fixture, and the part is correct as it was held. Reference it to a functional face, and the part is correct as it will be used.
Those are not the same, and choosing between them is a decision about what failure matters. A part that is dimensionally correct relative to a bowed face and wrong relative to the bolt holes will not fit the vehicle, no matter what the inspection report says. The bolt pattern is the datum that matters, and everything else should be referenced to it.
The most useful tolerance strategy for a weldment is to spend the tolerance where it is needed. The interfaces, the flange faces, the bolt patterns and the tube ends that must seal, are the features that have to be held. The path between them can be allowed to wander, because nothing depends on it.
This is the opposite of how drawings are often dimensioned, with a general tolerance applied everywhere and no distinction between a face that bolts to something and a face that is only seen. On a weldment that approach guarantees that the tolerance is used up in the places where it does not matter and unavailable where it does.
Where the interfaces are far apart, a weldment may not be able to hold them both at once. In that case the honest design answer is a slotted hole, a shim, or an adjustment built in, rather than a tighter tolerance that the process cannot meet.
Aluminium moves more than steel for the same heat input, because it expands and contracts more and conducts heat away faster. That combination means the shrinkage is distributed over a wider area, so the pull is less local and the whole assembly tends to move rather than just the joint.
Aluminium weldments also lose strength in the heat-affected zone if they are made from a heat-treatable alloy, and that has a tolerance consequence: the part may be straight and still not perform, because the material beside the weld is no longer the temper it was. Straightening afterwards is limited for the same reason, since the alloy will not recover its properties without a full heat treatment.
A weldment continues to move as it cools. Measuring it while the last joint is still warm, or while it is still clamped in the fixture, gives a reading that will not survive the next hour. Parts have been signed off in a fixture and rejected on a bench, and the difference was time.
The practical rule is to let the assembly come to room temperature, out of the fixture, before anything is measured. Where the part is large, that can mean leaving it overnight. Where it is small, it still means waiting until it can be handled without gloves.
For work that has to hold position after welding, the sequence and the checking routine are agreed with the customer before a batch starts, which is the same conversation described on our customization page.
The questions that decide whether a weldment can be held to tolerance are mostly about what the part has to do, not about how tight the numbers are.
Welded assemblies in aluminium are grouped under TIG aluminum welding, and the stainless equivalents under stainless steel exhaust system and TIG stainless steel welding. The fabricated side sits under titanium fabrication components, and the tube that feeds it is grouped as exhaust pipe.
