Everything a welder can do to a joint is limited by how it was presented. If the gap is uneven, the arc will wander. If the parts are twisted against each other, the weld will pull the assembly further out as it cools. Dry fitting is the stage where those problems are found, and doing it properly is faster than fixing the result afterwards.
Dry fitting means assembling the joint with no weld at all, checking that every edge meets as intended, and confirming the overall shape before anything is joined permanently.
A tack is a small weld placed to hold alignment while the rest of the joint is completed. It is not a structural weld and it is not a way to pull a bad fit into position. A tack that bridges a wide gap will pull the parts together as it cools, and the assembly will hold that distortion afterwards.
Good practice is small, matched tacks, placed so that the final weld can consume them, with the gap between the parts unchanged from the dry fit.
Tack size deserves the same discipline as the finish weld. A heavy tack on thin tube puts a local heat concentration into the joint before the main passes begin, and on stainless and titanium that is exactly where a distortion problem starts.
Heat shrinks metal as it cools, and every weld therefore pulls. The way to manage that is to plan where the welds go and in what order, so the pulls balance instead of accumulating in one direction. Tacking opposite sides before welding a long seam is the simplest version of the idea.
On a closed frame or a box section the sequence matters even more, because the part has nowhere to move and the stress stays in the material. A balanced sequence spreads the pull; a convenient one concentrates it.
Alignment is checked against something. On a fabricated part that something is usually a fixture, a flat surface or a measurement to a defined feature. Checking by eye alone tends to confirm what the person checking expects to see, which is why a straight edge, a square or a gauge is worth the extra minute.
Where the part has to fit a vehicle or a mating assembly, the important dimensions are the ones that touch something else. Those are the ones worth verifying before the weld is finished, because they are the hardest to change afterwards.
It also helps to check in the same order every time. If the first dimension measured is always the same, a drift shows up as a pattern rather than as an isolated surprise.
Both materials want a clean joint and a clean back side, and both are less forgiving of a tack that has to be cut out and redone. Titanium in particular reacts with the atmosphere when hot, so the tack and the following weld have to be protected, and the interval between them matters because the joint is open to the air again the moment the torch stops.
The practical consequence is that dry fitting should be finished before the first tack goes down. Making adjustments after the arc has been struck means either grinding out a tack or welding over a joint that is no longer clean.
If the dry fit shows a mismatch, the answer is to correct the parts rather than to close the gap with weld. A formed edge can be adjusted, a tube end can be trimmed or re-cut, and a flange can be faced. Where the mismatch comes from the drawing or from the sample the parts were made to, that is an issue to raise rather than to solve quietly at the bench.
Weld metal used as filler for a poor fit is the single most common source of a part that cracks later, because the joint ends up with a section that was never designed to carry load.
A mismatch that only appears after tacking usually means the parts moved rather than that they were made wrong, and the first thing to check is what the tack pulled before anything is cut.
Before the main passes go on, the assembly is checked once more: the tacks are sound, the alignment still holds, and the purge is in place where it is needed. This is a short step, but it is the point of no return for the geometry.
After that the welder's attention belongs on the pool, the travel speed and the heat, and the part cannot be moved without losing the fit that was just established.
If you are commissioning a fabricated assembly, these are the fit-related points that decide how it comes out.
Every assembly on this site went through a dry fit first. Our finished titanium exhaust systems, our formed tube work, our fabricated metalwork and our TIG welded components all depend on it, the custom work page explains how a job's fit is agreed, and our material pages cover how each metal behaves at the joint.
