Capillary action only works in a narrow space. The filler is drawn into the joint because surface tension pulls liquid towards the region where the surfaces are closest, and that pull depends on the gap being small and even. A brazed joint is therefore a designed clearance, not an accidental fit.
This is the part of the process most often improvised. Two parts that fit together loosely will not braze, however clean they are and however well the heat is managed, because there is nothing to draw the filler inward. The gap is the joint.
If the fit is an interference or a press, the filler has nowhere to go. It sits at the mouth of the joint, forms a cosmetic fillet, and leaves the interior starved. The assembly passes a visual check and fails under load, which is the worst combination available.
A tight fit also traps air. As the assembly heats, that trapped volume expands and pushes back against the incoming filler, so even a joint that looks wetted can be hollow a short distance in. The surface appearance tells you nothing about what happened inside.
A wide gap removes the driving force. The filler bridges the opening at the edge and stops, leaving a joint held by a fillet rather than by a filled annulus. Strength drops sharply because the load now travels through a small area at the joint mouth.
Wide gaps also demand more filler, more heat and a longer time at temperature, all of which work against thin components. On tube, an oversized fit is usually a sign that the wrong fitting was selected rather than that the technique was wrong.
Control comes from the parts, not from the torch. Machined fittings, drawn tube and accurate cut lengths give a consistent clearance; a filed or hand-fitted joint gives a variable one. Where the joint is critical, trial-fit a sample from the same batch before committing the assembly.
Temperature changes the gap as the parts expand, and dissimilar materials expand differently. A fit that is correct cold can close or open as the assembly reaches working temperature, so the material pair has to be considered before the clearance is chosen. Our fabricated components are dimensioned with that in mind, since a joint designed on the bench can tighten in service.
Tube joints are annular and self-centring, which helps: a fitting with a short land will align itself and hold a reasonably even gap around the circumference. Plate joints have no such geometry and rely on clamping, so a lap joint in sheet needs support along its whole length.
Titanium adds its own difficulty. It wets reluctantly and reacts with atmospheric gases when hot, so the gap has to be right and the shield has to be effective, as described in our TIG titanium welding parts work. A generous gap is no substitute for atmosphere control.
Assemble dry, mark the parts, and look at the fit. The joint should go together with light hand pressure and stay put without rocking. If it drops on with no resistance it is too loose; if it needs a press it is too tight for capillary flow.
Confirm the check on a scrap pair from the same stock. Cut the test joint open after brazing and look at the section: a full, even fill along the land is the only evidence that the clearance was right. That single destructive test settles more arguments than any amount of visual inspection afterwards.
