Question: How can I keep a tube centered in a fitting for brazing when the dimensions of the tubes and fittings have such broad, loose commercial tolerances and the tube tends to “wobble” inside the fitting?
Answer: This question pops up again and again as new people come into the brazing world, and it’s a good question. One of the ways to save money today is to only purchase products using standard commercial tolerances that are available off-the-shelf. People who are somewhat new to brazing (and some old-timers too!), as well as many of their managers who are not brazers, don’t understand that proper brazing requires tight joint clearances for maximum joint effectiveness (strength, corrosion resistance, fatigue life, etc.).
Wide gaps in brazing often lead to weak joints, leaking joints and brazements that are not able to last in service. Depending on the base metals and brazing filler metals (BFMs) involved, proper gap clearances should range from 0.001 inch (0.025 mm) to no more than about 0.005 inch (0.15 mm) at brazing temperature.
Problem: When joint fit-up is sloppy and loose, the inner tube/rod may drift off to one side inside the outer tube/fitting, resulting in uneven braze clearance around the joint (Fig. 1). It is not uncommon to find that when the inner tube/rod drifts off to one side of the fitting, it might leave a large gap on one side and metal-to-metal contact on the other. The large gap can be as large as 0.030 inch (0.75 mm) in some cases or even larger. This huge gap can be VERY difficult to fill with molten BFM, resulting in large voids remaining in the joint after brazing.
How can a brazer prevent that problem from happening? We will look at the solution in part 2.