We discussed a problem last time, and we will look at the solution today.
Solution: One method that has been very successfully used over the years to help keep the inner member centered in the outer member (Fig. 2) is to use a prick punch to move (“upset”) some of the metal on the surface (OD) of the inner member so that the tube will fit more uniformly in the outer member tube/fitting. Notice how the prick punch is used to “upset” or “dimple” the outside surface of the tube/rod, causing some of the metal to pile up a little bit around the punched hole.
By placing these punches around the OD of the tube/rod being inserted into the fitting (perhaps at about 120-degree intervals), as shown in Fig. 3, it could help to center the tube better inside the fitting since these indents will cause raised mounds of base metal, which will help center the tube/rod in the fitting. These are not burrs. Please do not use the word “burr” to describe these raised mounds of metal around the indents. The word “burr” generally has negative connotations, indicating a metal chip or sharp extension usually caused by improper machining or stamping.
For accurate centering of a tube/pipe in a hole or fitting, all you need is three such punch marks around the circumference of the tubing/piping evenly spaced around the circumference at approximately 120-degree intervals. Such a series of punch marks can be done approximately halfway down the length of the tubular joint.
For even greater accuracy, two circular rows of punch marks can be used. The first row would be approximately one-third of the way down into the joint, and the second row of circumferential punch marks can be applied to the tubing/piping about two-thirds of the way down the length of the joint. When the tube/pipe is pressed into the hole or fitting, those raised spots can very effectively keep the tube/pipe centered in the joint. Please note that when the punch marks become part of the brazed joint they are buried deep inside the joint itself and have absolutely no negative effect on the brazing.
Prick punching is better than tack welding. In my experience, prick punching surfaces for brazing can be much more effective than tack welding the tube/pipe in place. Too often the tack welds are large and can oxidize critical surfaces inside the joint if not done correctly. This often causes some distortion of the joint components, resulting in less-than-ideal clearances at the entrance to the joint being brazed. The prick-punch method has no such problems. It is a simple, clean technique for which clearances can be controlled quite nicely.
Prick punching eliminates the need for external fixturing. With practice, this mounded material can be controlled dimensionally quite easily. Three such punches (placed about 120-degrees apart around the periphery of the OD of the tube/rod) keep the inner tube/rod nicely centered in the ID of the outer member without any type of external fixturing needed.
Prick punching has been used successfully by many companies over the years to compensate for poor joint fit-up. It is an easy, reliable method. I recommend that you give it a try.