Braze-Fixturing Tubing/piping Using Prick-punching

 By Dan Kay

Prick-punch sm
Fig. 1 --  A typical prick-punch with a very sharp point

A number of people have inquired about how to keep tubing or piping centered in holes or fittings prior to brazing, thinking (erroneously) that if the tubing/piping does not remain centered in the joint, but instead touches one surface or another inside the joint (due to lack of centering) that the joint therefore may be weakened thereby, or that the molten brazing filler metal (BFM) will not be able to penetrate the area where the tubing/piping contacts one of the surfaces inside the joint.  That is incorrect thinking, because molten brazing filler metal (BFM) is able to penetrate extremely tight joints, even when there is metal-to-metal contact in some portions of the joint.  The microscopic surface roughness of the mating surfaces inside the joint will allow the liquid BFM to penetrate completely.

But, if you are in that group that feels that you must take steps to keep the tubing or piping centered in the joint to be brazed, and want to take steps to prevent any joint surfaces from touching, then there is a simple way by which to insure that the tubing/piping will remain centered in the joint throughout the braze-cycle.

The simplest way is to "dimple" the OD surface of the tubing/piping using a prick-punch, a tool that is illustrated in Fig. 1.  

Note:  A prick-punch differs from a “center-punch” in that the prick-punch is ground to a steeper, tighter angle than a center-punch, so that it can create a smaller, deeper indent than a center-punch.  This is illustrated in Fig. 2.  For braze-fixturing purposes, always be sure you are using a prick-punch, and not a center-punch.

Fig-2 sm
Fig. 2 --  Comparison of tips for a prick-punch and a center-punch.

In order to properly center the tube/pipe in a hole or fitting, a prick-punch should be used to create several indents around the periphery, as shown in Fig. 3.  These indents will cause raised mounds of base-metal around each indent.

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.  

Fig-3 sm
Fig. 3 --  a prick-punch mark upsets the surface of the metal, displacing metal out of the hole created by the punch.  This displaced metal helps create the needed clearance in the joint to keep the component centered.

For accurate centering of a tube/pipe in a hole or fitting, all you need is three (3) 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 half-way down the length of the tubular-joint, or, for even greater accuracy, two circular-rows of punch-marks can be used, the first row approximately a 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.   Then when the tube/pipe is pressed into the hole or fitting, those raised spots can keep the tube/pipe centered in the joint very effectively.

Please note that the punch-marks become part of the brazed-joint, are buried deep inside the joint itself, and have absolutely no negative effect on the brazing whatsoever.

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Fig. 4 --  Knurling the circumference of a component allows it to center itself nicely in a joint, while still allowing BFM flow through the knurled surface.

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, if not done correctly, can oxidize critical surfaces inside the joint, and often cause 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.

Another option, somewhat similar to the prick-punch method, is the use of knurling, as shown in Fig. 4.  By knurling the OD surface of the tube that is pushed into the fitting, it can act in a similar way as
prick-punching, with each vertical knurl on the circumference acting to keep the component centered in the joint, while allowing liquid BFM to flow between them.

Knurling can be automated, if desired, and, as with prick-punching, has no negative effect inside the joint whatsoever.

Bear in mind, too, that it is not critical that the gap be absolutely uniform all around the joint.  The critical things are cleanliness of the joint, the correct amount of filler metal, and the good evidence of a complete meniscus (fillet) on each end of the joint when the brazing is completed.

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Dan Kay - Tel: (860) 651-5595 - Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. CLICK HERE to contact Dan Kay.

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