Fig. 4 shows what happens when the hole is punched rather than drilled. Notice that a punched hole is often bent downwards slightly, and may often have a burr on it. This can significantly hurt the ability of that joint to be brazed, forcing the brazing shop personnel to build up large fillets (castings) of BFM around the joint instead. These cast fillets can be porous enough to cause leakers through the joint when the part is placed in service.
Take a look now at Fig. 5, which shows a threaded-fitting that is to be brazed into a drilled hole in a tube. I took this photo in a production plant where they were actually trying to torch-braze these fittings into the tube-holes. Please notice the large open gap along the bottom of the inserted fitting that needs to be filled with BFM in order to seal the connection between the fitting and tube. I was very surprised to see such a joint configuration actually being used, but the engineers told me that they didn’t know how to create a joint any other way. To join the fitting and tube, they had to use a braze-welding technique (which I discussed in last month’s article). Using the braze-welding method, large BFM-fillets were deposited around the periphery of the joint, which took a number of circular passes around the joint to finalize. Unfortunately, as I just mentioned, large braze-fillets are castings, and some of them leaked when tested prior to shipment to customers and then had to be re-brazed or scrapped.