How should fillets be specified on drawings or in specs?

1. Fillets should not be dimensioned on drawings – Be very careful about specifying on a drawing the size of a braze fillet. Once specified, it means they must then be measured, and this becomes a meaningless waste of time in the shop and can be very costly to a manufacturer. If a drawing specified that a fillet needed to be a minimum of let's say 0.0625” in size, and upon inspection after brazing it was found that the fillet was only about 0.050” in size, what then? Theoretically, it is supposed to be rejected and rebrazed to add more BFM to the fillet, a time-consuming waste of time. Therefore, I strongly recommend against anyone specifying fillet size on drawings unless they can come up with a truly magnificent reason for so requiring. I can think of none.

2. Preferred “visual” wording – Instead of specifying that fillets are needed and that all lack-of-fill is forbidden, or that only a certain number of voids are allowed, it would often be better to merely indicate the following: “There shall be evidence of brazing filler metal at all edges of the joint.”

Conclusions

Fillets are a natural outcome of most brazing processes and should not be dimensioned on drawings nor used as critical accept/reject criteria for brazed joints. Fillets exist, but they aren’t truly relevant when it comes to joint integrity or strength. Merely look to see that BFM is indeed present all around the joint and that any filleting is concave in shape and preferably as small as possible. That can often be achieved by following the brazing rule of thumb: “If a little BFM is good, then less is probably even better!” That’s because the volume of a brazed joint can be filled only once. Any extra BFM added beyond that amount will result in large external fillets (without adding to joint strength) or run down the outside of the parts by gravity, causing a lot of extra work to clean up the parts.