Fig. 1. Vertical Test Specimen (VTS) – The dimensions and the materials shown can both be varied according to the materials you are brazing and the dimensions of your furnace. The taller the specimen, the more accurate the results. The base material used MUST be the same as the material being brazed (aluminum, titanium, stainless, etc.), and the BFM must also be the same that is being used in that particular furnace cycle.
An interesting question I have often encountered over the years with furnace brazers is this: "How do I know if a particular gap-clearance will work in my brazing furnace?"
Please understand that each brazing furnace is unique and behaves in its own unique way. By this, I mean that even two furnaces of the same model number are not actually identical. Each one has its own personality, and the furnace operator needs to try to understand and work with each "personality." NOTE: Your comments about this would be appreciated!
Brazing clearances are very important for proper flow of brazing filler metals (BFMs) into a braze joint. However, many production shops that build parts to be brazed utilize different standards for the clearances they allow for brazing fitups, and normal tool wear can often impact this to the chagrin of many brazing shops. Some brazing shops tend to "cross their fingers," so to speak, hoping that the gaps they are given will be successfully brazed since they really don't know what their furnaces are truly capable of. If someone were to tell them, "Hmm, these parts have a brazing gap of 0.007" (0.178 mm). Can our furnace handle that?" The typical answer is often: "I'm not sure. Let's give it a try, and see what happens." That kind of answer is not good and not necessary.
You CAN find out what your furnace is capable of by using a special Vertical Test Specimen (VTS) in your furnace, along the lines of that which is shown in Figure 1 below. This VTS can be used in any furnace (batch-type or continuous), with any atmosphere (including vacuum), and with any base metal and BFM combination.
Next time, we will discuss how the VTS works and how to interpret the results obtained from it.