When a layer of insulation was then placed between each vice-face and a similar tubular assembly, the torch-brazing process was literally accomplished in half the time!
This simple illustration shows why it is important for people to take this topic seriously, and not just brush it off as if it were insignificant. Shops performing furnace-brazing can be some of the worst offenders in this respect. I have often heard from them: “Because the fixturing goes into the furnace, and therefore, isn’t “stealing the heat” as a vice might do, we don’t need to be worried about the amount of fixturing we use in our furnaces. Everything is heated and brought up to temp together.”
Such thinking completely misunderstands the negative effect that all the fixturing inside the furnace has on operating costs and brazing-cycle time.
Note: There is no free ride! every pound (kilogram) you put inside the furnace absorbs BTU’s (calories) as it is heated to brazing temp. therefore, every BTU (calorie) used to heat fixtures is costing you money and extending the cycle time in the furnace!
What’s the answer?
Simply this — keep the weight of any fixturing to an absolute minimum. If you are using any “external fixtures”, i.e., dead-weights, or some kind of metal assembly (such as metal bars, clamps, etc.) to hold assembly components in proper alignment while they go through the furnace brazing cycle, try to redesign the assembly to eliminate the need for such extra fixturing, or keep the design and weight of such fixturing to an absolute minimum, if at all possible.
What method of fixturing is best?
The best method of fixturing is NO fixturing — try to make the assemblies “self-fixturing/self-jigging”, i.e., design them so that they sit on top of each other in such a manner that gravity holds them in proper alignment. A few examples of this are shown in Fig. 2.