If you don’t know what your furnace is capable of (i.e., how uniform in temp your brazing zones are) prior to putting any heavy brazing loads into the furnace, then you may have a hard time trying to control temperature-uniformity when the furnace is fully loaded with parts to be brazed!
A number of years ago I was discussing temperature-uniformity with the owner of a company who manufactured specialized vacuum-furnaces for brazing aluminum. He told me that their goal was to manufacture their furnaces in such a way that the brazing-chamber would be controllable to +/- 1°F front to back, and top to bottom, in the empty state, during a test cycle of heating/cooling. And because 1°F is smaller than 1°C, he was certainly setting a very high standard for his company! He truly understood that because brazing temps for aluminum were so very close to the melting points of many of the aluminum base metals that were being joined, he therefore needed to be sure that the expected temperature spread in a heavily-loaded vacuum-furnace hot zone needed to be kept within a very narrow tolerance band from the outside surfaces of the load to the center of the load being brazed. Thus, he built his furnaces to be able to maintain a very narrow, uniform temperature spread in the empty furnace chamber, which would then enable maintenance of closely controlled, uniform temperatures when using appropriate heating/cooling rates during the brazing of heavily loaded furnaces.
Obviously, maintaining a temperature control of +/- 1°F front to back, and top to bottom, in the empty state would be very difficult when you take into consideration the “stack-up of tolerances” that exists during each furnace run, including the furnace heating-equipment tolerances, thermocouple (TC) tolerances, the temperature monitoring/recording instrumentation tolerances, etc., to name just a few.
But, as mentioned earlier, I strongly believe it is essential to know what you are starting with, i.e., what kind of temperature uniformity your furnace is capable of in the “empty-state” (i.e., when there is no significant load in the furnace chamber) as determined by a Temperature Uniformity Survey (TUS) of your furnace chamber.
Figures 1 and 2 show lightweight tubular frame structures put together as TUS-fixtures, constructed so that thermocouples (TC’s) can be attached to each of the corners of the TUS-fixture, as well as inside the frame near its center, etc. By using lightweight tubular TUS-fixtures, the very small amount of fixture-mass inside the chamber will minimize any temperature-distortion that might be caused in that zone due to the absorption of heat by large thermal-masses inside the chamber that could cause “cold spots” to be registered during heating, or “hot spots” to be registered during cooling.
By contrast, the TUS-fixture shown in Fig. 3, being much more massive than those shown in Fig. 1 or 2, can be an effective fixture to help develop optimal heating and cooling rates for a particular brazing process that might involve similar thermal-mass during a brazing cycle.