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Brake drum forges are often built as a first project at many blacksmith schools and beginner workshops. Then every student has a forge to take home. Note that a brakedrum forge has almost no coal reserve and is only useful for small projects as a hobbiest or starter forge.
Brake drum forges vary with the diameter of the brake drum. 10" to 16" (25 to 40cm) will work. They make a pretty decent firepot but are not nearly as heavy as commercial firepots therefore will not last as long. Brake drums are typically a cast iron rim with a pressed or formed in steel center of 1/8" plate. A few are solid cast. Some wheels are suitable for the same purpose. See My first forge
Fittings also vary according to your budget (new or scrounged). New are not prohibitive but it all depends on your view point. Current cost runs about $18 US. The drum I have left over from the JYH is 12" (30cm) diameter (inside) and about 6" (15cm) deep. It has a 3" (7.6cm) center hole and 5" (12.7cm) bolt circle. Fittings will be a 2" heavy steam flange (about 5/8" thick), a 2" close nipple, a 2" x 6" long nipple, a 2 x 2 x 3" reducing T, and a 3" x 6" nipple. Schedule 40 pipe is standard but 80 or 120 will last longer IF you can get them. The two long nipples have one end sawn off or may be cut from a longer piece of pipe. The 3" piece is cut about 1/2" beyond the threads and the 2" x 6" has the threads cut off OR left on depending on the blower type. A counter weighted ash dump is fabricated and hinged from the 3" pipe. A sliding door held on by a single screw pivot point also works and is easier to fabricate. Although most people prefer the dump type many commercial forges came with a sliding door. A pull made from heavy wire can be rigged so that you don't need to crawl under the forge to dump the ashes.
The legs on the above drawing should be spread out more for stability. They are attached by flattening the end of the tube, drilling or punching a 3/8" hole and bolting on with the bolts holding the flange to the brake drum "pot". The blower shown is a little 120 CFM model I use on gas forges. A hair dryer, furnace blower, or any type of squirrel cage fan will work. Blowers with a lot of pressure and CFM such as those from vacuum cleaners or leaf blowers will need an air control. The little 1/30th HP shaded pole motor on the blower above can be controlled by a room lighting control.
NOTE: A grate should be made to reduce the amount of coal that falls into the tuyeer. One or two pieces of 1/2" (13mm) steel or stainless steel bar will sufice. If you get fancy and fabricate a clinker breaker type gate a grate is not required.