In the past few days myself and our two welders, Sidney and Ramiro set out to weld the latest 3 6kg roasting drums. The drums is where the real “hand-crafted” bit comes into play; it requires a great amount of concentration and skill to weld accurately and neatly in such a confined space. So this is also where I hand over the welding to our master welder Sidney, who has been welding for well over 20 years. I doubt if anyone has the amount of insight and knowledge in the complex science of welding that Sidney has.
Welding a roasting drum is much, much more complicated than simply pressing a button on a CO2 torch and holding on for dear life. It requires weeks of preparation and a lot of people to get the job done right.
Before we can even think of welding, we need to spec and order the correct roasting drums. Our drums are from a stockpile of pipes that is used in the heavy mining industry. We use a 5mm thick mild steel pipe that is cut by the steel yard with high-abrasion cutters. This causes the ends of the drums to work-harden, which is of great concern to us when it comes to even heat-distribution over the entire surface of the drum.
The drums then arrive rusted and dirty at our workshop (See The Birth of a Roaster). Cleaning off the rust is done by a special supplier who bathes the steel in several acid baths for a few days which cleans off any rusts and coats the steel with a rust-preventing layer of powder. Thereafter we send the drums to an industrial lathe company who specializes in making train wheels. They where the only ones we could find with a lathe big enough to skim our drums perfectly flat on either side. Their secret process also removes the work-hardened edges and ensures that the drum will run perfectly flush with the front panel.
While the drum is being prepared, we start preparing the shop for their arrival. The first issue is to get the laser cut mild steel fins bent to fit into the curvature of the drum cylinder. We apply a “helix flex” to all the fins which ensures that the beans will always run along the surface at the correct angle in relation to the drum wall. This helps the beans to agitate as much as possible within the drum, which prevents scorching and tipping. Our goal is transfer heat gently and evenly throughout the entire surface of bean-drum contact with the emphasis on convection heat (air-to-bean) rather than conduction (drum-to-bean). After the flexes are in, we weld the center bushing to the cross-bar supports and add a few more fins to the cross bars and perforated back plate.
Welding the drum is truly an art. I do mostly alignment and help to check that everything is lining up correctly. Our CAD model gives us a great idea of exactly where everything needs to go, but in the end it all comes down to feeling; feeling the parts, the drum, the welding and the visualizing the path that the beans will take through the drum. This really is where everything comes together and needs a certain level of human touch rather that laser precision.
We start by marking the spots where the fins will go with a marking pen. Each fin is uniquely bent and has to sit perfectly flush with the drum at the right angle and elevation. The position of each fin relative the the other is as important, so one needs to plan ahead and mark out the entire drum before welding. In total, there are 12 outer fins, 4 inner fins and 12 paddle fins; each with a different design and function.
After marking, the rear perforated plate goes in with a slight tap of the hammer and weak welding tacks on the outside. This gives us a base point from where to begin our outer helix of 1.6mm fins. The fins are then tacked into place and I ensure that they are all spaced correctly. It is also important not to have any small spaces in the drum that could trap beans, so we take great care to weld every little hole shut and ensure a smooth path for the beans. With the outer helix done, we start welding in the inner helix supports. The outer helix moves beans towards the front, while the inner helix moves the layer of beans closest to the shaft toward the rear. This ensures a constant rotation of beans from the front to the rear and back. The 12 extra paddles are put in to help agitate beans that are in the very front and back of the drum towards the center. I learnt this the hard way and we now ensure that no beans are trapped around the shaft without any agitators to push them back into the stream of beans that are already moving back and forth. Luckily this has all bean accounted for and is now part of the standard design.
With the fins in place, we can start aligning the shaft bushings and cross-bar supports. This is a really tricky bit, as you have to relay on good old hand-eye coordination to get the two bushings to align perfectly. After some gentle persuasion with a grinder and a hammer, we finally welded in the cross bars in and did such a good job that the shafts simply slides in and secures to the bushing with a pair of grub-screws on either bushing.
With the welding done, we had some time left in the day to check each weld and remove some weld-spatter inside the drum. After quality control we can finally send the drums off to ceramic coating. This is the last step in their life cycle before being installed and seasoned. But that is a story for another day!
By: Neil Maree