In the manufacturing process for any AM part, questions about the steps after additive have to be faced at once. For example, how will the part be removed from the build plate after the additive cycle is done? And what features will be left for subtractive processing (machining) instead of being completed additively? These questions are crucial, because they shape the additive cycle. They determine the form of the part sent to the additive machine. In Delcam’s case, the engineers involved in the research expected to remove the parts from the plate via wire EDM, so they added 2 mm of additional stock between the part and the plate to allow room for this cut. In addition, they expected to drill holes (because the AM process is poor at generating precisely round holes) and also to use CNC machining to finish critical mating surfaces. As a result of these decisions, designing the AM part included a measure of “undesigning,” as engineers removed holes from the model and added milling stock onto mating surfaces to arrive at the form that was ultimately grown.
An important exception to this “undesigning” was a hole connecting the two chambers inside the manifold. This hole also could have been machined (a drill could reach it), but the hole was instead left in place to be produced additively. The reason: That hole provided a drain for unfused powder metal to escape from the chamber beneath it, once the build plate and the part were removed from the additive machine and inverted. Without this release, says Gunnink, powder accumulating in that chamber combined with the air held in the sealed chamber could have created a fire hazard during EDM.