July 20209MANUFACTURING TECHNOLOGY INSIGHTSagainst and around a fixed workpiece (milling), or by moving a stationary cutting tool against and around a rotating workpiece (turning). There's far more to the machining process than this micro-explanation, but what's important to know right now is that machining picks up where DMLS leaves off. In other words, DMLS adds material in single layers. Machining removes material, sometimes in large hunks, but sometimes very lightly in order to attain fine surface finishes.Accuracy Considerations for Metal PartsAlthough DMLS can create extremely complex shapes that might otherwise be un-manufacturable, it's not without its limitations. For starters, significant heating and cooling of the metal takes place as the laser does its work, creating internal stresses that must be removed via post-build heat-treating. This means little to the people designing the part, except that stress relief equates to some amount of part movement and therefore some loss of accuracy. This is one reason--though not the only one--why even a well-designed DMLS-produced part requires machining of any part feature where tolerances tighter than ±0.003 in. (0.076mm) is required, plus ±0.001 in./in. (0.001mm/mm) for each additional inch of build height.Finishing Where DMLS Leaves OffAnother reason for combining DMLS and machining is surface finish. On a vertical or horizontal surface, DMLS produces part roughness about equal to a sand casting. All other surfaces will see some amount of stair-stepping, an effect that's largely dependent on how the part is situated in the build chamber. If your part design requires a smooth finish, it will need to be blasted, sanded, or quite possibly machined. This last part is no big deal, unless your part design calls for a fine finish on a surface that the end mill, drill, or turning tool can't reach. Whatever the case, be sure to call out such critical features on your CAD model when submitting to your manufacture, so the features needing secondary processing, including machining, can be identified.Removing DMLS supportsSupport structures should also be considered when designing metal parts in additive manufacturing. DMLS is a little like building a metal sandcastle--without some seashells and twigs to hold the thing together, the ramparts will fall, the architraves crumble. With DMLS, scaffold-like supports are needed to keep the semi-molten metal from drooping, curling, or otherwise misbehaving. Oftentimes, these supports can be removed with a Dremel tool, but machining may be the preferred method where larger part volumes are called for, or when the workpiece is headed to the machine shop anyway for one of the drilling, milling, or turning operations mentioned previously.Fixturing Printed PartsUnlike DMLS, which requires nothing more than a simple "build plate" to carry the workpiece through to completion, machined parts must be clamped, bolted, or otherwise securely fixtured to the machine to prevent cutting tool-induced movement. If your 3D-printed workpiece is composed entirely of curved, organic shapes (which is one of 3D printing's greatest appeals), how will the machinist hang on to it for turning or milling? Check with an applications engineer, but you might need to design in a pair of parallel surfaces or some mounting holes by which to clamp the 3D-printed workpiece for machining.Mulling Over MachinabilityLastly, there's the metal to think about. The lasers used by DMLS don't really "care" how hard or tough a metal is, but cutting tools sure do. DMLS is known for its ability to 3D print aerospace- and medical-grade metals like titanium, Inconel, cobalt chrome, and others, and even though different laser parameters and build speeds may be called for, it does so with relative impunity. Machining those same metals, on the other hand, requires lighter depths of cut, slower speeds and feeds (a little machining-speak here), and will consume more cutting tools and machining time. To see a board range of metal options for machining and 3D printing, head over to Protolabs' material comparison guide. Combining Complex Metal Manufacturing ProcessesThe overall point is this: You can in fact leverage the best of both worlds--3D printing and machining--together for metal parts, but carefully consider the design options covered in this design tip. Machining and metal 3D printing are deep, complex technologies, and it's only by understanding how each will affect your design project that success will be achieved. Ask questions, embrace each process, and understand that both are close-knit partners in manufacturing. Machining and metal 3D printing are deep, complex technologies, and it's only by understanding how each will affect your design project that success will be achieved
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