Producing functional parts via additive manufacturing (AM) enables us to design and manufacture products that we just can’t make any other way. With the flexibility of 3D printing, we can realize part-performance benefits such as lightweighting and thermal efficiency, we can integrate separate components into consolidated designs with complex forms, and we can do all this within a build process that is often highly automated and highly efficient it its use of materials, producing little waste.
AM has a dark side, however. The benefits above are offset against some very real and potentially painful challenges with postprocessing. Finish machining of AM parts can be challenging directly because of their light weight and their complex forms. Both attributes can lead to problems with workholding and vibration and can result in poor process yields. Furthermore, there is the additional problem of aligning complex components when they lack precise geometric datums in the as-built state.
This article will look at the challenge of making lightweight parts stiff enough for effective finish machining. We will explore how to realize effective workholding solutions to make non-rigid additive parts machinable. We will also demonstrate how machine tool probing can be used to perform sophisticated alignments of AM parts, enabling us to “find the good part” within the shape that we have built and produce the critical datum surfaces correctly.
The Need for Machining
As versatile as it is, an additive or 3D-printing-style process, particularly in metal, cannot produce features with very fine tolerances. Post-process machining is often needed to produce precise round holes and smooth, flat surfaces for interfacing with other parts. Yet lightweighting often has the effect of reducing the stiffness of AM parts, which can mean that they do not stand up well to the machining process. The complex forms of AM parts also make them hard to grip securely without causing damage. And finally, it is common to produce the datum features on additive parts after the build itself, so setting up components for finishing can be tricky.
There are a lot of similarities here to the challenges that manufacturers of composite and super-plastically formed parts face—namely, complex shapes that may have some distortion onto which precision features must be machined. Users of AM can learn from the best practices in these other sectors while adding an AM twist of their own.
Case Study: Microwave Guide
Among the machining challenges characteristic of AM, the first consideration is whether the part is likely to be stiff enough to cope with the loads it must bear during machining. Many questions arise from this. Will the part deflect away from the cutter, and will it vibrate such that we get tool chatter and poor finish on machined surfaces? If yes, what can we do about it? Can we design the part differently to make it stiffer? Or if that is not an option, how can we hold the part to support it sufficiently that it will not deflect or vibrate excessively? We will use a case study to explore these questions.