The other day I got an email update from Rob Oliver, a machinist in Brooklyn who's helping me post process the EBM printed titanium parts that I got from Addaero recently. There's still a bunch of work to be done, but I wanted to write a bit about how we're thinking of the manufacturing plan - and the constraints that we're facing in the process.
While electron beam melting tends to produce much lower internal stress than laser powder bed fusion does, it's still a decidedly near net shape process. In further iterations I hope to get the as-printed part much closer to the final dimensions, but at this stage the parts I have deviate significantly from the intended tolerances. Specifically, most outer dimensions seem to have grown, most inner dimensions seem to have shrunk, and there are a number of locations where support structures have left unacceptable surface finishes.
My main focus right now is getting both inner diameters to within .006" of their designed size. It's difficult to get a reliable measurement of where they are now (due mostly to surface finish, and the presence of leftover support material), but they both appear to be about .040" undersized. In addition, I suspect that the shorter cylinder is slighly ovalized - though not to the extent that it'll be an issue in the end.
Although there are other conceivable options, the most obvious way to get the IDs within tolerance is milling. By using either a CNC toolpath on an end mill, or a boring bar on a conventional mill, it should be very easy to get well within .006" of nominal dimensions on both areas of the part. However, the issue of fixturing is nontrivial. I designed this part with T-splines, and its outer surfaces aren't orthogonal at all. As a result, we'll need custom tooling to hold the parts in a milling vise.
As an aside: Anyone who says that additive manufacturing eliminates the need for custom tooling has no idea what they're talking about.
In order to securely fixture this part, Rob is machining its negative into a set of aluminum blocks, which can then be clamped securely into a milling machine vise. This technique (which I'll refer to here as "soft jaws," although technically what we're making is more of a coped fixturing block) is used extensively in subtractive manufacturing to hold irregularly shaped parts.
The process of making soft jaws is relatively straightforward, but designing them for this part is somewhat complicated by the dimensional variation that EBM produces. Put simply, feature sizes in EBM parts tend to deviate from the design in the X and Y axes, but stay relatively true to size in the Z. That's because the Z axis is at least partly controlled by the Z stage drive system; the powder bed is lowered a predictable amount with each new layer, keeping features in the Z close to their designed dimensions. But in the X and Y, deviations in feature size are partly driven by the electron beam diameter, and partly driven by the distance that the feature is from the center of the build platform.
As a result, my part has grown anisotropically, and Rob will need (to some extent or another; soft jaws tend to be somewhat forgiving) to compensate for the as-printed dimensions differently in the XY plane than he does in the Z.
In the end, though, the most practical way of determining the final dimensions of the soft jaws is to make a set, test them on the as-printed part, and iterate as necessary. It's conceivable that the first try will work, and it's also possible that if we make the negative a bit too big in all directions, we could use a piece of soft material (for instance, blue tape) to take up the gap.
It's also worth noting that there's an alternative path that I decided *not* to take. A common way to make parts - both with additive manufacturing and conventional - is to design fixturing features into an intermediate stage of the part. These can then be used to hold the part while secondary operations are performed; they can then be removed in a subsequent step. I considered this option, but find it undesirable for the simple reason that it would likely result in more post processing steps. Worse yet, it would probably require the surface of the part to be blended where the fixturing element had been removed, which would be either labor intensive, or unattractive, or both.
Regardless, we should have the first iteration of our soft jaws machined shortly. Expect updates!