Knee Joint Struggles (day 81)

It’s been about a week since the last update: part of that was because on the weekend I went to an electronics flea market: prep for that kept me busy for a bit, as did testing out the stuff I got afterward. But beyond that, I started working on the knee joint, and it took me some time to make headway on it. While this part is well-defined at this point (being one of the parts for which I have already made a 3D printable model) in some ways the structure is a bit complex: being a joint, it needs some degree of mechanical precision. It’s also got kind of a sparse interior structure and irregular contours. After struggling with it for a bit I finally powered through enough to create the first part of the joint.


Way, way back in the history of this project I wanted to figure out how to do this knee joint right. I tend not to like it when Bandai makes a kit and makes armor parts around the knee or elbow shift around with the joint: They’ve done this since early MG kits like the Gundam GP01, and these days it’s all over the MG, RG, and even HG lines. However, designs like the Zaku FZ or Kampfer or Zaku F2 tend to have the opposite problem: While the reference art clearly shows knee armor integrated with the joint (and to be fair the anime can be inconsistent about such details), the kits tend to have the knee covered by a separate flap of armor. I wanted the knee armor to not only appear integrated with the knee joint but also to move in a way that might actually be beneficial, and not open up a million little gaps in the armor as it moves.
What I came up with, so long ago, was knee armor that would shift as the joint bends, but shift in such a way that it continues to cover and protect much of the knee joint even when the joint is fully flexed. Put another way, the armor plate covering the knee is made to cover it in the bent position. When the knee joint is extended, the armor plate shifts forward a bit so it doesn’t obstruct the joint movement.

As I said there were some things in the construction of this joint part that I’ve found tricky, despite the fact that I have a completely realized and working design for it: the joint part doesn’t have a lot of solid interior structure to it because so much space is hollowed out to make room for the moving parts of the joint and the sliding arm which carries the knee armor. It’s a part that demands some precision but it hasn’t got a lot in the way of simple orthogonal structures to serve as the starting point for a build. I worked through it, and it was a bit of a slow process. As usual the grid-printed styrene helped me quite a bit.
I started by looking at a side view of the joint part in Blender and cutting the same shape out of styrene. I decided to keep the layout of the grid on the part the same as the grid layout for the part as I’d placed it in Blender. (That is, the solid lines running through the grid are in the same places relative to the part as the bolder grid lines are in Blender). This way, as I make different segments of this joint part, I can align them to each other correctly by lining up the grids. The knee joint also has some tapered end-caps that go on the sides of the joint: this is a part that I scratch built in the past, fortunately I still have the molds to make more.

However, this part of the knee joint tapers at the front. I figured probably it’d be easier to build the taper with epoxy putty rather than carving it down from a solid plate, so I cut away the tapered section from the plates I’d just made.

Next I started work on the inner layer of the knee block: This is where it gets a little more tricky. The structure is a bit sparse because the moving portions of the knee joint take up space in the back, while the relatively small amount of material in the front needs to provide alignment holes to connect the halves of the knee block, and a track in which the knee armor can slide:

Initially I cut the inner layer parts from 1mm plate: however that layer is supposed to be 1.5mm thick on each side, so I duplicated the part in 0.5mm styrene to thicken it up. I could have used unprinted styrene here, just glue down the existing parts and use them as a guide to cut the two new ones, but I decided to use grid plate, and keep the grids on the two layers aligned as well as possible. The upshot of that decision is that the combined plate would have grids printed on both sides, making it easier to line things up. The down-side is that if the grids aren’t line up well enough, it’ll create confusion about what the correct alignment actually is…

I glued the inner layer to the outer layer, lining them up with the grids… But unfortunately I screwed it up, attached the thing upside-down. I didn’t realize this until I started reassembling the joint – the parts of the inner layer that are cut around the cylindrical moving joint parts were sitting above those parts instead of between them… So I had to cut it apart and glue it back, correctly this time…

With most of the knee block together I started building the 3mm spacing layer. With all the space needed for moving parts of the joint, there’s actually very little solid structure in the middle of this joint block. I also cut the axis holes for the joint, and alignment holes that will be used to connect the two sides of the knee block together during assembly. It’s hard to drill holes like this precisely, so instead I drilled starter holes of a smaller diameter, then cut the holes larger with a hobby knife, using the grids to determine the correct position.

As simple as it seems, to me this part was a bit daunting simply because of the minimal amount of interior structure in the part. I think it’s turned out well so far, though, and having the part fully designed ahead of time definitely made it easier.