Knees vs. Want (day 85)

I’ve been continuing work on the Zaku’s knee joint parts. The components are simple in principle but it is a bit tricky to build joint parts with the appropriate mechanical precision. Today I made a lot of progress on the lower portion of the joint: the portion of the hinge that attaches to the lower leg, and the strut which will carry the knee armor.

I started by building the structure for the lower portion of the joint: apart from connecting the knee joint to the lower leg, this part will also hold the pivot point for the knee armor.

At this point I felt a bit more comfortable moving on to something else, so I started work on the strut for the knee joint armor. This part provides the mechanism which allows the knee armor to shift as the knee joint bends. Because the strut has to move around the whole knee joint mechanism, its shape is a bit irregular and its dimensions need to be fairly precise for it to fit properly. I started by gluing together 3 layers of styrene plate: initially I did two 1mm plates and one 0.5mm plate, thinking I would need a half millimeter of clearance for the part to slide in the 3mm gaps of the knee joint parts. As usual in recent work, I was careful to align the grids printed on the diferent layers of plate, so I could use the grid as reference to check the part’s symmetry on both sides. (It’s a bit hard to align the grids, though, so I’m never entirely sure if I’ve got it right. I may need to come up with a more reliable procedure for aligning these things…) However, in a test fit I determined that the strut should actually be 3mm wide, as the gaps in the knee parts actually are slightly wider than 3mm, so I added another 0.5mm plate to the strut. I drilled holes and glued in rods to connect the strut, and the knee armor mechanism was working.

Among the knee joint parts I made last time were a couple of 6mm lengths of 9mm diameter styrene tube. These tubes form the outer surface of the area of the joint that encloses the polycaps. I integrated one of these into the lower knee joint structure I’ve been building, the other will be incorporated into the upper knee joint structure when I build that.
Whenever I build a hinge joint like this I like to include some overlap between the different parts. The alignment on these parts is never quite perfect, so as the joint turns there is a possibility of visible gaps showing between the different parts of the hinge. Having a bit of overlap between the parts diminishes this effect. To achieve that here I cut some 6mm diameter circles from 0.5mm styrene plate, with 3mm holes through the middle… It’s hard to get that sort of thing precise. I mostly just eyeballed it, using a piece or styrene tube, centered according to the printed grids, to cut the basic shape, and then did my best to tweak the drilled holes to the right position. Things do wind up a little bit off but I think it’s good enough. These circles were then glued to the interior surface of the knee joint, so the joint cylinders can overlap them a bit. In the mean time, the presence of these discs helps to hold the different parts of the knee joint together when I do a test.

Recent progress on the project continues to be a bit slow and spotty… Although the recent knee work is good progress it’s hard to feel a lot of satisfaction from it somehow. I think maybe at the moment working on joint parts is not real enjoyable for me. Though it’s also possible that as I get the parts closer to completion and I’m able to use them to connect the upper and lower leg, I might feel differently.

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