Building a Better Knee

Knee_Detail_FinalThis week I’ve been working on the design for the knee, and I’ve finally got a working design.  As I said in the last post, most figures I’ve seen have knee joints that could be improved upon, at that’s what I intended to with this figure.  Often they will have an oversized mid-joint area that looks awkward when fully bent.  Double joints also has have a tendency to bend fully at one joint before bending at the other, which is a relatively minor issue, but it’s something that I feel could be improved.  And improve it I shall.

I started with this basic concept of putting geared-teeth inside the mid-joint, and in theory it should make both joints bend consistently and at the same angles every time.  Before going forward, I thought it might be more effective to change the angle of the individual joints.  Looking at the way an actual knee bends, it seems like the kneecap stays with the bottom half of the leg, bending a lot more at the top of the knee.  To simulate this, I made the bottom joint larger than the top, making the lower joint turn more slowly than the top.

It took a few tries to get the gears to work right.  My first attempt used relatively small triangular teeth, and while they looked like they should work on the computer, I once again learned that doesn’t always translate to a physical product.  These teeth didn’t mesh at all and would grind a little bit, but that was it.  They felt more like ridges used for grip than gears.

KneePrint1

For a second attempt, I tried to adjust the size of the teeth and make them big enough to work in the physical world.  Once again, Looked great digitally.  Physically…

KneePrint2

Still just as ineffective.  They occasionally would mesh a bit, but not consistently, and therefor were still useless to me.  I reevaluated the design some, and realized the main issue was I was designing these gears with pointed, triangular teeth, and the printer was not able to handle sharp angles like this at such a small scale.  They came out rounded and prone to slipping and grinding.  I needed a design that wouldn’t do that.  So instead of the sly and shifty triangle, I moved on to the noble, sturdy square.

KneePrint3

And BAM!  Working small-scale gears.  Fewer teeth with larger gaps, and every tooth is securely fitted into its complementary gap with no room to slip.  As long as the housing holds everything in place, they should work consistently, every time.  I printed some quick leg parts to better illustrate the full working leg.  This design works just the way I planned.  These are the bones for the final product, now I just need to add the meat.  Extra Knee_Exploded_Finalfeatures will surely make their way in before then, most likely a ratcheting mechanism somewhere to help them hold any pose.  I would like a somewhat complex knee pad with some movement as well.  But all things in time.  That’s for another day.

-Nick

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