Yet another articulation thread
Posted: Thu Apr 28, 2005 5:38 am
Background:
I started to make some elbows, using "Constructing Articulated Joint Armor" from http://www.armourarchive.org/essays/gun ... ed_armour/ . I figured "Oh boy - patterns based on body measurements."
Measured my elbow as instructed, did the calculations (B=45% of A, etc.) drew the template, cut the steel into a nice little flat fish, filed corners and edges, and started dishing. It came out nicely, except that it was way too big for my elbow. Hey, guess what, it fit my knees perfectly, which I needed anyway.
I drilled the holes in the cop as recommended, such-and-such distance from ends and sides. I cut and shaped the lames, with just a hint of dishing, did a test fit with plastic spring clamps (similar to Sasha's G-clamp method - http://www.armourarchive.org/essays/sasha_articulation/ ) drilled the lames, and installed temporary nuts and bolts.
With a bit of tapping and bending, I had a cop and two lames that articulated wonderfully, from 180 to 315 defgrees with very little gapping or sticking. I was impressed with my first such effort.
Then things went horribly wrong. I put them on my knee, and discovered that when fully bent (as if I was kneeling and sitting on my heels) the top edge of the upper lame was about three inches off my leg. Not good. So I made some narrower lames out of cardboard, two on top and two below, and tried them in place of the steel lames. Better, but still not good. I taped on bits of cardboard to the inside of the wing and the inner end of the cop, so as to provide an expendible surface to punch "rivet" holes into, and moved the holes closer together (ie bottom holes at either side went up a bit, top holes at either side went down a bit. A tad better, but still a huge gap between the upper edge of the upper lame and my leg when bent.
So, after a much thought, I figured out that the human knee does not hinge around a line on the back of the knee (which about where the holes are now), but around some line running transversely near the middle of the knee. Therefore I reasoned that it would be necessary to move the articulation holes farther from the ends of the cop, so that the lames rotate through a much smaller radius.
So, I guess what I'm asking is if there is an easy way to find the correct radius by, say, wrapping your knee in two pieces of cardboard and marking up something as you bend your knee, so as to find the two spots on each piece that do not shift or move around relative to each other? Maybe taping two hinged sticks to you upper and lower leg to see which placement results in the smoothest flex? Is there a rule of thumb (better than Gundo's) to find the optimum radius (eg. some percentage of some tape measurement)?
The only thing I can think of is to make a mockup in cardboard and punch holes until you find ones that work, but that seem ... unscientific.
As I said before - the articulation was smooth, it was just in the wrong place.
I started to make some elbows, using "Constructing Articulated Joint Armor" from http://www.armourarchive.org/essays/gun ... ed_armour/ . I figured "Oh boy - patterns based on body measurements."
Measured my elbow as instructed, did the calculations (B=45% of A, etc.) drew the template, cut the steel into a nice little flat fish, filed corners and edges, and started dishing. It came out nicely, except that it was way too big for my elbow. Hey, guess what, it fit my knees perfectly, which I needed anyway.
I drilled the holes in the cop as recommended, such-and-such distance from ends and sides. I cut and shaped the lames, with just a hint of dishing, did a test fit with plastic spring clamps (similar to Sasha's G-clamp method - http://www.armourarchive.org/essays/sasha_articulation/ ) drilled the lames, and installed temporary nuts and bolts.
With a bit of tapping and bending, I had a cop and two lames that articulated wonderfully, from 180 to 315 defgrees with very little gapping or sticking. I was impressed with my first such effort.
Then things went horribly wrong. I put them on my knee, and discovered that when fully bent (as if I was kneeling and sitting on my heels) the top edge of the upper lame was about three inches off my leg. Not good. So I made some narrower lames out of cardboard, two on top and two below, and tried them in place of the steel lames. Better, but still not good. I taped on bits of cardboard to the inside of the wing and the inner end of the cop, so as to provide an expendible surface to punch "rivet" holes into, and moved the holes closer together (ie bottom holes at either side went up a bit, top holes at either side went down a bit. A tad better, but still a huge gap between the upper edge of the upper lame and my leg when bent.
So, after a much thought, I figured out that the human knee does not hinge around a line on the back of the knee (which about where the holes are now), but around some line running transversely near the middle of the knee. Therefore I reasoned that it would be necessary to move the articulation holes farther from the ends of the cop, so that the lames rotate through a much smaller radius.
So, I guess what I'm asking is if there is an easy way to find the correct radius by, say, wrapping your knee in two pieces of cardboard and marking up something as you bend your knee, so as to find the two spots on each piece that do not shift or move around relative to each other? Maybe taping two hinged sticks to you upper and lower leg to see which placement results in the smoothest flex? Is there a rule of thumb (better than Gundo's) to find the optimum radius (eg. some percentage of some tape measurement)?
The only thing I can think of is to make a mockup in cardboard and punch holes until you find ones that work, but that seem ... unscientific.
As I said before - the articulation was smooth, it was just in the wrong place.