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[quote="Conqueror_Worm, post: 3029520"]The metacarpal dimensions aren't missing, you just have to do a little math to get them. My drafting professor always stressed that you should never over dimension your drawings.
If I were going to make one, I would start with a piece 9/16" x 9/32" x 3 1/16". Turn it on a lathe, off center in a four jaw chuck, to create the 5/16 diameter round portion. Mill the slot and the radius at the top. Drill and tap the top hole. Lastly, fabricate the side lug separately and braze it on.
I guess I'm still not entirely clear on your elbow design, would you mind drawing your design over top of my drawings? That way I will know exactly where your pieces are, and how they relate to the other pieces.[/QUOTE]
Conqueror_Worm;
I just attempted to make the metacarpal by turning a chuck-centered aluminum bar that was 3/4 square. Doesn't work. You get the finger mounting tab but not the cable mounting tab. The off-center lathe thing is the next experiment except I hate off-center turning because it's really easy to screw up if the work isn't chucked exactly correct. It's like trying to make a crankshaft out of billet. Also, it seems to me (but I won't know until I try it) that a 5/8 square bar will do the trick with .0015 extra on both dimensions, at least that's how it looks when I dyekem dye the top and scrawl it out on the end of the bar. I get both the finger-mount point and the cable tab without having to weld any tabs on. It is all one piece. Also your drawing shows two flat sides on the dimension which measures .281. I think if you off-set the bar so that one corner of the bar becomes the rod of metacarpal under the tabs after turning, the flat spots automatically form if you cut the rod to be the dimension of .311. It knocks the outer corner off and the inside is also round but the back side stays flat. At least that's how my dyekem measurements seem to be working out. We'll find out once I cut it. I broke my parting tool and there is no local source for mini-lathe tools here in San Diego. Waiting on the replacement.
On a second topic, the lumbrical pistons I think can be made to be air-springs by taking advantage of a natural vacuum that occurs, negating the need to try and stuff a spring into a 1/8 inch hole. I stumbled onto that idea when trying to make little-bitty pistons out of brass and flange the bottom for that ball point pen spring idea I had. To keep from bending the piston shaft, I took a block of mild steel and drilled an 1/8 inch hole, the same dimension of the brass rod, with the intention of inserting the rod into the hole and then carefully peening a flare onto the end. What happened was I destroyed the piston trying to get it out of the block of steel because it had such a vacuum that the rod refused to be pulled out without vice grips. I repeated this experiment enough times that I wasted a foot of brass rod trying to make a spring-loaded piston at one-inch at a time. I took a second look at the photos of those things and noticed they look like miniature versions of a pneumatic cylinder I have on my desk, right down to the inlets, if you look at the set-screws as inlets. I'm thinking now of drilling a 1/8 inch hole all the way thru a quarter-inch diameter rod of stainless steel, bore one end to .155 for the Du-bro clevis, slide the piston in with no vacuum resistance, set the length as needed then the set-screws and the Du-Bro for the seal to make a vacuum. The real question is will it be strong enough to hold a vacuum but not too strong that it loses a spring effect like my little pneumatic cylinder on my desk has when you plug the inlets. If it works, it simplifies that part of the build. I'll keep you posted.
Also did your software give you any dimensions for those single-helix torsion springs you drew for the joint-returns? There is some math there, like wire thickness, inner-diameter, outer-diameter, leg length and of course tension in Lb's. Obviously this affects the fingers' ability to return to straight, but also there is a depth to be cut inside the knuckle to compensate for the wire, plus if the loop is too small, the legs don't rest against the flat-spots inside the knuckle and if too large a diameter, there needs to either be a larger diameter joint pin or more realistically, a shoulder on the side of the pulley to keep the spring centered so the joint doesn't bind. I've found a couple sources for torsion springs but they all need those dimensions. Did your software happen to work that out? What you drew looks really good and functional.
Update on above lumbrical pistons: Just noticed the Du-bro 303 clevis has an opening between the tines of the fork. Not air tight. One may have to solder in some of the Du-bro connecting rod into the hole to seal it. I'm trying to make the hand and arm functional, not just pose able. So far the pistons, the wrist plate and the spade bolts are easy and straight forward. With the pistons, getting a smooth bore is important for functionality. The drill bit used seems to make a big difference. Look for a bit with flutes longer than 1.66 in length. When cutting the longest piston, a shorter flute length causes the aluminum (or whatever material you're using) to get jammed up in the bore, scoring the inside of the bore. Also, I'm told, even though the bore is 1/8 inch (.125), stepping up with smaller bits till you get to .125 will yield smoother results.