Workshop Notes

Paper Globe

I spent a diverting evening looking round the polyhedron map website then printing out and making one of their models. They have numerous different polyhedrons with maps ranging from cubes and tetrahedrons to truncated icosahedron, the picture left.
The truncated icosahedron was fun to make. It did, however, get tricky towards the end, I could do with long thin delicate fingers instead of the sausage digits supplied. Perhaps the tetrahedron will be easier.
Interestingly, the truncated icosahedron is the same shape as a standard soccer ball and the carbon 60 molecule - buckminsterfullerene. It consists of twelve pentagons, each surrounded hexagons. Sixty vertices, one for each carbon atom in the C60 molecule.

Pushrod Problems

Pushrods, central to many mechanisms. I'm working on a new design (more of which at a later date) that requires a wing to be moved, right up then right down again. Much further that the Flying Pig, left. Ideally I'd like 180 degrees of movement. The problem is it just doesn't work using a pushrod. Let me explain...

Time for a diagram, it usually helps clear up the problem. So, I'm trying to move a wing from straight down to straight up using a pushrod. Look at the geometry. Move the pushrod up by one unit, the wing turns by forty five degrees. To turn the wing by the remaining forty five degrees only .41 units of vertical movement. (Its a root two thing) The higher the wing is raised the less the vertical component of the movement, The higher the pushrod, the less the force applied to the wing so the harder it becomes to raise the wing. Somewhere before vertical the force on the pushrod is not enough to overcome the stiffness of the paper joints and the wing stops moving. Realistically on this type of movement I can only rely on ninety to one hundred degrees of motion. What if I want more? What can I do? I can't think of a simple solution. Rack and pinion gears seem to be the obvious solution but tricky to make out of card, Especially when compared to a pushrod. I think I need to go for a bike ride, I do all my best thinking whilst pedaling :-)

Snip Snap

Found this on the beach. A crab's claw. Engineering perfection, it is a fabulous design. The moving claw is hinged around a ball and cup joint on each side of the claw body. Inside, there's a large fixing point for the muscle. Fascinating!

Notice the ball and cup on the drawing. The fitting is very accurate, the parts look machined or cast rather than grown. The design is nicely optimized. See the muscle attachment points on my drawing left. The close claw attachment point almost twice as wide as the open claw point, allowing for twice the muscle and hence increased strength. In a similar vein, it is more than twice the distance from the pivot, again, the crab needs to be able to close the claw with great strength but only a little effort is needed to open the claw. I wouldn't fancy being nipped by one of these!