I've been making changes to the design of the double crank mech that I started yesterday.

There were a few problems with the design that needed a bit of tweaking. Firstly, as the crank turns there is a linkage moving the central push-rod up and down. The linkage is very short which means that as the crank turns it leans over at quite an angle and pushes the push-rod to the side in the direction of the arrow. This causes the push-rod to stick a little.

I could extend the linkage but this would make the box base taller than I want. I've got a couple of things to try. To start with, the linkage currently joins to the edge of the push-rod. If I centre it, it should limit the range of angles quite a bit.

Starting with the linkage. I made a linkage with the joint part centred by making a little roof shape. (Left) I then realised that this was geometrically identical to a straight box shaped linkage (centre) and ended up with pointed linkage (right) which is easy to make, neat and effective.

How's that?!

Okay, so that did nothing apart from neaten up the design. Next the push-rod itself. I want to be able to join the linkage to the centre of the push-rod. I made a simple fold of card which slips into the push-rod and gives a fixing point.

The linkage joins on here. Everything nicely centred. So now, I need to remake the full thing and see how it works.

The other thing I need to check out is whether I need a left handed or right handed crank!

Octopus model? How does that look?