So, first, what is going to make my dragon fly?

As a said before I think that the best mechanism to replicate the movement of the Pegasus will be a crank. A crank is basically a lever attached to a rotating shaft, the diagrams below are from a great book ‘Cabaret Mechanical Movement’ by Aidan Onn and Gary Alexander, highly recommended if you want to do any work with mechanisms!

(scan of crank images, label parts)

I don’t think I’ll have enough time to cut my parts out of brass, no matter how beautiful the end result will be. To save time it would be quickest to draw the components in 2D Design and manufacture them on the laser cutter.

Fist things first, what do I have to make the pivots for everything to move around? Looks like I’ve got 3mm acrylic rod to work with, so what size hole do I need for this to work? I’ll need a tight fit for fixed points and one for moving points but not too loose, I don’t want any rattle:

Using 2D Design it was easy to manufacture test pieces, looks like it’ll be 3.2mm for the moving points and 3mm for the fixed points. Even though the results seem obvious you never know if the laser cutter will melt a little extra away from the cutting line, it all depends on how the machine is set up.

Next, the length of the lever connected to the dragon. This is all linked together with the size and position of the cam for the crank so…

The part of the drawing to the right is an accurate representation of the dragon at it’s lowest point on the crank. There appears to be enough room for everything to move so I think I can go ahead an design the rest of the dragon:

The additional pieces are all to make up the layers that move against each other. So now I have my dragon, the box size and shape and the cam drawn, I think I’m about ready to start construction!

Clang!! first problem, I’ve not left enough space for the vertical rod to fit through the top of the box! Back to the drawing board:

Right got there in the end, time to do a test fit:

Looking good till I tried the mechanism

Crud, he’s all floppy and moving way too far back and forth, I tried fitting some scraps to close the hole in the top up a bit to see if it made a difference:

Hmm it’s a bit better but not by much, I’ll need to alter the mechanism if I’m going to make much of a difference. To reduce the ellipse that the crank describes at the top there are a few things you can do, make the cam smaller, move the shaft and pin points closer together on the cam or to move the guide higher up. As I’ve already cut the box and I don’t want to waste material re-cutting it I’m going to ry a combination of the first two:

The one on the left is my new design, I’ve also included a couple of new components to limit the size of the guide hole in the box lid, time to test again:

Much better! I’ve made the hinges for the wings out of some acrylic tube cut into three pieces, the middle stuck to the wing and the other two sides stuck to the body with a piece of rod in the middle for the pivot, to stop it falling out I ran a little acrylic cement in the end of the tube at each end. It’s still a little clunky but hopefully the linkages will reduce a lot of that.

Glues:

Solvent Cement: As I have used in the past I could use a solvent cement to join the acrylic components. This glue works via capillary action so the edges to be joined must fit together perfectly, it shouldn’t be difficult to get perfect edges using the laser cutter but I don’t think this join will be strong enough for the safe.

Araldite: This would give a strong joint with edges that may not fit together exactly, it can fill small gaps. Hmmm on testing it’s quite messy and does not look very good on clear acrylic.

Joins:

A mechanical join combined with the solvent cement could give the strength needed:

This edge would require a channel to be ground in the top and bottom edges to fit the side panels, I could achieve this using the CNC miller.

For this one I would cut tabs and holes for the pieces to fit together, this is the most promising method as it gives strength, makes it easy to fit accuratly without glueing and also adds to the asthetics. I wouls also make the manufacture simpler as all the pieces could be drawn in 2D design and cut using the laser cutter.

Hinges:

I need to get the door to open (otherwise the safe will be a little too safe…) so hinges will be needed. As the case is to be made from clear acrylic it would be nice if the hings matched.

It would be quite elegant if the hinge could be part of the door itself though. After a discussion with Dean I have decided to use a tab on the top left hand and bottom left hand corners of the door, corresponding to two holes in the case to act a pivots, testing to come in my next post.

Components:

After putting the circuit together I can see that there are no spare pins to add in new sounders or lights (humph) I can exchange the bi-colour LED for one green and one red by wiring both to the same point in the PCB. The Bi colour LED works by lighting up red when the current runs through it in one direction and green when it reverses. With the red wired anode to anode and cathode to cathode and with the green wired the opposite way round it should work in the same way.

I did find a really cool component that I would have loved to use (and still might in another project) called EL wire (electroluminesent wire). It’s a thin copper wire coated in a phospher that glows when alternating current is passed through it. it’s flexible and can be run off battery or mains power.

From my drawing I can make out the I need a gap of at lease 6.5mm for the door to clear, I’ve used 7mm just in case (hey I may be accurate but there’s no point in going overboard ).

So I’ve now got the dimensions of the inner case, the outer case and the door, time to start construction…

I’ve included holes to allow me to attach the PCB, buzzer, keypad and the LEDs as well as ports in the door panels to allow me to access the stereo PICAXE connector, for the battery cable and for the solenoid to allow the door to lock.