Eggbot in action: http://youtu.be/w5YWRMa1uJk. Make your own and have fun designing and plotting your own designs onto eggs, balls or light bulbs.
Remixed from earlier designs, I have added some improvements of my own as follows:
End plates have been changed to allow more pen travel. The top front hole for an M8 stud on the original design was redundant because an M8 stud fitted there would interfere with the pen when lifted. The height of the end plates also limited the possible pen travel left and right. It would be possible to reduce the overall height of the end plates to 75mm and correspondingly reduce the height of the back plate by 25mm, but I chose not to do this because the back plate offers some useful protection to the pen arm when not in use. It also makes for more vertical space to mount the electronics. I added angle brackets to either side at the front to take the flex out of the end plates and therefore better hold the axes of the drive shaft and idler shaft in line.
Drive and idler cups have been designed to have nitrile O-rings inserted into the grooves in the end faces. I figured that an O-ring would be a sensible means of accurately holding the egg on centre. The O-rings I sized them for are ARP-206 (3/4"OD x 1/2" ID). Having the drive and idler a constant outer diameter allows you to use a short straight edge to assist with lining them up with each other.
I used an HXT900 9gr servo and found that when fitted to Finucane's arm the servo arm lined up with the centre of the spring rather than sitting either side of it. I made it fit together by carefully sawing and filing a slot in the spring so the servo arm could fit into the middle of the spring end.
I decided to buy the genuine EBB electronics (from http://shop.evilmadscientist.com/) because it is a tidy one-board solution with good software support and documentation and it was about the same price as an Arduino Uno plus an Adafruit motor shield. The genuine EBB came pre-loaded with firmware - all I needed to do was plug it in, install the software and start egg-boting.
See instructions from source things. You can see from the photos how the diagonal braces are intended to fit. I have started with studding 300mm long but 200mm long would be adequate unless you have huge eggs, or want to print light bulbs.
You can use cup suggestions from earlier designs if you wish. My O-ring cup design seems to work pretty reliably, but I have nothing to compare it with.
The EBB electronics are fitted to the bracket with 4 M3 or similarly small screws. You may need to drill out the holes to suit. EBB bracket is held in place by M8 nuts up against either end plate.
If you go the EBB route (which I can thoroughly recommend), you will find all you need to know about using it on http://egg-bot.com/. I like that there are extensions published for Inkscape that allow you to design artwork, control your eggbot, and plot to an egg all from within the one free package.
Now I just need to buy a bundle of coloured ultra fine sharpies and I'll be all set for Easter!
Extra: I had some M4 hex head screws in stock so made one of these M4 thumbwheels http://www.thingiverse.com/thing:13807 to make a convenient screw for holding the pen. Now I don't need to reach for a screwdriver when I'm changing pens.