Hi Ryan, welcome to the forum 🙂

The stuff you've got/ordered looks good. (As an aside, you mention the multiples for a 3d printer - using multiple L293D shields in a concerted way won't be easy I think - if you haven't already, have a look at http://store.amberspyglass.co.uk/buildlog-net-stepper-shield-complete-kit.html which is a very nice little shield designed to run GRBL (G-code interpreter) for things like the shapeoko. For polargraph it is overkill, but great for a little CNC kit.)

The PSUs I use are pretty much these ones:
http://www.amazon.co.uk/CostMad-Universal-Universally-Voltage-Replacement/dp/B004YBLD1G

but there's plenty on ebay that are pretty similar - Ex-Pro is a popular brand:
http://www.ebay.co.uk/itm/Ex-Pro-Worldwide-Variable-Voltage-AC-DC-Power-Plug-27w-Regulated-3v-12v-2250mA-/370823607471?pt=UK_ConsumerElectronics_PowerAdaptors_SM&hash=item5656cf34af

I like something with variable voltage because there's no other way of adjusting the power with the adafruit shield. I usually run a motorshield at 6v or 7.5v. You have slightly lower current motors so you can probably go a bit higher, but it's nice to be able to tune it. Plenty of people just bung a 9v (or more) supply on though, so it's not essential by any means. Most of this is covered in the Polargraph instructable actually.

The drivers on the shield can draw 600mA each, so a power supply that is capable of supplying 1200mA would be ideal, but I never had any problems with a 1000mA supply.

The motorshield has a jumper on it that unites the motor power circuits with the logic power circuits. So if the jumper is there, you can plug the PSU into the main power socket on the arduino and it will supply the motors too. Not recommended by Adafruit necessarily, but I've never had any problems with that arrangement.

The more conventional setup is to remove the jumper, and wire the PSU into the EXT_PWR terminals on the motorshield. That powers the motors, and let the +5v from the USB connection power the logic.

Re amount of cord - For each side, you need enough to stretch from one corner to the diagonally opposite corner, plus a bit extra to hang down the side. Doing very large drawings you are more likely to run into problems with the counterweight drop, or the lines sagging under their own weight on long stretches.

sn

The machine you make needs to be about 20-40% bigger than the biggest drawing you want to make, because you need a decent margin around the page. So a A0 page will need a surface about 1100x1500mm, which has a diagonal of around 1800mm. 4m of cord will probably do you because it doesn't actually need to reach to the very bottom edge of the surface, but 5 would give you a bit of extra room.

Remember that regardless of whether the machine is landscape or portrait there needs to be at least 1800mm drop from the motors to the floor, for the counterweights. That's fairly tall.

All the pulleys are spoken for, but I'll be making more.

More weight on the gondola will indeed help sagging lines. I sometimes tape a little bag of change to the gondola when I'm doing very fine work, particularly on rough paper. A few judicially placed weights (small change) can work wonders for sharpness when you've got a pen and a surface fine enough to express it, but there is a corresponding increase in load on the motors, power used, heat etc. Not to mention more likely to pull the brackets off the wall, ha! Usually not an issue, but something to be aware of.

sn

Some stuff like the sprockets I usually print on demand unless I've got nothing else to do. My little 3d printer is busy printing parts for my new big 3d printer at the moment, so unless there's a reason to stop it (ie a customer shouting at me), it will not be diverted from that.

sn

Ahh ok then didnt know they were available elsewhere.

Sounds like a good plan Ryan. Ideally, there's a bit of a slant, but you can get away with it on a flat surface. It depends a bit on your pen and your surface. You'll need to be a bit more careful about balancing the gondola, and be prepared for a little bit of skitteryness. There's less weight on the pen tip, but then there's also less friction.

I've done it on windows and on walls, and in both cases I have just stuck the motor bracket directly to the surface with sticky pads (command strip type stuff) and not worried about the slant. I have just been looking at a pic from another polargraph pioneer of a 2.2m tall machine, and that's just with the motor mounts screwed to a piece of timber attached at the top of the wall, and came out pretty nice.

sn

Hey I think that looks great. I can't see there being a problem with the gap between the surface and the motors. If the surface is thick enough, then the depth of it will help pull the pen against it too - equivalent to having a sloped surface. Unless you need to (for aesthetics), I wouldn't bother cutting the surface down - the bigger it is, the less chance you've got of ending up scribbling all over your walls, and the more safe margin you have.

sn

Cool stuff, that was quick. Yep the pen gripper thing is good. Let us know how you get on!