Welcome! If you try and build Contraptor, we'd really love to see your progress and get your feedback. Do you have a site/blog or Twitter?
Thanks for great comments, they are very helpful for the project.
1. Drilling aluminum angle.
This is true, drilling angle is perhaps the most time consuming part of DIY, and we're trying to discover as many ways as possible to do it efficiently. Pegboard + automatic center punch is one; Riley has lasercut a jig and is working on another one, CNC-milled. One way we've dealt with lack accuracy is using 10-24 screws in 1/4" holes.
How did you make your jig? We try to keep the DIY documentation on this site and Thingiverse, so please feel free to contribute and share your experience making Contraptor. Editing the Contribute page is a great start; then the information can be moved to the right pages.
2. Sourcing aluminum.
In my experience you can typically get aluminum online much cheaper than in hardware stores. At OnlineMetals.com, it costs about $0.75/ft in 8-ft lengths. Hardware store can work when you run out of angle for a project, need a little more and can't wait. If you have someone cutting it, keep in mind that perforated angles should be 1/16" shorter than the whole number of inches.
Motor mount can definitely be improved and vibration is one of the primary concerns. Cooling is secondary, perhaps mounting a PC fan next to motor can work. Again, we'd love to see what you come up with, so please share your work.
The stepper motors we're using are very cheap ($10) but seem to be able to take a lot of abuse and perform far beyond their specs (though how reliably is still the question). They are rated for 2.3A, which is too much for EasyDriver. Right now we're waiting for production of Makerbot Stepper Motor Driver v3, which exists only in schematics but has microstepping. It's surface mount though, so some people won't be comfortable with soldering it. We're open to alternative electronic options, especially if they're cheaper (3 axis electronics+motors + Arduino cost about $140). The HobbyCNC kit looks good, can it be made to work with Arduino? One of the reasons we'd like the kit to use Arduino is the ability to write your own firmware and simplicity of use.
5. Linear motion.
Right now we have only 2 options for linear motion - sliding elements on angle and linear bearings on linear rails. Sliding elements on angle have higher friction (adjustable) and work pretty well with leadscrews drives. They're also extremely cheap. BuildyourCNC.com previously used steel bearings on aluminum rails, but we're using UHMWPE pads on aluminum, so wear does not seem to be an issue.
The other option, linear bearings on linear rails basically has ball bearings rolling on drill rods. This is low friction option that's fairly new and not very well tested. It's possible that ball bearings will wear quickly under heavy uneven axial loads, however we think light loads should be tolerable.
Another obvious option we're tinkering with - sleeve bearings on drill rods which would only require a standard set of adapters - cubes to convert them to Contraptor 1" grid. The adapters could be lasercut or milled from plywood/plastic on a miniCNC bootstrapped from Contraptor.
Again, the more options the better - it would be great if Contraptor could offer several simple and inexpensive linear motion concepts, each having its own advantages, so that tradeoffs could be made depending on design goals. As long as something can be DIY'd without heavy, expensive or high-precision machinery, we'd like to consider it, so by all means bring it up!
6. Limit switches.
Sure, the existing solution is more of a hack that allows to get started and then add better options - microswitches, optoswitches etc. What are the ones you're thinking of using?