Brianorca.com
Aluminum RepStrap
I'm currently working on a "repstrap" 3D printer. Here's an early CG rendering of my design:
Repstrap 3D design
This image is missing a few parts, but gives you a feel for the placement of the main parts.

The concept is loosely based on the "Mendel" design of RepRap. I ordered the electronics from Makerbot, but the frame design is my own. The Y axis moves the table. The Z axis moves the X rail up and down. The extruder is mounted on the X axis. All axis are driven by 2x10 pitch 1/2" lead screw. Since I don't have a RepRap machine available to me, I designed this "RepStrap" out of parts I could machine with hand tools and a drill press. The device is approximately a cube 20" on each side. This size was largely dictated because I decided to cut a pair of the 3' lead screws in half, resulting in 18" lead screws. I think I will get a build area of about 10" x 12" x 14", which is large compared to most RepRap designs. In retrospect, I think I could have extended the table in the Y axis to get an extra 2 inches. I suppose if I need it bad enough, I could, without much trouble, make the table depth 12" or even 16" by extending the Y rails a just bit. Changing any of the other axis would require much more rework.

3-16-2010: Major assembly is complete, and I'm testing the axis movement. My first batch of stepper motors only had 20oz/in of torque, which is barely enough for X and Y, and not enough for my Z axis, which has to drive TWO lead screws with a belt. Now waiting for a few parts to beef it up. Here's the real thing so far:
I built the extruder by using some 1/4" acrylic sheet. I used 3 layers, topped off with some aluminum sheet on one side, and the motor on the other. This assembly is then bolted to the X-axis carrier with 2 dowel nuts, which extend through all 3 layers.
layer 2 of the extruder assembly. I had to Dremel the head of the bearing bolt so it would clear the stepper motor. The motor's shaft will fit the small hole in the middle, pinching the plastic feed stock between the shaft and the bearing.
layer 3, with the unfinished insulator and tip. The tip is a welding tip, which already comes with a 0.6mm hole and hollow interior. I tapped the insulator so the tip would screw in. This layer requires a larger hole for the motor, due to an extended bearing housing.
(Almost) completed extruder assembly. I was looking high and low for 3mm screws to match the stepper motor, but realized that the 1/8" screws that Home Depot sold cheaply by the bag actually fit quite nicely. (This after buying some expensive 3mm metric screws that had the wrong pitch thread!)
Extruder mounted to X-axis. The front bearings are also mounted using a dowel nut, and can be adjusted so they are not too tight, just enough to keep the rear bearings on the rail.
rear view, showing the rail bearings and lead nut. This is also a good view of one of the bearings. In order to reduce the load on the motors, each end of the lead screw has a bearing. This is then mounted between 2 sheets of aluminum, each with a hole smaller than the bearing. Between the 2 sheets, there's a spacer of 1/4" acrilic (because it was handy) and another sheet of aluminum (just to add some thickness.)
I found using a "step-drill" was best for making the large holes, after trying other methods.

I intend to replace most of the wood parts you see here with repraped parts when I get it working.
Here is the entire frame. The build surface is acrilic, but still has the protective paper on it. The power supply and motherboard is at top right.
The zip ties holding the circuits will eventually be replaced by repraped mounts.
another view
The rear. You can see the bracket I made for the power supply, which also acts to stiffen the frame, not that it needs much. When the T-slot rails are tightened down, they are quite stiff, even with no diagonal bracing. Just make sure you cut the rail ends square!