Check out my improved version: https://www.thingiverse.com/thing:2492939
This low-friction slide uses 6mm plastic balls (airsoft pellets) in recirculating channels to provide unlimited rolling-contact linear motion. There are two ball circuits in each half that bolt together around a 1" square aluminum tube. The carriage can support side loads as well as moment and torque loads. This is a great low-cost alternative to using rollers and bearings on more costly (and heavy) extruded aluminum profiles.
This can also be scaled to 75% for use with standard .177 cal steel BB's and 3/4" steel tube. (Thanks, brw_racing! http://www.thingiverse.com/make:151352)
Update: This can also be scaled to 75% and used with standard .177 cal steel BB's running on 3/4" steel tube. Use M3 screws instead of the 8-32 screws.
The top and bottom halves are identical except that the top has thru-holes for 8-32 screws and the bottom has holes that can be tapped with 8-32 threads. Print both with 0.2mm layers, 3 perimeters and 35% infill. I printed them in PLA, but ABS should work fine. Make sure that your printer is printing the recirculating channels cleanly because there is no good way to clean out any blobs after the parts are printed. Tap the 4 holes in the bottom half with 8-32 threads. You may need to tap from both sides.
Fill each ball channel with 16 6mm airsoft plastic BB's. Make sure the balls all recirculate freely. Lubricate with a few drops of oil. Place your 1" aluminum tube on the top half, and carefully holding these two pieces together, flip them over and place on the bottom half. Screw together with four 1 1/4" long, 8-32 socket head cap screws. Use calipers to make sure the gaps on both sides are uniform. Tighten until there is no longer any play, but no more than that. There should be enough thread depth left in the holes on the bottom side to mount your load.
Additional printing tips:
There will be a small patch of solid infill lines at the very bottom of the bearing surface. Orient the solid infill so that these infill lines are parallel to the ball channels. You'll need to check the g-code preview to make sure you've got it right.
Set the starting point for the perimeter lines to be towards one end of the ball channels. This will prevent perimeter start/stop irregularities from appearing in the middle of the bearing surface.