garageFab.cc

While I still need to test a few things such as vacuum paper pickup/placement tool and a vacuum table made out of cutting mat, I've been thinking about the coordinate robot that would drive the rotary cutter. I had started drawing sketches of the frame and the robot, which would be built from MDF sheets, shafts, pulleys, belts, etc, but then realized that it would take a lot of work to actually build something like that and then, if something doesn't work as expected, to change the design and re-build it, partially or entirely, again. What I need at this point is a quick rig to test the cutting of straight lines, circles and curves, precision of paper pickup and placement - with minimum time investment. Something like CNC machine but a bit more flexible than that.

Couple of weeks ago I was researching homemade 3D printers and I came across this MAKE magazine article. Basically some guys in Russia put together an Erector-style set which includes linear slides and 5 stepper motors (page in Russian). A dozen or so devices can be built using the set, including plotter, CNC mill, hot wire cutter, and a 3D printer, sintering powder paint or ground plastic - which is the subject of the original MAKE article. I figured this is exactly what I needed. I emailed them asking to buy the kit, but they were apparently sold out and said that they plan to sell their products overseas in 2008. In the meantime, wanting to change some things in their design, I decided to build my own kit like that. The idea of robots built from Erector sets is certainly not new, though I have not seen CNC type of kit before. One idea, which may or may not be original, but I thought was interesting, is that they use Teflon, not bearings in the linear slides.

Instead of steel, I decided to use 1"x1" aluminium angle, for a couple of reasons. First, aluminium is easier and faster to work with using minimum tools - vise, hacksaw, file and a small power drill. Second, I have seen plenty of 3', 4' and 8' aluminium angles in Home Depot, but have not seen a steel angle with smooth finish. Aluminium is not as stiff as steel but as long as the forces on the robot are small, I think it should do fine. Lack of stiffness is also a problem with longer slides - off-axis play in the middle of a 4' slide is a good quarter to half an inch, depending on the load - about 3 times that of steel. If aluminium slides prove inadequate for accurate paper cutting, I will look for smooth steel angle. I already tried a couple of steel angles from a hardware store and online (McMaster.com) - but both have very rough finish resulting in a lot of friction.

I also changed the design of the linear sliding element - it now uses ball bearings and Teflon angle. The bearings are held in place by 10-24 screws which can be tightened at the adjustable distance from the slide. 1" pieces of Teflon angle are held by ~1/4" hole in each leg, which fits over the head of 6-32 screw going inside out the tube. Finally, a 3/4" 10-24 coupling nut with two slots on one edge is held in the inside corner of the tube by four 6-32 screws. I tried to make the design to be forgiving to the tolerance errors unavoidable with simple tools.

The ball bearings seem like another reason to use steel slides: they will eat away aluminium over time. However, for applications such as tests and trials (which is the idea behind the kit) it's not a big concern.

I built several sliding elements over the last couple of weeks, and the sliding motion is pretty smooth, which is a good thing. The bearings can be adjusted from the tight fit with considerable friction to almost frictionless roll, but the adjustment process currently does not produce predictable results. I'm trying out different things to reliably achieve various amounts of friction while keeping the bearings in contact with the slide.

This is the current design of the sliding element (also see attached Sketchup file).

Parts

Bearings: McMaster.com part# 60355K861
Square Tube: McMaster.com part# 6546K221
PTFE angle: McMaster.com part# 8604K121
Aluminium Angle: 1"x1", 1/8" thick, Home Depot
Fasteners: 10-24 (3/4") and 6-32 (3/8") screws