garageFab.cc

Well, partial success again. On the up side, Z-axis assembled from the linear rail and bearing sliding elements does work; there is no play and the motion is nice and smooth - see the video.

On the down side it is still not pulling out the drill bit from the mat completely. I have two possible explanations - since it's a belt drive, Z-axis with mounted Dremel flex shaft slides down from vibration at the short moment the steppers are disabled; or the motor simply doesn't have enough juice to lift the axis and pull the drill bit out of the mat at the same time (there is apparently some resistance).

Another attempt to kill several birds with one stone:  

• Test belt-driven mini-router design
• Test solutions for motor coupling backlash (Oldham coupling, pulley on motor shaft)
• Drill the cutting mat in a few hours

The mat drilling contraption is basically the new belt-driven design of the mini-router, with the exception of Z-axis which is also belt-driven (mini-router still uses leadscrew on Z-axis). Dremel flex shaft with the drill chuck and 1/32" bit is mounted to Z axis. The results are rather mixed. 

Vincent Rouilly from OpenWetWare.org is using Contraptor plotter to build an open source robot for microbiology lab. Needless to say, I am pretty excited about this! We've been exchanging emails, modding Arduino GCode Interpreter and figuring out how to drive a plotter head with a cheap servo. He made a great suggestion to use different colors for different components in Sketchup files. Up till now, the 3D model of a contraption would be pretty much required to look at during assembly. With components in color, assembly from Sketchup screenshots (or printed pages) is a whole lot easier.

The current way of making perforated angle is fairly time consuming: print, cut, puncture and attach labels, saw long angle into pieces, and then center punch the pieces from which lables will be removed - for example, those that will be used as linear rails. Most printers do not print the labels to exact dimensions, forcing to resort to cheats and workarounds. Then there is also drilling, however the current approach is to drill holes on demand.

One problem affecting the accuracy in Contraptor is angular backlash in the motor coupling coming from the rubber spider in between two coupling jaws. While it's not noticeable in leadscrew drive, the error can reach 1/16" in the belt drive because of the low transmission ratio, especially if the stage is heavy. Needless to say, this is way too much. There are several ways to deal with this problem. First, the pulley can be mounted directly on the motor shaft, which conveniently is 1¼" long in Alltronics steppers.

To investigate the plotter stalling problems that were encountered during recent tests, I had ordered Keling stepper motors - one of the official choices of the Reprap project. They are rated for 166 oz*in of holding torque (vs 60 oz*in in Alltronics). They also cost about as much more. When the Keling steppers arrived, I assembled the plotter again. 

For some reason, the maximum feedrate that Keling steppers gave without stalling was 300 IPM, and that was on single-axis movements (like "G1 X10" or "G1 Y6"). Alltronics steppers could do about 600 IPM on single-axis movements in previous tests. I thought this was strange and decided to install Alltronics back to compare.

One of the expensive materials used in Contraptor set is Teflon angle. I had tried to find it cheaper than on Mcmaster.com, unsuccessfully - it is only available in one other place online, and it's even more expensive there, around $30/ft. Ultra High Molecular Weight Polyethylene (UHMWPE), which has coefficient of friction fairly close to that of PTFE, might be an inexpensive replacement. I ordered some UHMWPE to test out and built a couple of sliding elements. Turns out, the performance is as good if not better than with Teflon. 

There are couple of things that are different in making sliding elements with it though:

After I modified the XY plotter design to use new linear motion components, increasing the work area along the way, I needed to actually assemble and test it. During the build, I had to add a couple of shaft mounts with bearings, supporting the middle section of the threaded rods - they were bending too much. These changes are reflected in Sketchup files for the XY plotter. I also used version 1.2 of stepper motor driver boards. The good thing is that SMD v1.2 uses IDC header with 10-pin ribbon cable for Arduino interface as opposed to CAT5 in SMD v1.1 (which is soldered directly to the board and can break from bending). Unfortunately there is only 8 inches of the ribbon cable included with the SMD v1.2 kit, so I was constrained in where I can place the SMD boards and Arduino. This resulted in the weird "Γ" arrangement of heatsinks, as seen in the pics.

After milling tests, I tried drilling out small hole pattern in the cutting mat (which would make a vacuum table surface for the paper prototyper). Initially I used tiny 0.01" drill bit and drilled a short sequence of holes, only to realize that there would have to be 50x50 grid of 0.01" holes spaced at 0.01", i.e. 2500 holes per square inch. Even without doing the math it was clear that it was going to take forever to drill out 8.5"x11" rectangle on the mini-router, so I switched to 1/32" drill bit, with more realistic 16x16=256 holes per square inch. This would take forever/10 in theory. In reality, one square inch took 10 minutes to drill at 12 IPM which wasn't that bad. Still, at 6 sq.in/hr it was going to take about 20 hours to drill the letter paper size area + 1/2" outside margin.

Here come more mini-router tests - mostly milling soft wood, trying some Delrin. I've run into several issues along the way - they slow down the progress some, but on the other hand they are good learning experiences. 

Dremel runout 

First, the grooves that I initially milled with high speed cutters were about 0.02" wider than the tools. I googled "dremel runout" and found various bits of information, ranging from "Dremel is no good because of runout", to "Runout is much better with chuck as opposed to collet".