garageFab.cc

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. Nothing that can't be fixed by getting longer pieces of the ribbon cable. 

Due to occasional wrong turns and deviations from design, plotter assembly took two evenings which is longer than I expected. Ideally, I think that assembly for most contraptions should take a couple of hours, and I believe this is achievable with visual step-by-step instructions which would eliminate wrong turns.

The testing reminded me of the backlash in the motor jaw coupling - it became visible in the belt drive because of the much lower transmission ratio. The backlash comes from the rubber spider in between the two coupling jaws. It's in sub-millimeter range on heavier Y axis, somewhat less on X axis. Millimeter may be acceptable for some applications, but for others it would be a severe limitation. I've been thinking about ways to eliminate it. There is a thread on CNCZone where someone milled the center spider of the jaw coupling from plastic, but I wonder if this defeats the purpose of the jaw coupling, where the rubber spider accomodates misalignment of the shafts. A common recommendation appears to be Oldham couplings, but they are pricey - pair of hubs for 1/4" shaft costs $15 on Mcmaster.com. Maybe I'll find them elsewhere cheaper - that would effectively solve the backlash problem.

Another issue that I've seen during testing is occasional stall/misbehavior of an axis during 2-axis move, e.g. something like "G1 X16 Y9". Basically, on some moves heavier Y axis would stall, and on other moves it would arrive to its intended destination much later than X axis, moving much slower than the default feedrate and pausing along the way. This is really puzzling because the latter looks like the firmware problem.

Stalls might happen because of insufficient torque. However, adjusting trimpot on the board didn't seem to matter much. To rule out the weak motors, I ordered a couple of Keling steppers. Keling KL23H51-24-08B is one of the recommended motors used by the Reprap project. According to the spec it delivers ~166 oz*in of holding torque in bipolar (series) configuration, which is more than twice that of current Alltronics 44A501711.