Sunday, 12 February 2017

Experiments with Flying Extruders and Deltas

Mini Kossel with flying extruder
     Not long after finishing the Re-ARM upgrade, my geared extruder suffered a failed bowden coupler, so rather than reprint the entire thing, I decided to switch to an alternate configuration that I'd read about called a 'flying extruder', basically suspending the extruder motor over the effector/hot-end, allowing for better print quality with a wider range of materials.


MK8 extruder with suspension bracket attached
     I started by replacing the failed extruder with a MK8 variant that was in my spare parts pile, then tracked down one of the designs for the suspension mounts from Thingiverse (thing:1295606). It's a fairly good starting point, but it is missing a couple of parts for mounting on a standard Kossel frame, so I had to design some custom parts for the pulley mounts, the files are here.


Moving extruder power cable to the effector wire harness
Heat-damaged PLA fan duct skirt
     One unexpected issue cropped up during the process of changing the Bowden tube, which on an E3D Lite6 requires a full stripdown and rebuild, so I had to remove the lower half of the effector I've been using (thing:1569106) and found that part of the hot-end had rotated and melted part of the lower half of the effector. My solution was to rework part of the design and add a small loop on one side to allow use of a zip-tie to secure the heater wires, thus locking the rotation and preventing a recurrence. The file for the reworked part is on Thingiverse (thing:2103488).

Remixed and improved ducting skirt, note the zip-tie on the right
    With that issue corrected, I designed some custom brackets to use a couple of spare 608zz bearings and the MK8's old idler bearing as pulleys. Other parts use were some old washers and pipe fittings from the workshop junk bin for a counter weight, a roughly 30cm piece of 1/4-inch steel rod for the main shaft across the top, a 1.5m length of thin rope/string, and about 30 zip-ties. I also added some temporary loops to the carriages allow using some zip-ties to hold the extruder in place while I installed the upper pulleys.

Pulley bearings, 1/4-inch shaft and part of the counterweight
Using the cable to loosely suspend the extruder during install
Temporary anchor for zip-ties
      Initially, I had thought to try running with just zip-ties holding the extruder block to each of the carriages, but that proved unworkable due to the zip-ties restricting the range of motion. The basic idea of suspending from the carriages would probably work if you used some form of elastic for the cables, but that still leaves a 400-gram weight sitting in the middle of the frame, so it would definitely lower the maximum attainable speed.

Initial configuration idea, doesn't work with rigid cables
Initial Z-tower pulley mount 
Upper pulleys 2/3 installed
    As you can see, I'd initially thought to use just the zip-ties to hold things in place, but that resulted in some strange glitches during homing, so I switched to the full counterweight option. After I'd installed the first couple of pulleys, I realized that the counterweight would hit the back of the z-tower carriage, so I designed a short arm with a 608zz on the end to act as a spacer. Other hardware used in the spacer arm was 3 M5x35mm bolts, 4 M5 nuts, and 1 M5x10mm bolt.

Spacer arm fully assembled
Spacer arm fully installed just below the top triangle
     As for the counterweight, I just used a small scale to measure the approximate weight of the extruder assembly, roughly 395 grams, then found a combination of old washers and stuff that was reasonably close to that weight to use for the counterweight, ended up at 405 grams, then tied the lot to the end of the string/rope. As for the print quality, I still need to retune the retraction settings, but it's a drastic improvement over the original configuration, with a much finer surface finish. Here's a couple of #3DBenchy prints for comparison, the Green one is from the filament comparison post with the original configuration, the gold one is done with the new configuration.

#3DBenchy 32-bit on the left, 8-bit on the right
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