So after working ~30 hours on this project over the last 3 days we finally printed a part on the Hydra platform! Some details on the print:
Printing speed: 40 IPM
Stepper driver movement method: Half-stepping
Resulting resolution: 0.00125"
Printing medium: ABS plastic
Nozzle temperature: 240C
Print bed material: plexiglass
Firmware: Hydra-MMM v1.4 (to be released to the public soon)
Host Software: Hydra-MMM v1.3 GUI
Gcode generation software: Skeinforge
Total time for print: 9 minutes
And without further ado, here is a picture of the test print in progress
We were actually able to extrude the top layer of the dodecahedron just fine which was surprising from past experience. I think our extruder speed is a bit on the slow side so we are actually stretching the filament a bit as the machine moves from point A to point B. This stretching effect puts it in tension and allowed to to span the void across the top of the object without drooping down into the interior of the part. I suspect that voids up to ~1 inch would be possible to span at the moment with further gains possible using active cooling or from further tweaking of the print speeds. You can also see from the video that we have a heated platform setup, but it was not used for this print as we are still waiting for our 1" wide roll of Kapton tape to arrive.
First impressions of the part were very impressive. The machine has incredible accuracy and rigidity compared to my first repstrap I built using a belt drive mechanism with a threaded rod frame. We still need to figure out how to get Skeinforge to vary the start point of each layer. For this print, each layer started in the exact same spot so we have a relatively prominent seam on the side of the part. I know the setting exists, but Skeinforge is a little cryptic and every time we think we found it, we export the gcode and the problem still persists. We got it to vary the start point on the base layers, but the walls all started at the same location. We will keep investigating, but if anyone knows a solution feel free to shout out in the comments! Even with this problem, the surface accuracy was astounding, but should probably be expected with a machine as accurate as Hydra. I can't wait to add some microstepping drivers and see if we can improve the already impressive 1.25 mil accuracy that we have with half-stepping.
As far as the software and firmware, this was the first real test of the Hydra-MMM firmware and it's handling of the stepper-driven extruder. Up until this point, most of the testing centered around X, Y, and Z moves. For prototyping, the extruder stepper motor is added and needs to be controlled by the firmware. The Hydra v1.4 firmware has the ability to independently move the extruder motor, which is useful at the start of a layer when you want to get the filament flowing before moving and at the end of the layer when you want to pull the filament back and stop the ooze. Before starting each layer we moved the extruder stepper forward a few steps to get the filament to start coming out of the nozzle before beginning the XY moves of that layer. This helped ensure that we didn't have any gaps in the part where the plastic wasn't being deposited. Currently, the extruder motor speed is defined by a variable E_STEPS_PER_INCH, which was first found theoretically and then slightly tweaked to match the real work results. We simply varied the number until 1" of movement resulted in 1" of filament being printed. This number could be decreased to stretch the filament (smaller filament) or increased if you wanted a thicker extrusion. Changing the feedrate of the XY moves will change the speed of the extruder motor to match this velocity, but the relation between these two remains constant. Later versions of the firmware may allow for variable speeds of the extruder motor, but at the moment the results are good enough that I am not sure this is required.
We will hopefully be getting the heated bed up and going tomorrow so we may have some better prints to show off, but in closing, we also printed a traditional test object on the machine before shutting it down for the night. Cheers!
Labels: ABS, extruder, firmware, heated bed, host software, hydra, hydra-mmm, leadscrew, minimug