CycloidalExtruderDrive

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Cycloidal Extruder Drive

Release status: Experimental

CycloExtruder v04-3.png
Description Cycloidal gearbox for a lightweight & compact direct drive extruder
License GPL
Author Powdermetal
Contributors
Based-on [[]]
Categories Extruders Cold End
CAD Models Media:CycloExtruder v04.zip
External Link Build v0.4 Google Album - images and movies


Contents

Intro

Running a delta printer with a Bowden-style type extruder, many people have been looking into alternatives for a more direct filament-feed response (especially when using flexible materials) while still keeping the dynamics of a lightweight effector system. There are a bunch of options out there and each one has it's specific strengths...


Project Target

  • Develop a lightweight, compact and powerful direct drive extruder for 3mm (and 1.75mm) filament with a MK8 drive gear
  • Utilize a NEMA8/11/14-stepper for weight reasons
  • Minimize gearbox package through choosing a cycloidal drive system (gearbox ratio probably between 36:1 and 10:1)
  • Potentially setup an attractive alternative to the outphased NMB PG35L-048


Build v0.4

Objectives (see development history)

  • Improve overall robustness:
    • Avoid loosing steps while extruding manually from the printer controller
    • Stiffen the hotend clamp - v0.3 shows some visual flexing while extruding hard
    • Improve printability of parts (especially eccentric) and make the assembly process less tricky
  • Do some minor changes to the periphery


Design (see development history)

  • Gearbox ratio is changed to 16:1 -> no of teeth: 16, eccentricity: 0.8mm, radius: 16mm, roller radius: 1.6mm
  • Improve fit of centering bearing on motor shaft: Gain axial space by using bearings without shields
  • Make assembly process less critical by adding some axial clearances
  • Improve rigidity of hotend clamp by going back to the v0.2 concept
  • Add 3 connecting points for bolting the effector (M2 screws)
  • Replace the two idler springs with O-rings
  • Fix a minor bug in the v0.3 cycloidal curve equation
  • STEP-files of the assembly with detailed components and simplified geometry for the core parts (for simplifying mods): Media:CycloExtruder_v04.zip
  • STL-files: Media:CycloExtruder_v04_stl.zip


BOM


Make & Assemble

  • Print the parts: 2 perimeters, 100% infill, layer height 0.1mm, use ABS/PETG, go slowly for the gears (15mm/sec or less)
  • Cleanup the gears for running smoothly before assembly - remove any blobs and strings carefully - take your time for precision later on
  • Tune the eccentric's bore to make it a slight press fit to the motor shaft and remove it gently again
  • Tune the eccentric's outer faces so you can assemble the MR128 bearings
  • Press fit the MR128 bearings into the wobble gears, push them over the eccentric
  • Bolt the ring gear to the stepper motor
  • Mount the eccentric/wobble-gears over the motor shaft as follows:
    • Make the flat side of the D-shaft look to the right side
    • Align the marking dot of the lower wobble gear exactly to the left side
    • Align the marking dot of the upper wobble gear exactly to the right side
    • Make sure this position is kept, while the eccentric and the wobble gears enter the ring gear
    • Push the eccentric onto the shaft until the motor shaft is 2.0mm above the eccentric's endface (pushing too far will make the eccentric touch the stepper motor and rub inside the gearbox - you have 0.5mm safety...)
  • Add grease to the gears and to the holes for the pins
  • Press fit and superglue the 5mm shaft into the output carrier (using the shaft-mask tool for perpendicular assembly)
  • Press fit the 12 2mm dowel pins into the output carrier (the pin-mask tool and the pin-support-ring tool will help you)
  • Assemble all the rest


Test & Validate

  • Attached to an Azteeg X5, v1.1
  • Motor current: 0.45A, 32 microsteps, 9500 E-Steps (400 steps/rev * 32 microsteps / 21 mm/rev * i=16:1 -> 9752 steps/mm)
  • Retract: 0.5mm, 10mm/sec, 4000mm/sec^2
  • Speed: 80mm/sec, 0.4mm nozzle, 0.2mm layer height, material: PLA, 2.85mm, temperature: 210°C
  • Testing: 10x10x5 blocks, Dia30 cylinders, some larger parts 1-3 hours each, in summary 10 hours printing


Results

  • Works well
  • No lost steps while extruding and retracting during prints, no more lost steps when manually extruding
  • Motor temperature: approx. 50°C
  • Slightly wavy surface appearance, seems to get better after some run-in period, still not matching my Bowden setup
  • No specific noise
  • No oozing/strings in retract situations
  • Eccentric is printable with 0.4mm nozzle without problems (smallest wall thickness 0.67mm)
  • Assembly is easier, but still demanding
  • v0.4 objectives met


Todo

  1. Continue testing - confirm durability
  2. There is potential for better surface appearance of printed parts from this unit - needs precision-machined gears, eccentric and output hub


Links