User:Powdermetal
Revision as of 18:10, 8 July 2016 by Powdermetal (talk | contribs)
Release status: Experimental
| Description | Cycloidal gearbox for a lightweight & compact direct drive extruder
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| License | GPL
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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 driven extruder for 3mm 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
BOM
- Printed parts from some more heat resistant material than PLA (like ABS/PETG/..., I use ColorFabb's ngen)
- 1 x StepperOnline Nema 14 .9deg Thin Stepper Motor 12.5mm 0.5A 7Ncm(10oz.in) 14HR05-0504S (50g)
- 1 x MK8 Drive Gear
- 1 x E3D v6 HotEnd - 3mm Direct (12v)
- 3 x MR105-ZZ (5x10x4mm) Ball Bearings
- 2 x MR128 (8x12x2.5mm) Ball Bearings
- 1 x MR85 (5x8x2mm) Ball Bearing
- 1 x 5mm aluminium shaft (from your local hardware store - make sure you go shopping with your caliper: 4.97..4.99mm is fine)
- 12 x DIN6325 2x10 Steel Dowel Pins
- 7 x DIN912 M2x8 Hex Socket Head Cap Screws
- 2 x DIN912 M3x8 Hex Socket Head Cap Screws
- 1 x DIN912 M3x10 Hex Socket Head Cap Screw
- 2 x DIN912 M3x16 Hex Socket Head Cap Screws
- 2 x O-Rings 3x1 (ID: 3mm, OD: 5mm)
- Ball Bearing Grease
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 go gently (!) over the motor shaft and remove it gently again
- Tune the eccentric 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 gear exactly to the left side
- Align the marking dot of the upper gear exactly to the right side
- Make sure this position is kept, while the excenter and the excenter 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 it to touch the stepper motor inside the gearbox...)
- Add grease to the gears and to the holes for the pins
- Press fit 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 -> 9.752 steps/mm)
- Retract: 0.5mm, 10mm/sec
- Speed: 80mm/sec, 0.4mm nozzle, 0.2mm layer height, material: PLA, temperature: 210°C
- Testing: 10x10x5 blocks, Dia30 cylinders, some larger parts 1-3 hours each, in summary 12 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
- Continue testing - confirm durability
- There is potential for better surface appearance of printed parts - needs precision-machined gears & eccentric
- Review filament guiding
- ?
Links
- The development history of the Cycloidal Extruder Drive)
- Wikipedia: Cycloidal drive
- Design of a Planetary-Cyclo-Drive Speed Reducer Cycloid Stage, Geometry, Element Analyses
- Building a Cycloidal Drive with SOLIDWORKS
- Generate DXFs of hypocycloid cams for cycloid drives
- RepRap Forum: This engineer claims 100 grams direct extruder
- Build v0.3: Google Album
