CycloidalExtruderDrive-History

Build v0.1

Objective

Quickly check general feasibility using some existing components:

Design

Gearbox ratio: Some simple engineering calculations based on required feed-forces for 3mm filament and comparisons with existing extruder solutions propose for the 4Ncm stepper a gearbox ratio of approx. 20:1. Having an existing CAD model of a small cycloidal gearbox with a ratio of 36:1, this one was simply adopted.
STEP-files of the assembly with detailed components, simplified geometry for the core parts (for simplifying mods) and some tools/jigs which help you to assemble this thing: Media:CycloExtruder v01.zip

BOM

Printed parts
1 x StepperOnline Nema 8 Bipolar 0.6A 4Ncm(5.7oz.in) mini stepper motor 8HS15-0604S
1 x MK8 Drive Gear
1 x E3D v6 HotEnd - 3mm Direct (12v)
3 x MR105-ZZ (5x10x4mm) Ball Bearings
2 x MR128-ZZ (8x12x3.5mm) Ball Bearings
1 x 5mm Stainless Steel Precision Shaft (or a 5mm aluminium shaft from your local hardware store - make sure you go shopping with your caliper: 4.97..4.99mm is sufficient if you are on a hurry)
12 x DIN6325 2x10 Steel Dowel Pins
4 x DIN912 M2x4 Hex Socket Head Cap Screws (for fixing the stepper)
2 x Springs
Various DIN912 M3 Hex Socket Head Cap Screws

Make & Assemble (not easy...)

Print the parts: PLA, 2 perimeters, 100% infill, layer height 0.1mm, go slowly for the gears (15mm/sec or less)
Tune the excenter's bore to make it go gently (!) over the motor shaft and you can remove it gently again (this is a design weakness...)
Tune the excenter so you can assemble the bearings smoothly
Press fit the bearings into the excenter gears, push them over the excenter
Bolt the ring gear with the M2 screws to the stepper
Mount the excenter/excenter gears over the motor shaft as follows:
- Make the D-shaft look to the left side
- Align the marking dot of the lower gear exactly to the right side
- Align the marking dot of the upper gear exactly to the left side
- Make sure this position is kept, while the excenter gears enter the ring gear

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: 0.55A, 32 microsteps, 10500 steps/mm
Retract: 0.5mm, 5mm/sec
Speed: up to 80mm/sec, 0.4mm nozzle, 0.2mm layer height
Did some very basic tests (10x10x5 blocks, Dia30 cylinders), in summary 1 hour printing

Results

It prints
No lost steps while extruding and retracting
Some waivy surface appearance (compared to Bowden setup), probably related to poor tolerances
Motor temperature: reasonable warm
No specific noise
No signs of gear wear after tear down - some normal surface flattening, running without grease
Works basically - v0.1 objectives met

Todo

Objective

Design

From the motor characteristics, a gearbox ratio 12:1 was calculated. However to keep the v0.1 output hub, a ratio of 16:1 is implemented
No of teeth: 16, eccentricity: 0.8mm, radius: 17mm, roller radius: 1.5mm
Tooth clearance tightened from 0.2mm (v0.1) to 0.1mm, aiming for less waviness while keeping printability with FDM machines
STEP-files of the assembly with detailed components and simplified geometry for the core parts (for simplifying mods): Media:CycloExtruder v02.zip

BOM

Make & Assemble

Basicaly like v0.1 - printing and assembly simpler due to larger features and aligning of some of the components' endfaces
Hint: Cleanup the gears for running smoothly before assembly - remove any blobs and strings carefully

Test & Validate

Attached to an Azteeg X5, v1.1
Motor current: 0.45A, 32 microsteps, 10200 steps/mm
Retract: 0.5mm, 5mm/sec
Speed: up to 80mm/sec, 0.4mm nozzle, 0.2mm layer height
Testing: 10x10x5 blocks, Dia30 cylinders, some larger parts, in summary 4 hours printing

Results

Works basically
No lost steps while extruding and retracting
Started to slowly loose steps after 1 hour when printing a larger object - made further tests fail
Less waivy surface appearance than v0.1, but still worse than Bowden setup
Motor temperature: approx. 70°C (finger sensor...)
No specific noise
After disassembly:
- Excenter and excenter gears lost their press fit with the bearings, probably due to motor-heat induced material creep. This causes locking of the gears and loss of steps
- Some normal surface flattening, small amounts of worn-out particles - seems to be protruding material from the print being removed while in operation
Weight (drive system + E3D v6 HotEnd): 138g
v0.2 objectives met