BowTie Extruder Gearbox

Improve the extrusion accuracy of your direct-drive printhead while making it significantly lighter too.

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Features

• NEMA17 gearbox
• Gear-ratio, two options:
  • ~3:1 (50/17T, so 2.94 to be exact, same as BMG)
  • ~4:1 (53/14T, so 3.79 to be exact)
• Easy to print
• Very lightweight*
• The exposed gear doubles as a visualizer and thumb screw for manually (un)loading filament
• Minimal amount of assembly-tools required (1.5mm & 2mm hex-wrench)

(*) All plastic and metal parts weigh a combined 33g. Combined with a pancake motor (~120-150g), it allows for significant weight saving over the stock motor (280g). With it's compactness and thus minimal use of filament, it is the lightest of any current NEMA17-pancake gearboxes out there (Or at least: the ones I'm aware of).

Necessary hardware (BOM)

Printing Instructions

4 perimeters, 20% infill, 5 top/bottom-layers, PrusaSlicer: “detect bridging perimeters" enabled for the body. 

• Body
  • Material: PETG.
• Gear
  • Material, in order of preference: Nylon, Igus I150-PF, PETG.
  • Nylon can't really be printed accurately if you don't have a dryer & drybox, but it provides the highest heat resistance & inherently provides low friction.

Assembly

• Check to see if the gear fits the shaft without too much trouble. If not, you can use a 5mm drill bit to make the bore bigger for the first few mm's.
• Install the four hex-nuts using the “screw pulling technique”.
• Press the two bearings in place.
• Install the grub-screw onto the reduction-gear.
• Shaft assembly:
  • Look at the pictures for orientation of the gears
  • Insert the shaft through the front until it just protrudes past the front-bearing.
  • Put a single washer onto the shaft.
  • Put the gear onto the shaft - don't press the shaft completely through.
  • Press the second washer between the gear & back-bearing.
  • Fully push the shaft up to the end - take it back very slightly so it won't touch the motor.
  • Tighten the reduction-gear it's grub-screw
• Install the motor onto the gearbox with the four M3-screws.
• Install the drive-gear onto the motor.

Done! Now check to see if everything rotates smoothly. If yes, you are good to go, install it onto your printer! If not completely smooth, the printed gear might have some elephants foot or similar. Try rotating the gear a few times, the gear should wear in a bit. If it's really hard to rotate, undo all the M3-screws two-to-three turns. Press onto the indent on the motor-side of the body, towards the reduction gear, and tighten the M3-screws at the same time. Everything should rotate smooth now. The reduction gear may now have a tiny bit of backlash, but this poses no practical problems.

Firmware changes

For this mod on the MK3(S), make a custom .gcode-file with the lines below (put it on the SD & run on the printer) or run these over an USB-connection

50/17T

• M350 E16 - Set microstepping to 1/16
• M92 E415 - Change the E-steps to 415
• M500 - Save to eeprom

53/14T

• M350 E16 - Set microstepping to 1/16
• M92 E535 - Change the E-steps to 535
• M500 - Save to eeprom

If needed, to revert back to stock MK3(S)

• M350 E32 - rollback to 1/32 microstepping
• M92 E280 - rollback default esteps
• M500 - Save to eeprom

PS: Depending on the used motor/cable, the E-direction may need to be changed too. With the Prusa firmware, this needs recompiling of the firmware. So, I recommend to reverse the direction of the stepper motor wiring.

Background

This is an updated, refined version of my earlier design. Compared to the earlier design, the following notable changes have been made:

• Make use of two identical bearings
• Simplification of the reduction gear
• Add washers between the reduction gear and bearings
• Improved printability
• Improved tolerances
• Improved fitment
• Additional weight reduction
• Due to better fitment/tolerances, no more need for pliers to install the bearings