BowTie Pulleybox / Gearbox

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

This is a spin on my "BowTie"-gearbox, but made to use a belt drive this time. It features a very slightly bigger gear ratio - but it's main advantage is that the printed spur-gear from that design requires a very accurate/calibrated 3D-printer while this doesn't.

Features

• NEMA17 gearbox
• 3:1 gear-ratio (38/12T, so a reduction of 3.16 to be more exact)
• Easy to print
• Very lightweight*
• The exposed pulley doubles as a visualizer and thumb screw for manually (un)loading filament

(*) All plastic and metal parts weigh a combined 34g. 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 pretty much as lightweight as a NEMA17 gearbox can go.

Printing Instructions

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

Necessary hardware (BOM)

Assembly

• Pre-assembly:
  • Check to see if the D-shaft fits the printed pulley. If it is very tight (some tightness is desired), you can carefully run a 5mm drill bit through it.
  • Refer to the images for orientation of the different parts.
• Insert the hex nuts into the body, using the “screw pulling” technique (With use of a 12mm screw & preferrably a big washer too)
• Press the bearings into the body.
• Insert the square nut - with a screw installed - into the printed pulley.
  • “untighten” the screw through the hole in the pulley, until it gets a little bit tight, this will make it stay put.
• Put the belt around the printed pulley.
• Put the printed pulley in the body between the bearings.
• Insert the D-shaft through the front, about halfway into the pulley.
  • Make sure the cutout of the shaft matches the location of the square nut in the pulley. Otherwise the shaft will not fit.
• Put the nylon washer between the bearing and printed pulley.
• Fully push the shaft up to the end - take it back very slightly so it won't touch the motor.
• Tighten the screw of the printed pulley through the hole in the pulley.
• Insert the 12T pulley into the belt.
  • This is a bit tight, take care doing so, it'll fit with some playing around!
• Put the motor on (insert it into the 12T pulley), without tightening the screws yet.
  • Make sure the orientation of the wires/connector is as desired.
• Put the motor-screws through the holes in the body, screw them in about halfway.
• Press, and keep pressing the indent in the body with your thumb, to put tension on the belt. Tighten the motor-screws while the belt is under tension.
• Finally, position the pulley on the motor, so that the belt runs in the middle of both pulleys. Then, tighten it down.

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

• M350 E16 - Set microstepping to 1/16
• M92 E445 - Change the E-steps to 445
• 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, this can only happen in firmware, so I recommend to change the direction through wiring.