Geared extruder mod for stock Prusa MK3S extruder improves extruder uniformity via gearing like Bondtech BMG or Skelestruder. Rather than a complete extruder replacement, this is a quick upgrade for the stock Prusa R4 MK3S extruder.
This is intended for those who wish to improve uniformity, but do not want to replace their entire MK3S R4 extruder. This upgrade replaces JUST the motor plate and idler with geared Bunny Science versions. Remainder of the stock R4 MK3S extruder is left intact and even remains installed on the printer during the upgrade.
NOTE: You CANNOT put this motor plate onto the Bunny and Butterworth (BNBTR) extruder bodies. This will only fit the stock Prusa R4 extruder body. The BNBTR extruders also implement a filament path correction and tighter air gap at the bottom of the extruder. To remain compatible with the stock extruder body, this upgrade does NOT alter filament path nor air gap. Indexing pins on the BNBTR extruder body help prevent accidental installation of this motor plate on the wrong extruder body.
Advantages of Bunny Sciense MK3S Extruder Gearing Upgrade
Improve extruder uniformity via gearing like Bunny and Bear, Skelestruder or BMG extruders.
Reduce extruder mass by about 70 gm compared to stock motor.
Reduces moment arm by moving motor closer to x-axis than stock motor.
Upgrade replaces motor plate and idler door.
Leave the rest of your MK3S extruder installed.
Motor plate / gearbox MUST be printed with supports. 3mf file supplied with suggested plating and support enforcement block. Specify support only for the gearbox portion and up to the 2nd bearing. This leaves filament path unmarred by support material. 3mf's are set for 0.15 mm layers for nicer finish.
Removing support material from inside gearbox cavity takes 10 to 15 minutes. Take care to completely clean out mounting holes for bearings. Push shaft rod into gearbox from outside to remove support material within bearing mounts. If you do not clear out bearing mount holes completely, bearings will not spin freely.
Some support scarring will remain, but is largely hidden by installed extruder cover.
Build with two shaft bearings. There isn't a 3rd bearing for this upgrade.
5 mm diameter shaft rod
Length 61 mm
Pulley Flat 0 to 9 mm
Bondtech Flat 50 mm to end
see https://www.thingiverse.com/thing:3595205 for shaft cutting guide and instructions
2 x 1050ZZ bearings
1 x continuous loop 140-2GT-6 Timing Belt
3 x M3 square nuts
3 x M3 set screws (3 to 4 mm length)
M3 hex nuts & bolts (various lengths from 12 to 30 mm long. Some must be ground to custom length during assembly)
1 x 16T drive pulley (to simplify alignment, use one for a 10 mm wide GT2 belt despite the belt being 6 mm wide)
Motor - 1.8 degree, STEPPERONLINE Nema 17 Bipolar Stepper Motor 0.7A 13Ncm (18.4oz.in) 17HS10-0704S. This is the ONLY short body motor that worked well in my testing.
Wiring harness for motor.
You MUST also set e-steps by sending g-codes to printer
Pulley gearing originally inspired by Skelestruder
Extended flats on shaft to ends of shaft for easier alignment during assembly
Remove all support material, paying particular attention to bearing mount recesses. Stray support material left inside the recesses prevents free bearing function.
Once the majority of support material is removed, pushing shaft rod into each bearing hole to pop support material into gearbox.
Pass a small, flat screwdriver through opposite hole to clean out bearing mounts.
Flatten mating surfaces and motor mount. Uneven surfaces like the main extruder body and motor mount can be flattened with a file. Rear of motor plate can be lapped flat with wet sanding on a glass plate. Remove just high spots.
Draw in 2 M3 nuts at top of motor plate using standard bolt and washer
Press in two motor plate bearings with a bolt, nut and two washers. Outside washer should be 20 to 30 mm diameter. Inside washer (which presses agains bearing) should be just a bit larger than the bearing. Reach into gear cage with needle nose pliers or small wrench to stabilize nut while tightening bolt. Press just flush. Don't over tighten to point of deforming plastics.
Prep bearings by degreasing shipment oil and re-lubricate with light machine oil
Press in one bearing inside rear of extruder body. This one you can push in directly using a 10 mm Allen wrench or a similar diameter tool with flat end.
Check each bearing for free motion. The front bearing of the gearbox is the most prone to debris preventing free motion. Use special care cleaning out that mounting recess. If you need to remove a bearing, they can be gently tapped back out using a 5 mm Allen wrench as knockout pin.
Leave main MK3S extruder body on printer.
Lower two motor mounting bolts are not used.
IMPORTANT: Most 5 mm shafts are slightly too large in diameter. If diameter of shaft is too large to easily slide through bearings, slightly decrease its diameter by chucking in drill and using fine sandpaper while spinning it. Make it just slide fit, not loose. The entire length of the shaft should be tested for easy passage.
- Shaft flats for pulley and Bondtech drive must be ground to depth of 0.38 to 0.4 mm. Grind lightly with Dremel and measure frequently. Notching ends of shaft to indicate flat positions greatly simplifies later assembly.
Retrograde Shaft and Pulley
Prepare motor by grinding a flat on its shaft but don't attaching pulley yet.
Insert motor into gear box.
Insert rear end of shaft through front of large pulley. Front end of shaft should be flush with pulley front surface.
Place one spacer onto shaft behind pulley. Orient spacer so it's narrow diameter side is away from pulley.
Advance shaft through 1st bearing.
Add 1st shaft lock. Orient narrow tip end of lock towards front bearing.
Advance shaft further and add 2nd shaft lock. Second shaft lock should have its narrow tip towards second bearing.
Advance shaft through 2nd bearing. Keep advancing until large pulley is about length of motor shaft away from gearbox.
Add 2GT belt and motor drive pulley.
Advance shaft further while also slipping motor drive pulley onto motor shaft.
Position shaft to leave just a narrow space between spacer front of gearbox.
- Use shaft locks to to secure position of shaft. Each lock is to be placed against its corresponding bearing and set screwed in place. Once both locks are engaged, the shaft and bearings will no longer be able to slide in/out.
Continue with General Build
Secure pulley set screw against shaft flat.
Secure motor in position to take up belt slack, but not "stretched" tight.
Slide Bondtech drive gear onto shaft. Align its filament gripping teeth centered with rear surface of motor plate.
Cut two upper motor plate M3 bolts to proper lengths to not extend into belt path. Too short and they won't engage with embedded nuts. Too long and the ones on the left interfere with belt path. Bolt and shaft lengths are documented later in these instructions.
Attach motor plate to main extruder body. Motor plate assembly slides straight in. Alignment is easier if all mounting bolts are backed out flush with extruder body front surface.
- Recheck alignment of Bondtech is centered on plane between motor plate and main extruder body. Filament path is centered in that plane.
The below are best estimates for custom motor plate bolt lengths for flush fit.
Please check actual fit before grinding bolts to length.
BS MK3S geared upgrade, retrograde
upper right bolt 28 mm
upper left bolt 24 mm
Cut upper left motor plate mounting bolt so it does not extend from extruder, through motor plate, and impinge upon body of motor. This is vital to check before installing motor.
This upgraded requires plugging in motor cable flipped 180 degrees at EINSY to reverse motor direction.