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BS6RHD E3D fan shroud, hot fins Polymaker PC-Max plated.3mf
BNBSX Main Components B08i5 Polymaker PC-Max plated.3mf
BNBSX Extruder PETG Plated B08 i5.3mf
BS6RHD E3D standard fan shroud with support.stl

This BNBSX "Short Ears" extruder for MK3S sensor is the edition needed for upgrading Prusa MK3S and MMU2S printers.

For MK3 printers, see instead short ears for MK3 printers. https://www.thingiverse.com/thing:3606189

Build guide is being written, but not yet complete. Complex and Bunny Science specific portions of BNBSX assembly are already in guide. Remaining, generic steps should be easy for experienced builders.

https://bunnyscience.dozuki.com/Guide/Bunny+and+Bear+(BNBSX)+MKS3+Geared+Extruder+Assembly/2?lang=en

Advantages of this extruder over stock MK3S R4

• Geared 54:16 for more consistent extrusion. (54:16)
• Indirect drive isolates filament from extruder motor heat
• Corrected filament path/Bontech alignment (as in Bear extruder)
• Lower total mass by about 80 gm
• Extreme rearward motor position reduces moment arm
• Continuous PTFE path deep into extruder for MMU2S and Palette 2
• Two, swappable filament sensor systems (armature for single filament, internal lever for MMU2S)
• Internal filament sensor lever eliminates Prusa filament sensor chimney on MMU2S
• Internal filament sensor lever allows full opening of idler door
• Single screw tuning of MMU2S filament sensing
• New MK3S sensor armature design eliminates steel ball rattling
• Provisioned for LED filament sensor indicator mod
• Only one magnet needed for filament sensor
• Easy motor plate and hot end removal for service
• Separately printable hot fins for heat resistance
• Hot fins tightly encircle hot end.
• Replace PTFE, heater cartridge or filament sensor without full tear down.
• Fits E3DV6, Volcano, or Mosquito hot ends
• Air plenum optimum whether E3D or Mosquito installed

For comparison, the following are how far various extruders place motor from rear of extruder body.

MK3 R3 30.5 mm
MK3S R4 28 mm
Bear 27 mm
BNB Retrograde 23 mm
BNB Short Ears Edition 17.5 mm (with Bunny Science Universal x-carriage, 19 mm otherwise)

Because extruder motor is pushed so far back, this must be mounted on Short Ears version of Bear x-axis to clear the motor x-end. Normal Bear x-axis mounts will impinge against motor.

You can also mount on Prusa x-axis using the BNB Universal X-carriage, but the Prusa axis is much more difficult to tension properly.

Videos of BNBSX in operation
https://youtu.be/z7LmkS7mqpA
https://youtu.be/9z_EihvkaD8

Filament armature detecting above Bondtechs. Optional LED mod indicates when filament is detected by either armature or MMU2S internal lever
https://youtu.be/ReESJ-j2wwU

Internal lever MMU2S sensor (M7 revision) undergoing bench test. I am manually turning shaft to move filament in/out of Bondtechs.
https://www.youtube.com/watch?v=YfuO-00C6_s

Essential to use the BNB Universal X-carriage when doing this upgrade. https://www.thingiverse.com/thing:3610593. BNB Universal X-carriage has cable management features to reduce risk of hot end cable damage during XYZ calibration

That carriage also lets you remove the motor plate WITHOUT taking off the back of the extruder.

**** Double check lead dress of hot end wires with x-carriage max left and print bed max forward BEFORE XYZ calibration. Verify cables do not contact print bed cable cover.

Build differences from the regular Bunny and Bear Geared Extruder...

• Short Ears X-Ends must be installed to allow motor to clear
• Compact shaft locks required
• Bunny and Bear Short Ears Shaft is different
  Length 55 mm for 2 bearings
  Pulley Flat 0 to 9 mm
  Bondtech Flat 42 mm to end

If you do not already have a hot-end collet clip, print one from Bear extruder (https://www.thingiverse.com/thing:3226689). That STL is included here for your convenience.

Motor plate prints with intrinsic supports that must be removed. Also, trim away overhanging layer of filament that covers inner bearing mount. Time spent neatening up the layer printed as a overhang saves grief later.

Some support scarring will remain, but is largely hidden by installed extruder cover.

Build with two shaft bearings. No 3rd bearing is used for this extruder

Materials needed...

5 mm diameter shaft rod
Length 55 mm
Pulley Flat 0 to 9 mm
Bondtech Flat 42 mm to end
see https://www.thingiverse.com/thing:3595205 for shaft cutting guide and instructions

5 of Bearings - 3x6x2.5 mm Miniature Steel Bearings MR63ZZ
https://www.amazon.com/gp/product/B00ZHSQX42

M2 x 16 mm socket end cap screw, in black alloy to connect MMU2S internal lever to its optical interruptor arm. THIS SCREW CANNOT BE STAINLESS STEEL. MUST BE STRONGLY ATTRACTED BY MAGNET )
https://www.amazon.com/gp/product/B07MN9X2KN

Remainder of screws are hex head, stainless steel

I suggest these sets of multiple sizes as you will may sizes of M3 from short to long.
Not the strongest hardware, but adequate for plastic to plastic work.
https://www.amazon.com/gp/product/B071KBVZVV
https://www.amazon.com/gp/product/B07C9F2347

MK3S IR filament sensor kit

5 x M2 screws x 12 mm for hot fins and lower x-carriage cover
1 x M2 screw x 6 mm for MK3S IR sensor (Screw supplied with MK3S sensor is too long)
1 x M2 screw x 8 mm for optical path insert
2 x M3 screws x 8 mm for optical armature

2 x 1050ZZ bearings
1 x continuous loop 140-2GT-6 Timing Belt
4 x M3 square nuts
3 x M3 set screws
M3 hex nuts & bolts (various lengths from 12 to 30 mm long. Some must be cut to custom length during assembly)
M3 washers
M3 Nylock nuts

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. Must plug into EINSY upside down to reverse motor direction.

--
You must also set microstepping and e-steps. Depending on which type of motor you have installed (usually 1.8 degree) the e-steps differ.

geared extruder, 1.8 degree motor (usual)
M350 E16
M92 E473
M500

geared extruder, 0.9 degree motor
M350 E8
M92 E473
M500

Note that M92 command must be issued AFTER M350 because M350 may alter prior M92 value.

Follow with power off/on and M503 to verify settings are correct.

--
Pulley gearing originally inspired by Skelestruder

Reversible motor x-end option inspired by Cristian2281 https://www.thingiverse.com/make:667387

--
Issues:
Check motor x-end top clamp bolt. Depending on build tolerances, your motor may come into slight contact with the head of that bolt. If that is happening, file off a bit of the bolt head or change to button cap bolt (with smaller head profile)

Upper bolt tab of print fan likely to contact drive belt. Use x-acto knife to shave a bit of plastic from fan's bolt tab to clear belt. Takes just a few minutes to perfrom.

--
Recent Change Log

B03M6 - PINDA mounting clamp adjusted to better clear idler x-end.

B04M - Extruder main and extruder E3D pane modified to fit thicker Sunon 2510 fan on Mosquito

X-end idler modified to better clear older PINDA clamps.

B04M1 - Changed pulley to 54 teeth to reduce interference with parts cooling fan. This requires slightly different e-steps value (detailed above). 56 tooth pulley remains in files set because older gear plates may not have enough belt adjustment range to accommodate 54 tooth pulley.

Multiple 54 tooth pulleys with varying shaft hole diameters are supplied so users can select one that best works with their printer's extrusion characteristics. Best fit is one that just slips on shaft without loose wobble nor excessive tightness. Usually about 5.20 mm will work. Note that hole on flat side (printed on bottom) will likely be lower diameter than top due to slight elephant foot. Check fitment from top and carefully ream bottom to enable best fit without creating eccentricity

Motor plate BO4M1 has larger motor excursion range to allow 54 or 56 tooth pulley.

B05 - Motor plate gearbox prints with intrinsic supports instead of slicer generated supports. Simplifies post-print processing.

Added BSC RHD fan shroud.stl - a high volume fan shroud with front vent like that in BS4RHD fan shroud successful with Mosquito hot ends.

B06 - Motor plate reinforced to improve PETG resistance against warping under belt tension.
Hex pockets in motor plate and extruder main slightly enlarged to better accept nuts especially if printed in stiffer materials like polycarbonate.

Added bearing pressing aid to support washer against uneven motor plate surface during pressing of rear bearing.

Fixed internal void in hot end e3d hot fin

B07 - Bearing pockets modified for easier pressing.

Motor plate has pads at left end motor mounts to compensate for tilt of motor created by belt tension.

B08 - Extruder now has more supportive, longer tips to constrain filament path. Combined with PTFE tubing sculpted for tight constraint, the more constrained filament path improves flex filament handling.

Improved motor plate gearbox printability with more intrinsic supports. Critical geometries inside motor bay and gear pockets should print more cleanly. Overhangs are also shortened by intrinsic supports. Bearing pockets reduced in diameter by 0.2 mm because new prints are so clean.

Overhanging bolt pocket geometry now print with sacrificial membrane. This produces a nicer print result as a complete overhang surface. Drill through sacrificial membranes to open bolt holes.

B08i5 has minor shortening cable protection hook by 2 mm. Please check that noctua fan mounting screws do not impact Z-screw with this smaller hook. If so, change to smaller head screw for noctua fan.

Fan shroud has intrinsic support which improves print geometry.