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Bunny Science MK3S Geared Extruder Upgrade

by BunnyScience Apr 28, 2019
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do you have any suggestion as to how to mount the part cooling fan with this setup? it seems to be lacking the mounting point

This replaces ONLY the motor plate and idler door.

You still use the rest of the stock extruder including the normal hot end cover (which has the cooling fan mount point)

I need some advice. I printed the parts in PETG with 4 perimeters and 20% gyroid infill but the motor mount is flexible enough that the motor can warp. The tension of the belt is pulling the motor towards the gear, causing the motor shaft and gear shaft to not be perfectly parallel. My belt is wearing down a lot with small pieces of rubber coming off and I think its because the 2 shafts aren't parallel.

The drive shaft is fine, it's held perfectly straight thanks to the 2 bearings. It's the motor mount that is warping.

I thought about using a stiffer material like carbon fiber PETG or PC, but first I want to see if you have any other suggestions. Thanks.

Yes, polycarb greatly reduces the motor plate warp potential. However, you really should not be at so high a belt tension that you are losing bits of belt. If actual rubber is coming off, please check that the belt isn't rubbing against anything.

Tension on this should be about 1/2 that on the x-axis. it really doesn't need much tension to work properly.

If you want a really definitive solution, the more advanced BNBSX design has a stiffer motor plate design.

Did you need to change microstepping on the MK3S for this extruder?

No - you don't need to change microstepping unless you are running special 0.9 degree motor or a modified microstepping firmware instead of Prusa's.

You DO need to change e-steps to match the gearing.
M92 E980

Could you post the STEP files? I want to try to modify the body to print in 2 parts and weld or screw together to reduce the amount of supports. Thanks.

STEP file for motor plate gearbox is in files. Look for..
Bunny Science MK3S retrograde motor plate gearbox.step

Bunnies did consider several approaches to breaking up the gearbox for screw together avoidance of supports. Abandoned that due to concerns about bearing alignment precision and adequate stiffness with limited plastic footprint on motor plate.

Support removal turned out quite doable and unified print piece yields very good alignment and rigidity. That's why bunnies keep using supports on the motor plate gearboxes.

Go ahead and have a play, though.

Is there a construction video online yet?
Would be nice to have one.

Unlikely to happen. This geared MK3S extruder is the most stripped down of the Bunny Science extruders. This is a quick and dirty, one and done. Just a single step above the bolt on gearbox.

Nearly all development work and advanced feature set additions are happening in the Short Ears extruders - particularly the BNBSX.

What are the specs for the M3 set screws? Min/max length? Thanks.

3 to 4 mm length. Longer may impinge gearbox legs during shaft rotation.

Thanks. Also will this work with regular Bear X-axis and/or with the short ears version? (Assuming the MK3S R4 extruder carriage supports Bear).

Cannot use Prusa MK3S carriage on Bear nor Short Ears axis. the belt path isn’t compatible.
If you want this on a Bear style axis use the BNB Universal x-carriage. That has dual belt paths so you can use either style axis

This should also work on the mk2.5s, doesn't it?

I don't know in specific. It will work anywhere a regular R4 extruder works.

If a regular Prusa MK3S R4 extruder will run on a mk2.5s, then the answer is yes.

[EDIT: Yes, it should work. The MK2.5S uses R4 extruder]

Sorry, No. That LDO motor yields delayed extrusion restarts post retractions. I tested two units of the LDO's - no joy.
The ONLY motor that has consistently worked well is the STEPPERONLINE Nema 17 Bipolar Stepper Motor 0.7A 13Ncm (18.4oz.in) 17HS10-0704S

I saw your long thread on the prusa forum. What motors do you recommend for x and y out of all of your searching. I want to get VFA to a minimum.

LDO 0.9º LDO-42STH40-1684MAC
Must have linearity correction set to 1.130 for this motor to outperform the Moon's

Moons 0.9 MS17HA2P4100
Leave linearity correction OFF for the motor.
Moons here because it is easier to get homing working correctly with the Moons on X.

You will also need my 0.9 degree motor firmware https://github.com/guykuo/Prusa-Firmware/tree/0.9-Degree-Stepper-Support
More instructions are on that page.

X-belt tension absolutely must be neither too tight nor too loose for homing to work. Once set correctly, it is reliable, but out of range and homing won't work.