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Bondtech Prusa 3 Extruder - FDM Printable - MK3 \ MK2 BMG

by bondy007 Jan 10, 2019
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Hi there, how do you place supports? There are some nasty overhangs in the object itself in the extruder body? IOn some spaces you don't want them, so what is your recommendation? Thx in advance.

I have printed this and I have some suggestions:

When the filament heater is installed, the locking ring on the PTFE tube in compressed resulting in unlocking. Is that OK? Could the recess for the locking ring be enlarged so that it does not unlock?

The connector for the Pinda probe prints as a separate very small piece until the end of the print when it is connected by bridging filament to the main body. The small footprint of the connector tends to break off from the bed before the print is completed. If you could add a 1mm tab connecting the main body with the Pinda connector at the first 2 layers of printing (about 0.4mm) this would help stabilize the delicate part until the print is finished. This small tab could be easily cut off when the print is finished.

will this work with the Bear Upgrade parts?

I realy like this design. Only one thing I do not understand:

The reason mk3 users mostly want a Bondtech BMG is issue 602 inconstitant extrusion which can appear after some time.
The exact reason for 602 is not known, but as users found ot it can be solved with a geared extruder.
Original stepper motor with Bondtech BMG means about 3-times the torque and 3-time the full step resolution. Both aspects obviosly help if the filament flow is not constant between full steps.

So I do not understand why to use an pencake stepper motor.

Normaly you use a geared extruder like Titan or Bondtech BMG to get the same torque with a light stepper to increase speed without getting ringing. This makes sense, if the head is moved in x and y direction like on Ultimaker-style printers.
But to get a constant flow of filament, you should use same max acceleration in x and y. So on an i3-style printer the normal reason to get a geared drive does not apply - the print bed limits max. accelleration and speed anyway. Only if you print an object ther ringing in y does not matter but in x it does, you could increase speed becouse ofthe pancake. A very special case.
So normaly no reason to reduce weight of the print head on an mk3.

Another reason for using geared extruders: Yor printer is not constructed für the weight of an non geared extruder. Like on an ender 3 with only one z-leadsrew a heavy direct extruder may cause a momentum wearing out the v-slot-wheels faster.
But the mk3 is constructed to take the weight of a normal extruder motor.

The reason to use BMG on a mk3 ist to get higher full-step resolution and more toque. So a pencake is contraproductive. You only get higher fullstep but not higher torque. So why changing to a pencake stepper?

Slim stepper motor is about 130-150 grams while 40mm stepper motor is 380 grams + internal rotor weight inside motor is also lighter in the slim steppers.
These 2 will reduce ringing in the X axis movement and will show less surface issues when there is fast rotation direction changing in the extruder (fast retractions etc).

Pancakes stepper motor has less torque but in 1:3 gears there torque is enough and u gain higher print quality with more resolution, less ringing on X movements and less surface marks in retractions and fast extruder motors moves.

This extruder design will not help the 602 (AKA inconsistent extrusion) but it will help on surface uniform lines .
I personally seen many problems with 0.9 degree motors in the Einsy 8bit boards so I recommend sticking with 1.8 degree pancake steppers as Bondtech selling with there kits.

Thanks for your answer.

Yes, ringing on x-axis become less on same accelleration if you reduce weight, that I understand. But a print with less ringing in x but much in y normaly look bad anyway.

I think its wrong to say less mass => less ringing. Ringing depends on the mass, the acceleration and printing speed (and of course the mechanics...). Normaly you choose the acelleration and printing speed to get a god result. So less mass means more printing speed with same quality as before.

In a cube design (like ultimaker) it makes perfect sense (so they are most bowden, but if you need direct extruder you need a geared one to add as low mass as possible). If you reduce weight of the head you reduce ringing (or allow more acceleration/speed with same ringing) both in x and y, so you can print faster with same quality.

But there is an other aspect: Molten plastic do not respont instantly to extruder acting. This is not only a problem at begin and end of printing a polygon (there linear advance is helpfull), but on every change in filament flow. So mostly the flow should be constant while printing. If you print a polygon, different speed/acceleration in x and y would lead to significant more problems in the "perfect" flow of filament.
So nearly all printsetting I have seen use same acceleration and speed in both x and y.

So if you have optimiced acceleration and speed for your desired quality, you can not increase it only in x because you move less mass there. Same values in both axis... If you increase acceleration and speed you only will get same quality in x, but massive ringing in y.
So in an prusa-style printer reducing weight of the printhead has no effect, exept on objects there y-ringing does not matter.

The surface quality of a geared extruder has to do nothing with the weight, I think.

A stepper motor has the highest torque at full step. A microstep has much less torque. And mk3 uses trynamic drivers with 256 microstepping... Microsteps reduce noice, making the motors movement "smother". But you can´t hold exact positions in a microstep. You can not be sure on microstep 64 you have moved a quarter of a full-step. You do not hear clicking like skipping a full step, as it is no skipping at all. Its delayed action, accumulating the force. Maybe on microstep 96 the force got big enough to get to to the coresponding position. Maybe on microstep 102. Or at 65...

So while the amount of plastic extruded between two full steps is correct, between theese full step it may not be a constant flow. This is exactly issue #602. Inconstistant extrusion. With less microstepping (32 or even 16) much more regulary the same microsteps fail. So you see salomon skin / moiree (You might can call it "constant inconsistantcy". It will be increased by not properly handled self induction of a motor on cheap drivers (there the TL-smoother help) or decreased by using a bowden system (it smooths out the effect acting as a spring. I think thats the reason chineese companies mostly use bowden systems, so the can use cheap drivers...))

By using a geared extruder you tripple the resolution of full steps. So you have two effects:

  1. You are 3 times as often on a certain printed legth at the correct position. So inconstancys reduced to a third. As the molten plastic in the heatzone is elastic and smooth out a bit, this may be enough to solve the problem at all.
  2. The stepper rotates 3 times as fast, so the rotating mass inside the stepper has 3 times the momentum. So the friction for example in the heatbreak is 3 times easier to overcome (thanks for that inspiration - I had not thought about that)

But all this has nothing to do with the weight of the stepper motor - exept more weight of the inner part of the stepper means even more momentum and more constant rotation.

The aspect of slower speed/direction changes is interesting; I have`nt thought about that before. You may be right, that can improve the quality at beginning and end of a line.

Thanks for the information with the 0.9° steppers at the Einsy. Do you know ift its caused by the 8bit Boards in special or due to the driver used (with 256 microsteps)? I have a (normal sized) 0.9° stepper, but also other printers with Duet Wifi. If its a problem using 8 bit, I can use the stepper. If its a problem with 256 microstepping, I can not...

The reason I asked why to use a pencake stepper is Teaching Techs new video. He testet the bondtech mk3 upgrade and saw only a very slight improvement. The comments there are very angry "105$ for nearly no effect".
For myself I changed my extruder in november 2018 to https://www.thingiverse.com/thing:3165551. That solved the issue 602. Before I had it, after changing its gone. Others report the same.

As the Bondtech mk3 upgrade is simply a "merged" version, it should solve the problem also. Only other real difference: in http://www.thingiverse.com/thing:3165551 most users seem to use their original motor(as there is a modified fanduct an fanholder to keep it).

So in my opinion using a pencake would be less good than using the original stepper. But you make your point with retractions. I haven´t seen any problems using http://www.thingiverse.com/thing:3165551 , but I have´nt look for them.

The only reason I want to change form http://www.thingiverse.com/thing:3165551 to this design: thing 3165551 have a small gap between filament sensor and Bondtech BMG. A small tube is used. In 1 of 10 times inserting new filament you do not hit this tube on the first try (but on the second). So no problem in normal operation - but of course you can not use an MMU2. My arrived in October and I haven´t assembled it now...
As others reported this thing works with mmu2, I hope it will work with normal stepper and solve the 602 issue as good as http://www.thingiverse.com/thing:3165551 . Because else it would be the dissision between 602 or MMU...

One aditional question: Prusa upgraded coldend cooling to dyson principle. This Bondtech design seems to base on the old design without dyson. Has this any specific reason?

P.S.: I am not a native english speaker (as you may already have discovered) - quite the oposite, my grades in school for english were very bad. So if any sentence is not understandable or may even seem rude, this is not intended an I appologice. To avoid any missunderstanding, please tell me, if something is missleading, I will correct it then.

Bondtech BMG Extruder Carriege for Prusa i3 MK3
by drodfro

Is it possible to add an indirect filament sensor in this design, like this one https://www.thingiverse.com/thing:3223513 ?

Prusa Mk3 indirect filament sensor for all filaments (transparent, black, shiny, whatever).
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I will try work on that this week hopefully will finish in few days

Prusa Bear-BMG fusion (indirect filament sensor) for MK3 / MK2.5
Comments deleted.

Is there a fan mount to fit larger motor?

Unfortunately there is no room for larger motor , I will need to expand the fan duct for it to fit and in that part cooling fan set up will be poor performance .
Slim motor is cheap and will cost u less then 10$
I use this one:

You can use it with the original motor. You can use https://www.thingiverse.com/thing:3347150 (fan_kepping_prusa_motor-DissEd_v2.stl) as part cooling fan. You have also to modify the fan mount; I quick & dirty did it (see attachment).
You can see it in the attached image, the yellow part is the modifyed fan holder, the orange part is from http://www.thingiverse.com/thing:3347150 .

If you use original stepper, you have to change firmware or wireing to change steppers direction. I choosed to reverse the wires.

As you can see in the picture, I used pet-g cf20 (printament) for the housing. It looks and feels nearly exact like the SLS prints from bondtech (to compare: I used the door hinge from an original bondtech bmg), but of course some postprocessing on overhangs is needed. But anyway, printing the head is only about 6€ of material, much cheaper as

Bondtech Prusa 3 Extruder - FDM Printable - MK3 \ MK2 BMG

The Bearings fits much better in the new design (01.13.19), Thanks

You are welcome :)

what were the last revisions?

After some feedbacks I changed the bearings holes to 8.12 instead of 8.08 or 8.2mm .

With 8.08 its too tight for FDM and 8.2 was little loose .

Is there a forum thread on this design? Checked FB Prusa Community little bit there but not alot. Have you printed with this design yet?

I printed the parts & need to find some time to install it on my printer - This design was tested by Bondtech its there official l design they selling.
U can keep tracking on FB Prusa Community feedbacks - this design is totally new so its need some printing time so ppl can really test it.

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i had a question if someone has already printed this parts. can you please measure the size of the holes of the bearings? It seems for me that they are not 8mm. is it possible?

The bearing hole size is 8.08mm for 8mm bearing

okay that is strange because everything else seems to fit but the bearing hole on my print with petg is ~7.48mm. i'll later check my extrusion multiplier if there is something wrong. can you post me the size of the hole on top of this that i've something to compare.

thank you

You can carefully drill it with 8mm drill bit.

I modified it to 8.2 , my bearing was hard to insert as well.

Edit: Final holes sizing changed to 8.12mm

still not well printable, you have to increase the thickness in some points

Sorry I dont understand , can you explain what do you mean?

In some parts is better to add some thickness on the wall, check the photo i attached. Is not very important but if you want to remix better you can do some modifications for helping print better.


You should changed the perimeters width to 0.4 in this print (BTW u should check every print if the perimeters width is ok)

Got it. Thanks

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