BearExxa V2 New Beta Release (Patreon)

Finally time for a new beta release :)

New Features

• Add MMU2S, compatible with MMU2S and ERCF
• Improve wiring for filament sensor, Pinda and print fan cables
• Add protection for the print fan wires
• Nozzle fan duct knob screw is now standard (no more optional part)
• Nozzle fan duct has a better mounting system to avoid it getting angled
• Improve print gears cooling (useful for long PLA prints in enclosure)
• Tensioner is stiffer and the tension as been slightly adjusted
• No more E3D collet needed for the filament entry. Still possible to use it thanks to optional reverse bowden
• Improved reverse bowden optional parts, now support two ways of holding the PTFE
• Stiffer Pinda mount
• Improve several tolerances for easier assembly

What needs to be re-printed?

Unfortunately everything except the X ends need to be re-printed. There are too much modifications and improvements and parts are not compatible anymore.

For information, the next release will probably come with a custom motor and so will require to reprint a lot of the parts as well (new motor will change the gear distance and so the motor plate, extruder body and carriage size). This is why it is still a beta ;)

MMU2S / ERCF

In the optional parts you can now find all the parts to make the extruder compatible with MMU2S or ERCF. I don’t have a MMU2S unit nor ERCF so I couldn’t test it "in real use case" but I can confirm I could calibrate easily some wire of 1.5mm as well as flexible. Any feedback on this would be very welcome :)

Testing, lots of testing

Thanks to several precious feedback I have decided to make extended testing on several topics : motor, print gears, extrusion force, tensioner adjustment, etc. I have developed new test cases as well and improved my data and knowledge. In the following sections you can find some interesting points I have discovered during those tests.

I have shared few pictures of some tests (sorry I didn't save much images of those). The last one is a 1 hour print, yep just few lines :D

Motor

We have evaluated 3 steppers made by LDO (in addition to the Bondtech one) :

• Optimized 25mm pancake closer to the Bondtech extruder motor specifications. Works great but we think we can make a lighter/better steppers
• Custom 23mm pancake, lighter and more compact. While we can have higher extrusion force (motor torque and filament grip) than BearExxa V1, we found that we need a bit more safety margin.
• Nema 14 with 10 teeth gear coming from Orbiter V2 extruder (which makes a 5:1 gear ratio). While this motor is very well built and optimized we found that it still get too hot when doing long PLA prints (tested on Einsy with Prusa firmware).

In addition to all those tests we also found that the motor gear isn’t really rotating perfectly circular which might create some extrusion variations. The problem is due to the gear mounting system on the shaft and we don’t think it can be improved much. We have then decided to work on a custom stepper with a gear fixed on the shaft.

This is having other several advantages like weight, easier assembly and more room for the filament sensor. Moreover, we can also improve the gear ratio and, if everything goes as expected, the next version will get a bigger one. It will increase torque and makes the stepper rotating faster for smoother layers.

Print gears

I have made new test cases for the extrusion force and I could observe that, while using the same print gears, we can get higher force than the BearExxa V1. This is probably due to a stiffer shaft assembly, smoother rotation (improved gear distance, smoother bearings) and other smaller details of the design. With BearExxa V1 we did not observed any lack of extrusion force (when using realistic print settings), this means that going to higher print gear diameter does not make sense. It does confirm my choice of smaller Bondtech gears and save some weight and space.

When I was testing the stepper temperature on long prints (in 3 different scenarios), I was also measuring the temperature of the print gears (can you see the thermocouple in the picture number 10?). As those are in contact with the filament (as well as the air in the whole extruder cavity) they could preheat it and reduce print quality and reliability on long PLA print in enclosure. As we are using gearing and the big 50 plastic gear I was expecting that the temperature of the print gear to be quite low in comparison to the stepper but it was actually not the case. I found that there is only around 7°C of difference between the stepper and the print gear (with a variation of 0.5°C across all motors we have tested). So if the motor is at 60°C the print gears (and the internal cavity of the extruder) is at 53°C which is too close to PLA melting temperature. We could also validate that the hotend does not affect much the print gear temperature, it is mostly the stepper and the bed (in enclosure) that are responsible for this. After some design improvements to take advantage of the natural convection and extruder movements, I could reduce the print gear temperature by approx 1°C (not much but it is free).

Tensioner

I tried to find a reliable way to test the best pressure to apply on the idler print gear and unfortunately I couldn’t achieve it. I have reduce a tiny bit the pressure compared to the previous extruder judging my experience with different materials. If anyone has some inputs regarding this I am very interested.

Otherwise, some tolerances of the tensioner have been improved for a smoother rotation as well as stiffness, it has a slightly different shape than the previous one.

Source file

This release is not including the source file nor any open source license. I can’t use an open source license if I I restrict the files to my Patreons, it would be incompatible. Obviously when it will be in release candidate or final version, the source files will be available with open source license on my GitHub. All the current files have been made from scratch and are not forked from Prusa nor BearExxa V1 files with the exception of the X ends (those source files are available on my GitHub) so to my understanding it is all compliant. It would be too risky for my work to be shared with source file and no license at the moment. To be honest, I don’t know another way to give early access to my Patreons. I am happy to share sources individually in case it is needed and if only for few parts. Please don’t hesitate to write to me if you think this is wrong or not nice or if you have a better idea on how to manage this.