Nextruder Filament Sensor Magnet Mod

After completing the side filament sensor magnet mod, originally by @tg73, on my Prusa XL, it lead to my remix of @INVESTIGATE' s remix of that mod, I still experienced issues getting filament past the Nextruder's filament sensor.

https://www.printables.com/model/1262215-prusa-xl-side-filament-mod-for-5mm-od3mm-id-ptfe-t

Flexible filaments were especially troublesome.

Just like the side filament sensor, the Nextruder filament sensor uses a steel spring. This spring applies way more tension on the ball bearing that is used to trigger a hall effect sensor.

It was surprising to see Prusa's departure from the solution that was implemented in the MK3S and carried into the MK3S+. That is, to use the opposing poles of magnets to act as a spring.

Inspired by @tg73's work on the side filament sensor mod, I went looking to see if anyone had a similar mod for the Nextruder. Alas, none was found, so I set about doing it myself.

As it turns out, the mod is so simple (although a little fiddly) to do; it beggars belief that this is not how they come standard from the factory?! We simply replace the spring with two more D3x3 neodymium magnets.

I've made a quick video to demonstrate the difference in the force required to load 82A TPU.

I can load 82A through the side sensor and all the way into the extruder without having to disconnect the Bowden tube now.
 

If you have done the side filament sensor mod on your XL, you will likely have a bunch of the little magnets left over. Now you can put them to good use. :)

If you haven't done the mod, then knock them both out in one go.

WARNING!

Do not attempt this mod if you are not comfortable with potentially having to reassemble the planetary gearbox on the Nextruder. You will only have to do this if you mess up majorly. I strongly suggest you print the pg-assembly-adapter before attempting this mod just in case you need it.

And then following the planetary gearbox assembly guide:

https://help.prusa3d.com/guide/5-nextruder-assembly_434014#465212
 

BOM

• 2 x D3x3 N52 neodymium magnets per tool head. Just get the 50 pack.
  https://www.aliexpress.com/item/1005007169304340.html
• IPIND_3D Maker Coin   I needed a model for download to be able to post these instructions. Please download the file and give us a LIKE. ;)
• Prusa planetary gearbox assembly adapter )pg-assembly-adapter)
  https://www.printables.com/model/451500-mk4smk39s-printable-parts/files
  
  
   

That's it!

Disassembly

You can do this without removing the tools heads from the printer. On and XL you can sit it on on a filament box on the bed. Alternately, you can remove it completely to work on it on a workbench. Regardless, you want a means to try and keep the tool head upright. It will make reassembly a lot easier as the extruder motor will be ‘floating’ one you remove the gearbox.

1. Ensure the printer is powered off.
2. Lift a tool head off a dock.
3. Open the Dwarf board cover and the filament tensioner.
   
4. Completely undo the 3 bolts on the gearbox, but do not remove them.
   
5. Carefully jiggle the gearbox off in one piece. You want to keep all 3 parts sandwiched together. If you don't, you will need to go through the planetary gearbox assembly guide with the tool to put it back together properly.
   
6. Place the gearbox face down on a flat surface and be careful not to lose the shim. Now is also a good time to look for any debris in the extruder body and filament path.
   
7. Gently remove the filament sensor assembly from the tool. The needle nose pliers that came with the printer work great.
      
8. Remove the ball/magnet/sprint assembly from the housing. It will just tip out.
   
9. Remove the spring and then add two of the D3x3 magnets. Don't remove the magnet from the ball bearing due to how the filament sensor logic in the printer appears to work. I'll explain this in the troubleshooting section.
   
10.  Take the last magnet off (the far right one in the pic above) and insert it into the sensor housing. The end going into the housing should be the end that was in contact with the magnet ahead of it. Look down the hole in the housing to make sure the magnet is oriented correctly. There is enough room in the hole that is can fall on it's side.
      
11. Then put the ball bearing/magnets into the housing as well and hold it in a ‘pinch’ fashion with your thumb holding the ball bearing in. If you did these steps right, you should feel the resistance of the magnets pushing away from each other.
    
12. Carefully insert the housing back into the tool head ensuring it ends up flush.
13. Grab the shim off the gearbox and put it on the extruder shaft.
    
14. Carefully jiggle the gearbox back onto the extruder shaft. You want to get it on far enough that you can start doing up the bolts.
    
    If the extruder motor gets pushed back, the bolts are not long enough to take up and pull it back. If there is a big gap in the area shown in this pic, you will need to stop and close it back up. You can do this by holding the tool in one hand and pulling the extruder shaft with the other.
    
    If you are having trouble and have a longer M3 bolt on hand, you can remove the top right bolt from the gearbox and insert the long bolt. Then you can put the gearbox back on the shaft and start doing up the long bolt to pull the extruder motor in enough to get the other two bolts started, then you can remove it and put the original bolt back in and tighten all the bolts up.
        
        
15. Test feeding a piece of filament through the sensor to make sure the path is clear.
    
16. That's it. Rinse and repeat if you have other tool heads.
    
    You can power the printer back on. If you have an XL, you will need to run the filament sensor calibration routine. On the LCD go to Control > Calibrations and Tests > 9 Filament Sensor Calibration
     

Troubleshooting

I did experience and issue with the filament sensor calibration on one of my 5 tool heads. Get the printer to pick up the tool head you're having issues with, disconnect the Bowden tube, on the LCD screen, go to Info > Sensor Info and scroll down to Tool Filament Sensor and look at the value. Insert a piece of filament into the extruder and see whether the value rises or falls.

It would appear that the firmware is looking for a significant reduction in the value to recognise that filament is loaded. It does not appear to just be looking for a significant change either up or down.

If you accidentally removed the magnet from the ball bearing when removing the spring (this is what I did on the tool I had the issue with), you may have put it back with the polarity in the wrong direction. This causes a rise in the filament sensor value instead of a drop. You will need to pull the housing back out and flip all the magnets over.

If you see no change, then the magnets have possibly been inserted with the polarity all the same way, so the two attached to the ball bearing are not opposing the single magnet in the housing or the first magnet you put in the housing fell on it's side. Again, you'll need to pull the housing out and redo those steps.

THE END

As with all my designs, there are no affiliate links, etc. I get nothing out of them other than the satisfaction of helping the community and some Prusameters if you are kind enough to give my design a LIKE. :)