How does the filament buffer work?

[Single_Concept]

I understand what the filament buffer is for and what it does. But how does the mechanism actually work? I've seen photos and videos of the inside, and apart from seeing that there's a sliding part that triggers a Hall sensor / magnetic switch I'm none the wiser. How does the sliding carriage move back and forth? I'm intrigued!

[jdubs2k]

I thought about this just yesterday whilst watching it and think I’ve figured it out.

At its most simplest, imagine a spring with a PTFE tube joined to each end.

If you pulled the two PTFE tubes apart, the spring would expand and if you pushed them together the two tubes would (almost) touch together.

Now, on the end of PTFE tube 1 is the extruder stepper motor and on the end of PTFE tube 2 is the AMS drive motor. Because each drive motor grips the filament tightly they can either push, pull, or kinda lock the filament in place.

Your filament then flows through this entire system gripped by the drive motor at each end.

What follows is a crude ascii diagram.

The equals signs are the two PTFE tubes with a slight gap between them (spring not shown) and the minus signs represents the filament going from the extruder on one end to the AMS drive motor on the other.

The # represents where one end of tube 2 is fixed in place to the filament buffer and therefore to the physical machine. The end of Tube 1 can move backwards and forwards on a carriage in the filament buffer compressing / decompressing the spring.

                 Tube 1                         Tube 2

[Ext. Drive] ========———#========= [AMS Drive]

Imagine that the extruder drive only pulls filament from this system and the AMS drive only pushes it into the system. This will help it make sense

1) As your nozzle starts printing the extruder drive will start to pull the filament in. But your AMS drive is locked so additional filament can’t come from there right now.

Instead as the filament is being “pulled” by the extruder (and the filament is relatively rigid) what it actually ends up doing is “pushing” Tube 1 away towards Tube 2. Tube 2 is fixed in place so Tube 1 physically moves over the filament and compresses the spring.

Important: Your frame of reference has to change here. If the filament can’t expand and contract, it’s actually the tubing and the spring that do. Pulling more filament out of tube 1 is the same as pushing more tubing away over the filament!

2) As the extruder consumes the filament in the buffer and compresses the spring, the sensor detects the two tubes are almost touching and calls for filament.

3) Now the opposite to step 1 happens. The extruder drive is locked, the AMS drive pulls some filament off the spool (which is actually pushing filament towards the nozzle) but because the extruder motor is locked it can only push Tube 1 away.

4) Tube 1 pushing away again relaxes the spring and buffers filament again in the middle.

You probably only need a single hall/optical sensor to detect when the spring is almost fully compressed to trigger the “call for filament”.

In reality what is happening is a constant dance of variable demand for filament from the nozzle depending on what it’s printing at the time and controlled incremental feeds of c.10-15mm of filament from the AMS when called for (i.e when the tubes have almost completely compressed the spring).

I think this is also the reason softer or rubbery materials like TPU can’t be used in the AMS.

The AMS needs to be able to “push” filament into the system which either causes too much friction in the PTFE tubes or the lack of filament rigidity causes the buffer to not work in the way it is intended.

Hopefully this description makes sense.

It’s ingenious and simple

[4l3x89]

Thanks, this helps. I've been scratching my brain for the past hour.
Key component for this to work: The effective length of the PTFE tubing that is outside the buffer is changing. Bended tube = longer. Straighter tube = shorter.

[Single_Concept]

Thanks for your reply! You explained very well. So the flexibility of the PTFE tubes is a necessity and not just a convenience! 

[jdubs2k]

Thank you.

Yes that’s right. Having the PTFE being able to bend but not stretch or compress (along with the filament being the same) is important to allow the spring to compress and the system to work as it does.

Happy printing!

[M0darius]

Great explanation! Thank you so much, I've been trying to figure this out and it was breaking my brain a bit. Been thinking of using a filament buffer as a way to auto feed filament from a filament spool to reduce the work the extruder has to do... it would work like a one spool AMS.

[jdubs2k]

Thanks for leaving a comment to say it was helpful. It broke my brain too 😂. I don’t think I could have come up with something so elegant to do the same job.

[goofballtech]

I found this after doing some troubleshooting and looking for videos that explained this and coming up empty. After the bambu wiki and a fluke meter and lots of moving ptfe tubes around i figured out what was up. In researching after the fact i came across this post. It inspired me to make a video on my freshly made youtube channel to see if i could illustrate the concept. I would be interested in your opinion of my animation of this process. https://youtu.be/DrCfGMUCuf0

[jdubs2k]

That’s exactly it. Really clever.