This is a bowden tube. Its job is to guide the filament from the
extruder into the hotend so that extruder can drive the filament over here, but the
actual pushing force happens over here at the hotend. If this printer didn’t have one, the filament
would get pushed just the same, but the driving force wouldn’t be able to push it into the
hotend. Makes sense, right? But not every 3D printer is a bowden system,
in fact, this guy isn’t, but I’m going to argue it’s still going to profit from
adding a bowden tube - a “reverse bowden” of sorts. So follow along as we try and figure out how
much the wobbly toolhead is going to deflect from filament pulling up, whether we can fix
these ripples on my prints and how you can add a reverse bowden to your own printer properly. Right after a message from today’s sponsor,
Anker! Anker has been my choice for powerbanks, USB
cables and USB power supplies for years now and frankly, they’ve been fantastic! These small power banks are good for about
one full phone recharge. This guy is good for about 120 charges - and
does a lot more than just recharge your phone. The Anker 757 PowerHouse essentially replaces
a 2HP gasoline generator with its 1500W of AC output - 230V here, 120V for the US. It’s also got USB and power delivery outputs
as well as a 12V DC car output, and with 1.2kWh of capacity, it’s enough to run a typical
3D printer off-grid for 6-10 hours, or even power a table or a chop saw for well over
an hour of non-stop cutting. But what makes this really cool is the way
you recharge it - you can either plug it into the wall and charge it to 80% in just one
hour, and it doesn’t actually slow down much on its way to 100%, or you can directly
connect a solar panel to the PowerHouse and use the built-in 30V, 10A MPPT tracker. So it’s 3 use cases in one - replacing a
gasoline generator, running a full off-grid setup, or simply as an uninterruptible power
supply that has a passthrough mode and under 20ms of switchover time. They’ve also got smaller models that are
perfect for powering a NAS for hours at a time or for bridging that 30-minute power
hiccup on a 30-hour print. The Anker PowerHouse comes with a five-year
warranty and uses safe and long-lasting LFP batteries that are rated for 3000 full charge
cycles until they start losing a noticeable amount of capacity. I really like how versatile the Anker 757
PowerHouse is and if you think you could make use of it in whatever way fits your use case,
you can check it out at the links below. To understand what’s going on here, we need
to start with a bit of physics - every action has an equal and opposite reaction. So the forces in a Bowden setup look something
like this: At the extruder, the filament gets pushed into the tube by the extruder, and
that pushing force is equal to the resistance the hotend puts up. I’m just ignoring friction for now and assuming
Pi is equal to 4, as one does. Now, the force on the filament has its equal
and opposite reaction, but for the toolhead and the extruder, we still only have a single
force - basically like pushing down filament into the hotend by hand. For that force to find an opposite reaction,
it would need to travel all the way down through the printer’s feet, applying forces to everything
in that path, and since those parts aren’t infinitely rigid, they’re going to flex. Not good. So to more directly transfer those forces,
we add a bowden tube. Since the tube is fixed on both ends, it can
apply a counter-force both on the side of the hotend and on the side of the extruder,
so for each of the systems of just the toolhead, just the tube or just the extruder, as well
as the entire printer as a unit, all the forces cancel each other out nicely and we don’t
have to worry about any forces having to travel through the motion system and flexing it out
of shape. Which, of course, would be somewhat bad for
print quality. But, with a direct-drive printer, the situation
is a little different, as all those parts that generate forces of some kind are now
one single unit contained in a plastic or metal shell, so the force of the filament
pushing into the hotend is immediately transferred back to the extruder through the toolhead
body. But there still is an externality, which we
ignored before - the filament itself on its way from the spool. I mean, you’ve surely watched the filament
drag on the toolhead as it’s getting pulled in or as the toolhead moves around. This is a non-zero force on the toolhead that
needs to be transferred all the way through the frame up to the spool mount in this case. So to quantify how much of a difference that
drag force actually makes, I tried to measure its effects. It’s super hard to measure how much the
toolhead is flexing when it janks on the filament, because everything’s moving, but I can measure
pretty easily how much it moves from the drag the filament creates as it’s being pulled
in. Mag base, magic arm, and a genuine Miluloyo
test indicator, not to be confused at all with the Japanese brand Mitutoyo. I’m putting the feeler right against the
hotend, aaaand. Nothing. There’s just no movement at all. Which is surprising, considering that you
can flop the Prusa’s toolhead around quite easily by hand and even just slightly tugging
on the loaded filament visibly changes the position of the nozzle, but it seems that
the filament actually unspools easily enough to not cause issues here. And a quick test print confirms this: The
pattern I thought the reverse bowden would get rid off is still there, just as visible
or invisible as before, depending on the lighting. Thankfully, adding a reverse bowden isn’t
hard at all, it’s not a permanent mod and it comes with some other advantages that allow
you to use other spool positions than the one on top of the machine in the case of the
Prusa. So all I needed to add a reverse bowden to
my machine are these two tiny printed parts and a piece of 4mm OD, 2mm ID Teflon tube,
which is the same as the Bowden tube that’s commonly used for 1.75mm filament. The first piece snaps over the extruder cover,
the second one screws onto one of the Z-rod top clamp screws with a longer M3 screw and
a nut, and the (reverse) Bowden tube just runs between those two. Mine is maybe a little too long, even. Overall, a reverse bowden is much more forgiving
in how you install it than a “real” Bowden - regular Bowden printers have the Teflon
tube under constant tension as they push filament, so it always wants to pull out of either end
it’s mounted to. But a reverse bowden tube instead is under
slight compression, so it will naturally want to push itself further into any loosely fitting
couplers you’re using. There are two use cases that this enables:
First, printing out of a drybox. This is an Eibos box they sent me a while
ago and I’ve been using it for drying and printing PETG and other moisture sensitive
materials in a controlled environment, and because the filament exit for that is either
on top or on the side, feeding that into a moving extruder just calls for trouble. When the spool is off to the side and the
filament tightens up at the wrong time, it’s super easy to lose steps when the X-axis tries
to do a quick move and suddenly has the whole filament spool dragging on it. You can work around this by routing the filament
over the top of the printer, but then it’s going to be pushing on the frame, which, again,
could cause some deflection. So either use this sort of integrated reverse
bowden and then add another bit of tube from the fixed point to the drybox or go straight
from the extruder to the drybox with a single piece of tube. Protip: You still want both ends of the reverse
bowden to be fixed, so with holes like these on the Eibos drybox where the teflon tube
could just easily slide through, I just add a large M3 washer over the filament that blocks
the tube from going in. These DIN 9021 washers are a bit thicker than
fender washers, but fender washers would work just as well. As an added bonus, the reverse bowden system
also mostly covers the filament outside of the drybox and the extruder, which is great
for filaments like Nylon, which are extremely good at absorbing humidity from the ambient
air, so it greatly helps in keeping those dry during and in between prints. For other setups, like printing in a full
enclosure, this is the Boxomo one, there’s actually almost no way around using a reverse
bowden to route your filament without it getting snagged - even when you use the filament mounts
inside the enclosure. So even though the reverse Bowden didn’t
manage to directly improve print quality, it’s still a super useful tool to have at
your disposal when you need it and it’s going to make your machine more reliable in
those situations. Huge thank you to my Patrons and YouTube members
for enabling me to create these videos, if you want to join in too, we do regular Q&A
hangouts there, and every supporter helps keep this channel running, so thank you all. Link here or in the description below. Thanks everyone for watching, keep on making
and I’ll see you in the next one!
