Hello everyone, i’m Tom, and as a follow-up
to last week’s miscalibration video, today
i’ll show you how to use the Prusa calculator
as that has been requested, and in the process
of that, i’ll explain what the things you
are entering there actually mean.
So the Prusa calculator takes many of the
calculations that would go into setting up
a printer from scratch and conveniently does
them for you.
And that ranges from filament volume calculations
(hint: 1kg of PLA actually has quite a bit
less printing range than 1kg of ABS) over
steps per millimeter calculations, which is
the part we’ll be looking at today, for
both belt and leadscrew systems, so typically
XY as the belt axes and Z as the leadscrew
axis.
Then it goes over optimal layer heights to
hit full steps on you stepper motor - more
on that later - which can give you more consistent
layers if you’re not using an auto-paralleling
setup with a Z-probe.
And the last part is a calculation of the
actual speed an axis can reach for given travel
lengths and acceleration settings.
You’ll routinely see people proclaiming
that they are printing at insane speeds, but
in reality, their printer can’t actually
accelerate to those speeds for the short moves
that it’s doing.
And that’s what you can verify with this
last bit down here.
But we’re going to stick with the steps
per millimeter calculations for now.
So to start out, what does that number, say
100 steps per millimeter for a fairly normal
setup, actually mean?
You see, everything past the microcontroller
on your printer’s control board only has
a very limited idea of what it’s doing.
The stepper motors and their drivers only
get the command to move forward or backward
one increment at a time - this is a step.
And because of the way your drive system is
set up, each step moves the corresponding
axis by a set distance.
Now, when the microcontroller, or rather,
the firmware running on that microcontroller
gets the command to move an axis by, for example,
one millimeter, it uses the steps per millimeter
setting and calculates how many electrical
pulses - one for each step - it needs to send
to the stepper driver to get the desired distance
in the end.
Now, when we look at the Prusa calculator,
you’ll see that there are four variables
that determine the steps per millimeter value
in the end.
Now, in the simplest case, the stepper motor
would rotate by one step for each pulse the
microcontroller sends out and its driver receives.
The step angle or steps per revolution is
simply a data-sheet value that determines
how many full steps the motor has until its
shaft makes a complete rotation, 360°.
Typically, our motors have 200 steps per revolution,
some more precise ones use # 4 00, while cheaper
or salvaged motors somtimes have only 48.
Which, of course, means that the steps are
laid out much coarser throughout a revolution
than in a 200 or 400 step motor, reducing
the final resolution of that axis by quite
a bit.
So, typically, that setting is 200, but you
can also easily look it up if you know the
part number of your motor.
Now, because 200 steps per revolution actually
means that each individual step is still rather
large, stepper drivers can add some electrical
trickery to split each step up into finer
/# microsteps.
So, for example, when your driver chip, typically
an Allegro A4988, is set to one sixteenth
microstepping, the motor will only move approximately
one sixteenth as far per step pulse as without
any microstepping.
Now, i’m saying approximately because microstepping,
especially in the Allegro driver can be somewhat
unpredictable and doesn’t always position
the motor at the exact sub-position it intends
to.
Still, it’s a nice way to give stepper motors
a bit extra resolution and, as a bonus, it
also means that the motor will be a bit less
noisy.
The microstep setting for each axis is typically
set with jumpers next to each stepper driver,
but newer boards often make them software-configurable
instead.
Which jumper settings mean which microstepping
setting is explained pretty well on Pololu’s
site, which i’ve linked to in the video’s
description.
Typically, that setting is one sixteenth with
all jumpers installed, if you’re using Texas
instruments’ DRV8825 drivers, you can go
up to 32 times microstepping, but that might
mean that the puny Atmega microprocessor on
your control boards runs out of processing
power and slows the entire printer down.
So right now, we have the microcontroller
# receiving move instructions and # sending
pulses to the driver, the driver doing its
thing and splitting up the motor’s physical
steps into microsteps and the motor’s shaft
finally rotating in accordance to what currents
the driver is sending its way.
That poor motor never had a choice.
So basically, we have the entire software
and electric side covered of what goes into
the steps per millimeter figure.
So the last two things are the belt pitch
and the pulley tooth count.
Now the belt pitch is manufactured extremely
accurately, so that’s actually a figure
that we can rely on.
Nowadays, pretty much the only belt used is
GT2-2M, so a belt with a profile that’s
compatible with HTD, GT, GT2 and GT3 pulleys
and with a 2mm pitch.
Meaning each tooth is precisely 2mm from the
last one.
Earlier printers used T5 belt with a 5mm pitch,
I use HTD-3M, so a 3mm pitch, on my big Mendel90,
and some US manufacturers also use imperial
belt with the XL or MXL profiles, which have
a 5.08mm or 2.03mm pitch, respectively.
They are not compatible with pulleys for metric
GT profiles and are actually rather rare these
days with everyone moving to GT2-2M.
Anyways, it’s pretty easy to figure out
which belt and pitch you have, as it usually
says so on the back of the belt.
If not, well, the article description should
have at least included that bit of information
when you bought the belt.
Also, in the 3D printing sphere, GT2 is often
used as an abbreviation for 2mm pitch GT2-2M
belt, even though it’s technically incorrect
or at least not precise.
And the last part of the puzzle is the tooth
count of the pulleys.
And that is sometimes noted on the side of
the pulley, but it also something that you
can simply count.
You know, how many teeth there are on the
pulley.
Mark one and then make your way around.
So when you have gathered all that information,
you are ready to punch it into Prusa’s calculator,
which gives you a couple of presets for most
parameters.
The output can be used with the M92 command
to temporarily set steps per millimeter for
each axis or used in the firmware’s configuration
to permanently use it.
Now, there is a note that you might still
need to calibrate this further, but like i’ve
said in the previous video, that will likely
make things worse instead of improving them.
The steps per millimeter value we just calculated
is extremely close to what your printer is
actually doing, and for regular printers,
any tiny error here is within the tolerances
of the FFF process anyways.
So that was the process of the belt-driven
axes, and for the Z-axis, which is usually
driven by a threaded rod or, in better printers,
a real leadscrew, it’s even simpler.
You don’t need to know a belt pitch or pulley
tooth count, but instead simply the pitch
of your leadscrew.
If that’s a threaded rod, it’s usually
an M8 or M6 one with a 1.25 or 1mm pitch,
while leadscrews can have all kinds of pitches.
Look it up in your printer’s documentation
if you don’t know it.
And that’s it for today, one more thing,
i used to have a Surveymonkey survey under
each video, but as it turns out, Surveymonkey
is pretty greedy and was like “hey, cool
survey you have there, shame if anything would
happen to it.
You know what, just pay us an 24€ a month
and you’ll actually get to see the new answer
to that survey.”.
And i wasn’t really happy with that, so
i moved that over to Google forms.
Which is free.
And unlimited.
And awesome.
So, don’t use Surverymonkey, use Google
forms.
But it also means that i lost all responses
from that survey, so if you’ve already answered
the Surverymonkey one, please do it again
on the Google Forms one.
Linked in the description.
And that survey is really helpful to me for
picking topics that you actually want to see.
And now, that’s really it.
Feel free to use the like and share buttons
if you feel like it, thanks for watching and
i’ll see you next week.
Or rather, you’ll see me next week.
Since i don’t actually see my viewers.
But that’s ok.
I guess.
