Hey, how’s it going, everyone? Have you
ever wondered why 3D printers and actual CNC
machines seem to be so similar, but rarely
you ever see anyone successfully using a 3D
printer as a milling machine and vice-versa?
Well, today we’re going to look at why that
is and maybe at what a compromise could look
like to handle both!
[Intro]
So the three printers, well, the three machines
I have here today are the CR-10, a fairly
common and actually surprisingly okay 3D printer,
my converted MendelMax 3, which used to be
an oversized 3D printer and is now a super-light-duty
engraver, mill, whatever you want to call
it, you can watch the conversion process here,
and yes, it’s not perfect by any means,
and lastly, the Sienci CNC, which from now
on I’m just going to call Bob because I’m
sick of every time having to clarify whether
I’m talking about the CNC category or the
Sienci product (hrmpf, “Slic3r”), ‘scuse
me, and this is a light-duty mill.
I know, I’m using the terms “milling machine”,
“router” and “engraver” somewhat interchangeably
here, at least for these machines, because
they are neither, actually. They’re not
built like a router, which typically has a
moving portal, which is this part right here,
and that’s great for cutting sheet goods;
they’re also not built like a milling machine,
which has a smaller work surface, but much
sturdier construction with a rear column and
often a moving work surface, which is heavy
and slow, but great for machining metals.
Neither Bob nor the MendelMax are either of
those, but both of them are built like your
typical Mendel-style 3D printer with a fixed
portal and a moving bed, the MendelMax has
the X-axis mounted on top of the Z-axis, and
Bob has the Z-axis on top of the X-axis, but
otherwise, their basic kinematics are the
same.
And while we’re at it, let’s actually
take a look at how these two are built differently.
First difference: The frame. Both 3D printers
here use machine profiles and both are actually
using the openbuilds-style V-slot, which already
is a good bit sturdier than your regular 8mm
smooth, unsupported round bars, and Bob has
these massive aluminum angle profiles with
the wheels running on the edges of that. Proper
milling machines would either have linear
carriages like these or sliding dovetails,
which look like this. And why? Because rigidity.
On a 3D printer, there are no forces between
the toolhead, that is your hotend, and your
workpiece, so the printbed and your print
because the hotend is basically just skimming
over the part and lays down molten plastic
as it goes. On a milling machine, you have
the cutting edges of your tool actually digging
into the material you’re cutting and that
does, in fact, generate quite a significant
amount of force. Let’s see what that force
does with these machines: For reference, here’s
a Prusa MK2… The CR-10… Mendel Max 3…
Bob… and my glorified drill press “milling
machine”. That’s quite a difference. The
floppier a machine is, the more it is going
to deflect when cutting and that means the
more likely it is going to start ringing or
doing some other weird stuff that is completely
inappropriate for milling. What also plays
into this is that 3D printers typically use
belts, and especially the cheaper belts are
often incredibly elastic - which is the same
as having a non-rigid frame. Some CNC routers
use steel-reinforced belts, but typically
you see trapezoid leadscrews or even ballscrews.
Simply speaking, these don’t flex at all.
And you always have to keep the key difference
in mind when looking at 3D printers vs. CNC
machines: 3D printing is, by definition, an
additive process, meaning you’re adding
material to your workpiece as it gets processed;
milling, turning, etc, on the other hand removes
material as the part gets processed. So with
3D printing, you only put in as much material
as the finished piece is going to use, plus
supports, brim, priming, whatever, that’s
a tiny amount of waste, but with any subtractive
process, you actually need a larger blank
of solid material that then gets gradually
shaved away. So basically, with a milling
process, any part is already in here in the
blank, it just needs to be freed from all
the waste material around it, which, is actually
a pretty cute way to think about it. Where
does that extra material go? Well, shavings.
If you’re machining wood, you’re often
also getting a ton of dust, but aluminum shavings
aren’t much nicer to deal with, either,
because they are sharp, pointy, small, and
large in numbers. It’s one of the things
you’re very likely to underestimate if you’ve
never used a CNC and are thinking about converting
a 3D printer, for example. There’s going
to be a lot more of these than you expect
and they will get everywhere. Oh, and the
noise. It’s pretty intense.
So, let’s actually make something with these
machines, shall we?
[Timelapse]
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[/Skillshare spot, duration about 55 seconds]
So in this case, the milling job was already
a bit faster than the 3D print, and this just
gets amplified the larger a job is. If it’s
really intricate parts, a 3D printer does
have the upper hand as complexity doesn’t
nearly “cost” as much as with a subtractive
process, but because a 3D printer will need
to lay down every bit of material instead
of just not shaving it away, it’s not quite
as scalable as a milling process.
Now, a 3D printer, at this point in time,
is pretty much always built in the same, very
simple sequence: The machine starts at the
bottom, works its way up vertically and once
it reaches the tallest spot of the part you
want to make, it’s done! With a CNC, it’s
not always that simple. What you often see
with these smaller machines is just a single
tool being used for the entire job - often
that’s an ⅛” square endmill - but on
nicer machines, you have all sorts of tools
from a cutter head over roughing endmills
to finishing endmills, chamfering tools, taps,
drills, reamers etc. In which order those
tools are used and for what areas is really
up to the user, so the process of preparing
a file for machining is typically much less
automated than the process of preparing a
3D model for 3D printing - which just uses
a single nozzle size for the entire print,
typically. In both cases, how the input file
is processed has a huge impact over how the
finished part is going to turn out, but with
CNC’ing, there’s no simple one-size-fits-all-solution.
3D printers in general are quite the rigid
concept - it’s three movement axes, X, Y,
Z, plus an arbitrary amount of extruders that
basically work as extra linear axes along
the length of the filament. On a CNC machine,
you usually also have X, Y and Z that position
the tool relative to the workpiece, but you
might also have, for example, a B and C rotational
axis that rotate the tool or workpiece about
the Y or Z axis in a four- or five-axis machine.
But let’s not forget there’s not just
mills, there’s a CNC lathes, combination
machines and completely different types as
well.
Other “details” are actually surprisingly
similar between 3D printers and other CNC
machines, like the part cooling fan control
on a 3D printer and a coolant pump on a CNC,
temperature control of a hotend and RPM control
on a spindle, etc, etc, so it’s no surprise
that DIY CNC routers are increasingly using
electronics, firmwares and just general components
that might not be directly 3D-printer specific,
but at least based on the developments from
3D printing. Like with Bob, all the electronics
are based on 3D printing boards, the motors
are standard NEMA17, the computer interface
is the same software serial port - it’s
basically a beefed-up 3D printer.
But let’s not forget that 3D printers originally
did originate from the larger CNC machines.
Things like G-code are directly carried over
that, and many of the early 3D printing developments
were built to suit both CNC applications and
3D printing - think firmwares and even the
back-then slicer, Skeinforge. What also carried
over is that these machines are maybe not
dumb, but at least blind. If a cutting tool
breaks, the workpiece dislodges or anything
else happens, the machines are often not going
to notice it and will just keep going. That
is just now getting better for both 3D printers
and mills, and bigger CNC machines are using
things like load monitoring, the same thing
the Trinamic drivers are doing on 3D printers,
as well as Servo motors that have a feedback
loop etc.
So yes, 3D printers and CNC milling machines
are actually quite similar and knowing one
of them well will help you get a head start
with the other. But the differences in machine
construction, especially when it comes to
rigidity and top speed, don’t really make
combination machines all too viable, at least
for “real” applications. Maybe with a
somewhat rigid frame and high-pitch leadscrews,
you could build something that does both,
but it would neither be a great 3D printer
nor a great CNC mill.
Ok, so with that, I think we’re set for
trying some more CNC machines! I hope this
video clarified some of the differences for
you, if it did, give it a thumbs up, get subscribed
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products we used in this video - like the
CR-10! And that’s it for today, thanks for
watching and I’ll see you in the next one!
