In this episode I want to show you why you should be using this setting in cura,
and what it actually does. Let's go.
Slicing tolerance in cura is tucked away in the experimental settings section,
where it has been for quite a while now, at least since 2017 where it was changed from being called
slicing mode. Neither of these particularly explain what it actually does so I will attempt,
from a combination of poking around in the code, to reading the documentation,
to explain how cura actually slices and how this makes things fit.
Because fundamentally that's what this setting does in practice.
I have here three trays, which correspond to the three settings available for slicing tolerance.
The default is "middle", with "inclusive" and "exclusive" being the settings either side.
I printed out a machine screw and nut using all three settings, and on middle it's
tight and largely unusable, on inclusive it's worse, but on exclusive, it works fine.
Similarly with this print in place bearing, on inclusive setting it's
pretty thoroughly welded together, but exclusive actually works fine.
But on the flip side, this puzzle with a ring on it actually works better with inclusive mode than
exclusive mode, so it's clearly not as simple as a simple fix everything setting. Nor is it
simply a setting that increases the spacing between elements. it doesn't work like that,
that's actually a side effect of what it's really doing. So what is it really doing?
I will attempt to explain through the medium of awful drawings. And play doh.
Cura is a slicer. Slicers slice models into...slices. Cura is no different. If
you take any model and slice it into slices, you get...unsurprisingly, slices of the model.
Following so far? good.
Now, in the real world, cura needs to work out how to translate each slice
into effectively a 2D shape, a cross section is notably not the same thing as a 2d shape
when 3d printing because a 2d shape has no 3rd dimension, but a 3d printed slice does,
because each slice has its own layer height. So each slice occupies its own portion of
the height of the model. It's not just a flat cross section of the model. This is important.
A spanner in the works of 3d printing these slices is of course that extrusion isn't the
process of laying down cubic lines as much as cura's maths might wish it does. If you
look at extruded lines close up they are very much not straight sided rectangles,
they probably are inside a model but at the edges where the outer walls meet the world outside,
they are very much curved. This already means that your cross section aka slice
is not what you think it is, it's less this shape and more this shape.
For a cylinder, this is all very straightforward. Slices of a cylinder are tiny cylinders. Stack a
bunch of objects this shape ontop of each other and you'll get a cylinder, broadly speaking.
Sure if you attack it with calipers you'll be able to pick out wider and shallower parts
but this is fine, if you set the resolution small enough.
But what about literally anything else that isn't straight sided?
Most of what we 3d print has what could be referred to as complicated geometry,
meaning your slice might not look like something from a can of ham,
but might look more like this. How, indeed, do you turn this into one of these donut things?
The first problem is that even the most basic slice from a sloping sided cylindrical
object has two distinct radii. You have a wider end and a narrower end. This...isn't this.
And this is where slicing tolerance comes in. Cura has decided to tackle this problem
by giving you three choices. First, the default, is "middle". When you choose middle, you're just
slicing based on the radius straight down the middle and taking that to be the edge of the
slice. What the cura documentation shows is clear enough once you get the concept but if you've seen
my flow math video you'll know that cura extrudes cuboids so I assume it's doing this with the edge
of the rectangular extrusion profile touching the edge of the slice at the middle. Meh whatever.
Choosing "Inclusive" uses the widest point of the 3d model slice as the edge of the printed
slice, and as you probably can guess, "Exclusive" uses the narrowest point.
** me from the future here, I realised during editing that if I don't add a caveat here I
will get corrected in the comments. What I just said *IS* broadly correct except what actually
happens is slightly more complicated, if you see the diagram here, you will see that inclusive
and exclusive use boolean intersections between top and bottom, so the size of a slice is kind
of the smaller of all dimensions which is quite complicated, but a distinction worth stressing.
So what does this mean in reality? Well, only one of the three settings will end
up with a physical model that fits in *less* space than the CAD design
and that's the exclusive setting. Middle will be "on average" the same size as your
cad designed model, and inclusive will be outright larger because every part of
every slice where there's any kind of tapering is taking more width than the model.
You can see how this setting is useful then for a screw thread. For screw threads to clear each
other, if your model has no tolerances built in, and your printer is spot on accurate,
as I understand it you can only make threads work using "exclusive".
I've printed out a square based pyramid and cut it exactly in half and sliced and printed each
half to demonstrate just exactly how stark the difference is between these slicing modes. One is
exclusive, one inclusive, and I have printed this in I think 3mm line width - yes on a 0.4 nozzle,
we've been over this, this is absolutely normal, but anyway yes this shows the layer shapes well,
and you can see that one model is significantly larger than the other.
So what's the takeaway here? Well I've seen it said that you should slice with "exclusive"
for parts that need to mesh, middle with parts that don't, and "inclusive" with parts that you
plan to sand down. I think that's a good way to think about it actually. Of course that's not
the whole story, because some people design in tolerances to make settings like "middle" work
and "inclusive" would be too loose, so you have to deal with the setting on a piece by piece basis.
I hope that made sense, as always please check the description for any corrections,
additional info, links to designs. Also if you feel like sponsoring the channel then
patreon links below - patreon sponsors see this video first as I mentioned,
something I'm trialling. Also join our discord, we're up to about 120
members now I think which is way more than I expected so come down and talk 3d printing.
Otherwise I will see you next time! Thank you for watching!