Do you ever print two things that are supposed to go together and
they don't?
It happens a lot, so I think it's about time we talk about it, let's go.
This video is sponsored by PCBway, more on that later.
Also a trigger warning, words like tolerance, fit and clearance are going to be used interchangeably
to basically mean the wrong things throughout this video.
I am not an engineer as will become painfully obvious in the next few minutes.
So this thought process started really with prusaslicer and the new pegs feature, in case
you didn't know prusaslicer now has these automatic pegs you can make, but the problem
is, on the default settings, they almost certainly won't fit.
The reason why is pretty straightforward.
FDM is a process of pushing plastic through a round nozzle into what's often called a
bead, and it's somewhat predictable but it's not square, not at the edges.
You are effectively guessing where the edge of the filament will land based on the shape
it makes as it is laid down.
Add to that the variances between filaments and you have, well, you have exactly the problem
we need to solve - stuff doesn't fit, stuff fits on the designer's machine but not yours,
and so on.
Before we get onto what I made I want to just fly by a couple of important concepts, the
first is about wall ordering.
Cura calls it wall ordering, prusaslicer calls it "External perimeters first" but it's the
same thing.
If you imagine the process of extrusion but larger, the number of walls is typically more
than one unless you're doing weird stuff, and the default for a slicer is to draw the
center line first then move to the outside.
But as you probably guessed the aforementioned setting reverses that and prints the outer
walls first.
The issue is, if you print the inside wall first, then this is a whole messy process
in real life where the inside wall will squish, settle, and cool, and when you come to print
the outer wall, it's like finding a seat in the cinema but the person next to you has
already sprawled over the arm rest, and you have to kind of politely try to lean over
the other way..we've all been there.
The way a slicer works is to assume when you place the nozzle in the center of the line
width and extrude, you get equal material deposited each side, but in reality due to
the guy in the next seat hogging the arm rest phenomenon, the second and subsequent lines
in a perimeter won't sit perfectly centered like the ideal scenario the slicer will have
you believe, they will be constrained on one side.
The result, seems obvious now we think about it, the edge will overfill.
In my experience at least this gets worse the more walls you have.
This is why outer wall first exists, to try to direct that squish the other way.
I know it sounds like you ought to have outer wall first as a default and maybe that's true
but it's not default in slicers for a very good reason, if you try doing outer walls
first with any overhangs you will soon see the problem.
So at least on models without overhangs, until slicers get smarter about wall ordering, it's
a good idea to set outer wall first on models you want to be dimensionally accurate.
So that is all well and good but being dimensionally accurate is only half the story, parts still
need to fit, and even dimensionally accurate parts won't fit if you don't allow for the
real world, if a peg is the same size as the hole it is meant to go into then it seems
like common sense that it won't fit, we need to allow a clearance between parts.
Even theoretically you need to do this, you can't have two sets of edges occupying the
same space.
but how much tolerance you need to allow in your design... is pretty much guesswork and
highly subjective.
I'll show you what I did though after the sponsor segment which is of course for PCBWay,
and I certainly imagine their PCB tolerance is a lot better than you can get on an FDM
printer, that gives me an idea actually.
PCBWay can make parts using methods you can't if you have an FDM printer, so if you want
or need tolerances far more consistent than you can achieve at home then you could consider
using say DLP without having to deal with resin at home.
You can choose the service for 3d printing, send your stl file to PCBWay and they will
take care of everything.
There's also CNC machining, sheet metal processing, injection molding, and of course PCB manufacture,
and there's extensive guides on each of those if you click on the respective services so
even if you aren't familiar with a service you can get the information you need to design
for that service, yes, including tolerances.
Go check out PCBWay's website at the link below in the description.
And thank you pcbway for sponsoring this episode.
So back to the video.
I made this.
Some of the stuff I make is useless, some of it people find useful, its quite early
to call it on this one but even if it's totally useless then at least hopefully you leave
with some knowledge about wall ordering and linear advance, we'll get to that stay tuned,
but this gauge is for finding the right tolerance for the filament you're using.
You see, if you're printing parts on multiple printers, with different filaments that swell
in different amounts, and silk really does swell if you haven't noticed, then you may
think you have the right spacing between parts, but you often don't, and there's no way to
gauge that without having to print your part, fail, print again which is a huge waste of
filament.
Until now, because this gauge hopefully solves that.
What we have is a gauge piece that you print on one printer or filament, and then some
pegs that you print on the other printer or filament.
It's imperative, very imperative, that for these two parts you use exactly the settings
and conditions, meaning filament and printers and speeds, that you plan to print with.
The pegs have dots on the bottom, this indicates the clearance that has been built into the
peg to gauge distance.
So a peg with two dots has a 0.2mm built in clearancer, as in, built into the design.
If you know your design has 2mm tolerance, use the peg with two dots.
You then want to try to fit that peg into the center hole of the gauge.
If it fits perfectly in the round and square holes, then you're good to go, your part will
hopefully print with the right spacing.
But this is also a diagnostic for when, inevitably, it doesn't.
You have two choices when it doesn't, you can remain on the same peg, ie 0.2mm tolerance,
and move left or right on the gauge until you get a good fit, and this will tell you
the percent reduction, or who knows, increase, you might think about adding to get a good
fit.
Finding that the peg fits in the +2% hole would mean you need to make one part 2% bigger,
at least, presumably.
Or if you are designing the part yourself you might want to control clearance in which
case you can swap out the pegs until you find one you like, and then you know what tolerance
you want to design for.
Of course bear in mind that there are different desired "fits", you might need an interference
fit which uses friction to hold parts together and those are I think by far the hardest to
pull off in FDM, or you might want varying levels of clearance, it's all down to the
intended use.
If you don't know the designed clearances on the part you're printing then remember
that slicers have tools in them that allow you to measure parts, you have the new prusaslicer
measure tool and there's a plugin in cura for it too.
I think it's quite a cool idea but I've only just made it, so I really have no idea how
well it works in the real world, it might be absolutely useless, so I hope you will
consider trying it out and giving me some feedback, I hopefully will remember to link
to it below.
I think there is probably a need for a larger one for larger line widths
so I'll be working on that too.
So finally what about the situation where the round peg fits but the square one doesn't?
This is kind of inherent to how FDM printing works, so you should expect it in most cases.
FDM doesn't make perfect squares, it's actually far better at circles, and this is something
linear advance actually addresses, so if you find this a huge problem where the corners
are particularly out compared to the sides making squares look like cushion shapes then
that is almost certainly something from poor pressure control and if you have linear advance
on your printer then that's where you need to look first, but even that aside, I think
it's inevitable that parts with straight edges and corners like squares will need more tolerance
than round parts, meaning round pegs will always fit square holes.
So I guess we know now why the saying is the other way round.
Of course, this is an ongoing discussion, I'm interested to see where this goes, as
usual comment below and let me know what you think, any feedback and so on.
Remember to subscribe if you're into this kind of thing, and I will see you next time,
thank you for watching!