Why your 3d printed stuff doesn't fit together and how to fix it!

Fed up of 3d printed stuff not fitting together after you print it? Me too. Let's do something about it.

Or...you can visit amazon using my store links below to support the channel:

Models used:
The tests on the thumbnail image: coming soon

[DoctorZombo]

One setting that's kind of hidden by default and makes an enormous difference on Cura is slicing tolerance: it can be set to "exclusive", "inclusive", or "middle", which basically means wether the slicer will place your filament path entirely inside the geometry, outside the perimeter, or average its placement.
By default it's set to "middle" which means it will always make your holes slightly too small, and your pegs slightly too thick.

Edit: after a quick google search as a refresher I see I learned about that feature on this very channel! What were the odds haha

[Slide]

Thanks for this! I can't remember the number of times I've had this conversation with myself: "Ok, the screw is 3mm, and 3.5 for the hole -should- be ok" 🙂

[JBMetalShop]

Honestly i am an engineer and i think your channel does a magnificent job and explaining concepts and discussing important topics for people interested in 3D printing, it was a HUGE help on my 3D printing journey.

[rickharriss]

As a D&T teacher (retired) I used to do a short exercise with 11 year old students where they were asked to mark, cut and file a square of 3mm Acrylic to some given dimensions. When they presented their work as finished I tested it in a go / nogo gauge that had +/- 0.25mm sized holes in it. Generally first time none of the samples fitted but after a few tries they got the understanding of what tolerance was.
When I fit 2 printed parts together I usually print the inside part 0.25mm smaller and then use fine wet and dry paper to get a perfect fit. Fettling, as my engineer friends would call it.

[KeepYourself]

I would recommend using “Hole Horizontal Expansion” in Cura or the equivalent for Prusa. Scaling the object to fit causes all other features to scale. Hole Horizontal Expansion allows you to adjust holes with affecting the rest of the model.

[matgggg]

As a mechanical engineer who has also been 3d printing for 7 years you did a great job explaining everything that’s necessary to this topic! some times people go off on tangents that aren’t really relatable and it’s interesting but some people may find it confusing. It’s good you didn’t.

[TheThrustProject]

Wouldn't it be a great way to integrate PCBway as a sponsor by letting them print this gauge model in their different printing technologies and comparing them?
Would love to see something like this.

[okflyer]

I would suggest adding a feature to the pegs to indicate their X-Y print orientation. That would allow the user to diagnose X vs Y axis fit issues if they are present. Good job.

[j---land]

In Prusa slicer, a modifier can be added with the setting "External Perimeters First", so you could apply that setting only to the sections of the model that need higher accuracy.

[cls]

I print always "perimeters first" up to 45° overhang. Not only because of dimensional accuracy but it also gives you a nicer surface finish as imperfections (from the usually faster printed) inner perimeters don't transfer so easily to the outer wall.

[haavard]

For anything other than cylindrical hole fitment, I always put a radius/fillet on the edges of the male part. Works every time

[samadler]

I took a 3D printing class in my MET degree as an elective, this info is spot on and in more detail about tolerances than that course. We had learned about the expansion coefficient of the layers, but were just given estimated tolerances for common materials, for typ layer heights for the machines we used in the lab. We had a demonstration print exactly like you made @5:19, this allowed us to see how tight/loose certain tolerances were in real life. great video! earned a sub from this guy!

[gsdtdeaux]

By FAR my favorite printing channel! I wish this guy had a channel called “lost in history” where he did documentaries lol i swear i could listen to him talk about paint drying lol

[randomviewer]

When I need precise tolerances on something I usually mess with the horizontal expansion setting. I have a workflow for this where I duplicate the parts meant to fit together in CAD, but I only duplicate the pegs/hole sections of the two parts while being mindful of their orientations. I then print them, adjust the horizontal expansion (or slicing tolerance) and try again. Sometimes I have to iterate this two or three times to get the perfect clutch strength for what I want.

Interference fits are counterintuitively more easy to do this for, assuming you don't care about the parts being removable in the future. I just print one of the parts with very few perimeter lines (usually two) and a higher infill density to make up for the weaker shell. The perimeters will stretch into the infill when pressed together, permanently deforming things and making a rigid joint.

[darrennew]

I found that if you have a "big" peg to go into a "big" hole (like making something out of two colors of plastic that you want to fit together) you can make the inside thing significantly smaller and then give it a wrap of teflon plumbing tape to hold it in place.

[reverse_engineered]

What you mentioned applies to holes along the X/Y plane, facing into the Z (e.g. holes in top or bottom). However, holes in the sides suffer from yet other issues. First, a layer has a thickness and the wall at that layer will be basically straight - it won't follow a circular path. Therefore, the wall will be either too close or too wide as compared to the circle you are trying to make. By default, slicers will make the wall at the middle of whatever line/curve is crossing as you go up, which means it's both too close and too far. You can set it to print on the inside or outside instead, which will make it always too close or always too far, but neither is ideal. Really, you don't want round holes on anything but top/bottom if you need dimensional accuracy.

In addition, the top of a hole has very shallow angles - approaching horizontal at the top. This amounts to a very sharp overhang. Slicers won't try to make this extremely shallow overhang. Instead, they will create a bridge across the top. Again, this means that either you have filament where you don't want it, or you are missing filament where you do want it.

Another issue for top/bottom holes is the seam where the circle starts and ends. Again, you will run into issues at this seam. Either you will be missing a fit of filament or you will have some extra. The shape of the seam also depends on the direction that the extruder leaves when it joins up the seam - something you rarely have any direct control over. So you'll more than likely end up with bumps along the inside wall of your hole that make it too narrow.

Getting a perfect fit for any shape is nearly impossible. The easiest way to get accurate and consistent round holes is to undersize them a bit in your design, then drill them out with a drill. We use the same technique in subtractive methods like milling because it suffers many of the same problems - you can't make perfectly square shapes and you can't make consistently round holes. When we need a perfect hole, we will drill (come directly down from above) rather than milling (circling around the perimeter). For any other shape, leave clearance, and provide relief in tight inside corners, such as using dog bones.

[PhilWare]

If I'm needing exact tolerances I usually do a couple of test prints on a cut-down version of the part/tolerance in question. I've found that even if you think you have allowed for the issue you will still be surprised. To be honest given the relative low cost of modern 3D printers its actually surprising how good the tolerance/consistency of prints are so I'm not complaining!

[bensutton]

I regret that I have but one upvote to give to this video. This solved probably 90% of my dimensional accuracy problems immediately.

[teamscarletdevil]

nice gauge bro, will certainly give it a try.
and my way of dealing with this is the lower flow at the outer wall. my usual setting is something like 2mm shell thickness with 0.7mm line width(from a 0.4mm nozzle) the 2 inner pass of wall is set at 112 flow(varies by materials, 112 is for pacf) while the outer most pass is like 92. the higher flow on the inside is to have higher extrusion pressure and presumably higher strength, and the outer most wall is just to give a smush so its mostly dimentionally correct.
at the end of the day i find it very helpful to have a rubber mallet and a tiny file on hand, which can solve like 90% of fitting problem.

[reverse_engineered]

This is a nice gauge and certainly useful. I think one thing that's missing though is how to account for these offsets and clearances. You briefly mentioned resizing the part, but that's not going to give you dimensional accuracy: it will not only make the holes larger but it will scale up the entire part, causing the outsides to be even larger too. Instead, you probably want a setting like horizontal expansion which will keep the overall scale of the part, but will subtract a bit from the size of each edge, like shaving a bit off of every face with sandpaper. You can use your gauge to determine how much too large your pegs and how too small your holes are, then put that amount in for horizontal expansion and it will correct for it, while maintaining the same overall size. You should never have to change your steps/mm or scale your part to make clearances!