When you print something that should fit inside something else (like another printed part

When you print something that should fit inside something else (like another printed part or a screw), what happens?

I ask this because despite my best efforts, it’s very rare to get parts that actually fit how it should and I think the bigger problem is that people design parts way too precise.
Considering the approximations made by the slicer combined with all the imprecision of my homemade printer, could makes it impossible to actually fit a part into another.

But if it’s just me, than I should keep looking for a better mechanical explanation?!

try this: https://github.com/mzbotreprap/VORON/wiki/Slicer---Extrusion-Multiplier-Calibration

So here’s the thing:

Different printers have different tolerances. If I design for SLA, I need to use different tolerance than when I’m using my FDM. It also sometimes comes down to the material used.

My general rule of thumb is a .2-.3mm tolerance on parts to have a tight fit with my FDM printers

Each material has it’s own coefficient of expansion and contraction . The use of lubricants helps to reduce friction between moving components . The stress of expansion and contraction may cause components to crack and break and malfunction . Professional fitters and turners are knowledgeable about fitting parts together , so that moving parts will last as long as possible .

When doing this the design gets tweeked for the short comings of the printer, so that they do fit after a little trial and error

I usually add about 0.1 to 0.2mm to parts which have to fit together. Works quite well. The thing is, when you know how much your material will shrink after printing you can easily counter this while designing.

Openscad (or any other parametric CAD software) helps a lot here: I print it with 0.1 or 0.2 mm margins according to the printer I use, then I tweak the value by 0.05mm and iterate 1 or 2 times. After all the printer is more willing to spend time on making another part, than me filing or sanding it.
When the design is not mine in the first place, it happened more than once that I redesign the part…

Changing the design does not work well if you want to download things. I want my slicer to be able to figure out how much it goes wrong in various situations and account for that. Any slicer does that?

@Lars_Clausen no, not that I know of. Shrinkage of material isn’t easy to figure out. ZWCAD can do this for injection molding, but I haven’t seen this for fdm in any software I know. When you download things you can try to overcome non fitting parts by scaling them properly. No need to change the entire design. I have done this today with the you axis belt tensioner for the x3s from thingiverse. I took the end cap from the y axis tensioner and scaled up so it fits. Worked very well.

Wow…I’m shocked that so many people don’t need any kind of work to get parts to fit!!!
But as I expected, most people have better printers than me.

Anyway, for all the tips, I built my printer (it’s an i3 with wood frame) so I"m familiar with all the parts and most of the problem sources.
I also have little if any problems with the parts I design myself, but I don’t have crazy 3D modeling skills and depend on downloaded models for more complex things.

Anyway, thanks for participating, it helped with the frustration :3

I never have issues with PLA I just add 0.1mm to each side. ABS, now that’s a different story. It shrinks way more so I do have loads of grief with that.

depends how fussy I want to get with measurements. If I measure it carefully then usually a little bit of sanding or a file is all that’s needed. Often I’ll deliberately make holes too small so I can drill them out to the proper size, so does that still count as needing work since it’s planned work and designed to not fit?

I have no problem mixing and matching multipart assemblies together from different printers and filaments straight off my printers.

Two things need to be considered.

  1. The tolerance your printer can hold to mechanically.

Most high quality FDM hobby printers can hold to 0.1mm or even 0.05 mm of tolerance. If you put at least 0.2mm of tolerance between your contacting faces of the parts in your design this takes care of the mechanical side of the multi-part fit issue. Keep in mind that surfaces contacting the bed already have the squish to take into account which provides at least 0.05 - 0.75mm of the clearance.

  1. The thermal properties of the FDM filament you are using. This is the one that most people ignore or try to compensate with mechanical design changes and micro-step changes. I cannot stress the importance of this one enough.

By thermally calibrating your filament you can know how to adjust your filament instead of changing your design for each filament you use. I use these calibration clips because they tell me exactly how the filament is behaving for interlocking features.


Settings that result in a fit that is too loose (under extruding slightly but still filling in the surface) can be used on other peoples assemblies that have no clearance built in so I can get them to fit together right off the printer as well.


something it fits other Times is doesn’t, but my printer usually messes things up, I have had to do up. to 5mm to get it to git

Nice calibration pieces, @Jeff_Parish , and in particular nice that you include instructions on what to adjust based on how they come out. Trying to apply them to my printer. I think having some hole-matching would also be good, for adjusting the speed parameters…

+1 to preplanning to drill (or tap) in most designs to get holes just perfect if I’m really concerned about fit. If slop is ok I’ll add half to one nozzle width tolerance or so depending on how little I care :slightly_smiling_face:

I have a moded wanhoa i3 with fake e3d v6 and bowden extruder with fake smothieboard (mks sbase 1.3). First I calibrated all the axis with a dial indicator (10$ cheap Chinese one). Also I use Craftware slicer (nice slicer btw) with a outer perimeter offset of 0.1mm. This gives me a tight fits with ball bearings.

@Lars_Clausen Thanks. If you have any questions about how to use them just ask. Once you get a feel for how your filament is behaving you can adjust the temp and filament size (or extrusion multiplier) to make it do exactly what you want.

For holes that you want something to spin inside, like an axel, bump the clearance to 0.3mm and the parts will move freely.

Slower speeds will result in better surfaces in general but you need to calibrate for the print speeds and layer height you are using. I find I require different settings based on the nozzle size as well.

Depending on the material and the direction of printing you have to add some spacing between the different parts if they should fit together. How much spacing you have to add depends on your printer, the direction the object is printed and the material you are currently printing and of course the part you are printing.
The direction in which the part is printed counts because the
X/Y axis have a much higher resolution that the Z axis.

If you want to print screws(>=M8x1.25 for fully printed screws):
Screw axis along Z (upright, does not matter if inner or outer thread).
additional spacing ~ -0.2mm (nGen on an Ultimaker2/3) for all thread surfaces .
layer height of 0.15mm at most, better 0.1mm. and clean with a thread cutter.

One of the first tests back in 2015: https://spirou42.tumblr.com/post/117417084747/printed-m8-screw-threads-material-xt-layer

Those screws are used in: https://www.thingiverse.com/thing:930626