How to take in account tolerances when coupling

Question

I'm quite new to 3D CAD and printing. I own a Dremel 3D45 and I use FreeCad / Ultimaker Cura as softwares.

My question is pretty simple. Say you have to make one object with a pin and another with a hole. They should be coupled together. Of course if you set the diameters of the pin and the hole equal the won't fit!

Right now I'm setting the hole larger of 0.2 mm and the pin smaller of 0.2 mm. This allow a quite good coupling (not so hard but with some resistance).

I guess this tolerance (0.4 mm in my example) depends on a lot of variables: 3D printer settings, material, etc... so it may change using different setup.

How to correctly handle this?

Should I add a variable in my CAD spreadsheet and use it to change the nominal diameter of the coupling items?

I don't think so, but anyway: is there a settings in Ultimaker Cura that allow to compensate an hole or a pin by a specified amount?

Any other suggestion is gladly accepted.

Answer 1

I guess this tolerance (0.4 mm in my example) depends on a lot of variables: 3D printer settings, material, etc... so it may change using different setup.

Tolerances required depend on the geometry you're printing. A hole that is horizontal, vertical, or diagonal will need different tolerance (as I found out in a project that used the same steel dowel pins in three different orientations). And, vertical holes and pins are different from flat-sided shapes: the plastic will be pulled toward the center of a curve, so diameters come out small (and more so for holes since there's no material further inward to resist the movement).

That said, I think you can expect that the distortions of printing are fairly consistent, if your printer is functioning well. I have a Prusa i3 MK3S, and I have printed many parts other people have designed, and when those parts have been designed carefully, I almost always get very good fits between parts. So, my experience suggests that models do not necessarily need tuning for specific printers.

Should I add a variable in my CAD spreadsheet and use it to change the nominal diameter of the coupling items?

Yes. If nothing else, this allows you to define how the two parts should fit together separately for the printing error. Use separate numbers for different shapes/fits, so that you can adjust one without messing up another.

Answer 2

Once you understand how the parts will need to fit together to meet their purpose, you will need to define allowances on your parts in order to create clearance between them.

You will need to understand your printer's capabilities and accuracy by printing some test parts and measuring them. Tolerance is the amount of variation from the specified dimension that is acceptable on a part.

If your printer isn't accurate enough to achieve the tolerances specified on the part, you'll have to find some way to improve the parts so that they are within tolerance. Often this is done as a finishing step: filing, sanding, or grinding an oversized printed part; drilling out an undersized hole; etc. You should also consider other options: buy a more accurate resin printer, redesign the part to make the pin out of a commercially available metal rod or tube, pay someone else to make the parts for you, etc.

Armed with this information you can "design for manufacturing". That means you alter the design of your parts enough so they can be successfully produced with the tooling available to you.

I just posted a little Q&A that discusses this very topic of the difference between allowances and tolerances, and ways to achieve that.

Answer 3

I guess this tolerance (0.4 mm in my example) depends on a lot of variables: 3D printer settings, material, etc... so it may change using different setup.

Yes, this is true, you need to find out for yourself on your rig. Fine tune the printer. Note that filament also shrinks, although some less than others.

Once you figured it out, you can address the tolerance in the CAD design. E.g. I used to print with a material that has a lot of shrinkage, once you established the level of shrinkage, I scaled the complete model accordingly.