# how do you define tolerances for moving parts so that they work properly?

I have a mechanism in CAD that I want to either 3D print or CNC mill. When I went to the school's shop the tech asked me what the tolerances of my parts where and I had no idea. How do I go about assigning tolerances to parts that are supposed to fit together?

• What "fit" do you need? So they are fixed or locked together? So they can move with a lubricated joint? So they have pressure lubrication? So one goes inside the other without any point of contact? Commented Apr 13, 2022 at 5:56
• I added 2 pictures to the post. I used an old compass to practice CAD. Now I want to CNC a copy out of aluminum and brass, and 3D print another. The tech at my school said my drawings did not have any tolerances on them and that a zero tolerance drawing is impossible. I haven't learned how to apply tolerances yet and don't know where to start Commented Apr 13, 2022 at 23:32
• you can ask: what is the furthest off that a surface could be and still have this thing function as I want? Some people don't realize but you can often get something made faster if your tolerances are large.
– Abel
Commented Apr 14, 2022 at 0:14

You basically need experience with real things in your hands. It depends on material and surface finish as well. You to do things like drill and ream holes of different tolerances see how pins fit when the fit is 0.0000", 0.0002", 0.0005", 0.001", 0.002", and 0.005". Or measure a piece with a micrometer and then back off on the jaws by the amounts mentioned above and see how easily the parts slide out. For smooth metal surfaces, 0.005" is visibly loose enough that you can feel it wobble with a lot of play and slide freely. 0.001" is not really wobbly loose but will slide easily and is by no means tight. A non-wobbling fit is located somewhere between 0.0005" and 0.0001" and is such that if it is something like a shaft sliding into a hole, you can't get it in or out without an arbour press even though it is not truly a press fit because the tolerances are so small it need to be square or it will jam.

But I'm talking about metal which can hold nice smooth, flat, straight surfaces which don't expand much with temperature or at all with humidity. 3D printed plastic is terrible by comparison, but at the same time can give.

Get a micrometer then at least you'll be able to hold it up to the light and see what 0.001" or 0.005" looks like and stick pieces of stock in the jaws and see how easily it slides as you widen the jaws. You can stick feeler gauges, pins, drill rod, and other things in the jaws.