People new to ordering machined or printed parts tend to do one of two things: leave tolerance off the drawing entirely, or mark everything ±0.01mm because tighter sounds more serious. Both cause the same problem from opposite directions. The first gets you a part that's technically "correct" and doesn't fit anything. The second gets you a quote that costs three times what the part needed to, because a tolerance that tight has to be held on features that never needed it.
What the number is for
A tolerance of ±0.1mm on a 20mm dimension means: this feature can measure anywhere from 19.9mm to 20.1mm and the part still works as designed. Most CNC mills can comfortably hold much tighter than that on a simple feature. The tolerance is there to tell the machinist which features can be cut fast and checked loosely, and which ones need a slower pass, a sharper tool, and a real measurement before the part ships.
As a rough, starting-point reference, not a guarantee: FDM 3D printing typically holds ±0.2–0.3mm, varying with part size, orientation, and cooling. SLA/resin printing does better on small parts, around ±0.05–0.1mm, though it's still not a substitute for a ground fit. Standard 3-axis CNC milling comfortably holds ±0.05–0.1mm on most functional parts without special setup. Precision CNC milling can reach ±0.01–0.02mm, but only with a proven fixture, sharp tooling, and real inspection, and both cost and time go up accordingly.
How to decide what a feature needs
Three questions cover most of it:
- What kind of fit is this? A shaft that has to spin freely in a bore (a running fit) needs real clearance and a moderate tolerance. A pin that's meant to be pressed in and stay put (a press fit) needs a tighter tolerance on a much smaller target, often just a few hundredths of a millimetre of interference.
- Does this dimension stack with others? If four machined faces each have to land within tolerance for a fifth part to bolt on, the error at each face adds up. A tolerance that's fine on a single feature can produce an assembly that won't go together once four of them compound.
- Does temperature or material movement matter here? Aluminum and PETG both expand and contract with temperature more than most people assume. A press fit machined and checked at a cold bench can measure differently once it's sat in direct sun.
Where this goes wrong in practice
The most common failure isn't a part machined out of tolerance. It's a tolerance applied to the wrong feature: a decorative chamfer held to the same precision as a bearing bore, or an entire drawing marked ±0.05mm as a default because nobody wanted to think about which features needed it. Both waste money on precision the part doesn't use, and both usually mean the feature that really did need attention wasn't called out any differently than the one that didn't.
Takeaway
Tolerance describes what the part has to do. Figure out which features genuinely mate with something else, tolerance those deliberately, and leave the rest reasonable. A part with a clear, honest tolerance on the features that matter is both cheaper to make and more likely to fit.
