Ask for a 10 millimetre pin to go into a 10 millimetre hole and you will be disappointed every time. In the real world nothing is ever made to an exact size, and two features sharing the same nominal dimension will either jam solid or refuse to meet at all. Designing in the right tolerance and the right fit is how you avoid that frustrating and avoidable outcome.
This is core to good CAD work, and it changes significantly with both the material and the process being used. What gives a perfect sliding fit on a precision machined metal part will be completely wrong for a printed plastic one, so the numbers have to be chosen with the manufacturing route firmly in mind, not copied blindly from a textbook that assumed metal throughout.
Nominal size is not enough
Every dimension on a real part has a tolerance, which is the band within which the finished feature is actually allowed to fall. When two parts meet, it is the relationship between their two tolerance bands, not their ideal sizes, that decides whether you end up with a loose fit, a sliding fit or a tight press fit that needs real force to assemble.
Designing a fit therefore means choosing that relationship deliberately, then setting the dimensions so the parts behave the way you want across the entire range of variation, not just at the one perfect nominal size that no real part will ever actually hit. Get this right and the assembly works whether you grabbed the first part off the bench or the hundredth one of the run.
Process changes the rules
A precision machined part can hold tolerances measured in hundredths of a millimetre, which means fits can be tight, repeatable and confidently designed close to the limit. A printed part is a different story, because it shrinks as it cools and varies more from one print to the next, so it needs noticeably more generous clearance just to assemble reliably without seizing.
That is exactly why we tune clearances to the process rather than applying a single rule everywhere. A clearance that produces a beautiful sliding fit in CNC machining would, if the same numbers were used, often leave a pair of printed parts effectively fused solid and impossible to separate. The geometry has to suit how the part is actually being made, every time.
How we settle on a number
Choosing a tolerance is a balance between how the part has to behave and how much accuracy you are willing to pay for. Tighter tolerances mean slower production, more inspection and more rejected parts, so we never specify them out of habit. We start from the function, decide how the parts must fit, then work back to the loosest tolerance that still guarantees that fit across normal variation.
Material and process come into it at every step. A part heading to CNC machining can hold a close fit confidently, while a printed part needs more room and is best proven with a test piece first. Where it matters, we make a sample through prototype development, measure the real result and adjust the model, so the production run assembles every time rather than just on paper.
Common fits and where they belong
Choosing a fit is really about deciding how the parts should behave once they are together in service. Naming a fit is just shorthand for that behaviour, and matching the right one to the job is what keeps an assembly working smoothly rather than seizing up or rattling loose.
- Clearance fit for parts that must slide or rotate freely.
- Transition fit for parts that locate accurately but still come apart.
- Interference fit for permanent press together joints.
- Generous clearance for printed assemblies that must move.