FDM is accurate enough for a huge range of jobs, but it is not magic. Plastic shrinks as it cools, holes print slightly undersize, and tight press fits can be unforgiving. If your part has to mate with another, a little planning saves a lot of reprints.
What affects accuracy
Material shrinkage, layer height, cooling and part geometry all feed into the final dimensions. Round holes tend to come out a touch small, and outside features a touch large, so designing in the right clearance is more reliable than printing to nominal and hoping.
Larger parts accumulate more shrinkage across their length, which is why big flat assemblies need extra attention.
Designing for a good fit
For moving or assembled parts, we build in clearances that suit the material and the machine, and we can tune holes and slots so fasteners and pins seat correctly. If tolerances are critical, CNC machining may be the better path, and we will tell you when that is the case.