Accuracy, Precision & Tolerancing
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3D printing to dimensional accuracy can be tricky for a number of reasons.
The molten plastic shrinks as it cools to room temperature, guaranteeing dimensial accuracy is therefore hard to dial in.
Printers have a tendency to 'overshoot' slightly in corners as the printer head is forced to change direction. So corners have a tendency to be larger than they should be.
If dimensional accuracy is important, you may have to adjust the size of the model being printed by incrementally changing the size, printing and measuring.
FDM 3D Printing inherently prints slightly larger than when modelled using a computer. It is therefore important to account for this overprinting when designing parts that are intended to fit together.
While it may be possible to sand the parts down to fit together accurately, this is often a lot of work and the fact that achieving this fit by sanding may actually wear through the shell of the print revealing the infill.
Instead, to allow for this overprinting it is best to undersize the parts by a collective 0.5mm. For example if you are attempting to fit a round dowel-like shaped 3D printed part into a larger 3D print, reduce the diamater of the dowel by 0.25mm and increase the diameter of the hole in the larger print by 0.25mm; for a combined tolerance of 0.5mm.