When you are designing a part you need to keep this in mind. PLA shrinks as it cools, very slightly, but enough that it makes things not fit. My new printer is using 8mm rods and linear bearings for the carriage. To mount the rods I had made 8mm holes in the model where they needed to be. The rods do not fit, it is close but not great. To resolve this currently I am just going to buy an 8mm drill bit and enlarge the holes slightly.