Hello everyone I am designing a parts that needs to rotate so I am planning to insert a round bushing.

But I am curious of how much space I need to leave between the round bushing and the parts that need to rotate? (Assuming the round bushing will be fixed by tighten the screw)

The outer diameter of the round bushing is 3.5mm so how many mm hole should I design for that 3.5mm round bushing? If produce by a CNC machine how accurate can it mill?

Thank you very muchenter image description here


1 Answer 1


This is one of those questions that can be answered by "you get what you pay for." Ignoring the poor grammar of that statement, one does have to spend money for quality performance.

In the 3D printer world, you'd have to have as much as 0.2 mm spacing to ensure the rotation you seek. In the CNC world, it's not that large. A friend who owned a machine shop created parts with his CNC with tolerances of as small as 0.0005". That's five ten-thousandths of an inch. In these circumstances, one must ensure temperatures are uniform for both parts. If the parts were machined in 65°F conditions, those parts might not fit together in colder or warmer conditions.

You can get a good sliding fit with room for lubrication by having a clearance of two or three thousandths of an inch. When I want a clean sliding fit for tubing, I'll purchase a "standard" size from Aircraft Spruce and Specialty, say, 0.375 outside diameter. The matching tubing, next size up would be 7/16" in the list, and I'd have to pick a wall thickness to match the 0.375, which in this case is 0.035, giving me an inside diameter of 0.368, a difference of 0.007". The next size down would cause too much slop and the next size up would result in an impossible-to-fit set of tubes.

Seven thousandths of an inch is tight enough that a small burr will prevent a fit. One last consideration is your bushing size. Typically a manufacturer will specify that part's tolerance as well. Is the 3.5 mm listed as +/- 0.1 or something similar?

  • $\begingroup$ thank you very much for your very detailed answer! $\endgroup$
    – tempo3010
    Mar 29, 2019 at 3:54

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