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3DP Tolerances and Fits

3DP Tolerances and Fits

Owned by Mike Angus, created
Last updated: 15 Nov, 2024
2 min read

To a limited extent, it is possible to design for specific tolerances when 3D printing. This is particularly important when parts need to be inserted into others with high locational accuracy or an interference fit.

This page is to record results of specific fit tests that have been conducted in the past, to provide a starting point for appropriate dimensional offsets to use when designing new parts.

For reference, here are links to a useful table of ISO tolerance designations, and some guidance on the tolerance pairings to use for different kinds of fits

At this level of precision, the exact outcome is highly dependent on the machine, its calibration, the material used and the geometry of the part. As such, the results recorded on this page are just to provide a starting point, and do not guarantee a particular tolerance.

 

F170 - Bearing Push Fits

Testing a press-fit pocket for a 26mm plastic and glass Igus bearing. Using F170 printers and ASA material, 0.254mm layer height. Printed in horizontal orientation.

The printed pocket was slightly elliptical, so measurement with an internal micrometer produced a pair of results rather than a single number.

The pocket will deform when the bearing is pressed in, so the exact final diameter of the pocket is undefined and will lie somewhere between the measured values

I’m focusing on the tolerance ranges with a 7 (N7, P7) since these seem to be commonly cited in fit tables. In some cases the measurements could have been classified in multiple categories.

  • Pocket diameter 25.9mm produced the following results (two different machines, same test part geometry):

    • 25.84mm-25.87mm (-13 -19) and 25.83mm-25.9mm (-10 -17)

    • So in this case a -0.1mm offset from the nominal diameter produced an N7 tolerance.

    • The plastic Igus bearing pushed in with a small amount of force by hand, and ran freely in this pocket

  • Pocket diameter 25.85mm produced the following results (two different part geometries, same machine)

    • 25.71mm-25.81mm (-19 -29) corresponding to a P7 tolerance

    • 25.76mm-25.97mm (-9 -24) corresponding to an N7 tolerance

    • So in this case a -0.15mm offset from the nominal diameter produced an N7 or a P7 tolerance, depending on part geometry.

    • The plastic Igus bearing pushed in with a reasonable amount of force by hand, and ran mostly freely in this pocket, with a couple of higher-friction points

  • Pocket diameter 25.8mm produced the following result (single test piece only)

    • 25.67-25.78 (-22 -33) which is in the P7 range

    • So in this case a -0.2mm offset from the nominal diameter produced a P7 tolerance

    • The plastic Igus bearing took a lot of force to push into this pocket by hand, and ran with noticeable extra resistance to rotation.

Note that the above results do not account for the print seam which can be present on the inside face of the pocket, which would of course change the diameter at that single point. I found that the seam can be removed quite easily with a straight deburring tool.

 

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