# Incorrect inner dimensions of 3D prints with Ender 3

I have problem with my Ender 3. Whenever I print, I have proper outer dimensions (accuracy usually better than 0.1 mm, sometimes a bit worse), but every hole, pit or any inside dimension is inaccurate (0.6 - 0.8 mm never less or more).

I have tried extruder calibration. Calibrated extruder voltage and e-steps (in Ender firmware), but this does not change a lot.

Here you can see test dimensions I modeled:

And it will come out with these dimensions:

Aaand the final print looks like this:

As you can see, my estimation (second picture) was almost perfect. 19.3 mm and results are weirdly consistent. Always holes are 0.6 - 0.8 mm smaller than holes in model.

Do you have idea why this is happening?

Specs:

• Printer - Ender 3 with original circuit board
• Printer software - Marlin 2.0.5
• Model cutting software - Cura 4.8.0
• Modelling software - Fusion 360
• Found a solution in Cura – Trish Mar 3 at 10:16
• What's your line width? If the wall width isn't a multiple of your line width you can end up with a bigger or smaller wall width depending on the way the model is sliced. It shouldn't cause a big error but maybe it adds to something else. – Yareon Mar 4 at 12:43

There are 3 effects at work, and you misread your micrometer: the measurement is 19.35 in the picture.

# You have a little lip

There's a little lip at the top and bottom of the print. You'd need to clean that up with a sharp knife or sandpaper. That is the biggest part of the error you measure.

# Movement errors accumulate

Errors also collect on the center of holes due to the order in which walls are usually placed, resulting in outer walls having the correct diameters but inner holes having a small offset.

# Plastic shrinks when it cools

A smaller part of the error is the plastic shrinking as it cools, but that can be compensated for by the slicer - if your printer allows for it: Under Materials, there is a Shrinkage Ratio setting.

# There's compensation for that in Cura

The option is under Shell and called Hole Horizontal Expansion. Setting that value to 0.6 mm to 0.7 mm should solve the hole sizing error.

• The error here is way too large to be shrinkage with PLA, especially as it's a thickening of the part not thinning. I'd suspect loose belts here. – R.. GitHub STOP HELPING ICE Jan 1 at 17:29
• I read 19.3 mm from my micrometer, proper value (will not argue about 0.05 mm). On this side of 3D print there is no "elephant foot". I am still missing 0.7 mm (or 0.65 mm with your readings). – Jan Kowalski Jan 2 at 12:35
• So there is no little lip and I think that PLA does not shrink that much. 0.6 mm is a lot when working with holes as big as 5 mm for example. I guess my problem is "Movement errors accumulate". Is there a way to improve that? – Jan Kowalski Jan 2 at 19:19
• @JanKowalski usually try to use multiples of wall thicknesses to get that small... or you might compensate by demanding a slightly larger hole... – Trish Jan 2 at 23:39