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±19% or ±10% or something completely different?

For small objects printed with PLA filament, it may vary significantly depending on the orientation of the object.  We did a test with a stepped print, modeled in TinkerCAD, that varies in size as a 1mm, 2mm, 3mm,4mm and 5mm rectangle that is 15mm X20 mm wide.  This was then used to create a keyhole in a duplicate of the object.

This was then printed with a 0.3mm layer height. The printers have a 0.4mm nozzle. The one with the hole was printed horizontally, so that no supports were needed.   The  “key” was printed vertically.  The vertically printed dimensions measure 1.18mm, 1.94 mm, 2.81mm, 3.90mm, and 4.94mm.  This ranged 19% larger-19% smaller then the model dimensions.   The total length and width(height in this case) were 15.29mm X 19.83mm or roughly 2% larger and 1% shorter .

When it was printed horizontally, the steps were 1.11mm, 1.80mm, 2.78mm, 3.82mm, and 4.98mm.  This ranged from 11% larger to 10% smaller. Length 15.55mm X width 20.2mm or  3.5% larger and only 1% wider.

The interior keyhole measured 0.79mm, 1.88mm, 2.82mm, 3.9mm, and 4.58mm.    This ranged from 26% smaller to 6% smaller. The smallest measurement was difficult(14.38mm X 4.33mm).

This test was only printed once and there my be more variability between individual printers as well as variance based on the uncalibrated Harbor Freight caliper used for the measurements. The layer height may also affect these results.

Dremel additionally claims in their FAQ at https://digilab.dremel.com/support/faq the following:

The 3D40 and 3D45 Printers have a maximum resolution of 50 microns (0.05mm), the 3D20 has max resolution of 100 microns (0.1mm). All printers have a minimum resolution of 300 microns (0.30mm). The higher the resolution, the smaller each layer and the better your object will look. The layer resolution depends on the setting: Low – 0.3mm Medium – 0.2mm High – 0.1mm Ultra – 0.05mm (3D40 and 3D45 only)”