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Is there a convention for the scale of the 3D printing STL files that are shared online and/or expected by 3-D printers?

I found a model on Thingiverse that is useful for my project, a connector that must fit with other parts, so precision is important. Eyeballing, it seems that each STL unit corresponds to 1 millimeter, is this a safe/conventional assumption?

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The STL File format is unitless, It only contains relative positions for the vertices of the mesh which makes up the part. As such, there is no true "standard." However, most 3D printing focused slicers will import STL files on the millimeter scale.

The vast majority of STL files you find online will be in mm units, however, It's important to note that such is not always the case.

Typically however, a file imported in the wrong unit will be very obvious, and unless the model author has done something very unusual, scaling should be trivial within your slicer software of choice.

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    $\begingroup$ This. By convention, STL units are mm, but this is a human convention not conveyed in any way through the data. Every once in a while you find files where the author has done something else (inches is probably the most common) or (mostly in pure-art/non-functional models) where the author designed freehand with no care whatsoever for the scale of the numbers and you just need to pick a scale you like for printing. I find the latter case are almost always very-low-quality because the author paid no attention to printability affects of detail resolution. $\endgroup$ Jan 9, 2023 at 8:24
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STL files are based on coordinates rather than individual units of measurement as they're supposed to be completely unit agnostic in order to allow them to be evenly scaled on any platform, and to not be bogged down with unit translations.

Typically, the units are set by whoever makes the model, and by whatever the default of the software that they use is. In most cases, creators use a one-to-one scale.

So, the answer to your question is that the conventional scale is "100%". Though probably isn't the answer that you're looking for.

If you print it at 100% it will be whatever size the creator intended it to be. For example, if you were printing a Warhammer miniature and it has a base attached to it, and you print it at 100% then that base will be 25mm wide, and the figure will be the exact height that the creator intended it to be. Whatever that height is.

For the purpose of your question, if the part "looks" like it was created in MM then it almost certainly was.

If you take that STL file and put it into your slicer, then use the default scale (100%), you can use the slicer to determine how its width or height or length in MM. Just set the scale of the model to whatever makes that dimension the same as the dimension you need it to be and the rest of the model will be scaled accordingly.

Your biggest problem might actually be the material that you use, as some will shrink after printing, or shrink unevenly.

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