Internal dimension accuracy
Seems there's been a lot of stuff written about this, but I can't find what I'm looking for.
Specifically, I had a computer crash. I still have the hard drive but since the OS was Win 7, I pretty much had to reinstall everything with a Win 11 machine. Mk3s. So, fresh install, all my slicer settings from scratch.
I guess I have become complacent. I design my own precision fitting parts to print quite often so I've gotten used to padding a small amount of tolerance to get good fitting parts. I designed a part with an internal diameter of about 18mm. It printed at about 17mm. Part looked good otherwise, no elephant foot. I went back to double check my model and the measurement was correct. I checked the model in PS. It too was correct. This is where I designed a test part. Simple. 12.5 mm square hole and a round hole with the corresponding male part. I enlarged the holes 0.3mm, my typical tolerance for a slip fit. The parts fit perfectly, just like I was used to with my old computer and slicer settings!
Could perimeter settings have been the cause for the discrepancy? I had 8 perimeters on the mis-sized part and only 2 on the test parts. The 8 perimeters resulted in no infill between the inner and outer diameters of the print. I appreciate any input.
STL is the common transfer format. It's triangles describe the nearest approximation to any curve so all curves are effectively made of a sequence of straight lines. Draw one and put every point precisely on the curve allowing the lines to form chords. All curves tend to the concave side. Gcode tends to follow the line exactly so the extrusion will spread on both sides of the line by half an extrusion width.
Where both parts are 3D printed the error is similar on both so the fit is usually OK with tolerance around half an extrusion. When one part is made by a different technology 3D printed holes will tend to be tight and inserts loose.
Your CAD application probably has some sort of precision setting that will reduce this tendency but test printing and calibration is often the best method.
Cheerio,