Am finishing up a board design, my first one. A lot of the work was done while units of measure in KiCAD chosen was “mm”. So, when switched over to “inches” notice some oddball dimensions. Meaning, when I laid out plated hole diameter to be 1mm, it converts over to 39 mils. I did not find at couple boardhouses FAQs mention of preferred/default units of measure.
Am asking if I should go through and convert everything to mils, as in above example, change to 40 mil hole diameter? I guess cause I see a boardhouse having a 40 mil endmill, not a 39 mil endmill. I am inexperienced in these matters.
When you switch in KiCad between in of mm, it does not change anything about KiCad’s internal resolution. If it did you would get a stackup of rounding errors.
KiCad works with an internal resolution of 1nm or 10nm (Cant rember, there is also a difference in what part of KiCad).
Gerber files seem to be only in metric:
Pcbnew / Plot / Plot format: Gerber.
While the drill files can be metric / imperial, but one of those does not work properly I think:
Pcbnew / Plot / Generate Drill Files …
There is probably a variation in what board hoses do with your design.
Drills may very wel be rounded to the nearest 2mil, 3mil or to metric drills.
Note also that plated hole diameter is usually interpreted as the diameter of a finished hole. The drill uses is often bigger, so the finished hole is (close to) your specified diameter after plating.