One of the busiest
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https://reprap.org/forum/
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https://www.reddit.com/r/3Dprinting/
pretty busy
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https://forum.e3d-online.com/
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https://forum.duet3d.com/
but depends on your wish to discuss most can be rather useles or not .. depends on many things, least of them is how "busiest/largest" they are... universal tech stuff, self building etc .. e3d and reprap are good ones, specific problem for specific printer, as already said, search for the community around your printer... the PRC ones usually center around FB groups, other ones usually have each their own community (teartime, ultimaker, prusa, craftbot..)
and you can find solution to your problem on strangest of places, like here, or on octoprint forum
.. you just need to spill out the problem
gcodestat integrates with Simplify3D and allow you to
Calculate print time accurately (acceleration, max speed, junction deviation all taken into consideration)
Embed M117 codes into G-Code
Upload your G-Code directly to Octoprint
open source and unlicence