I've been meaning to try my hand at CAD and designing models to print but I haven't quite made the jump.
One thing that has given me pause is a good CAD program for Linux, does anyone has any good tips for a complete Newbie where to begin?
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Downsides are that the CAM plugin is paid-only (irrelevant for 3D printing) and you're obviously trapping yourself in a commercial, proprietary walled garden that might start charging subscription fees or otherwise rug-pull you once it gets popular enough. I've decided that the ease of use benefit is high enough to warrant the risk - I'd rather risk not being able to edit my models in the future than not creating them in the first place because the alternative software is too painful to use.
It's helpful to understand how the software works, because it's different from what you might have experienced from other software: It essentially stores operations, like "start with this sketch, then extrude this part of it to a height of 10 mm, then add a fillet". You can go back and edit previous steps and the following steps will be directly re-applied.
In sketch mode, you can just draw, but you can also add arbitrary constraints, e.g. "these points have to be exactly 3 cm away" and it will adjust your sketch to match the (new) constraints. This makes it really easy to change some aspect of the part later. This is common in CAD software, although OnShape's implementation seems more intuitive to me than e.g. Fusion 360.
If you want to do actual 3D CAM (for CNC machining), Fusion360 seems to be the only free option (not available for Linux).
In general, with all CAD software, the common "just poke at it until you figure out how it works" approach doesn't work well, although once you've understood the basic concepts that I've explained above and know some CAD terms/concepts like creating 3D parts by extruding or rotating 2d drawings, Onshape will mostly let you get away with that approach. You probably should still watch tutorials before you start.
The free CAM available in F360 has been artificially limited to only allow extremely slow travel speed. It's almost useless.
You certainly won't want to use it for mass production, but for hobbyist use where getting the model and CAM config right, setting up the machine etc. are the biggest time sink and most parts are made in quantity 1, I found it acceptable.