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Sketch-up us is useless toy from engineering point of view. Good for "sketchy" modeling. FreeCAD is fully parametric 3D cad system. It's great for precision design. Unfortunately buggy as hell.

It may not be open source, but DraftSight is free to use (commercial use included) and has 99% of the functionality of autoCAD for 2D stuff, but its a bit anemic if you are doing 3D.

I use it all the time at work, very easy to pick up if you are used to autoCAD (similar interface).

https://www.3ds.com/products-services/draftsight-cad-softwar...

I haven't used it in a while but there is QCAD. It doesn't do everything that AutoCAD does by no means but they have been working on it for a while.

QCAD is great, but I would not consider it free: without paid package is not that usable. But this package is just 40 bucks, which is really cheap.

LibreCAD is a good QCAD near-clone that is open source.

I'm all for free but if you're doing actual engineering drawings have you checked out AutoCAD LT? Much cheaper than full blown AutoCAD.