I would like to download a free symbol library of common bolts, nuts, hex cap screws, allen cap screws etc. Does anyone have a reference to one?
2D or 3D? Do a web search. You'll find many examples, for "free" or, for a fee.
I would suggest, as it has been my own experience, that you might be better creating your own selection, as suggested by Roland.
It's a good way to learn, and, is very quick, once you've got the hang of it.
You can design without engineering, but you cannot engineer without design.
Using Win 10 with Designer 2016/2017 and TurboCAD Pro. Plat. 2018/2019 + Lightworks (64-bit versions) + AnimationLab 5.2.
Windows 10 Pro 1903 (Build 18362.418) 64-bit