Bdtyktl

FreeCAD

If you haven't tried FreeCAD, you're missing out. It's a fully parametric 3-D modelling package that can be extended with Python. The interface is reasonably user-friendly, and feels like a "younger" version of Solidworks.

Installation: Just install freecad package , or newer version using PPA. Also you have an AppImage.

OpenSCAD

OpenSCAD is a software for creating solid 3D CAD models. It is free
software and available for Linux/UNIX, Windows and Mac OS X. Unlike
most free software for creating 3D models (such as Blender) it does
not focus on the artistic aspects of 3D modelling but instead on the
CAD aspects. Thus it might be the application you are looking for when
you are planning to create 3D models of machine parts but pretty sure
is not what you are looking for when you are more interested in
creating computer-animated movies.

To install OpenSCAD, just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:

sudo add-apt-repository ppa:openscad/releases

sudo apt update

sudo apt install openscad

For more CAD apps see 8 Best CAD Apps for Linux

BRL-CAD

Maybe BRL-CAD would suit:

BRL-CAD is a powerful cross-platform Open Source combinatorial Constructive Solid Geometry (CSG) solid modeling system that includes interactive 3D solid geometry editing, high-performance ray-tracing support for rendering and geometric analysis, network-distributed framebuffer support, image and signal-processing tools, path-tracing and photon mapping support for realistic image synthesis, a system performance analysis benchmark suite, an embedded scripting interface, and libraries for robust high-performance geometric representation and analysis.

There is no free 3D solid modelling CAD software worth using.

FreeCAD is the only one coming close, but "close" in a relative sense... It does not do fillets well, boolean operators do not work, you can't even create sketch planes!

So unless you want trivially simple, blocky models for free then you are out of luck.

Sometimes you have to pay for real software. Varicad is a very good linux compatible system and the price is very decent indeed. I have tested it against Solidworks and it performs just fine. It does not have built in FEA but there are many such programs for Linux. Try the free trial and see.

Try Nanocad with the Wine emulator. Nanocad is the most similar software to Autocad I have found; I haven't tried it with Ubuntu but you can take a look at Wine's page for more information on how to run the Windows version on Linux.

You must be looking for something like MEDUSA Mechanical CAD: http://www.cad-schroer.com/products.html. I don't know how good it is, never used it, but it's the only specialized CAD i found in linux.

use SIEMENS PLM NX. Although its not free and there is no ubuntu inbuilt support but u can use it via adding some libraries and applying some modifications to your os as it is supported in redhat and suse linux

EQUINOX-3D has advanced freeform and numeric CAD, photorealistic / physically based rendering, animation, and tons of other features.

We use it heavily and exclusively for advanced CAD modeling / 3D printing very complex, organic shapes, like robot parts.
Here's the list of the latest features, movies, etc.

All images and movies on the Web site were a 100% created with EQUINOX-3D