I am looking for a way to create some complex surfaces, does anybody know if there’s an Add-in or a free (and friendly) software to create it?
Will mutliple equation driven curve in 3D sketch work for you?
Someone did create a macro years ago according to my bookmark but it had gotten deleted… https://www.3dcontentcentral.com/macros/download-Macro.aspx?id=6290&catalogid=171
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I’ve never used them, but isn’t that what MathCAD and Maplesoft do?
Yeah, did not find it here 
Need to be surface, I want to do something like these:
http://k3dsurf.sourceforge.net/
These software look pretty cool, but they also look too much for just some design modeling.
Thought I was missing some Add-in because I found 3DPlot for AutoCAD and Equation Driver Surfaces free for Fusion360; FreeCAD also has something because is written on Python, if I am not mistaken. 
What do you want to use the equation driven surface for?
Will it work if the surface is in STL?
There are various ways to mathematical surface in STL
Eg: MATLAB
You can also export the surface “points” and form the surface in SOLIDWORKS
Just make some design like these for now:
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You mean to generate the points and create Curves Through XYZ Points? Or something else to read the STL surface and generate it in Solidworks?
There’s also something in Blender, although I don’t know how to import this kind of surface to Solidworks yet.
