Convert 3D objects to .obj or .mtl format

CoSpaces imports 3D objects in .obj or .mtl format.
What free software do you recommend to convert a 3D object in one of these formats?

Perhaps using sketchup maker with a plugin? I have found that is a great repository

I create for my students 3D printable mechanisms with moving parts relative to each other (with rotations and / or translations). I use for this the professional software Onshape:
OnShape allows to model complex parts in 3D, to assemble them simply by choosing the type of mobility (kinematic joint), to simulate the kinematic behavior, to export these 3D modelizations in several formats, whose format .stl to be able to print the 3D parts, …
Like CoSpaces, Onshape is a web application, but Onshape also exists for mobile. The license for Onshape is free for educational use.

Onshape allows to import a file in .obj format but not to export to this format (not yet at least). So I tried to go through Sketchup Make 2017.
I managed to export my work from Onshape in .dae format (COLLADA).
With SketchUp Make 2017 I was able to import it in .dae format and then export it in .obj and .mtl format (in one go).
With CoSpaces, I could import it in .obj and .mtl format (in one pass).
But CoSpaces automatically changes scale and you can’t change the scale by choosing a numeric value. Si I can not assemble precisely the parts of my mechanism, because they are not at the right dimensions.
In addition, CoSpaces does not yet allow to simply assemble parts by choosing the type of mobility (kinematic joint). I have to do it with Onshape.
In short all this process is still complicated and I can not do what I want to do.

The .obj format is interesting because it keeps the color of the parts. But in my case, when pieces are different colors they are moving relative to each other. So It is easier for me to import each part (or set of parts) separately so that I can move them separately with Blockly. So in most cases, it’s easier for me to import my pieces in .stl format and then change the color manually from the CoSpaces UI.

There remains a major problem: the automatic change of the scale when I import an objet in CoSpaces.

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I am modeling a 3D printable model of this system:
Theater flying dynamic attractions (3min21).
It will be animated by two or three motors and automatically controled by a BBC micro:bit board (a very simple electronic programmable board).
I hope then to be able to simulate the behavior of this system with CoSpaces.

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I have the same problem of imported objects as you. If a scaling factor can be inputted before import, it would be nice.

In addition, if the scaling button in CoSpaces can be numerically inputted, an accurate and uniform scaling factor can be applied to several objects.