Reasons machines are made of metal ......
It's about production quantities. Stampings and casting are far easier to mass produce than wood pieces with possible exceptions to something CNC made. Welding can be automated as can surface grinding and boring. Mass is a big consideration for reducing vibration and stability.
Not to say that wood can't be made massive and substantial, but metal is better for larger quantities. Wear is another factor, like table surfaces. In the video shown above he laminates HPl for the table surfaces subject to wear, a good choice.
For a home shop on a budget, that DIY wood edge sander would be perfect. The same concepts could also be used in a platen or panel sander. I though using the router to round the drum was very creative. Then shimming each side to partially taper it was also slick.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)