Here's the main component for the new tensioning mechanism:


Got some extra bolts, and replaced the tracking adjusting rod with a bolt and knob:

The tensioning mechanism works real nice and I calculated that I can take up to a 50x1000mm sanding belt. So this sander can take belts that are 790-1000mm long, a span of 210mm.

Because of this wide span of possible belt sizes, the upper and lower wheels will be in different positions depending on belt length. So I made the platen so it could be moved and adjusted for various belt lengths.

Platen is made from a piece of angle iron that I literally found rusting under my deck this summer (some of the house builders must've left it behind) and had forgotten about. Now to paint it, I am thinking black and yellow.