I am looking to build a 4×4 machine, large enough to slide an 4×8 sheet in, and may be run it twice for a sheet.

1/ The Vee Grove bearings only make point contact on the corners of the aluminium rails I can see this causing premiture wear and the Vee actualy grinding down the edges losing its acuracy. I would say instead of using one Vee bearing you should use 3 regular bearings, one for the downward forst and the other to keep it inline with the rail, one on each side.

2/ I like the idea of the Roller chain and sprocket but not sure how the repeatability is, will it always hit the same place every time.

3/ No one will answer my questions, now that generates huge doubt.

So how did your machine work out.

Let me know
Ray