belt grinder

I've been fooling around with building a belt grinder, mostly muddling it over. Finally decided (New Years resolution) to use it as a project to teach myself FreeCad. I came up with the following idea, combining a belt grinder and disc sander. Maybe a boondoggle? Anyway, thoughts/comments...

I'm thinking about building a 2X72 belt grinder for shaping/sharpening HSS lathe tooling and general shaping/deburring metal parts. I have a 6" bench grinder now and it works, but it's slow. But, I have some questions on the design and construction. Why are most belt grinders 3 or 4 wheels? I'm...