My Mark V began life as a Model 500, then was a 510, and finally has been upgraded to a 520.
The two places where you'll see the biggest difference between the 510 and 520 are in the table saw mode and horizontal boring mode.
Table Saw Mode
One very handy of the feature of the 520 is measuring tape. Simply lock down the fence at the appropriate distance and your wood will be the exact width.
The other nice feature is the heftier fence. It locks down more securely than the 510 fence, plus it has more slots for attaching things like hold-downs and featherboards.
Horizontal Boring Mode
One of my favorite uses for the Shopsmith is as a horizontal boring machine, but with the 510 there was just enough flex that it could be a problem when drilling a large hole in a hunk of hardwood. The 520 locks down with extraordinary pressure, and flex is not a factor.
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