Multiple reasons really. Mostly, I tried to build something like this
before using plywood, full extension drawer slides, and generally cheap
components, and it shreded my workpiece pretty good. I want to do it
right this time. An ME could maybe explain better, but the Acme
screws have a vertical face will push my table the direction I want it
to go, while the hardware store rods have the "V" shaped threads that
will transfer some component of the applied turning force in the wrong
direction for my application. I think the "V" threads are designed
more to hold a night at a high torque than they are to advance the bolt
on a rotating shaft. Plus a small threaded rod that my drill could
turn would bend when the router bites the wood, and a thicker rod would
smoke my drill trying to turn it.

Part of where I need the accuracy is in controlling the feed rate. If
the feed rate is to slow the bit will burn the wood deeper than can be
sanded away, and if it is too fast I will burn up router number 5.
Another place where you need accuracy is stopped dados and sliding
dovetails. If you look at something like the Incra jig for routers, it
uses a lead screw just for the fence. And mostly I want to might want
to use this sliding table as part of a stage some other CNC project.

I won the bid last night for linear bearings on 3' shafts, so parts are
on the way. I will try to post a link to a picture when this is
finished.

- James B