Initial design questions

Im a newbie In preliminary design phases of a gantry router about 4' x 6' or 4' x 8' The question is in the design of the X axis "long one" I was thinking of ball screws until I saw a post on castcnc which used a rack and pinion drive. My initial thought was 2 screw drives down each side. the question here was should they be tied together with a belt (to sync them) or can you get 2 motors to drive at the same time/speed.

In the castcnc design it looks as if they are using two motors on the RP but since they do not provide all the "critical motion parts" it is an unknown as to what the details are.

I do not like the idea of a single screw down the middle because of the table support issues. What do major manufactures use?

Screws have problems with whipping at the speeds you'll want when they're that long. 4' of travel is about the max unless you get some really large diameter screws. I'd highly recommend looking at rack and pinion. Check out cncrouterparts.com for example. You can make 2 motors drive at the same speed. That's how I run my router (with screws) and how many other people here have their machines set up. It's called 'slaving' motors together in Mach3. You also get the advantage of twice the force to push around the heaviest component on the machine. You can also set up individual home switches so you can square the gantry every time you home. It's a pretty cool way to go.