The turntables I have built in the past have used fixed direction wheels (instead of swivel casters). The wheels are placed on the bottom of the platform so they are perpendicular to a line between the piviot point and the center of the wheel. To put it another way the axels on the wheels should all point toward the pivot point.

The pivots where never anything special, just a piece of 2" diameter threaded pipe that was screwed into a pipe flange attached to the stage. On the bottom of the platform I drilled a coreponding hole into the frame in the center. The pivot wasn't doing anything more the preventing the platform from drifting when it was turned.

I have never build on bigger the 8' in diameter but the same principles should apply.

Dan

I'm webmaster for Marshalltown Community Theatre and we have a short video of a LARGE turntable we built for the sets of THE WOMEN.  The process was very similar to what has been stated above, but if you want to see the video, check out:

http://www.mctiowa.org/Season%20Bill%20Archive/Women/Archive 40-2.htm

Scroll down to the bottom of the page.  The file is about 2MB in size so it might help to have a DSL or cable connection as well.  Dial up will work, it's just kinda slow.  Enjoy!

You have probably produced the revolve by now?
But if you run into to any strife, with the central pivot - Try utilising an old car ?s(vehicle)  wheel stub axle assembly, with out the wheel. You can pick these up cheap form any  auto wreckers usually. Because the axil has a housed bearing the revolving action is very smooth. They are easy to modify as most of what you need is on it. The wheel studs are utilised to mount a modified plate to lock in each of the revolves wedged sections. The axils mounting plate is used to bolt to a stage plate.
The trick of inverting the fixed castors, suggested by arty, is an excellent idea.  However if your not allowed to screw or bolt anything to your stage. We used MDF boards under the revolve to hold the revolve, which because of the weight holding down the MDF to the stage, had no sideways slip at all. Further to turn the revolve, we ran a cable around the revolves plinth  & utilised a simple barrel windless to operate it. If you use wedged sections, a good idea is to make the tops or plates over lap on one side, so they interlock the next section. To hide the cable if your not using a false stage to surround the revolve. Can be easily masked by decolouring the cable or rope with dye or a thinned paint solution - the same colour as the plinth of course!