[NeoMoses]

Although some of your thinking on position feedback is sound, it does not eliminate the need to minimize backlash in an assembly. When machining, cutting forces vary and any backlash will allow the tool to 'wander' thus cause at a minimum gouges in the workpiece, possibly broken parts and/or tools, depending on the amount of backlash.

If you have elminated virtually all of the backlash in a system, the next logical step to improve its accuracy is to put the position feedback on the table, not the screws. If you look at most of the professionally made machines, they are done this way, usually with linear position encoders that are very accurate and very pricey.

As for the drive on your 8' router... a tensioned timing belt should work fairly well, but don't expect to be machining at 200+ IPM with it, the accel/decel will definately cause some stretching over that length. Other options, such as screws, are available, but at a higher cost. Take a look at the Cad Cut CNC router, they have a good drive method, too. Basically, with such a long span, you'll want the cross sectional are of any cable or belt to be maximized to reduce the stretching. Do some quick F=m*a calculations to see what your max acceleration forces should be, then size your cable/belt accordingly to give you the accuracy you desire.