Repeatability is very good for these machines and can be as low as .001" depending on the way you control the machine. If you use the spindle, you can be moderately aggressive, but you will want to use a finishing pass to make sure the edges are exactly what you would expect. since this is a kit, there will be varying levels of repeatibility depending on how well the kit is assembled and maintained. Calibration, as with all machines, is a critical aspect of keeping repeatibility over time.
Timing belts and chain are about the same when it comes to precision as long as the chain wraps most of the drive sprocket. There are differences to each of these systems as Kevlar with steel cables impregnated within the Kevlar and the steel used in roller chain have different coefficients of linear expansion (the expansion and contraction over temperature change), and roller chain may stretch over time, so it is important to maintain the machine by tensioning the chain and timing belt, and calibrating the machine over time and when the temperature changes significantly.
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What resolution and level of repeatability can one usually expect from a kit-built cnc, and is there a significant difference between timing belt and chain drive systems?