gear boxes are expensive and have backlash.. the commercial routers ive seen use big belts on 1/2" or larger rods to reduce.
also, 'helical rack and pinion' isnt exactly what you need. regular rack and pinion will be fine. if you use gearboxes as mentioned, helical gear boxes have less backlash, but also often add a 90degree bend.. but really, unless you have a good hookup for some nice gearboxes, why go through the trouble? mxl pulleys are industry standard, easily replaceable, comparably inexpensive, etc. very little backlash, easy to tension! so many advantages.
what do you mean how to calculate rapids? first, rack and pinion is fast. the ratio you need to calculate everything is driven by pinion size.
say you have 200step/rev motors, 4x microstepping = 800seps rev, say you reduce this down 4 times back to 200 step/rev. ok. now say this is driving a 1" pinion, each revolution will move the machine the pinions circumfrence, which is diameterx3.14 so, 3.14!" - imagine! this means that you have a resolution of 3.14inch/200steps = .0157 resolution, but not accuracy as you are using 4x microstepping. the accuracy will be the motor accuracy, parallelism of everything, right down to the runout on the spindle.
this was a simple example, i realize you want much higher accuracy, etc.
Here is good news, the speed! with the above example, lets say your stepper can reliably do 400RPM under full cutting load. 400RPM x 3.14 inch/rev Hah. 1256ipm! not bad, eh!
Using a horizontal shaft and belting instead of 2 motors will save $ over a driver board and motor. it sounds more complex, but it looked simple the times ive seen it..
someone please check my math.
http://www.mcmaster.com/
http://pergatory.mit.edu/2.75/softwa...and_Pinion.xls
may help