I am sure that this is not a new question. On my 4 axis mill I will be using rack and pinion on the X asis, 13' of travel, and life would be much easier if I could have the pioion mounted on the shaft of the stepper. The gantry will weigh less than 50 lbs, and use a 3.25 HP router, speeds will run about 200 IPM max and max cut will be .25". I have planned on dual 640 oz steppers with gear reduction but why can't I step up to large stepper motors without the gear reduction?

Cost & availability & viability of finding the EXACT motor specs you want/need.

A motor with the torque you want usually won't have the speed,

conversely,

A motor with the speed you want won't have the torque you need at low speed.

Once you start to review the torque/speed curves of the available motors, you'll see what I mean....

If you're using rack and pinion and direct drive stepper the smallest move your axis will reliably move, and hence its resolution will be one full step of the motor or 1.8 degrees. Whats the travel distance on your rack for 1.8 degrees of the pinion?

Gearing is usual in R&P to provide smaller resolution - ie one step = 1 thou or what ever, as well as to get the motors to work within their torque curves at appropriate speeds.

I think with the rack and pinion you'll easily get the speed you want. You said 4 axis mill, but I don't think NC CAMS realizes it's actually a 13 foot wood lathe, so you should also have plenty of torque for the light cuts you'll be taking.

The downside is a lack of resolution, which depends on your choice of pinion.

A friend of mine has a Shopbot and they do exactly what you're suggesting. Go to: http://www.shopbottools.com/prtalpha.htm and click on Explore the PRTalpha. On my friends machine, they hinge the stepper and load it against the rack with a spring. On the machine in this picture, they're just using a turnbuckle.

Walt