Gearing is totally dependant on the application, if your building a mill, your going to want more Lb-force and accuracy, while speed and acceleration are less of a concern. However, if your building a plasma, the speed and acceleration are your primary concerns while the accuracy is less of a primary concern and Lb-force for cutting is nill, your still going to need some level of force to get the acceleration rates up high, but not for sustained cutting. Gearing is used to take a motor and modify the output to fit the level of performance you want. Therefore a Mill will typically have more reduction than a plasma cutter would. You basically need to sit down and figure out what you want and via some easy math figure out what level of reduction you need.
Jerry [using a 10:1 reduction w/ 2" pinion on my X axis..]
JerryFlyGuy
The more I know... the more I realize I don't
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)