What Karl T says is the basic difference. In otherwords if you have a stepper that runs at 275 oz/in up to 1200 RPM and then power drops to its max RPM of say 100 oz/in (or even less) at 2000 RPM. So you gear it 2/1 and end up with a pathetic 50 oz/in at a max RPM of 1000.
1000RPM with a 5TPI screw gives you rapids of up to 200 IPM, but with 50 oz/in of torque its not very usefull.
Then you have a 4000 RPM servo with a 100 oz/in of torque throught its speed range. You gear it 4/1 and end up with 400 oz/in throughout its range up to a shaft max RPM of 1000. With that same lead screw you have 400 oz/in of torque from 0 - 200IPM and allmost instantaneous acceleration, and a method of insuring position accuracy.
Here are some pics of the new gantry I'm building. I am getting 600IPM rapids on the long screw with over 400 oz/in of torque and 400IPM rapids with 200 oz/in's on the short Z axis, while running Mac3 at 25KHZ. All that I need to do now is decde on how I'm going to put the X axis together. Iv'e got another 200watt servo and controller, like the Y axis shown to drive it, or two 100 watt drives and motors like the Z axis is using for a dual drive X axis.
Halfnutz
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)