oz/in. on a servo is it the same as on a stepper?

[CATCH22]

im looking at getting into servo motors for my lathe. it is 12x40 and im not sure what size of servo to go with. i see servos on ebay and the oz/in. seems way to low for the size of the servo motor.is there a difference between a 80 oz/in servo vs a 80 oz/in. stepper. this of course is just an example. any info would be appreciated. thanks mike

[Karl_T]

Steppers have more torque at very low RPM and can rapidly lose it at higher speeds. On servos, torque remains constant up to rated RPM. Therefore reduction gearing on a stepper to get more torque can be a bad idea, but it will work very well on a servo.

karl

[Halfnutz]

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.

[mcpltd]

Also the rated torque of a servo can be up to one third of the peak available.