Rick, the .2"/turn lead has been good, but I (personally) would go with something coarser next time around. For my machine, with a 25x32" work envelope, a .5"/turn lead (or probably 10mm, 12mm, 15mm, metric) would be a very good fit.

You're correct that is is a bit of a trade off in terms of speed/accuracy. With steppers you may lose some positional accuracy because you're dividing each screw revolution into 200 steps. Fewer revolutions per inch means less granularity with motion.

My servos have 1000 CPR (quad) encoders, which I am only using in 200 step/turn mode, so I could definitely use a coarser screw and adjust the step count to keep the same accuracy I'm used to. The accuracy is a function of the encoder and not the motor.

The downside of my current screws is top speed. Theoretically, I could drive them at 3000 RPM (600 IPM), but in reality I start to see resonance (whipping) at more like 300 IPM. Do I need to move that fast? No, not really - but it's kind of sad not to be taking full advantage of the great motors and electronics I have!

For a smaller machine, with 18x18" of travel, screw whip is not going to be an issue even at very high speeds. You can calculate the max speed with a few variables.

So, for your machine - I would probably go with the .2"/turn screws, which are a good balance. If you're using brushed DC servos, it's a good choice because you get increased mechanical advantage with the finer pitch and you can run the servos at very high RPM.

If you are using steppers, the max speed is dependent on the motors, electronics and screws. You may hit your max RPM with the steppers and .2"/lead, but it's (probably) going to be over 250 IPM. Pretty fast. If you really need more speed than that and want to keep the RPMs down into the higher torque range of a stepper, then a coarser screw would also work well, but you trade off some resolution.

Oh yes - I was using the aluminum helical couplers and then switched to the Lovejoy style with the urethane spiders when I went to the DC servos. I couldn't go from the 16mm motor shaft to the .25" screw end with anything else.

Steve