Thanks guys for your replies. By "ball screw stretch" I mean lack of stiffness. The screw compresses or stretches when pushing/pulling the load. Some people that have done RF45 conversions have complained about screw stretch.
I did get the High Performance Machining book. It has provided some valuable insights. I am definitely building an "under $100k" machine that the book talks about. Ha! But seriously, I will do another post about what I learned in that book.
I switched my design to using servos because the stepper's torque tops out <1000 RPM, and going with any larger ball screws puts you over what the steppers can do. If that wasn't an issue, everyone needing a powerful push could just use 1600 oz/inch steppers, but they are terrible at higher RPM's.
I was going to argue about total mechanical advantage including the step down pulleys, but OK, I get it. You want as much torque as you can get for quick small movements, which is what the machine is doing most of the time. I see both of you telling me the same things. I will go with 5mm pitch screws. It looks like:
Y (65"): 2505, max of 220 IPM. A C7 pair from LMB2008. I haven't seen anyone stocking larger ones.
X (41"): 2005, max of 470 IPM. A single screw, so I can try for a C5, or C7 LMB2008 if I have to.
Z (18.3") 1605, max of a gazillion IPM. A single also, C5 if I can.
I am planning on glass slide encoders for X and Z. The 2 60" for Y are quite expensive, so maybe not yet for Y. I will do another post showing the slides in my design. They should help precision even with C7 screws.
I did some research on maximum ball screw RPM. Looking at a PDF, they show that the max rpm varies based on the nut design.
The general rule is (root diameter in mm) * RPM < 50000. This max speed is allowed for "short periods". Here is the max speed for the common sizes based on that formula:
16mm = (50000 / 13.3) = 3759 RPM
20mm = (50000 / 17) = 2941 RPM
25mm = (50000 / 21) = 2381 RPM
At these high speeds the lubricant starts getting thrown off of the screw, and you have to monitor the lubrication and nut temperature.
This answers the question for my RM2005 Y. 2000 RPM for short periods is OK.
Assuming it didn't have to slow down, It would be going 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45 inches in 9 seconds. Traveling at 9 IPS or 540 IPM.Here's a math problem to work out: If your machine has a 4 foot wide table, is capable of 800 ipm rapids, but can only accelerate (torque) at 1 inch/second/second, how long will a rapid from one side of the table to the other take, and how fast will the spindle be moving when it gets there?
Yes, I have to look at accelerations. Servo sizing is next! I will be looking at gear ratios, kilowatts, brush vs brushless, motor inertia, encoders, drivers, tuning, and accelerations. I had it made when I was just looking at steppers!