[wizard]

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.

There is more that one way to look at stiffness. For most users stretch isn't a huge issue though the ram growth might be. In any event is should be fairly easy to calculate stretch due to mechanical loading. What may be a bigger issue is lead screw twist due to torsion applied. This is a very real issue when working on high precision machinery though I'm not convinced it is a huge issue with a router type machine.

Twist is a huge problem when trying to interpolate curves to a high degree of precision. I know of some cases where tool path modification is used to correct for the lead screw unwinding. This is done for high precision optical work but again I can't see it making a huge difference on a router with reasonable sized components.

The reason I bring this up is because if you know precisely what is causing your error you can correct for it given sufficient metrology hardware and the requisite programming skills.

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.

Servos use to be far more expensive than what we are seeing these days. Given that it isn't an automatic no in a custom router. Even then I'd still consider carefully what makes sense, higher leads or higher RPMs.

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.

We are saying approximately the same thing because it makes sense in the general case. You have to think about your specific use case though. If you aren't convinced this is the age of YouTube, plenty of videos machining related as such search for those that are similar to your interests. For me the question comes down to is it worth the extra expense to engineer for wild rapids? Obviously you don't want to fall asleep waiting for an axis to slew across its length. What you ultimately end up with, will likely be axis specific motors and transmissions.

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.

There are domestic manufactures if LMB2008 can't deliver what you want. Which brings up a question you are a awfully concerned about stretch and other accuracy issues but what about your lead screws?

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.

2 for the Y? How is that?

For a router type machine I'm not sure glass scales are really worth the investment. I guess it depends upon your goals and just how good your mechanics end up.

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.

I'd go so far as to say that if you don't have automatic lubrication don't even bother with those high speeds.

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.
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!

Err steppers still require proper sizing if you expect them to work correctly.