LNG1 AC Servo Speed Test

Following yesterday’s failure I just couldn’t let it rest, so I burned the midnight oil.

I did extensive research on the internals on ballscrews. Then removed the servo from Mill #2B and then tried to turn the ballscrew by hand. Lo and behold, the ballscrew turns freely in one direction, but “cogs” in the other direction. The only thing that can explain such an asymmetrical behavior is the asymmetrical failure of one of the ballnut’s deflectors (or several, if they are all damaged in the same way).

This behavior was made evident by using a servo instead of a big-assed stepper motor because the servo is so much more accurate and sensitive. The stepper motor simply powered through the bumps in the ballscrew and may even have aggravated the initial damage. In theory I could just pop the stepper motor back on and carry on using Mill #2B but I’ve lost confidence in this X axis.

Luckily I still have the old X axis. It has a shorter stroke but at this point it’s more important for me to keep M2B operational since I don’t have another CNC mill I can use until it’s fixed.

I took the opportunity to install the AC servo on LNG1 and verify its performance with that design. Here’s the video :

At 30 meters per minute the acceleration is so brutal that my crappy camcorder can’t even follow. The servo goes from 0 to 3,000 RPM in the blink of an eye. This speed test demonstrated something interesting : if you’ve read my previous posts about LNG1, you may remember that this ballscrew (and the identical one on the other side of the machine) are actually spares taken from another project. These were made in China and because of their length (and FEDEX apparently playing baseball with our packages) they arrived with a slight bend, about a couple degree over their entire length. I initially decided to write them off : they were cheap and not worth going to the trouble of find a way to straighten them.

On LNG1 they were only supposed to be placeholder until construction of the machine was far enough ahead that I could buy the longer (and hopefully straight) definitive ballscrews. But then realized that due to their length, the ballscrews were a bit springy. And the rest of the machine, especially the linear bearings, were mounted very rigidly, and very straight. Eventually it turned out, as you’ve seen in older videos, that these bent ballscrews actually work : the machine’s rigid construction keeps them straight under tension. They actually rotate so smoothly you can drive them in reverse, with the hand.

But they are still bent, and that made me wonder if they would wobble or vibrate at really high RPM.

LNG1 is a high-speed machining design, so one of the steps I took was to mount the ballscrews between pairs of angular contact ball bearings : this is the recommended setup for high-RPM applications. I also hoped it would contribute keeping the ballscrew from wobbling.

Turns out I was right : as you can see in the video, no shake, no wobble, just smooth motion from one end of the machine to the other.

Lesson learned : while it’s not a good idea to bend your ballscrew (or pay FEDEX to do it for you) that doesn’t render them unusable. They may have lot some accuracy, but it’s probably nothing that can’t be mapped and compensated in software.

This test also allowed me to verify that the cogging I observed on Mill #2B yesterday was not due to the servo.