Well, everything can’t always go perfectly. In my last post I demonstrated that my DIY CNC milling machine, upgraded with a 10mm-lead ballscrew and a 600W AC servo, could pull off linear speeds of 20 meters per minute.
While that sure is impressive for something built from slotted aluminum extrusion, it turns out the real challenge wasn’t in going fast, but in going slow. Really slow.
I couldn’t make a video of this that would clearly show what’s happening, but when moving at very low speeds, such as 0.1 meters per minute, something unexpected happened : going right to left, everything worked fine, but going left to right, the ballscrew started to stick every few millimeters, causing the axis to slow down and then jump ahead.
In the past I had already noticed that this ballscrew made a different noise going one way than it did going the other way, but I didn’t think it was problematic. The few parts I’ve machined since installing this ballscrew didn’t require ultra-low feed rates and didn’t include G2 or G3 (arc) operations. Somehow switching from a stepper motor to an AC servo made the problem more evident.
In an attempt to characterize the problem I decided to design a test program and cut it into a spare block of aluminum. The program basically cuts concentric circles at different feed rates. Circles require the X and Y axes to slow down progressively until they reverse and accelerate again : if anything was going to evidence a problem at low speed, that was it.
And here’s the result. I hope you haven’t eaten yet, it’s quite shocking :
Scale : this hole is 30 millimeters wide. It was cut with a worn-out 2-flute 3mm HSS end-mill I normally use for tests that have a chance of going wrong, so the surface finish isn’t the issue here. The picture was taken at exactly 90 degrees above the part, there is no perspective involved : you can see the wall all around the hole (click on it, it’s really high definition).
Now, to help you make sense of this picture : I used climb milling, so the cutter went in counter-clockwise motion around the center of the hole. The Y axis is still powered by its old stepper motor and still uses its old 5mm-lead ballscrew. The X axis is powered by an AC servo and uses the new (probably faulty) 10mm-lead ballscrew. Starting position is at 9 o’clock.
Note that as the cutting starts at the 9 o-clock position and the cutter moves towards the 6 o’clock position, at first it goes straight along Y, there is no movement on X. That’s why I call the ballscrew sticky. Evidently since stepper motors have all their torque even at zero speed, starting the ballscrew wasn’t a problem even if it stuck. With the servo however, it’s a different story : at some point the driver sends enough current to the motor to get it to move and then it must catch up on where it should be.
To a smaller extent the problem is also present at 3 o’clock.
Feed rates varied from 250 mm/minute to 2,500 mm/minute over the 12 concentric cuts, which had no discernible impact.
I have a variety of corrective measures planned : for one thing, I still have to tune the AC servo. I can also mount the old, shorter X axis (and its 5mm-lead ballscrew) and try machining this part again. I also intend to re-machine it using a stepper motor instead of the servo, and I think I’ll bite the bullet and try to repair the sticky ballscrew : I think one of its recirculation inserts may be damaged, causing the ball bearings to catch only on the direction where they enter the insert on its damaged side. Could be I can just replace the insert, or the whole the ballnut, I just hope I don’t need to replace the whole ballscrew.
Unfortunately duty calls, I’m attending a big industry convention all of next week, which means I won’t be able to try any of that as fast as I’d like. If you’re anything like me, knowing there’s a problem you should fix but can’t get to immediately… that’s psychological torture.
All I can do is but hope someone will read this post and offer insights. If you do, I’ll buy you a beer next time you’re in Paris !
Until then, take care everyone.