This is a story from the shop. It's delightfully full of shop and mechanical bla bla talk. If you're a nerd or enjoy a technical story, welcome!
A week or so ago I started having an odd problem with my CNC spindle. The spindle is kind of like a router, but it is a little more industrial, designed for continuous use.
What was happening was that I was hearing some odd fluctuations with the sound while cutting. I always wear ear protection and and never listen to music while the CNC is running because the sound is what tells me if something is wrong. I've spend many hours next to my machine so I know what sounds like what. It was making a periphery cut and I heard the sound change a little. Just a little, and then it went back to normal. But that was enough to worry me. Intermittent changes aren't random, they happen for a reason, but maybe it just hit a harder section of wood for a sec? A few minutes later the sound changed again and this time it got lower and more prolonged.
SUDDENLY THE MACHINE BOGGED DOWN AND STOPPED CUTTING.
This was a huge mess because the computer still thought everything was good to go and kept dragging the cutter and it was splintering the wood and I dove for the emergency stop button and finally everything was still.
Oh man. Obviously the part I was cutting was ruined but that wasn't the real bad part. It was that something was obviously very wrong, and now I had to fix it. I had some preliminary thoughts:
Maybe I was cutting too fast
Maybe I was cutting too deep
Maybe the cutting bit was getting dull
I spent the next few days reading more about all of the above, and I reprogrammed my part to cut better and lighter, then cautiously ran the program again. This time I sat next to the control box and watched the amps that the spindle used.
1.6 at idle,
2.1 while ramping into a cut
2.6 when cutting a 1/4" deep or so with a half inch cutter
seemed like 2.6 was about the usual. And then it started laboring. I heard the sound difference and it started using crazy amounts of amps 4.5-5.6-7.1....AAAAAAAHHH
Oh and this is a 3 phase motor so you have to times it by 3, so it was trying to pull 21 amps all of a sudden which is a bit and would be frying things if I allowed it.
I rapidly reduced the feedrate and it recovered and continued cutting but my confidence was shaken and now I watched and listened like a hawk. It did it again, pulling way to many amps and I had to adjust it again. I finished the part but I was a mental mess.
Day 2. I tried to run another part.
This time it drilled all the holes but when I switched cutters it refused to start spinning. what? Never had this happened before. I heard the power ramping up but nothing was turning. I quickly stopped it. I don't want power going to a motor with no rotation. Just for curiosity I put a glove on and gingerly spun the motor. It spun freely. I supplied power to it and while it was making the ramping up noise spun the motor with my gloved hand (this was scary and I made sure to touch it very quickly and lightly). But it worked! Once the motor had some help it took off spinning up to normal speed.
I read on the internets about this. On the the CNC forums others discussed having similar problems. When this happens it means your motor isn't phasing properly. I could go into 3 phase motor sine wave phasing but it's going to take too long for this post and I don't fully understand it myself. At any rate, this told me that one of the wires to the motor was either bad or the motor was bad. For some reason I started by taking apart the motor and everything looked OK. It was pretty simple inside. I tipped it to one side looking in it and some washers fell on the floor. awesome. now they had dirt on them. I cleaned them carefully because this motor spins at 24,000RPM and everything inside has to be perfect.
I used a multimeter to test the motor windings and they all had the same resistance which made me believe the motor was probably OK. Time to look at the wiring to the motor.
I used the multimeter to test the continuity between the ends of the wires and the connection at the VFD. one of the wires had no continuity. The motor was unable to start on it's own because only two of the three wires had power. Almost like a gas engine running on 3 cylinders when a 3phase motor is only running on two phases it's unable to start properly and doesn't have the proper amount of power. That's why it was bogging down during the cut. I checked the first place I'd expect a break, the joint between the main power cable and the motor. Yep. one of them just slid out of their shrink wrap sleeve. When I had soldered it I had not done a solid enough job and so the vibration and movement had eventually caused the connection to fail. All this trouble could have been prevented by a better connection when I had originally wired it.
Whew! It's all part of the journey of building and running your own CNC machine. Sometimes there's failures and there's nobody to blame but yourself. At least I know how everything was built. Time to go cut some more parts.