strange encoder behavior

[unsaint33]

Our CNC machine's spindle has a speed feedback encoder connected to it via a belt. The feedback rpm is displayed on the operator screen. When we first run the machine, everything works fine but after about 10 minutes ( about 5 parts later) the speed reading drops almost to zero rpm while the spindle actually is maintaining its idle speed (I say about 100 RPM). We checked the belt, setscrews, pulley, etc, which are all good. So, either the encoder or the cable is bad. I thought if the encoder or the cable was bad, it would show the problem right away, not after a while.

 

[ndzied1]

Some problems are heat related. After spinning for a while the bearing will get warmer and could show up as an error. A lot of other things could cause this too. Electronics do warm themselves up as well due to the current running through them.

 

[bill4807]

How are you sure the belt is not slipping? Can you put a tach on the encoder shaft to check.
Is their an encoder also on the spindle motor?
What type of control?
I am surprised there is no comparison for speed between the motor encoder and spindle encoder, which would trigger an alarm.
I think maybe the cable is fine.

 

[unsaint32]

A new encoder took care of it. Yes, it's strange that the machine builder put the spindle encoder speed displayed on the operator screen, yet the speed discrepancy is not throwing a fault.

 

[I_Automation]

Machine builders sometimes don't think of all probabilities and options.

I just had a CNC table router that kept showing Head Stopped Fault as soon as it was powered up. The spindle shouldn't move until all 3 axis' are homed and Z is full up - so the fault was weird.

On the third call to the serviceman that can't get here for 5 weeks he finally said the three sensors in the head were Open, Closed and Tool Present - all the other times he called the 3rd one Head Stopped. There was a tool in it. It saw the head was clamped closed, but the PLC input for sensor 3 was dead so the fault showed. But the fault should have read No Tool In Clamp.

And, just for fun, with the Head Stopped fault you could not unclamp the head to put a tool in it if it were a real fault - I had to disconnect the air lines going into the head and reverse them to open the head clamp.

I also mentioned to the service tech a few other wrong's on the machine. All the motor OLR's were N.O., the VFD fault was N.O., four MODBUS servo controllers were all address 1, and a few things on the HMI were not as a normal one of us would have put them.