Noise problems solved

Back in October I wrote about some phantom limit and e-stop triggers. The noise produced by the spindle motor is proportional to the speed. I wrote a simple program to monitor the limit and e-stop lines and turn on LEDs in response to state changes. With no debouncing on the lines, it was easy to see the effect of turning up the speed on the spindle on the noise.

The wiring for limit and e-stop is in two parts. Both use a standard 4-wire telephone line (the line that connects the phone to the wall jack). One part connects all the limit switches in series. The other part runs from the control panel circuit board to the stepper driver box. Two lines are the switches, the other 2 are the out and back of the stepper opto-circuit power.

All the other lines running from the control panel board to the buttons and LEDs are twisted pair bell wire. So I thought I'd try more of that. I ran the same stuff through all the limit switches. Since the 2 signal lines running to the stepper box were not "out and back" of the same signal, I thought the twisted pair might not solve the problem. So I picked up 50 feet of shielded microphone wire at Radio Shack and used it for the 2 signal lines.

Upon rerunning my noise test program I was able to run the spindle at full speed with no phantom limit or e-stop firing. Yay! I'm actually pretty happy with the Radio Shack wire. I'll probably redo the limit switches again with it.

The other problem I'm having is with thermal shutdown of the spindle motor. My original theory was that the thermal switch was faulty. The time to shutdown is proportional to speed. Under no load, 1500RPM leads to shutdown in about 30 minutes. At 2900RPM it's less than 20 minutes. I bypassed the thermal switch and connected it to a light bulb and battery so I could see when it was opening but not have it shut down. I ran the motor at 2900RPM past the point of the thermal switch opening. At the time of thermal shutdown the motor is warm to the touch but not uncomfortable. A few minutes past and it is noticeably warmer, I would say uncomfortably warm but I could still keep my hand on it.

I think that kills the theory of the bad thermal switch. I've talked to Sherline about it and they want me to send the whole motor and speed control assembly back to them. I'm not happy about that. It might be only a week or two delay, but it might cost close to $100, $100 I don't really feel like spending on the project right now.

It's possible the motor has always worked this way, I've just never run it long enough to suspect a problem, though I have experienced a couple thermal shutdowns in the past.