The motor in my milling machine has been working poorly for a long time. The problem dragged on for a really long time, mostly due to the limits of my diagnostic skills. The entire story might be worth a discussion (because I learned some useful stuff along the way), but I'd like to get to the end point right away because discussing it might save someone a really bad time trying to figure out what the heck is going on.
The issue I faced was that the mill's tool spindle became progressively more difficult to start. I mean that rotating the switch on the front panel labeled "F-S-R" from the "S" or stopped position to either the "F" or "R" position would sometimes result in the spindle rotating, or not. At first, I could flip the switch several times, and eventually the spindle would go.
Eventually the machine completely failed to work. I will pass over a lengthy section of work I did on the gearbox, because that's another story. I will also pass over another lengthy bit of work I did to figure out the motor-to-power switch wiring, because it turns out, that was also unrelated. Mostly.
What did turn out to be the underlying problem (assuming that my repair continues to work) it appears that a washer inside the motor was put in the wrong location at the factory. This is not just any old washer, but a special wavy washer that, I presume, is there to take up slack in the vertical location of the motor spindle. It sits between one of the spindle bearings and the bell housing.
Why, you may be asking yourself, would this have any effect on whether or not the motor starts? It's because the centrifugal actuator mechanism that controls the start winding cutoff switch has a limited range of motion that must be set correctly for the switch to 1) be closed while the motor starts and 2) open after the motor spins up so that the start winding doesn't burn out as the motor is run.
My mill's motor came with this washer at the bottom of the motor spindle. I believe it belongs at the top of the motor. With the washer in the factory supplied location, the centrifugal switch can be made to operate sporadically if the motor shaft is pushed hard all the way down, compressing the wavy washer. There is considerable friction between the lower bearing and the bell housing, and the motor would tend to stay this way, until perhaps it vibrated loose under use. With the washer relocated to the top of the motor, the centrifugal switch appears to operate robustly. Time will tell. A bit of looking at photos of electric motor centrifugal switch mechanisms shows that some manufacturers have a means of adjusting the location of the mechanism vertically on the motor spindle. Mine is pressed on and I would not attempt to move it, short of any alternative means of adjusting the mechanism.
Stop reading here if you don't want to hear my tale of woe explaining why this was so difficult for me to figure out.
To diagnose the failure of the centrifugal switch to close, it's simply a matter of detaching the switch leads at the motor terminal block and check for continuity. Doing this correctly would have saved me many hours of ineffective fiddling around with the motor electrical leads.
I had difficulty performing what should have been a simple diagnosis for several reasons. The main one is my inexperience in dealing with motors. Mine is limited to replacing a power cable on a fan motor. This inexperience led me to stumble on really basic issues like whether I could assume that the motor leads could be assumed to be paired between the two bunches of wires emerging from the case, or whether I needed to investigate the possibility that a winding might have both ends emerging from one of the two holes. I decided the issue by checking continuity between every pair or wires and tracking that information in a table. Yes, the windings have their ends in different holes. Assuming there wasn't some sort of short in the windings.
On my version of the PM-932M milling machine, the motor has 4 pairs of leads emerging from the motor housing, and none of them are labeled. There are 3 different colors used for the leads, but I couldn't find any source that mentioned motor lead coloring. The wiring diagram on the inside of the terminal block housing only shows terminals, jumpers and power input leads (maybe?). The wiring diagram in the owner's manual shows 4 leads coming out of what might be the main drive motor, but the there are only two labels, somewhat loosely associated with the leads, which, remember, are not labeled on the machine.
A more experienced person would have concluded that the two pairs of red wires having continuity, as did the yellow pair, but not the black pair was a solid indication that the black pair was the switch, and that it had failed open. I had no such confidence. Only after much time searching the internet did I deduce the explanation for the two run windings. The motor supplied with my mill can operate with either 120 or 240 volts. The main windings are connected serially for 240 volt operation, or in parallel for 120 volts. I did not find any mention of this in the owner's manual, or any indication in the wiring diagrams. The start winding is deducible because it has a higher resistance (2 ohms?) than either of the run windings (1 ohm) . Plus the leads are yellow for the start winding compared to the red leads for the run windings.
Underlying all this was the fear that I had damaged the windings of the motor by my various attempts to "fix" the wiring, and therefore any ohm-meter diagnostic readings from the leads was not to be trusted. I decided if that was the case, risking irreversible damage to the motor by opening it up was worthwhile. In hindsight, I should have either 1) trusted my ohm meter readings or 2) much earlier in the process, opened the motor up.
At any rate, my decision to open the motor did pay off. I was immediately able to verify that the black pair of wires was directly connected to either end of the centrifugal switch. With the bell housing separated from the motor body, it was straightforward to figure out that the centrifugal mechanism was not correctly positioned relative to the switch, and it was just a matter of sorting out where the misalignment was located.
I hope this helps someone with a similar problem. At any rate it helps me to de-stress by putting it all down in writing. I may put up a post regarding the issues I had with the motor reversing (drum) switch wiring. I'll leave it here with stating that the manufacturer had an unusual means of wiring that is not documented anywhere I was able to find, and is apparently unfamiliar to Precision Mathews too.
The issue I faced was that the mill's tool spindle became progressively more difficult to start. I mean that rotating the switch on the front panel labeled "F-S-R" from the "S" or stopped position to either the "F" or "R" position would sometimes result in the spindle rotating, or not. At first, I could flip the switch several times, and eventually the spindle would go.
Eventually the machine completely failed to work. I will pass over a lengthy section of work I did on the gearbox, because that's another story. I will also pass over another lengthy bit of work I did to figure out the motor-to-power switch wiring, because it turns out, that was also unrelated. Mostly.
What did turn out to be the underlying problem (assuming that my repair continues to work) it appears that a washer inside the motor was put in the wrong location at the factory. This is not just any old washer, but a special wavy washer that, I presume, is there to take up slack in the vertical location of the motor spindle. It sits between one of the spindle bearings and the bell housing.
Why, you may be asking yourself, would this have any effect on whether or not the motor starts? It's because the centrifugal actuator mechanism that controls the start winding cutoff switch has a limited range of motion that must be set correctly for the switch to 1) be closed while the motor starts and 2) open after the motor spins up so that the start winding doesn't burn out as the motor is run.
My mill's motor came with this washer at the bottom of the motor spindle. I believe it belongs at the top of the motor. With the washer in the factory supplied location, the centrifugal switch can be made to operate sporadically if the motor shaft is pushed hard all the way down, compressing the wavy washer. There is considerable friction between the lower bearing and the bell housing, and the motor would tend to stay this way, until perhaps it vibrated loose under use. With the washer relocated to the top of the motor, the centrifugal switch appears to operate robustly. Time will tell. A bit of looking at photos of electric motor centrifugal switch mechanisms shows that some manufacturers have a means of adjusting the location of the mechanism vertically on the motor spindle. Mine is pressed on and I would not attempt to move it, short of any alternative means of adjusting the mechanism.
Stop reading here if you don't want to hear my tale of woe explaining why this was so difficult for me to figure out.
To diagnose the failure of the centrifugal switch to close, it's simply a matter of detaching the switch leads at the motor terminal block and check for continuity. Doing this correctly would have saved me many hours of ineffective fiddling around with the motor electrical leads.
I had difficulty performing what should have been a simple diagnosis for several reasons. The main one is my inexperience in dealing with motors. Mine is limited to replacing a power cable on a fan motor. This inexperience led me to stumble on really basic issues like whether I could assume that the motor leads could be assumed to be paired between the two bunches of wires emerging from the case, or whether I needed to investigate the possibility that a winding might have both ends emerging from one of the two holes. I decided the issue by checking continuity between every pair or wires and tracking that information in a table. Yes, the windings have their ends in different holes. Assuming there wasn't some sort of short in the windings.
On my version of the PM-932M milling machine, the motor has 4 pairs of leads emerging from the motor housing, and none of them are labeled. There are 3 different colors used for the leads, but I couldn't find any source that mentioned motor lead coloring. The wiring diagram on the inside of the terminal block housing only shows terminals, jumpers and power input leads (maybe?). The wiring diagram in the owner's manual shows 4 leads coming out of what might be the main drive motor, but the there are only two labels, somewhat loosely associated with the leads, which, remember, are not labeled on the machine.
A more experienced person would have concluded that the two pairs of red wires having continuity, as did the yellow pair, but not the black pair was a solid indication that the black pair was the switch, and that it had failed open. I had no such confidence. Only after much time searching the internet did I deduce the explanation for the two run windings. The motor supplied with my mill can operate with either 120 or 240 volts. The main windings are connected serially for 240 volt operation, or in parallel for 120 volts. I did not find any mention of this in the owner's manual, or any indication in the wiring diagrams. The start winding is deducible because it has a higher resistance (2 ohms?) than either of the run windings (1 ohm) . Plus the leads are yellow for the start winding compared to the red leads for the run windings.
Underlying all this was the fear that I had damaged the windings of the motor by my various attempts to "fix" the wiring, and therefore any ohm-meter diagnostic readings from the leads was not to be trusted. I decided if that was the case, risking irreversible damage to the motor by opening it up was worthwhile. In hindsight, I should have either 1) trusted my ohm meter readings or 2) much earlier in the process, opened the motor up.
At any rate, my decision to open the motor did pay off. I was immediately able to verify that the black pair of wires was directly connected to either end of the centrifugal switch. With the bell housing separated from the motor body, it was straightforward to figure out that the centrifugal mechanism was not correctly positioned relative to the switch, and it was just a matter of sorting out where the misalignment was located.
I hope this helps someone with a similar problem. At any rate it helps me to de-stress by putting it all down in writing. I may put up a post regarding the issues I had with the motor reversing (drum) switch wiring. I'll leave it here with stating that the manufacturer had an unusual means of wiring that is not documented anywhere I was able to find, and is apparently unfamiliar to Precision Mathews too.
