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- Dec 25, 2011
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As far as whether you want a reversible motor on a threaded spindle lathe, my answer would be yes. You know not to try to do turning, facing or threading in reverse. But there are at least two scenarios where you must be able to reverse the spindle motor.
One is if you are ever doing OD grinding with the machine. The surface of the work piece and the surface of the grinding wheel must be moving in opposite directions. The way in which grinders are normally set up, the surface of the grinding wheel where it contacts the work is moving downward (so that the spark stream will be down toward the chip pan and not up towards the operator's face). And the front surface of the work with the spindle motor properly wired is also moving downward. So the relative surface feet per minute would be the difference between the SFM of the work and the SFM of the grinding wheel. It's never likely to actually happen but if the surface velocities just happened to be the same, the relative velocity would be zero and no grinding would ever be done. So the spindle motor would be put into reverse for any OD grinding.
The second one is if you are ever doing an operation where once the half nuts are closed and the lead screw is turning whenever the motor is running, you cannot open the half nuts for the duration of the job. So at the end of each pass, you stop the motor without disengaging the half nuts, back the cutter out with the compound, and reverse the motor to run the carriage back for another pass. The most common case would be cutting metric threads on an Imperial lathe.
There may be other cases that I haven't thought of.
Back to the drum switch wiring, Mark's thought that the red and black leads go only to the start winding and not to the start circuit could be the reason for the problem. the result would be that the start winding would always be energized. And will eventually burn up. The start winding is not intended to be energized all of the time that the motor is running and usually has fewer turns and smaller wire than the run windings. You should unplug the line cord and run the checks that Mark gave.
If that turns out to be the problem, you can still use the drum switch to reverse the motor but it does not have enough contacts to stop the motor in the OFF center position. Which means that if you do it that way, you must NEVER leave the drum switch in the OFF position because the main motor switch will still apply power to the run windings (only). And a few minutes of that will burn up the motor.
One is if you are ever doing OD grinding with the machine. The surface of the work piece and the surface of the grinding wheel must be moving in opposite directions. The way in which grinders are normally set up, the surface of the grinding wheel where it contacts the work is moving downward (so that the spark stream will be down toward the chip pan and not up towards the operator's face). And the front surface of the work with the spindle motor properly wired is also moving downward. So the relative surface feet per minute would be the difference between the SFM of the work and the SFM of the grinding wheel. It's never likely to actually happen but if the surface velocities just happened to be the same, the relative velocity would be zero and no grinding would ever be done. So the spindle motor would be put into reverse for any OD grinding.
The second one is if you are ever doing an operation where once the half nuts are closed and the lead screw is turning whenever the motor is running, you cannot open the half nuts for the duration of the job. So at the end of each pass, you stop the motor without disengaging the half nuts, back the cutter out with the compound, and reverse the motor to run the carriage back for another pass. The most common case would be cutting metric threads on an Imperial lathe.
There may be other cases that I haven't thought of.
Back to the drum switch wiring, Mark's thought that the red and black leads go only to the start winding and not to the start circuit could be the reason for the problem. the result would be that the start winding would always be energized. And will eventually burn up. The start winding is not intended to be energized all of the time that the motor is running and usually has fewer turns and smaller wire than the run windings. You should unplug the line cord and run the checks that Mark gave.
If that turns out to be the problem, you can still use the drum switch to reverse the motor but it does not have enough contacts to stop the motor in the OFF center position. Which means that if you do it that way, you must NEVER leave the drum switch in the OFF position because the main motor switch will still apply power to the run windings (only). And a few minutes of that will burn up the motor.