CNC Repair Tip #4: Magazine Index/ Overshoot Problems (3-Phase Magazine Motor)
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Recently I was asked about a problem with a magazine on a VMC like the one in the picture. The cause of the problem eneded up being one that I have seen several times in the past but had forgotten about. The cause of this particular problem is very elementary, but after years of doing cnc services, I still sometimes forget the answer. I thought I’d write about it as a tip for readers and a reminder to myself.
Again, this issue occurs with a 3-phase magazine index motor (not servo-driven type motors). The magazine count is usually done with count switches or an encoder. The problem that occurs is mis-positioning of the magazine after the index. The switches or encoder are checked and found to be working correctly, and yet the magazine mispositions and alarms out the machine or messes up the ATC cycle. Any guesses yet?
If you get to the point where you have checked the switches/ encoder and all looks well, take a second to look at the direction of rotation of the magazine. If you have CW and CCW buttons for manual indexing, does the actual turning direction agree with the switch you press? If not, it’s possible that the motor is phased backwards.
If you run into the unexplained magazine index misposition issue, find out if the machine was recently moved or had power disconnected/reconnected for some other reason. Find out if the magazine index motor or one of the contactors for the motor were recently replaced. A FWD and REV contactor are used in many cases.
Correcting this phasing issue is simple. Disconnect power sources from the machine for safety. Find the source of the incorrect phasing (main breaker, motor, contactor, etc.). (Note: If it is the main breaker that has been wired incorrectly, every 3-phase motor on the machine will probably run backwards. This is not a good thing for coolant pumps or hydraulic pumps.) The servo motors typically will not run anyway even if the machine is phased backwards, and will not resut in an alarm on the machine. Swap any 2 of the 3 power wires U-V-W to correct the problem.
Again, remove the power sources from the machine entirely before doing any service work. It’s not worth dying for.
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