Machining center can realize many kinds of automatic procedures like milling,drilling, boring, reaming and threading etc., while auto tool changing is the key link, which is the symbol of the machine. Because of its complicated structure, breakdown diagnosing & eliminating becomes quite difficult. What’s more, breakdown happening frequently makes it a big problem for maintenance personels. Here we conclude some problem-solving experience happened on auto tool changing, for maintenance personel’s reference.

Once, one of our vertical machining center ME850 adopted Siemens 808D system controller and round type tool magazine. When X axes moved to the nearest site to the magazine gate and was ready to get tool, during auto tool changing, the gate kept closed which made tool changing failed. Then we reset and changed again, but problem kept.
According to our years experience in cnc machinery, especially machining center, the causes of such issue would be: 1) X axes didn’t reach the tool changing place. 2) X axes reached the place, but no feedback signal to the system. 3) magazine gate broke down. 4) tool changing procedure or system broke down.
Firstly, we checked the X tool changing place and the preset place, they matched. Then we checked the inductive switch signal, it worked regularly. So this is not the cause. Next was the gate, which was droven by pneumatic system, so we inspected the system’s pressure, normal as usual. The gate could be open & closed normally when we tried it by manual operation. On the other hand, we found no problem on the inductive signal.
Here, we finished the peripheral troubleshooting, and next came to the system trouble finding. We displayed the Siemens system screen and changed from normal mode to professor clicking “ALT+N” at the same time and entering the command words. Click “machine manufacturer program”, and open the Tool Change file under the ”sub-program SPF”, this file is the program set by the manufacturer. We found two command codes missing in the tool changing program, comparing with previous program. Therefore, tool changing program missing was one of the reason causing auto tool changing failure. After we replenished those two codes and saved, we tried auto tool changing again, the machine worked as normal. 
Hope this case happened on machining center we met can give you some help when you happened to meet same or similar issue in your future work.