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Tool clamp signal going off during cycle on Mori seiki MH1000

Had a good problem that I chased my tail on the other week.  When the program was running occasionally the machine would alarm out with "tool not clamped". The machine was a Mori Seiki MH1000. I checked the typical prox switch on the back of the unclamp cylinder to make sure it was back which is the only clamp confirmation signal for this machine. I was able to see that it was not back all the way one time during the process. I tried taking a pry bar and to pull the cylinder back and it moved about a 1/8 inch. I tried cracking the hydraulic line that feeds it for extend only. Oil bleed out and the clamp cylinder did not retract. So I assumed there was no restriction or back pressure to tank.

I ordered up parts to rebuild the unclamp cylinder assuming their might be a rolled o-ring or spring broke. After rebuilding and testing I had the same problem.

I started to troubleshoot again. After monitoring the tool change cycle and watching the ATC cylinder shuttled over to magazine side I found the unclamp cylinder would move out about 1/4 inch then retract. Given that both the valve stacks were on their own feed I was not sure what could be happening. I checked the main oil pressure filters for return line for back pressure and they were all ok. I then looked at the hydraulic valve stack for the unclamp cylinder, there was one directional control valve, pressure reducing valve. As well as a lucifer valve after the valve stack inline with the unclamp cylinder. I looked at the lucifer valve. Then verified all output and IO signals from ladder to make sure they were firing at the right time and they were.

By this time the only thing left were some hydraulic valves and manifold blocks. I opened up the hydraulic tank and diverted the tank line so that I could see the amount of oil being discharged and at rest position. There was an excessive amount going back to tank, so I new something was dumping or bypassing.  I was able to track it to a bad pressure reducing valve. Some how when the tool changer shuttled the unclamp cylinder would be actuated.  The amount of flow or pressure spike would cause the pressure reducing valve to open up more or pressurize the unclamp cylinder. Then start dumping oil and more pressure to the unclamp cylinder causing it to unclamp for a couple seconds. 

Installed a new valve and all was good.