Head drop during tool change Fanuc 0MC

I have a Komo VMC144XT machine center with a Fanuc 0M-C control. The counterbalance chain
recently broke, causing the head to drop rather violently before the Z-servo alarm was triggered
and the brake kicked in to prevent the head from dropping further. After replacing the chains,
the machine then would not move to the proper Z reference zero. It would reference to a position
about 8" below the usual Z-reference position. I used it for a few jobs this way, but had to do
manual tool changes because of the improper z-reference.

After perusing advice from several online forums, I moved the head up to Z+ over travel, shut
the machine off, then held "P + Cancel" while powering up the control and then referenced the
machine. This appeared to return the Z reference to its normal position.

However, now the tool change does not function correctly. The head drops slightly (about .05 to
.07 inches) when the tool change arm is directly under the spindle. Two alarms are generated:
1270 Extreme -Z overtravel and 1290 Extreme +Z overtravel.

I did notice that the Z reference position did not perfectly match the tool changer (was out
about 0.015"). I moved the homing position/reference dog and changed parameter 0510 to fine
adjust the Z reference position to suit the tool change arm. This made no difference as the head
still drops and generates the alarms when I try a tool change.

The tool change routine is in the ladder, not a macro. It seems as if perhaps there is a
variable that got screwed up when the chain broke and that this variable has not been reset.

Anyone have any ideas or suggestions on what might be wrong and how to fix it?

With fanuc om on my machines the M6 command will take the machine to 2nd reference(G30) to do a toolchange......with P cancel you have reset the 1st reference(G28).....you just need to find the parameter to change the G30 Z value to one that lines up the toolchange arm......
I'm not back at work til 2nd Jan or I would look in our manuals or go through the machines parameters until I came across G30 and tell you what to change.....but I believe you will have sorted it by then.....the macros are fine don't mess with them....its just a minor G30 glitch.....
go through your parameters and you will come across it...
good luck...
Jep

P.S. I had a similar problem ten years ago when one of my guys had a high speed head to fixture crunch....I had to alter 2nd reference by over 7mm to get the tool arm back in line again.....but that sorted it and I still use the same machine now without problems...

Hi Jep,
Thanks for the reply. I did a G30 on my machine and it returns to exactly the same Z position as G28. Parameters 0735 to 0738 define the second reference position on each axis in the machine coordinate system, and these are all set to 0. G28 or G30 move the head to the proper position for a tool change. The head drops during the tool change and goes to E-stop with the 1270 and 1290 alarms.
Gord

Hi Gord,
With the time difference me being over the pond it can take a while for me to reply if I've gone to bed....

Anyway there is a line by line sequence to any toolchange.....are these codes visible on your monitor during toolchange? some of my machines they are visible others they are hidden.....

If a reference return is in mid toolchange cycle then just alter that reference Z value....it will alarm out until you get this value close to tool arm height.....

If this doesn't work then another possibility is that a proximity/micro switch has been knocked out of position when the head took the nosedive....this has also happened on one of my machines...that is how I had an E-stop alarm once.....the micro switch housing on top of the spindle for tool clamp/unclamp had split and was out of position...couple of temporary cable ties got us going again....

good luck

Jep

Hi Jep,
Thanks again for your help. I finally found the problem. Turns out there was an almost broken wire that supplied power to the Z prox switches. They were getting 9 volts instead of 24v. This was just enough to operate the proxes normally. However, when the toolchanger was operated, several more proxes powered from the same supply would change state, causing enough of a power disruption to cause the alarms. I fixed the wire and now everything is operating normally.
Also found that the air pressure to the head counterbalance was a bit low. This caused the head drop when the Z servo cut out due to the alarms.

Hi Gord
Glad you sorted it....not sure how I helped tho......apart from mentioning the words proximity switch lol

And thankyou for giving me something else for my memory banks reguarding proxis,frayed wires and voltage drops....as every little helps...

Cheers

Jep