CNC Repair Tip #8: Axis Interlock (No Axis Movement) on a CNC Machine with Fanuc Control
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When you are troubleshooting a problem involving an axis that will not respond to commands for movement, it can be frustrating. There are probably many possible causes for the problem.
Now, I’m not necessarily talking about a situation where no axis will move. This would involve a more general troubleshooting approach. Naturally, the first thing most people would go for is the obvious. This may seem incredibly obvious, but I’ve seen it: is the feedrate over-ride set to 0%? Hey, it happens. Am I in “E-Stop”? Pretty easy to verify by the letters “EMG” on the screen. Do I have “Machine Ready”? This can be verified by locating the MRDY signal in diagnostics or the ladder. Maybe your machine has a “Control On” or “Pump On” light that comes on when the control is fully up.
No, I’m talking about a situation where only one or two axes will not move. The cause for this situation can be tougher to isolate. Especially if you were working on a different problem to begin with and somehow caused the no-movement problem. It adds a frustrating twist to the original problem and usually must be addressed before you can continue with whatever you were doing to begin with.
In most cases I think you’ll find the cause of a “no axis movement” issue not to be mechanical or electrical in nature (not technically, anyway). You will likely find the cause to be “situational”. In other words, a situation exists (or does not exist) that results in an axis being “locked” or more corrctly stated, “interlocked”.
For example: let’s say you’re working on a 4-axis horizontal machine and no matter what you do, the B axis (table) will not rotate. Possible causes that come to mind might be a missing B axis unclamp signal, or maybe a pallet-changer that is not at it’s “home-position”. If the APC wasn’t actually home or the B axis really was not unclamped, you would not want the B axis moving. You could have a servo alarm, but this would usually be displayed on the screen. In our example, no servo alarms exists. The APC appears to be “home” and the B axis “looks like” it’s unclamped. So where would you go from here?
I would recommend you check the status of the axis interlock signal for the B axis using either the diagnostics or the ladder. On a Fanuc 16, 18, 21, 16i, 18i, 30i, 31i, 0i control, this would be found at the ladder address G130.
Typically, for the above mentioned controls, the axis interlock signals would be addressed in the ladder like this:
G130.0 = X axis (or 1st axis) interlock signal
G130.1 = Y axis ( or 2nd axis) interlock signal
G130.2 = Z axis (or 3rd axis) interlock signal
G130.3 = B axis (or 4th axis) interlock signal
For Fanuc 10,11,15 Controls:
G064.4 = X axis (or 1st axis) interlock signal
G068.4 = Y axis ( or 2nd axis) interlock signal
G072.4 = Z axis (or 3rd axis) interlock signal
G076.4 = B axis (or 4th axis) interlock signal
For Fanuc 0 Control:
G128.0 = X axis (or 1st axis) interlock signal
G128.1 = Y axis ( or 2nd axis) interlock signal
G128.2 = Z axis (or 3rd axis) interlock signal
G128.3 = B axis (or 4th axis) interlock signal
So for our example, it would be G130.3 (or G130, bit#3) that we are interested in (B axis interlock signal, or IT4). You look at this and find that G130.3 = 0. In order to have axis movement for the B axis ( or any axis), the interlock signals must = 1. In other words, if any of these signals = 0, that axis will not respond to movement commands. You are interlocked!
Somebody told me along time ago to remember this: “G” addresses are signals GOING to the Fanuc CNC from the machine-side (or ladder). “F” addresses are signals coming FROM the Fanuc CNC to the machine-side (or ladder). Goofy, but I always remembered it. So in this instance, we are looking at the signal GOING to the Fanuc control (G130.3) from the machine-side giving the “OK” for the control to move the axis, but unfortunately G130.3 = 0. B will not move.
Now you can use the ladder diagram to find the reason why G130.3 = 0.
Admittedly, you will have to be somewhat familiar with reading a ladder diagram to futher isolate the exact cause of your interlock condition. You probably need some understanding of a ladder diagram to even get this far. This post is primarily inteneded for those who already know something about how to use the ladder diagram and diagnostics available on the control. If you would like to learn the basics, I would recommend our Fanuc troubleshooting manual as a good place to start.
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