Some pretty strange things can happen on CNC machine tools. In the days of NC (before computers were part of machine controls), and in the early days of CNC, the electronic components used with the machine tools would often fail, resulting in unusual machine actions (and some pretty dramatic crashes). Indeed, the first thing we’d suspect in those days would be the electronics, even though operator mistakes could have easily been the cause.
We’ve come a long way since then. Today, machine malfunctions caused by failing electronic components are relatively rare—at least with name brand CNCs. Most mishaps can now be categorized as human errors of some kind, yet some pretty strange things can still happen when we make mistakes.
Consider, for example, a feature called “manual/absolute.” When equipped, it takes the form of an on/off switch. It could be an illuminated button or a toggle switch. While I’ve never been able to find a feasible application for the feature, there are machine tool builders that still include the switch on their machines. When the switch is on, the control keeps track of the machine’s absolute position, even during manual axis motion (using jog or the handwheel). When off, the control will not track the machine’s position in the absolute mode.
This switch has been the source of many crashes. If operators unknowingly have the switch turned off when they use the handwheel or jog the machine, the program zero point will be out of position by the amount of manual motion. Sending the machine to its zero return position will properly reset the program zero point, but if operators don’t send the machine home before restarting the program, the results could be disastrous.
This is most frustrating because operators may be performing procedures they have successfully repeated countless times before a mis-positioned manual/absolute switch triggers a crash. Also, it’s likely that they won’t be able to determine what went wrong, even after the crash. My suggestion here is to first determine whether any of your machines have the manual/absolute switch, and if they do, be sure all operators know its function and leave it in the on position. I’ve also seen companies remove the switch and hard-wire the system so this function is always on.
Similar problems can occur if your machine has a “machine lock” feature and/or a Z-axis feed neglect. Again, these features are controlled by on/off switches, and several machine tool builders equip one or both switches on their machines. When machine lock is on, it will keep all machine axes from moving. When the Z-axis feed neglect feature (a machining center feature) is on, it will keep only the Z axis from moving.
Some operators will turn machine lock on when they leave the machine for an extended period of time (breaks or lunch, for example). This ensures that the machine axes cannot move while they’re away.
Setup people may use machine lock or Z-axis feed neglect when verifying new programs. When machine lock is on, they can run the program and allow the control to check for syntax mistakes. While the program is running, they can rest assured that the machine won’t move. In a similar fashion, Z-axis feed neglect enables a setup person to run a machining center program and see only X- and Y-axis motions. This could help operators determine if a new program is moving properly in these axes. During the program’s execution, there will be no chance for a collision because the Z axis won’t move.
Like the manual/absolute function, machine lock and Z-axis feed neglect can cause serious problems if mistakes are made. When either of these functions is on, the control thinks the machine axes are moving, and it tracks absolute position accordingly. For example, say an operator turns on machine lock and then attempts to manually jog the X axis by 5 inches. The X axis will not physically move, but the control will think it has moved. The X-axis absolute-position display would confirm this. If the operator then attempts to run a program, the machine will be out of position by 5 inches in the X axis.
Again, these two features have been the cause of countless mishaps. Maybe machine lock can be useful for checking syntax mistakes, but the program doesn’t end with the machine axes in the same position as they started. Or, with either function, the program is stopped mid cycle. Either way, the machine will be out of position when the operator turns off these functions. As with manual/absolute, sending the machine to its zero return will reset the system and get the absolute position back on track. However, operators must thoroughly understand the implications of using these features in order to avoid costly mistakes.
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