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Soft overtravel limit alarms when Zero returning alarms - 500 & 501(soft over travel)

All to often I get phone support calls for soft overtravel alarms on fanuc controls. The soft overtravel limit alarms typically occur after a power outage, hitting an overtravel, or crashing a machine. Many times it is an Alarm 500 nth Axis stroke limit + or 501 nth Axis stroke limit  on the newer fanuc controls 16/18/21.

Definition of a softovertravel or soft over travel limit alarm.

Soft overtravel occurs when the axis moves outside of a grid set up in the machine parameters. Whether the machine looses it or the main travel area of the grid shifts by one or two revolutions of the ballscrew. Soft over travels are based off of home postion. So when you set home position on a fanuc control in another area you are moving the soft overtravels.

Hard over travels occur when an axis hits a fixed limit switch to prevent damage to the machine when the home position is set in an improperly place or the machine looses where it was at.

Many times people will notice the problem because they will not be able to zero return the machine. The machine will not even make it to the point where is starts to decel. Instead the over travel limit is reached.

How to recover from a soft over travel alarm. On fanuc controls 16,18,21 and 16i,18i,21i the best way is to hold in the Hard keys  "P" and "cancel" while hitting the power on button. keeping them held in until the control comes up then release. Be careful holding any keys on power up as this can blow out all parameters and machine data as well. Only hold the keys mentioned.