VMC Fanus axis overtravel hijinks

Got another used VMC in yesterday. 1990 Bridgeport Interact 720 with Fanuc OM control. At some point, previous owner removed the Y axis servo as it was poking out the rear too much to get the forklift close in to pick up the machine.

Anyhoo, I reinstall the servo and fire the machine up, home but get overtravel error on Y...doesn't move much at all before overtravel alarm..no where near the limit switches.

So I pull the "hold down P and cancel buttons during boot up" trick (which in my case was really a trick as I was by myself and had to rig up a rope so I could pull the main switch on from 6 feet away while pressing P and cancel), and then it would move to the limit switch but keep going unitl it hit "hard stop" and shut down !

So, then I experiment with relocating the disk that connects the servo to the screw to another position. And that fixed it...works great now.

The question is .... why did that fix it ? Seeing as P and cancel should have forced it to set limits based on the limit switches, why did the position of the connection from motor to screw matter ? All I can figure is the control had the exact degree of turn of the encoder in memory and that rotational angle had to agree with the exact point of limit switch contact. Is that it, or is it something else ?

An absolute encoder comes to mind. The other problem it could be is index pulse. They expect to find the index pulse after hitting the limit switch. If they see the index pulse before the limit switch it tweaks them out. I would assume they have 2 limit switches, a home and fine home. If the screw wasn't clocked properly then the index pulse would be seen before the fine home and generate an error.

Absolute encoders.

Usually the builder puts in a zero return function to resettle zero, or you can change the overtravel parameter to allow you to move to home. Then there is a procedure where you change the appropriate bit in parameter 22, cycle power, change the bit back, cycle power, and the new zero point is settled.