A tach does not normally have any effect on position, especially just homing, on this machine you can have either a resolver or encoder, a resolver has the appearance of a small sealed motor approx. 1" in dia. with 6 leads attached.
I would suspect that being on a Punch where a great deal of vibration is present that the resolver coupler may be loose or the resolver changing position due to the vibration, or the motor coupler could cause it also.
I also assume we are talking about one axis only.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.