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Sometimes when we zero the machine axis' at the start of the shift the 'z' axis mis-positions by what appears to be exactly 0.250". If we repeat the process several times it will finally position correctly. Is this caused by the ballscrew or the servo drive?

On most machines, when they are started after being shut down, the coordinate system needs to be re-calibrated to let the control know the exact position of the slides. This is achieved on some models by the use of an encoder (rotary pulse generator) in conjunction with a proximity or micro switch. One of the pulses in the encoder has a different wave form than the rest and can be described as a marker pulse. As the encoder turns, driven by the servo motor, the machine control searches for the marker pulse. The control stops the motor at the precise moment the marker pulse is detected. The problem is that theoretically the marker pulse occurs once every revolution of the encoder, so to alleviate this a switch is positioned on the slide with a trip dog that signals the control to start searching for the marker pulse. Using this method it is possible to tell the control exactly where the slides are when the machine is re-calibrated (zero referenced) after start-up.
Now to your problem - if the marker pulse trigger's the control at the same time as the proximity switch, it is possible for the control to miss the signal from the encoder, so the motor will continue to rotate the ball screw one extra turn. To cure the problem, mark the position of the trip dog and move it one-sixteenth inch one way and re-zero the machine. If you still have the error, move the trip dog back in the other direction.

Express Ball Screw Repair

112 Price Cir.
Ringgold, GA. 30736

Phone: (423) 559-2153
Fax: (423) 559-1044

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