5T G28 U0 one sided

[John3]

Hi,

Does anybody know if this behavior can be corrected or if its "normal" for a Fanuc 5T:

I put a G28 U0 in a new program expecting it to HOME (Machine Zero) the cross feed (X-axis). My machine has front and rear turrets, the carriage has zero in the center and has +8.0 and -8.0 inches of travel.

When in the negative X range the G28 U0 worked as expected, the X-axis traveled positive to the Machine Zero Position and stopped with the X Zero Return Lamp on. When already in the positive X range, the carriage just takes off to the Positive limit and the machine goes into an Alarm.

Is there a prameter that affects this, or some programming trick or another command that will let me send the X-axis to machine zero, no matter where it is currently positioned?

Thanks for any comments,

John

[Dan Fritz]

The G28 U0 command should zero out the X axis just fine, but the DIRECTION of zero return is fixed with a parameter setting. If the zero-return switch is in the center (instead of at the extreme X+ limit), the you must be on the X+ side of the switch if you zero-return. Otherwise, it will just move in the +X direction and (not) hit the switch, then overtravel.

If you are on the X- side of zero and give it a G28 U0, does it stop in the middle, or does it go to near the +X limit and stop at zero?
If it stops in the middle, then you're already PAST the switch when you're on the X+ side of center.

[John3]

Originally Posted by Dan Fritz The G28 U0 command should zero out the X axis just fine, but the DIRECTION of zero return is fixed with a parameter setting....

Are there any other commands available on a 5T that will cause the X-axis to travel directly to Machine Zero, from either + or - direction. (I don't need an intermediate point). What about G27 perhaps? (I surely don't understand it, even after reading Fanuc's Japenglish Manual)

John

[Dan Fritz]

You can use the intermediate point with G28 to go to an absolute position of a positive number, then zero-return. For example:

G28 X10000

This will move to an absolute position of X10000 (one inch) then zero return.

[John3]

Originally Posted by Dan Fritz You can use the intermediate point with G28 to go to an absolute position of a positive number, then zero-return. For example: G28 X10000 This will move to an absolute position of X10000 (one inch) then zero return.

Dan, as always, thanks for your quick and helpful answers.

Is that X=+1.000 inches in machine coordinates or X=+1.000 inches in current G50 coordinates?

John

[Dan Fritz]

If you use X with a G28, it moves to the absolute position (as defined by the G50 statement), then it zero-returns. If you use U with G28, it makes an INCREMENTAL move of the U amount, then it zero-returns. G28 U0 always goes straight home. G28 U-10000 will move minus one inch from where it is, then zero-return. G28 X-10000 will move to X minus 1 inch (which might be quite some distance), the it will zero-return.

[Paul Pippenger]

The U value tells the slide to move to a point first,,,, then zero out
you can do z+ some amount,,then command G28 u minus some amount to always get the slide on the correct side of the switch before it zero returns.