On Fri, Mar 06, 2009 at 09:31:17AM +0100, Robert von Knobloch wrote:
> Thanks for your solution, which worked perfectly.
> Just for my benefit, I would like to know if I misunderstood the
> command or does it not work as documented?
I think the documentation is accurate but maybe not terribly clear. The
key words are "the named axes"
To return one or more axes to the predefined position by way of the
programmed position, program G28 axes (or use G30). The path is made by
a traverse move from the current position to the programmed position,
followed by a traverse move of the named axes to the predefined
position.
This is also different than older versions of emc which did what you
expected: first moved to the programmed point, then moved all axes to
the predefined position. I know that this change was motivated by a
user request, but I don't remember the exact reason. Here's what the
docs say in the "differences from rs274ngc" section:
When G28 or G30 is programmed with only some axis words present,
EMC2 only moves the named axes. This is common on other machine
controls. To move some axes to an intermediate point and then move
all axes to the predefined point, write two lines of gcode:
G0 X- Y- (axes to move to intermediate point)
G28 (move all axes to predefined point)
http://linuxcnc.org/docs/html/gcode_main.html#r8_1_5
One that comes to mind are machines with rotary axes (though I don't
think this was the motivation for the change): until we do
something about the way rotaries have to undo every revolution they
performed to get back to 0, G28 would be really annoying on a rotary
machine.
Jeff
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