A friend who uses MACH3 and Fusion360 (free version) found that every G-Code 
file created by Fusion for the MACH environment added:
 
G28 G91 Z0 
G90
G28 G91 X0 Y0 
G90
 
He's since figured out how to tell Fusion not to do this but looking at:
http://linuxcnc.org/docs/html/gcode/g-code.html#gcode:g28-g28.1
 
I’m curious why there are two moves involved in this G-Code.  In this case the 
G91 changes to relative so the Z0 moves exactly 0 first and then to the machine 
coordinates Z0 position.  Same with XY.
 
If 5161-5166 have something other than 0 and the G91 is left out the system 
makes some interesting moves.  
 
My question is why would anyone want this kind of behavior?  Where would a G28 
be used without the G91?
 
Is it perhaps to move around an obstacle before it heads for 0,0,0?
 
Thanks 
John
 
 
 
"ELS! Nothing else works as well for your Lathe"
Automation Artisans Inc.
www dot autoartisans dot com 
 

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