G28 is a return to reference using an intermediate point G90 G28 z0 would bring the tool to absolute Z0 before returning to reference zero (machine zero in most cases). By using g91 g28 z0, you specify that the intermediate point is your current position and the machine will reference return from there. You can also use values such as g90 g28 z50. To use 50mm above your workpiece origin to be your intermediate point or you can use g91 g28 z10. To move 10mm up and use that as your intermediate.
Fanuc g code system a does not use g91, it uses u v w as their respective incremental axes for x y and z, which is why on a lathe you'll usually see g28 u0 w0 or something of that nature. You could use absolute values, but they come from your workpiece origin, so you'd have to say something like g28 x100 z100 to move to the absolute intermediate position above the part to not have a crash. The posted code in fusion is just ugly, no real reason to keep flopping back and forth like that. Fusion posts are JavaScript, so they're not terrible to modify. Phil T. The Feral Engineer Check out my LinuxCNC tutorials, machine builds and other antics at www.youtube.com/c/theferalengineer Help support my channel efforts and coffee addiction: www.patreon.com/theferalengineer On Fri, Jun 25, 2021, 12:28 PM John Dammeyer <[email protected]> wrote: > 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 > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
