G90 vs G91 Absoulte vs Relative

ive noticed the post processor for vectric products does not set either a G90 or g91 command. does it matter? easel appears to use G90 in its gcode, when would i need to use G91?

G90 = Positioning defined with reference to part zero. So all the points are relative to the position you just set.
Move to x1, y, z1, now move to x1, y2, z1, etc
G91 = Positioning defined with reference to previous position. Moves are added to the previous position
Move x+1, move y+1, move z+0, etc

I have always used G90. And I don’t know about grbl, but my other machines defaulted to G90 if a G90 or G91 was not sent in the code. [Edit] I think I will manually add a G90 to my vectric post processors just to be safe.

I have never seen a reason to use G91 with the software we have.
I always imagined it was more for doing recursive programing tricks to minimize gCode for older machines.
If I want to do 9 copies of a part at once I just lay them out and hit carve.
But you could generate a relative gCode for just one part, with a offset at the end and just run the same bit of code over and over and over, working down the length of your material.

Here is a quick example / explanation of the difference

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ok thats kind of what i figured. i saw a post somewhere that a guy was saying it was a bad idea not to have one or another of the commands in your gcode because the machine might do something unexpected. thats pretty much all mine does is unexpected things haha. anyways i guess ill just ad the G90 to the post processor to be on the safe side

BadWolf how would you go about doing that as I also have unexpected results most of the time with V carve. Is there a way to open the PP file and look?

Yep just open the file as a text file in notepad or whatever you use.

There will be a section called header and you will just add “G90” with the quotation marks. And then just save the file and thats it. It should be pretty self explanatory once you get in there but let me know if you need any more help