G90 or G91 is the question @ hand

[pbal]

I am going to mill a flat piece of Al (2 x 1.25) around the parameter w/t an
X shape mill from corner to corner in the middle. Should I write G90 - G91
combined or all G91 commands. depth of cut does not change.

[HuFlungDung]

Take your pick, one way is as good as the other.

G90 absolute mode requires that you take control of the machine position, and know how to define the current tool position, and the datum point on the part.

G91 incremental mode means that you can pretty well start anywhere that you happen to jog to, as each move builds as a simple vector starting on the endpoint from the previous movement. The gotcha, if any, with incremental, is the possibility of rounding errors creeping in. Thus, in order to go right around a closed chain of entities, it is necessary to calculate exactly where you wish to drive the tool, and keep track of where it already has travelled, so that the final movement on the closed chain ends up exactly where you began. The controller will not give you an error if it doesn't coincide, so after a day of making parts, it is entirely possible to have allowed the machine to drift considerably off position.

That scenario is why I like to use absolute mode. Even if conditions were favourable for an incremental subroutine, it would still be good practice at the end of the subroutine, to go back to absolute mode and send the tool to a known position in the absolute frame of reference. This will neutralize accumulation of errors.

[cdlenterprises]

99% of what I do is in G90. There are some good uses(tricks?) where G91 is handy but you have to be very careful, especially with Z-axis moves(ask me how I know) If it's just a simple, symmetric profile use G90.

[tobyaxis]

Ooops!!!!! my bad, wrong thread.

[jackson]

Use both I run some alum. plate that i mill a circule then mill the sides i use G13 G91 for the circule and G90 for the sides

[pinemartin]

Does anyone know where i mught find some free tutorials for beginners of cnc. I need to start with the basic. to understand what g-codes are how to undertsand what everything means as in spindle speeds, i just do not have a clue. but would like to learn before i get involed with building my own machine
thanks all

[dertsap]

http://www.majosoft.com/engraving/ht...are_tools.html
http://www.majosoft.com/engraving/ht..._with_g-c.html
good place to start ,search around this site there is lots of usefull stuff