When I laser cut out a shape, lets say a rectangle, with specific dimensions the actual cut is 15% larger than design dimensions.
I have set my steps/mm and feel confident I am within a half mm on all axis. I tried changing the ratio from 100% to 85%. The actual cut was smaller but still oversized. is there a setting I am missing or is my set up wrong?
These two statements contradict each other. The simple way to calibrate the step size on a machine is by drawing a shape of known size, then adjusting the current step value for each axis by the ratio of what you asked for vs what you got.
Are you drawing the rectangle in LightBurn with a known size, or importing it from somewhere else? (Imported files sometimes lose scale, like DXFs or certain SVGs exported from other software)
Drawing in LightBurn. When I use Fusion 360 and import it into LaserGRBL I have no issue. For a simple cut I thought using fusion was to much of a hassle vs LightBurn. I have not tried to import from Fusion 360 into LightBurn.
If you draw a 100mm square in LightBurn and run that, what do you get? GCode uses actual units, so LightBurn is not involved in the process of converting your measurements into proper scale on the machine - the controller does that.
If you draw a 100mm square at the origin in LightBurn, the GCode looks like this:
G0X0Y0 ; move to X0 Y0
G1Y100S100F6000 ; cut to X0, Y100
G1X100 ; cut to X100, Y100
G1Y0 ; cut to X100, Y0
G1X0 ; cut to X0, Y0
You can very plainly see the 100’s in there - there’s not much room for me to screw it up.
