Normal working area of Prusa i3 MK3S in XY plane is 250mm x 210mm. Tool center point of the 3D Printer is the nozzle tip but TCP of the laser head is its beam spot. Because the laser head is mounted in some distance from extruder, tool center point during work with laser is moved. It is shifted -86mm in Y axis and -45mm in Z axis (if focus distance is 60mm).
User can add G92 X0 Y-86 Z-45 command at the beginning of g-code file. In this case, working area is limited to 250mm x 124mm (see picture below).
It is also possible to not limit the working area. To do this, G92 X0 Y0 Z-45 command has to be used and the table should be enlarged by 86mm in the negative direction of Y axis.
Offset in Z axis depends on focus distance of the laser:
The lens of PLH3D-2W laser head are focused at 60mm when shipped:
