Athomas, thank you very much for the kind information.
I tried PCB-gcode. It did product G code from an example. I need to verify the G-code to see whether it is OK or not. In addition, a ulp plug-in is able to convert the eagle file to DXF file so you can edit the drawing through Autocad. Similarly, I am hesitating for the spindle in my CNC milling machine made by myself. It is working beautifully with 0 backslash.