Thank you,
To answer both of your questions, the last two engravings are 7.5 X 9.5 and running with a stepover of .007 at 50IPM and took about 3 hours and 15 minutes to engrave. Im in process on another build with dual 2.5W 445nm diodes that will speed the engraving process up considerably. Your correct, its not really shades of grey because of the color of the Birch Ply, but with my setup I have 256 levels of power from wood color to burnt dark. I use a US Digital MA3 10bit analog shaft encoder that outputs 0-5V for the analog modulation to the driver in a 360 degree rotation. In my image to gcode program, it interprets the image into 256 .0001 Z axis moves (depths of cut) which my encoder is tied by a timing belt to the Z axis.
There is another electronic means using a DAC that takes the step and direction pins from Mach3 to get the 256 steps (power levels) also. Here is a PDF that was written by the author of the program I use and explains in more detail how it is done. More information about my setup is in this PDF also.
http://picengrave.com/Laser%20Setups.pdf
John, the author of Picengrave Pro is in process of building a laser diode engraver using an X&Y stage from lightobject.com. He will be using a Max5451 digital potentiometer for the 256 power levels. His program lets you select A,B,C or Z axis for the Gcode generation to control the laser diode's power levels.
I have looked at a 405nm, but what I have read, the 445nm's have more wattage and durability. My laser diode engraver mounted on a CNC router has over 800 hours on it with no failures.
Here is a video of my mini laser engraver in action. If you watch the 0-5V analog volt meter, the voltage goes up and down with the power of the laser diode.
Jeff