Beeing Snoopy, I was able to turn my K8400 into a laser cutter, but 1st of all:
Laser light can be very dangerous for your eyes, so NEVER look at the laser without
the goggles, even though you are curios.
Attention: Plan another 7805 Voltage regulator to convert 15V to 5V for the laser ;)

This is what you need

• protection glasses: http://www.ebay.de/itm/292144122667
• 1W UV Laser : http://www.ebay.de/itm/232355088751
• The 3D Part given
• app 50cm Cable
• Soldering equipment
• Micro-switch(toggle)
• 2 pin jump header + connector
• m3x8 screw and nut
• 2x m3x10 screw

Build it

• install a tape to protect the laser lens
• Install an Additional Wire from the Printer Power Supply(Minus Port) up to the Hotend board
• Print the Part, install the Laser and Mount it with screw and Nut.
• Glue the toggle switch and jump header on either side of the plastic part
• Implement the laser side schematic
• Unmount the Hotend such you can access the bottom of the board and solder 2 cables to the 2 rightmost pins of the long connector. Black(rightmost)=Minus, White=Plus. This is FAN supply, mount the hotend again
• Mount the Laser on the 2nd's extruder place with the 2 M3x10 screws
• Implement the rest of the schematic

Operating it:

The Jumper is used to generally switch on/off the laser. Switch it off after laser cutting always
The Toggle switch is used to test-drive the laser. If its not pressed, its operated by the program.

Calibrate it

• Wear Glasses
• Remove the tape from the laser
• move Z=100mm on the printer
• Put Paper onto the print bed
• Use one hand to test-drive the printer while the other hand adjusts the focus such as
  the laser can acutally cut the paper

Use it

• Write special style gcode programs(sample rect.gcode attached)
• Wear Glasses
• Enable Laser with Jumper
• Print the File
• Disable Laser

The attached image shows the result of the program. its a 30x30 square.

Safe you Eyes!