L-Cheapo laser, like any other diode laser, has a focusing lens. This lens makes it possible to focus a laser beam right down to 1 cm. Below, we describe how to focus the laser beam in the inner coordinates of a 3D printer to a precision of a few mm. In addition to a computer, 3D printer and L-Cheapo laser, you will need a piece of plywood fitting the size of the printer table, clips to fix the plywood to the printer table, and a ruler.

We need to burn a straight line on a flat working surface evenly changing the height of the laser head above the surface along z axis. For a working surface we use a piece of plywood 200х200 мм in size and 4 mm thick. At the end of the burn process we take measures with a ruler.

1. For the burn program we define the range of the head movements along the axes. The range should not exceed the size of the plywood piece. For our 3D printer with an L-Cheapo laser Xmin= 0; Xmax= 200; Ymin = 77; Ymax= 200; Zmin= 0; Zmax= 140. In G Code these parameters are as follows: «…G1 X0 Y77 Z0….G1 X170 Y200 Z140…».
2. Observing the safety procedures, we start the burn program. Here are our codes:
   G28
   G90
   M107 ; Turn off fan
   G1 X0 Y77 Z0
   G4 P1
   M106 ;TurnONfan (our laser is installed in place of the extruder ventilator)
   G1 F1000
   G1 X170 Y200 Z140;
   G4 P1
   M107 ; Turn off fan
   G28
3. Then we take two measurements:
   • the length of the burn line L0;
   • the length between the beginning of the burn line and the point of the min. thickness of the burn line L1, estimated by eye to an approximation of 1-2 mm.
     The burn line looks exactly like the in-line profile of the laser beam: in the middle (focus center) the beam is thin and getting wider at the ends. This effect results from changing of the laser head lift (from Zmin to Zmax ) during the burn process, while the working surface remains at the same level.
   • To determine ZF coordinate of the laser focus we make a ratio:
     
4. And find the sought-for laser beam focus ZF=89/203*140=61 mm.