Homebrew PCBs

> What power CO2 laser do you need for cutting 3mm 
steel/brass/aluminium?
> I've got a two-stage pump that can go to 10^-4 torr. I'm designing a
> cnc x-y table...

I had a Mitsubishi CNC Laser for several years and have some 
knowledge of how they work. This machine was a model 1212HC with a 
3000 watt resonator and 48" x48" cutting table. The entire machine 
installation was the size of two pickup trucks. 

We used this machine mainly for cutting small stainless steel and 
acrylic plastic parts. When cutting .060" stainless steel the power 
levels would vary between approximately 400 and 700 watts depending 
on cutting speeds. The CNC program would change power levels when 
needed. Lower power and table speed is used for fine cuts with higher 
power and feed rates for everything else. 

A coaxial beam of cutting gas is always used with CNC CO2 lasers. We 
used oxygen for most steel and stainless and sometimes nitrogen for 
stainless cutting since it leaves a cleaner cut edge. Clean 
compressed air or nitrogen was used for cutting plastics.

Acrylic plastic cuts well with compressed air and 75 to 150 watts for 
up to .125" thick and around 200 watts for .250" thick. The power 
level, frequency, duty cycle and gas pressure was fully adjustable 
(even while cutting) and allowed precise control of the cut quality.

Since most of our work was small parts with fine detail we used short 
focal length lenses. A 5" focal length lense was used for most work 
and a 2" lens was used for super fine cutting on thin materials. A 
7.5" lens was used on materials over .25" thick and up to .5" thick.
The longer focal length gave a straight cut through thicker materials 
but had a wider kerf (cut width) in the material being processed.

One time we tried cutting some FR-4 double sided laminate and it 
didn't work very well. It took about 900 watts to pierce through the 
top copper layer and then the glass epoxy exploded into a blob since 
the power level was so high and was also being reflected back from 
the bottom copper layer while piercing it. 

Non-ferrous metals such as aluminum, copper and brass take much 
higher powers levels to pierce and cut on a CO2 laser. My experience 
was that the same power level to cut .375" steel could not even 
pierce through the copper on a circuit board. I don't think the CO2 
laser is the best choice for cutting PWB laminates. Maybe the YAG 
machines will do a better job on circuit boards.

Tom