Hey Dazza,
The beam is pretty straight throughout the path until it goes through the lens where it's focused to a very small dot then when it exits it expands. The aluminium does reflect the beam but because it's not a mirror surface it destabilizes it so it probably goes all over the place. The surface of the table is brushed martensitic stainless steel which also sends the beam all over the place.
I usually sit a few feet away and have had no problems with reflected beams but I still keep my polycarbonate glasses on just in case. You only get 1 chance with an eye.
Hey Herbert,
I haven't cut steel and my only laser is the 30 watt one I have but if I would take a guess at it after all the info I have read I would say that a 150 to 200 watt would be the way to go. You could probably cut it with less power but it would be like cutting through a tongue depressor with a magnifying glass. You could probably do it but you would fall asleep waiting for it to finish the cut.
YAG lasers seem to do a much better job than CO2 but CO2 lasers are readily available from time to time on eBay. There is currently a Synrad 400 watt laser on there for $40K
If I had more money I would not get anything less than a 60 watt laser for my type of work as the 30 watt does take a lot more time to cut things and leaves a charred edge that you have to brush off after. 60 watt would do it faster and probably not leave a charred edge due to the speed of the cut. The slower you cut the more it tends to burn the edge. Air assist is mandatory.
If you only want to engrave granite or marble then 10 watts is enough and there are 2 Firestar lasers for sale on eBay right now for less than $1000 US each.
Hope this helps
Richard.