Laser Engraving

Hi - couldn't find anything about this with a "search" so not sure if this is the right place to ask. But just some general questions to begin with - and see where it goes from there.
Has anybody tried laser engraving of wood?
You can get 300mW engraving lasers on ebay quite cheap - any good?

I'm not planning to cut with these - just to engrave wood, plastics.

There are information on laser engraving in the wiki,

But yes for burning wood 300mw should be ok,but use a large heatsink on the laser, the cooler it runs the better
The results.

I have mine water-cooled

Thanks cncbasher for your information.
I've ordered a 2W M140 blue laser + lens + body + driver from the US and a finned heatsink from China. I decided to go with a higher power laser to increase my options in the future. I realise that I will need to put in extra substantial mechanical cooling (water or fan cooling) to the heatsink and/or intermittent running when operating at the high power. This will all become apparent as I begin to test the laser at lower powers. I will probably embed a thermocouple into the heat sink close to the laser body to monitor body temperature. So several things now spring to mind and perhaps someone can point me in the direction for answers - personal experience, books, wikis, etc.

From an operational point of view, I'm guessing that the laser is focused either by manually jogging the z axis onto an impervious (i.e. non-reactive to laser light) material and setting "touch-off" or by just using the known focal length of the laser lens and setting an offset "touch-off".
Secondly, while normal cnc engraving involves moving the cutter away from the material in the z-plane, is this also how laser engraving works? Or alternatively do you instead just turn on and off the laser beam where you would normally move the cutter away from the material. There again, moving a constantly on laser away from the material loses focus and any engraving/cutting ability - but would this be fast enough to prevent engraving loss of precision at start and end of lines?
Thirdly, if you normally turn the laser on and off - I'm guessing that this is somehow controlled through the software onto one of the parallel port pins. And I guess you can also monitor other operations - on my case - the thermocouple temperature from the laser housing via the parallel port and linuxcnc software. But which pins and is there Gcode options to do this (not the temperature - I will probably have to monitor this externally).

Any pointers would be useful.

Thanks Andy,

Now I know where to go (in the nicest sense) I will investigate your suggestions. I have both a Raspberry Pi and a Arduino board - currently setup to investigate outputting, in real-time, guitar chords and scales fret finger placements to 24 x 6 led lights in a look-alike guitar neck. But it wouldn't take much to turn these to cnc use - they are sufficiently cheap - might just get another pair anyway (Christmas being just around the corner).

Regarding safety:
While I am prepared to put up with cheap goods until I experience reasons fro getting better items, laser light and eyes come into a different league. You can get appropriate wavelength blocking goggles for use with lasers on ebay from Asian sources reasonably cheaply. However, I'm not really prepared to trust these without positive recommendations from laser cnc users - what do experienced users recommend?
Once I am protected from the laser light - what are the safety concerns for people who inadvertently enter my workshop when I am laser engraving? Is there a normal square-law reduction in laser light power? Is it just direct beam light that damages the eyes or any incoherent light reflected off surfaces and entering the eye? Currently I have my cnc machine fully enclosed in a see-through box with an access window at the front. Would people recommend I black out the sides with plywood (say) and enclose a small video camera inside to monitor operations - or is this taking it all a bit too far?

given what Andy's beat me to , lol .
watch out for reflective light , more than anything , and as the diode is effectively pointing down and fixed you shouldn't have any problems . you'll find even a clear sheet of perspex between you and it will be fine ,
and of course NEVER EVER look directly into the diode , up at cutting wavelengths that becomes a different matter , as at 10.7 their invisible . and if your not careful will give you a good suntan after 20 mins or so .
but your using totally different lasers, which are only really good for burning ( some people think it's cutting but it's not , it's really just burning it's way through ) , one other tip dont have glossy surfaces around the laser it can bounce off .

whilst the laser is off the machine then always be careful of where it's pointing etc ,
your more at danger when it's being fitted or your adjusting etc , than any onlooker , always have the power off when your handling it
laser glasses are fine for these low power diodes , it's more serious when your working with 2 - 5Kw lasers , where i use ce marked glasses and i know their pedigree and get them checked every 2 years .
being behind say a 1/4" of clear acrilic will be fine for onlookers ,or tinted for example .

en.wikipedia.org/wiki/Laser_safety

I think, to make things easy in the first instance, I will just monitor the temperature with the thermocouple using my DVM input and investigate better heat dissipation with larger heatsinks or fans. I'm not sure I want to go down the water-cooled path if I can avoid it. I'm a great believer in Occam's razor. Having done all that I can then assess whether I need to control the output of the laser for high temperatures.