educa wrote:Ow,
You system really looks nice. However, I must say 50mm/min or even 200mm/min is quite slow. Does the system also work if you go ut to for example 600mm/min ? Thats more like 10mm/sec and thats a speed which my cnc router also likes.
Can I also ask you what this does about the width of the cut? I am currently reading your whole buildlog (so I'm still ok for some hours of reading fun), but I dunno yet what laser wattage you have and also not what focus distance.
I understand that for a deepre depth of cut, i need a longer focal distance, but that will also make my cut wider. For engraving its then better to have a shorter focal distance? So what would be the best compromise then ?
Thanks, I have put a lot of work into it and now it is paying off.
The speed question is a loaded one. There is no simple answer to that. You posted this before I could finish the post about getting good results with PPI. I can certainly make cuts faster, but nothing comes free. Getting good results with acrylic can be tricky because it melts at a low temperature, which means you can't put much power into it and expect the edges to be nice and crisp without bubbling/melting. Unfortunately, this means going slow. This is also why acrylic doesn't machine well... if you aren't careful you'll melt the chips that come off and end up with terrible cut results. If you can increase the heat transfer and disipation of the system, then you can cut much faster.
I can also cut acrylic faster, but it is not reasonable to expect such high quality edge results.
Also, you really have to watch out for sharp corners. The power delivered per unit volume of material is high at those points even if the cut energy density is linear. Accelerating into and out of corners will help with that.
With other materials, like plywood, I can cut very fast. I routinely cut 1/8", 3 ply baltic birch at 400-800mm/min.
Going to a higher power laser may help. You really want to get a faster responding laser though to get speed with PPI. I can't go much below 3ms pulses with a DC discharge laser because it doesn't turn on fast enough. Getting shorter pulses with a higher power laser would be beneficial to cut speed and power control with PPI. You would need a Q-switched laser and/or an external AOM to get much faster than what you can do with the DC discharge, but you are talking serious money for one of those.
With regards to focal length, it isn't that important. I go with short focal lengths because I can rely on self focusing in the material (that is the laser interacts with the off gasses to self focus) and that keeps the beam very narrow throughout the cut. A few guys at the lab I work at didn't even use a focus lens to cut 2x4 lumber during tests and the kerf width away from the impact point was consistant and narrow. It's a weird thing, but it does happen. I have found that the beam actually gets significantly less than 25 microns as it pierces through the material even though the beam spot is 150-200 microns.