TI worked on a “magic formula”. I found the biggest secret of making the MOPA work on MOST things is spacing your LI and pulse SPACING the same at about 0.7 of the spot size. Further apart and they have gaps. Closer and the overlapping pulses create a high localized surface temp and hot metal cuts WORSE. The galvo mopa “cuts” with explosive pulses that cause a particle of surface to fly off, often as a spark. If the metal gets near its melting temp, it becomes soft and elastic and pulses get absorbed as heat but no spark flies off. If you actually cause melting it will flow into the channel you’ve created and fill it in.
It’s not just metal. Ceramic chips fly off when cold. If it’s hot, little or nothing flies off, and it may melt into the cut.
Speed is always of value. Max you can get accurate performance at, always.
Heat is your enemy. Lower overlap minimizes heat burden per particle of material removed in a pulse.
First, make sure your timing is good. Doing a fill with no rotation and not using bidir is the least critical on timing, but ton/toff being off together in the same direction will offset the whole thing. If one one- ton or toff- is off, then the cut will be wider or narrower than commanded. I’d actually done the cal for vectors and thought I had it correct, then made up a new test for raster timing and found mine was totally wrong. I really need to investigate this, either I did that vector timing right but LB or this whole system may be doing something different when I changed from vector to raster, or maybe it’s different with different q-pulse types? I really have to investigate thoroughly soon.
So this is the “golden rule” of MOPA overall:
speed=max
LI is ~0.7 * spot size
Freq=speed/(LI), this makes the laser fire with the max spacing it can do without leaving gaps by using the same spacing as between the lines. e.g. your LI is 0.005mm, speed=7000mm/s, use 1400KHz. This spacing may leave little scallops on the edge. You can increase the freq further but material heat increases a lot here so you will have to hold back a lot to limit it and it all gets slow. LB can do an amazing fix here if the even passes offset by 1/2 pulse. It will make a smooth edge without actually making pulses overlap on one pass. And it will get even better if you can do it with 3 steps of 1/3rd of a pulse in groups of 3 passes. You would decrease the freq so each spot doesn’t even touch the adjacent ones within a pass thus no heat overlap
And in that same vein, LB needs to be able to do the same by line, “interlace” the passes. e.g. I have a 0.007 spot size and generally need to use a 0.005 LI. But I may want to get it smoother by reducing LI, but the overlapping heat makes that impossible. Interlacing would be to say, like, an LI of 0.002 but with 3x interlacing it will do one pass at 0.000, 0.006, 0.012, then 0.002, 0.008, 0.0014, then 0.004, 0.010, 0.016. So ultimately you do have lines 0.002 apart but on each pass they don’t even overlap so no overlapping heat patches.
Anyhow, nexy, go to your specific laser’s performance chart and pick the LONGEST q-pulse with a cutoff above that freq. This is your best starting point. Don’t have the performance chart? Oh… well, you totally MUST have that to make settings or you’re just shooting blind and will get nonsense results from testing. Get it from the mfgl
100% power always. If you need to reduce the output, I don’t think this is the way to do it. It will reduce amount of heat burden delivered to the stock, but also reduces the explosive shock that actually removes material more. The removal-to-heat-burden ratio is key, but reducing power makes that ratio worse.
I have a JPT 300W though. That’s a beast. If I do this, it will often melt anyways. Even if I increase LI and pulse freq spacing so much they have a gap between and thus unacceptable.
I found the next step to take is not all that intuitive- you need to keep the LI/pulse freq spacing meeting that criteria. We’re already rastering as fast as the head allows, no gain there. Reducing power will cost more in removal rate than it gains in heat load improvments.
Therefore, the next step is to reduce the Q-pulse to a shorter type. It works. To engrave a coin, usually the q-pulse has to be dropped by several types to get below the melting point. Sucks but any other measure just makes it worse. Either drastically slower cutting, or melts even easier.
One thing I did discover- I could melt a coin in just the wrong way and create a really shiny mirror in some spots. Then even if it cools down and reduce the settings, that spot does NOT want to cut at first. The surface condition matters. I’m seeing about an initial “roughing” pass to first, esp for stainless. Or maybe the other way, go slow, lowest q-pulse type that actually makes a mark, and with a freq signficantly higher than the spot size, to smooth it.