Fiber Laser Frequency

[Seann Fordham]

Hello,

Frequency is one of those things that requires a lot of experimentation to get certain results on plastics, annealing processes, etc. but have never given it much thought when etching metals as it is much more straight forward.

When etching metals I've always run the job at 20,000 frequency which is what everyone recommends. Because the less frequent the laser pulses, the more power that goes into each pulse and vice-versa if you run at 80,000, right?
Well, knowing this has really got me even more confused. Yesterday I engraved a stainless steel gun barrel at 80,000 to see what would happen, well all I can say is there was a lot more sparking, the sparks were a lot bigger, and the end result the engraving felt 2-3x deeper with a nice contrasting dark brown mark. The same seemed to apply with sterling silver jewellery, etc. just without the amount of sparking you get from stainless steel. It just felt like a much longer, more durable mark.

With a bit more experimenting however, I discovered a pretty big tradeoff, the depth of field decreased significantly.

I've always thought that 20,000 hz would provide a deeper mark from what I've read on the internet, but proved myself wrong by being a little bit curious.

Has anyone else done what I have just to see what would happen? It's weird because at 20,000 hz the mark sounds a lot more penetrating, but at 80,000 there is a lot more sparking (from my experience anyways).

[Gary Hair]

The settings for your gantry fiber will be very different than a galvo and, unfortunately, there isn't any way to correlate the two.

[Seann Fordham]

yes, while both types of machines are very different I would have thought frequency would play a similar role in marking metal. For instance if I were to mark a s/steel shiny watch back, I would want it to engrave with a mirror finish which would mean I'd want the frequency to be around 80,000 and a very fast speed. I'd imagine you would also have a high frequency on a galvo to get the same result, not including the power/speed, etc. as they are very different. Sorry if I'm wrong.

Just find it really strange that I'd get a deeper mark with a higher frequency, kinda goes against everything that I've read on the internet.

[Kev Williams]

If there's one absolute I've found with my fiber, it's that nothing is absolute.

Examples-- depending on a few speed, power and focus changes, I've found I can get a great black mark on stainless no matter what frequency I choose. The other day I 'wobbled' a nice black mark onto some 20 gauge flat 304 SS, so I used the same settings to mark a customers SS coffee to-go mugs. I literally fried the stainless- the blue/brown discoloration extended nearly 1/8" beyond the engraving. This was ONE pass at 50% power, 1500 speed (wobble slows that down dramatically), .01 wobble spacing both ways, and 100k frequency, on the rotary. This is the result after scrotchbrighting the thing. This came out totally different than my flat sample. Material thickness would be one reason, and possibly they're 2 different grades of SS...
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These mugs were for practice thankfully ;) -- off topic: the obvious rotation seams, this underscore the need to for testing and tweaking your rotary. In this case, the diameter I entered needs adjustment as the sections are spaced too far apart. I believe this means I need to enter a larger number as the working diameter, this would make the actual rotation move slightly less, closing that gap....

back to frequency-- Not sure if you're using EZcad with a gantry fiber? EZcad has a 'visual aid', that shows the relationship between the laser's firing frequency and the speed you're running. I've found this to be fairly important actually, because for the most part (not absolute!), when you find a frequency/speed ratio that works for a particular situation, you can work with the ratio's to save engraving time.

This is the visual aid. First up, 1500 speed and 40kz frequency, this is popular for anodized aluminum...
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This aid assumes a beam diameter of .05mm, and shows each spot the laser fires. The overlap is the important part, this is a good 'chewing' pattern...

Now, double the speed to 3000 and the frequency to 80kz-- you have the exact same spacing pattern...
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This means you can double the speed and get the same results! Or can you?
To a point, I've found this to be true. However, the beam is spending half the time in any one place, so there's power loss. Also, laser start/stop times come into play, and can create other differences. But depending on what you're engraving, you may well be able to save some time.

Another benefit, is the visual shows you can't increase speed AND frequency to get a totally different result. 3000 speed and 80kz likely won't anneal or 'light' engrave much better than 1500/40kz...

So, change one or the other, then check the results...
This is the fire pattern at 500 speed and 80kz-
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This is likely a good annealing pattern, but will likely require some experimenting with power and focus. Doubling up to 1000/160kz would produce the same pattern, but may produce totally different results...

Always a work in progress. :)

[Seann Fordham]

Cool, thanks for the input. This is the speedy 300 flexx we're using, so both laser sources run off job control, ezcad looks a bit more confusing, but I suppose every new software does when you first learn it.

Frequency seems like the harder setting to master on a fiber (for me anyways). Just means there's a lot more testing to be had and what else can be achieved.
Also, I tried engraving black anodised aluminium with the fiber and could not get a consistent result at all, no matter how much testing I did with frequency / power, the result was patchy, but not really a major concern as co2 works just fine with it.

A bit off topic, but not long ago I engraved a silver plated round music box on the laser, and to my surprise the engraving chipped and left an ugly border approximately 2mm around the text where the plating had melted (usually you would get a similar result with bad quality plated items engraved with diamond drag). This is pretty rare though as all other plated items will engrave perfectly with the fiber laser. Thankfully the item was ours, not too expensive and had another one in stock which I engraved with a diamond drag engraver. Mistakes do happen and it can make you feel like hell when they do happen, but I suppose that's what experience is and hopefully won't make the same mistake next time.