NEW! Power, Freq, Duty explanation please!

[collins8991]

Hi guys. New to this place and laser cutting in general. Working a new Amada FLC-AJ.

Looking for an explanation of how power, frequency Hz and duty effect a cut. Also how to calculate the power outcome when you adjust these. I understand that when duty is 100% you have a CW (continuous wave) and the frequency is then irrelevant as the wave is CW. However when the duty is dropped below 100% the freq comes into account? Is this correct? So duty is the percentage of how long the laser beam is on per cycle? And freq is the amount of cycles per second? And is the power effected by all this?

Sorry guys as you can tell im very new to this but trying to achieve tolerances of +/- 0.010mm 1.5mm holes on 1.2mm thick medical stainless so i need to get a grasp of the conditions! thanks

 

[IRstuff]

Duty cycle is the ratio of the ON time to the period of the cycle, and frequency is defined as cycles/second. The power is the peak energy FLOW, i.e., watts is joules/second. So what changes when a laser is pulsing, is the total amount of energy delivered. So, say, the power is 1kW, a CW laser delivers 1 kilojoule/second, while a 10% duty cycle of the same laser would deliver only 100 joule/second. A non-CW condition may be required to prevent burning, splatter, etc.

You should talk to the laser manufacturer's applications engineers, who would probably be able to tell you what conditions are required to do what you're trying to do.

TTFN
faq731-376

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[collins8991]

Thanks for the reply! Great, so the actual duty not only controls the % of time the laser is on/off per cycle but how much actual power is used? I didn't realise that, I thought the power was a constant through out - hence why I was getting a lot of burn marks. So as you said a 10% duty with a 1kW laser actually delivers 100 watts to the material but has the frequency setting a impact on the power out put or does that basically dictate the physical distance between pulses (mm) i.e. if you had a very low frequency (3Hz) along with a duty of 10% and a high feed rate you may actually be able to see the physical gaps the laser turned off?

Again sorry for repeating this thread and the confusing questions im just trying to get a good grasp of the concept and the engineers description was as clear as mud!

 

[JayMaechtlen]

The power setting of the laser is often the CW power setting, and if you're pulsing the delivered power is CW power * duty cycle %.

But it generally won't be that simple - the power monitor may show the net average power delivered, or something else?
Some lasers won't just chop on/off cleanly, the waveform may show a spike instead if a square wave.
The spike may be desirable, or not - depends on what you're trying to accomplish.
Depending on material and other factors, you may need some minimum power density at the focus spot. (to melt/vaporize/ablate the material, or just to get power into it. (Aluminum!))
If you need the high power density but low average power (due to contouring limitations or other process requirements) then you go for high ON-power and short pulse width. Frequency needs per speed and how close you want each ON-pulse to the next...

Hope this helps.

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[collins8991]

Ok so the power setting I have in the conditions e.g. 2000w is the power when I'm using a CW (Duty at 100%) ? And if I was to change the duty to let say 50% I would now be emitting 1000w?

And what do you mean by "minimum power density at the focus spot"?

A lot of our work has complex contours and small features - so this would be a case when I use a high power on, but not a CW?

Sorry, I'm a bit lost!

 

[IRstuff]

"And if I was to change the duty to let say 50% I would now be emitting 1000w?"
>> No, the power would still be 2000W, but the energy delivered is only 1000J per pulse

""minimum power density at the focus spot""
>> the optics in the laser are designed to produce a small laser spot on the target. Since the energy delivered is constant, if the spot is too large, there would be insufficient energy delivered to make anything happen. Since your power is fixed, increasing duty cycle, up to CW conditions, and focusing the spot into a smaller area would be ways of delivering more energy.


TTFN
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[JayMaechtlen]

"energy delivered is constant" - probably not.
Power (rate of energy delivery) can be varied in most lasers I'm aware of.
The OP has an Amada FLC-AJ - I didn't bother looking that up at first.
I see that's a 2 KW fiber laser.
energy delivered per pulse = (average power during pulse) * (duration of pulse).
Energy density at the spot is a function of the spot size, power, and power distribution within the beam. (spot size would be affected by delivery system, focusing optic(s), and relationship of focus point to the surface of the material.
With 2 kw fiber, energy density at the spot is probably not an issue, except maybe for Cu.

Anyway, my previous comments regarding set power and display power were generalized because I don't know how Amada's fiber lasers are set up. Those would be great questions to ask Amada's support / application staff.

Regards
Jay

Jay Maechtlen

http://www.laserpubs.com/techcomm