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Laser depth of focal point for thick aluminum

Strostkovy

Titanium
Joined
Oct 29, 2017
We've just gotten a used 4kw CO2 laser (made by HK) and have managed to cut 1/8" 5052 aluminum at 150 ipm with a 7.5" lens (can probably get to 165 and still be dross free, and maybe 200 with the 5" lens) but I'm having some trouble with 1/4" aluminum.

Basically it seems like either the focus is too deep and the cut quality suffers, or the focus is high enough that reflections ionize the assist gas above surface and attenuate the beam badly.

I've only spent a half hour or so on this material because our main concern was dross free 1/8" aluminum. Does anyone have an idea of where the focal point should sit within the metal? I'm not sure if I should work with the focus being very low, towards the bottom of the sheet and deal with reduced feeds, or if I should try to avoid the reflection problem by focusing directly at the top of the sheet and avoiding the beam overlap at focus all together. It doesn't help much that I don't have a very good factory condition to start with, and I'm brand new to this particular machine.

The beam is well centered in the nozzle and the focal point is set by the HK rep doing the recommission, but I'm not totally sure exactly where the focus is relative to the various focus settings. They seem to do everything quite a bit different than the Mitsubishis I've worked on in the past (though in many ways that's a good thing). To get a usable part we are crawling along at 30-40 ipm, at 4kw and around 14 bar of nitrogen through a 3mm nozzle. Nozzle gap is around 0.04", though I've varied all of those parameters to some extent.

I'll be back at it tomorrow and can probably figure it out, but any input is appreciated.
 
Thank you all for your input. I found out that focusing on the bottom of the sheet or just below is correct, but our auto focus head is pneumatically driven and has either a bad valve or a bad PLC card driving the valve, so we got intermittent or no focus control. Manually adjusting the lens assembly to bring the focus down gave great results on 1/8", and improved 1/4" to be passable, but I only had enough lens travel to get the focal point 1/8" into the sheet. Hopefully we can get the needed parts within a week.

Is it normal for the factory settings on lasers to suck? The steel conditions are low powered and slow while the aluminum conditions are heavy on the gas flow and call for a weird focus. We've been able to cut the gas flow in half and increase feed rate on all conditions. We currently get 2 sheets of 1/4" steel or 3 sheets of 10 gauge steel on a single bottle of oxygen using 0.5 bar instead of the recommended 3 bar, and a 1.2mm nozzle instead of 1.5mm, and with a 20-30% higher feed rate.

I recall similar issues on Mitsubishi lasers at my old job.
 








 
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