Dead zone in laser

[Bob Crimmins]

I've got an 80 watt Shenhui 900x600 with a Reci tube. It's got a dead zone where the laser cutting efficiency seems to drop off. My really bad estimate is that the efficiency drops by about 10%. I base that estimate on my observation of how closely a through-cut fails in that area. The orange area below is approximately where the power seems to drop off.

deadspot.png

As you can see in this pic, part of the circles cut just fine but then fail to cut completely through in the dead zone.

20141216_213003[1].jpg

It's behaved this way ever since I bought it used. Has anyone else seen this odd behavior in their laser or know a solution?

Thanks,

Bob

[Keith Colson]

Hi Bob

This "can" be typical of a mirror alignment issue. On my machine I have a mirror on the y axis and a mirror on the head. If either one of those mirrors are off alignment your laser can hit the edge of the lens in certain parts of the travel, which can cause your issue. Applying some masking tape to the holes where the beam goes you can establish if the laser is on taget. e.g. if you fire the laser for 0.2 of a second in the bad spot and look where the tape gets burnt. You can apply this method/tape to the optical holes before and after the mirrors.

Also if you have an air assist nozzle, that can be off target too. I place some masking tape over the nozzle and fire the laser for 0.1 seconds. I then pull the nozzle off and look through it to see if the masking tape is burnt through in a central location and adjust as necessary.

Beware that this test can put some vapour on your mirrors. I like to clean my mirrors directly after this procedure.

Cheers
Keith

[Bob Crimmins]

Thanks, Keith. I will give that a try. I could see where off-center alignment could be a cause. I will report back once I've tried.

Best,

Bob

[Bert Kemp]

Is it always in the same spot? Have you tried moving the material to a different location on the table. Have you tried bumping up the power a little, looks like it might be hitting hard spots in the glue.

[Bob Crimmins]

Ok, here's the verdict. My alignment was whacked. Not a single line was even close to centered. After several hours of adjusting every mirror in the machine, I got everything lined up. I bought the machine used and had never made a single adjustment in the 18 months that I've owned it. Nor had I cleaned the lens. I had always meant to, but was always worried something would get hosed and the laser would be out of commission. And it always worked well enough that it never felt like a huge issue. Now having taken everything apart and put it back together again, I'm confident I can do it again. Feels good.

Of course, my follow up project was to super charge my air assist by running a direct line to my compressor. More air is better, right? On the first product cut on 3/8 acrylic at 2mm/sec, the work caught fire and melted my drive belt. Hoping the lens isn't damaged too. Live and learn.

[Rich Harman]

On the first product cut on 3/8 acrylic at 2mm/sec, the work caught fire and melted my drive belt. Hoping the lens isn't damaged too. Live and learn.

You should be able to cut 3/8" at twice that speed. At 80w I cut 3/8" at 4.5mm/sec.

[Bob Crimmins]

Rich, thanks for that insight. Lucky that I mentioned the speed. Hmm... could my tube be waning? As you know, when I bought that laser from the SRND guys, it came is a second tube in a box. It's never been unpacked as far as I know.

[Rich Harman]

If it is aligned well, focused properly, the ammeter reads 26mA yet can't cut 3/8" acrylic at 4mm/sec then I would say there is a problem somewhere.

[Bob Crimmins]

Hmm... my ammeter reads ~24.5mA running at 99.9% power. It's an analog meter so I reckon the accuracy of the needle could account for the missing 1.5mA. Or does it? Should I pop it out and hook up the fluke? Could a 1.5mA variance account for this issue?

[Rich Harman]

Not sure. Probably the pot on the power supply needs adjusted. Still, 24mA ought to do better than 2mm/sec, can't imagine that 1.5mA difference is going to double the cutting speed. I'd be inclined to switch to the backup tube.