Laser cutting leaving a burr on only one side of a square test cut

[ozdan]

Hi,
Hope someone can help me, I am doing some test cutting out 2" square pieces. I Cant understand why 3 of the 4 sides of the square cuts are excellent and one leaves a burr. It is always the same side. At first I thought that the nozzle was not centred, but checked it several times and thats ok. Is there something that I am missing or need to check or adjust?

 

[jetrep]

Do you have another lens you could try?

 

[ozdan]

Ok, thanks. I will try a new lens, ironically thats my last 5inch focal so will get a new one and spare in and let you know if that solves issue.

 

[LASERNINJA]

This is an idea I am not certain of. I have been kicking it around for a while but would like to offer it as a possible cause for this situation and ask others if I am way off base.

Ideally, the focal point is directly in the center of the focus lens, just 5" closer to the material. In reality, I think some lenses are off just a bit, a little +X, a little +y, or the opposite. My idea is that when the nozzle is centered relative to the beam it may be done at one focal position, yet cutting done at another. The machine moves the focus to cut and draws the beam to one side of the nozzle, but when the operator runs the nozzle center program, the focal point is moved back to the center.

 

[Chally72]

LASERNINJA, wouldn't this be a result of the beam alignment farther up the chain rather than the lens itself? The first few mirrors in line past the output coupler aren't perfect, and the beam is angled as it comes into the lens, so centering the beam at one focal means it is off at another...

 

[LASERNINJA]

Good point. I was thinking about that right after I made the post. The real test is to find nozzle center behavior like this, then check up stream as you mentioned.

In either case, I think it would be a good practice to keep the focus position while centering the same as while cutting.

 

[Chally72]

Having chased the mirror alignment issue for several days after a tech botched it, I can attest to this being a cause of burrs on one or two sides of a part... On my Prima Platino, I always center the beam at the same head height and focus position as the cut. It helps immensely.

 

[krugtech]

Keep in mind ALL lasers have directional cutting tendencies. Example- as you approach the correct focal point, there will be a small window where you will see directional cutting and then if you keep going you will find a big window where it cuts great. As parameters drift, it will always show up directional first and looking at the usual suspects for directional cutting may not help.

Chris Krug

http://krugtech.com/

Maximum Up-time, Minimum BS

 

[SGAFFNEY]

Don't just look at the test square. Also look at the sheet the part came from. If the burr is due to nozzle alignment then the burr will usually be on the same side of the cut in the sheet as well. If the test square is oriented the way it was cut out of the sheet, then if the burr is on the left edge of the part look to see if there is a similar burr on the right side of the sheet in the hole where the part was cut (the same side of the cut). If so, then probably a nozzle alignment issue. If not look elsewhere.

 

[RPosty]

I have seen a lot of things cause this. First obviously is the Nozzle centering. second is the mode alignment. third Mode shape, maybe laser mode needs a tuning. Twice I had really weird things cause this. Once we had a Laser Mech head somehow get a little debris in the assist air chamber which caused the air to come out in not so perfect a cone. That one took us months to pin down because it is not really accessible. We changed the head, and that is how we found it to be the cause. Another time we had brand new Output couplers from II-VI for our PRC2200 that were causing the beam to be slightly uncollimated changing the focus and leaving a really weird edge. I put an old beat up optic in to figure that out. it was just a bad batch I guess. Also an external optic can be failing walking the beam, seen a few 1/4 waves do this. start the square 180 out see if it is happening more as a function of time as opposed to a function of side.

Oh, another weird experience was a freon leak in the compressor room which got into the beam purge air causing the mode to increase in size as the beam passed through it. found that by taking a mode at the top of the beam delivery, checked good. at the table bad. changed optic, same thing. jut off the air which comes in right above the lens to keep a positive flow to protect the lens. Mode was good. we use only nitrogen now there or nothing.