laser engraving on dome surface

[bryan henderson]

Hello

I am going to do a job on anodized aluminum that is domed. I am not sure what is the best way to do this. Should I get a 3d model of the domed surface with the engraving info already wrapped onto the surface say in DXF format and then bring it into Coreldraw. I am trying to do this on a 30W Speedy Trotec.

Bryan Henderson
Henderson Precision Inc.

[Michael Kowalczyk]

Hey Bryan,

I think it all depends on how much "Z" there is on your dome. I would call Trotec directly to see if it can do it. I use a 4" lens to do radiused items but never tried it on a dome. I know you can do z offsets by color but not sure how accurate it would be on what you are doing. The lens is about 400.00+- so in less you are doing many of them and plan on do more it may not be an cost effective option.

I think you should ask Amy at Trotec and that would be your best answer.

[Richard Rumancik]

If there is a significant difference between maximum and minimum "z" then I am afraid that this will not work too well. Even with a 4" lens you don't have a huge depth of field (the vertical distance in which we could consider the beam to be in-focus.) If you were doing a small round patch on one part of the dome it may work well enough. But at the edges (maximum z) it will tend to be very weak and you will get loss of clarity and contrast.

For a small graphic you would not need to compensate for the curvature but if you are really trying to wrap the graphic around the dome I have some doubt that a conventional laser will be able to do the job.

[Franklin Geiser]

If this is outside the focal length, I'd break it into several "donut shaped" jobs, so it could be refocused for each job. It still needs to be fairly flat for this to work at the outside edge, if it's a true hemisphere you're pretty much out of luck without some fancy jigs to hold it at an angle, and some way to register different parts.

Not exactly ideal if you have to run 100 of them though.

[John Barton]

We do billiard balls on our Universal 660 and I have about 1" diameter before there is significant drop off. We also do pool cues and before I got the rotary going I manually indexed them and that was a major pain.

My sugggestion is that you don't do it if you have to engrave over a large space on a curve. You will frustrate yourself trying to figure out how to get it right. But if you insist on trying (as I would) then read on.

The best way to think of it is by looking at the laser as if it were a pen. If you try to draw on a surface and it's uneven your hand will autmatically move with the material to keep the pen on the surface to mark it. A laser is a fixed pen, it cannot move up and down to compensate, only left and right and back and forth.

What I do for curved surfaces (billiard balls) is to focus in about the middle of my area from top to bottom. For example purposes let's say my focus depth is 2". On my billiard ball example my working area is a 1" circle. So I focus at .5 inches from the top center of the ball. This puts the focus depth of 2" right in the middle of the arc of my working area. With that I can get fairly good results on the edges of the 1" area.

Of course there are probably a lot more ways to do this. After doing hundreds of balls this is the best way I have found. If I had to do a logo that went longer than my 1" area I'd have to manually index the ball or use the rotary. It's the aligning on the manual indexing that will get you. Even when you think you are dead perfect and you run it and see that .00001 offset line you will go crazy.

Good luck with this. If you find an elegant solution that works please share it with us.

John

[Mike Null]

There isn't a really good way to do it but on occasion I have refocused the machine and run the art several times until the image was uniform.

The big problem is elongating the image.

I generally decline such jobs.

[Dave Fifield]

FWIW, I measured the usable focal length range on my Epilog 4" lens and found it to be only +/- 0.25" from dead-on focussed. I did this by placing a long strip of black anodized aluminum at a known angle to the focal work plane of the laser, lasering a very fine line the length of the aluminium, then judging (by eye) where the line became to unfocussed and weak, then using some simple trigonometry to calculate the usable focal depth.

HTH,
Dave F.