any idea of what is causing this...

[Don Corbeil]

I recently did an engraving on a piece of poplar, doing a few passes, both x and y axis orientation. After the second pass, in which I was going deeper on a y axis pass, I noticed a 'waviness' in most areas of the engraving. It wasn't the grain, which is running in the opposite direction. It looks very even, like an oscillation wave, on what should be a fairly flat surface. You can see it throughout the entire engraving. It was the first time I noticed this. Can anyone shed any light on what caused this? I've attached an overview and a closeup of the area. Thanks -

289136289135

[Kev Williams]

What you have there is one form of "banding"...

The pic below is one that I WAS going to enter in the photo contest, one reason I didn't-- notice up in the clouds, I have the same banding going on as you...


http://www.engraver1.com/erase2/boatspic.jpg


Many people may not realize that when rastering, the laser beam doesn't actually travel in a straight path. It wobbles. And as it wobbles, portions of the beam paths overlap more than in other portions. Sometimes this results in ugly vertical banding, and sometimes we get horizontal banding. Vertical banding seems more prevalent when doing large 'blackout' areas, horizontal banding seems more prevalent when photo engraving.


Here's a piece of plex I did just now-- the top stripe is a 'blackout', 100 power/100 speed, 500 ppi and 500 lines per inch. The light is reflecting off the banding, not good...
The second 2 stripes I did at 500 ppi but only 75 lines per inch. Middle stripe I did at 100 power/100 speed, bottom was 40 power/40 speed, to show that slowing down helps, but the beam is still wobbling.
However, while I didn't show it, doing a back and forth vector pass results in dead-straight lines. Only wobbles while rastering. The ONLY answer I've ever gotten as to why, was it's related to photo engraving... how true, I don't know...

http://www.engraver1.com/erase2/banding1.jpg


and a closeup--

http://www.engraver1.com/erase2/banding2.jpg


And I assume not all lasers are guilty of banding, as my little ULS has never left a band in its life. I can fully raster a 12 x 18" piece of plex and it'll look sandblasted...

[Don Corbeil]

Kev,

Thanks for your response. That helped explain it.
After reading your response I read up on 'banding' in the forum and found out it could be due to a number of things - engraving at different DPI than original photo, resizing, halftone pattern in software, etc, or (hopefully not) hardware issues. My guess is that it's an image file issue, since I worked the photo over pretty good during the process, and then resized in corel before engraving. I have never noticed it before in my trotec, so I will probably look more closely at my image processing steps.

[Scott Shepherd]

It's been told to me by of the top people at Universal that all lasers will have banding because of the way the beam is created, however, they did create a tube that did not have any banding in it. They did so by coating the inside of the tube with something very expensive. I think he said that tube cost them $40K, but it produced no banding. I've seen in on Epilog, Universal, and Trotec. There are techniques to work around it and they vary by the material being used. You can take it out of focus some times and it helps, other times you need to slow it down, other times none of that works. Different DPI, different frequency, etc. There's no one size fits all fix that I'm aware of.

[Rodne Gold]

Once we stuck regenerative UPS's (AC to DC to AC) on our lasers - no more banding...

[David Somers]

Good morning Rodney!

I am curious. When you said no more banding after putting on a regenerative UPS. Did you mean that before a regenerative UPS you had another form of UPS in place and had banding with that? Or that you had the lasers covered by some other form of surge suppression, or no surge suppression, just straight to the power source?

Dave

[Scott Shepherd]

Our power is good here and we've not found any reason to believe power issues have caused banding. My in depth conversation with the ULS guy told me exactly why it's caused, I just can't remember the details enough to say it back. He said it was a function of the tube and I consider him to be one the most knowledgeable people in the industry, not a sales rep.

[Don Corbeil]

He said it was a function of the tube

But is it also likely that image file issues are a prime suspect? Or is it that it just should not happen at all no matter what you feed the laser?

[Scott Shepherd]

But is it also likely that image file issues are a prime suspect? Or is it that it just should not happen at all no matter what you feed the laser?

It can be from a variety of issues. DPI mismatches between the file and the resolution you send it over at can play a role, certainly. It's a complicated issue with a large number of factors and getting rid of it isn't always an easy thing to do or figure out. If your photo dpi matches everything else, and it goes way, then great, but in general banding is an issue you'll deal with for a long time. Reverse engravable stock tends to really show banding worse than anything else I know of. Slowing the speed down doesn't solve the issue. It's not a speed issue, it's a laser issue, from my experience. I used to do almost exclusively reverse engraved stock. Now, not so much, so I don't see it much any more.

[Rodne Gold]

No power protection when it was banding , the UPS's only came after,
I was also sceptical about my agent/installers advice re these power regenerators , but they worked...

[David Somers]

Thanks Rodney!!!

Dave

[Don Corbeil]

what UPS do you use/recommend for an 80W?
I've been thinking about one anyway, although outages are rare here, surges may be occurring w/out my knowledge.

[Scott Shepherd]

what UPS do you use/recommend for an 80W?
I've been thinking about one anyway, although outages are rare here, surges may be occurring w/out my knowledge.

Keep in mind, Rodney is in a very different part of the world. I do not believe a UPS will fix your issues. He's not using RF lasers either. Two completely different things. Before I'd plug my laser into a UPS, I'd contact the manufacturer. There was talk on this forum years ago about it modifying the wave of the electricity in a way that may not be beneficial to the laser. Dan or Dave could comment more on it, but regardless, I don't think it's going to resolve your issue.

[Don Corbeil]

It seems to have been an image processing issue... I ran the file again making sure that DPI matched, and I didn't resize the image. I have not seen it show up again - yet.

[Michael Reilly]

There are different types of banding. I have seen it in the past with engraving large text on acrylic awards. It's like the engraving shifts depending on the amount of engraving in that line. So I would see variations at say the vertex of an M or would see it when going from an area of lots of engraving to an area of only a little. I believe this is caused, at least in part, by how laser tubes work. As I understand it, they have to build up a "population" of photons and as the laser fires, they are expended. It can only regenerate them at a constant rate, so if you're engraving a heavy area and using them up fast, it will result in slightly less power compared to lighter areas. I may be completely misunderstanding this, this is just my theory. One way to improve results in this case is to create a rectangle the height of your engraving and put it to the side or sides of your engraving (outside the media of course) so it has a more consistent consumption.

What Kev posted seems different... almost a hardware problem with Y-axis or maybe a tube/power supply problem.

One thing Don might want to try is changing the dither pattern used. If using halftones, try a diffusion dither (stucki, etc) or vise versa. I was engraving solid grays using a diffusion dither on an epilog and was seeing inconsistent patterns in it. Tech support explained that those patterns are intended for photos with lots of variation. Since they're using random math to generate the patterns, it's perfectly possible for those dots to form unwanted patterns in them, but you'd never see it in a photo with lots of variation.

Another thought is if the image was saved as a jpeg with very high compression, it could be a pattern in the image itself. Jpeg compression involves breaking the image into a grid and averaging the pixels within it to reduce the amount of data. Low compression results in no perceptible difference, but high compression rates can introduce defects. This seems unlikely here, but the pattern is so grid-like, it's something that came to mind.

[Scott Shepherd]

There are different types of banding. I have seen it in the past with engraving large text on acrylic awards. It's like the engraving shifts depending on the amount of engraving in that line. So I would see variations at say the vertex of an M or would see it when going from an area of lots of engraving to an area of only a little. I believe this is caused, at least in part, by how laser tubes work. As I understand it, they have to build up a "population" of photons and as the laser fires, they are expended. It can only regenerate them at a constant rate, so if you're engraving a heavy area and using them up fast, it will result in slightly less power compared to lighter areas. I may be completely misunderstanding this, this is just my theory. One way to improve results in this case is to create a rectangle the height of your engraving and put it to the side or sides of your engraving (outside the media of course) so it has a more consistent consumption.

Not all lasers do that. That's a problem with specific brands of lasers and their lack of ability to correct the issue. I'm very familiar with that issue and our Universal and Trotec both deal with that through their job control settings, and it eliminates the issue completely.

[Martin James]

The Bose noise canceling headphones use a microphone input to sample the surrounding ambient sound which is then processed so that the speakers play a reverse image of that sound so the 2 sets of sound waves cancel themselves out. One way to get rid of the banding is to try similar strategies in the image file.

Make sure there is no banding in the original image. Use blur, sharpen, find edges etc.
Rotate the any bit map screen a bit so it doesn't add to the problem.

Then add camo texture where ever you have an area that is prone to banding.

Always save image files at the size you plan to print, then do not resize, rotate etc in the later programs. IE photoshop 16"x24" @600dpi.

Some of the banding relates to the relationship between spot size and scan interval. If you try a small power grid that engraves a number of small squares using different dpi/scan gap/etc..

If you can make banding on a non photographic spot sample, then fix that file so it engraves without banding you will be able to rule out a lot of hardware issues.

There is no reason you have to have the file at right angles to the gantry. If you rotate the file 15 degrees then set the wood in the machine at 15 degree as well then the banding will be at a different orientation. If it is a very important project you could use jigs for placement and do a second pass from a different angle.

To me it looks like the problem is from the file.

Can you tell us more about the file and the settings, for example are you engraving using a 3d setting? And what programs.

Cheers Marty

[Don Corbeil]

Marty,
As I mentioned in an earlier post, the problem was with the file. I have had no problems since.
Thx