Laser Engraving Marble

I successfully engraved a picture of three kids on a piece of marble to create a gift for a family. I used an inexpensive, 40 Watt CO2 laser engraver and some image manipulation to make this work. I was pleasantly surprised by the quality of the final product. None of the Youtube videos I have seen of laser engraving marble talk about using such a low power laser for this job. I can verify that it works, and works well. This machine can definitely make more than stamps!

Laser rock spallation is a rock removal process that utilizes laser-induced thermal stress to fracture the rock into small fragments before melting of the rock occurs. High intensity laser energy, applied on a rock that normally has very low thermal conductivity, concentrates locally on the rock surface area and causes the local temperature to increase instantaneously. The maximum temperature just below the melting temperature can be obtained by carefully controlling the laser parameters. This results in a local thermal stress in subsurface that is enough to spall the rock. This process continues on a new rock surface with the aid of the high pressure gas purging blowing away the cracked fragments.

I notices that grey powder collected on the surface as I engraved the marble. The above described process must have been exactly what was happening.

http://en.wikipedia.org/wiki/Spallation

Laser Engraving Marble - Result

Laser Engraving Marble – Result

I can also say that there are many details that need to be adjusted correctly to make it work right. The engraved image can look pretty bad unless everything is set just right. I hope to describe the process that I used so that others with a similar system can also be successful. I purchased my new laser engraver directly from China for $660 shipped. I know I keep talking about that. Yes, I am really proud of the deal that I found! I was nervous about this deal being too good to be true, but so far I have really enjoyed using this laser for both engraving and cutting. I have made significant modifications to the system, but even with the additional money that I spent, this is a very reasonably priced system for such a capability. I have posted other articles about both laser engraving and laser cutting here and here.

I used Photoshop to process the image and Mosidraw, the software that came with the laser, to engrave. My version of Moshidraw works on Windows 7 with the “Micro Dog” dongle inserted, but the USB driver for the laser does not work in Windows 7. I used an old IBM Thinkpad to control the laser for the engraving. Note that I neglected to set the power to “Always On” on the laptop and ruined one piece of marble when the computer switched to standby mode part of the way through the print.

The image size was 135 mm x 60 mm. The “carving” speed was set to 5 cm (presumably per second), so each line should take 2.7 seconds to engrave. The stated resolution of the laser is 1016 dpi (dots per inch) which equals  40 dots per mm. if one dot is one line width, then there should have been 2400 lines. Minimum print time should therefore be 108 minutes plus turn-around time for the laser head. I did not time the process, but that duration sounds about right to me. I will time it next time to verify my calculation.

The first important part of this process is creating a monochromatic image that can be engraved on a surface and still look similar to the original image. The basis of the process I used relies on the Half Tone representation of a grey scale image, similar to what is used in newspapers to print images with only black ink on white paper. I first converted my original color image to grey scale, then to a Bitmap (1 bit monotone). Before Bitmap conversion I scaled the image size to match the desired engraving size and resampled the image to a resolution of 1016 pixels per inch, matching the best stated resolution capability of my laser engraver. When I converted to a Bitmap, I maintained 1016 pixels per inch, selected Half Tone conversion, 100 lines per inch, 45 degrees, and “Line” representation. I read about lines per inch versus pixels per inch here. Once the image is in this mode, zooming in and out will make it look like nothing but a black rectangle. It also looks this way in Mosidraw, but don’t be alarmed. If you have set the right resolution and the right settings for the Half Tone Bitmap, the engraving will look great.

Image Converted to Half Tone Bitmap

Image Converted to Half Tone Bitmap

Here are some important points about engraving on marble. I used black marble. I believe that using dark colors gives you the best chance to make an engraving that looks like a black and white image. Another really cool thing to do is to engrave light colored, thin material that is translucent. When you shine light through it, you will see your image. I have made a few Plexiglas image engravings as well, and will blog about those when I make that process work better.

It is very important to note that when the laser engraves black marble, it makes a white spot. Therefore a negative image is required to make the engraving look like a positive grey scale image. I simply inverted the Bitmap image, and it obviously worked pretty well.

Positive Image Wrong for Black Marble

Positive Image Wrong for Black Marble

Negative Bitmap Image

Negative Bitmap Image

Negative Image Looks Right on Black Marble

Negative Image Looks Right on Black Marble

Setting just the right engraving speed is also very important. I also ruined a piece of marble by trying to increase the speed from 5 to 7. I was trying to eliminate the spots that seemed too white in the engraving, but I think the only answer is to go back and adjust the image in software and maintain the right engraving speed.

Speed 5 works but speed 7 does not

Speed 5 works but speed 7 does not

Another important point is that marble is not very homogeneous. There can be different chunks of material in different places, and laser ablation will depend on material density, reflectivity, and probably heat capacity. This messed up my engraving a little bit, unfortunately on one of the kid’s faces, but having the engraving on marble instead of ceramic was pretty cool. This was only my first attempt, so I imagine that it’s just a matter of finding a more uniform marble blank to try.

Engraving on Heterogeneous Marble

Engraving on Heterogeneous Marble

As a side note, I am interested in re-polishing my failed engravings so that I can print new ones. The laser does not cut very deeply, and I know there are systems available for marble polishing. I will blog about that if I can make it work. I purchased a stack of beveled, polished black marble blanks on eBay for a pretty reasonable price. I thought about buying bigger tiles and cutting them myself, but for now I am happy just picking up what I can find pre-made. This is also the tactic I have sometimes used for Plexiglas engravings. It’s not usually difficult to find reasonably priced, pre-cut, beveled Plexiglas blocks.

Aligning the marble block in the laser was very important. I added a honeycomb bed to my laser engraver, so I used little pieces of wood veneer and foam tape to block in placement of the marble block. I tested alignment by first engraving a similar piece of wood veneer. That was a positive image engraving since the laser burns black spots. My laser has a Test button which fires the laser at full strength, so I can put a clear dot on my test piece. I cut the wood veneer to exactly match the dimensions of the marble block. That way I can size and center the image just right. Note also that the wood engraving itself is pretty cool. I have made outline engravings on wooden boxes, and now I see that I can additionally make image engravings similarly. That will almost certainly lead to a nice future gift for someone.

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