How to carve the moonís surface with CNC machines
Pre-reqs: This is an advanced process that requires previous knowledge of CNC usage and 3D modeling. I will not be instructing on how to run a CNC machine. Iíll try to cover this as generally as possible because the result can be achieved in many ways.
Iíve carved various hardwoods, plastics, resins, and solid aluminum.
NOTE: practice on a small piece first!!
Tools Required: CNC router, CNC mill, or 3d-printer.
Software Required: Photoshop (or good photo-editing DIM software), BobCAD with bobART (or any CAD/CAM software that can emboss a surface from a grey scale image: Rhino, artCAM, etc)
Step 1: Accessing the lunar surface information
NASA sent a laser scanner to orbit the moon. Yep. Sweet.
Theyíve done Mars, too, but in my opinion the moon has a more interesting surface for carving.
Hereís a page describing the mission: http://lunar.gsfc.nasa.gov/lola/
So, NASA 3D scanned the moon. What now? The information is publically available in several forms. They serve it up in various resolutions as TIF files or as IMG files (where IMG are a 16 bit signed type. I avoid these because they are a bit harder to deal with). The TIF files are 8-bit grey scale, and are equirectangular projections.
Global Topography maps:
I typically work with this image because it has a reasonable resolution and I get good scaled results with it: http://lroc.sese.asu.edu/data/LRO-L-LROC-5-RDR-V1.0/LROLRC_2001/EXTRAS/BROWSE/WAC_GLD100/WAC_GLD100_E000N1800_064P.PYR.TIF
You can find higher-res regional data sets here. Itís a bit weird to search. You really have to know your moon-stuff:
Step 2: Viewing the lunar surface information and choosing the area of interest
Open the TIF of your choice in your photo editor (hopefully Photoshop).
It should look something like IMAGE 1
Notice that this is NOT a photograph. The lighter greys are HIGHER elevations.
One of the reasons I work with the specific image above is that I can view the entire moon at once. It allows me to choose interesting topographical scenes.
Once open, look around, zoom in and out, take a look around and find something interesting to carve. I often look for a contrast of many craters to few. Once you have found something, select that area and copy it to a new file For example: IMAGE 2
I usually have a piece of material ready at this stage. The material helps me decide what size and area to select for carving. I also make my selected image a bit larger than my actual material to avoid weird edge conditions in the carving.
Step 3: Adjusting the image to exaggerate the depth of carving
The gradients in IMAGE 2 previous page are mild. While the depths can be exaggerated in your CAM process, I find that Photoshop does a better job in this area.
I use Levels in PS to expand the data into the entire 8 bit range.
Open the Levels editor in Photoshop. IMAGE 1
Pull the left and right arrows in toward the middle until they are close, but not right next to the indicated grey levels. Leaving a little room here leaves you headroom in your carving. IMAGE 2
Notice how the contrast improves. This process also makes it easier to see little details in the topography.
Save the file. If you are using BobCAD with the bobART plugin save the file as an uncompressed TIF. If you are using ArtCAM (Delcam) save the file as a BMP. With FeatureCAM (also Delcam) you need to create an STL file before you can work with it (this can be done in Rhino, BobCAD, or other softwares)
Step 4: Creating a surface from this image
One you have fine-tuned the greys in your selection, it can be worked with in a CAM software. I use BobCAD because I can actually afford it and I donít have patience or expertise to hack my own solutions. The processes that BobCAD uses are standard in computer aided manufacturing. Such as selecting tool sizes, limiting depth-per-pass, adjusting stepovers, etc. I will use BobCAD as example, but the features I use should be available in any CAD/CAM package. Again, this is not a tutorial on how to use your CAD/CAM/CNC system, itís about the moonís surface.
Open your CAM software that can create a 3D surface from an image (in this case BobCAD).
In BobCAD, this process is called "Emboss from Image" and can be achieved in the bobART plugin. In Rhino it is called "Height field from Image"
During the import, BobCAD asks me how I want to scale the image in X, Y, AND Z axes. Where scaling in the Z exaggerates the depth of the carve. I have found that in combination with the level adjustment in photoshop, I like my Z depths to be around 0.625". I tend not to scale much in X or Y: I make my selection in photoshop such that I donít have to scale in the CAD/CAM environment. IMAGE 2
I recommend you play with this until you get the results you are after.
Hereís what mine looks like: IMAGE 3
Once I have the surface created I can begin to generate tool paths.
At this stage I could also export an STL file for 3D printing. And from Rhino, once the Height Field has been created, an STL can be generated.
In ArtCAM a BMP file is imported as a layer with similar settings to exaggerate the Z depth.
Step 5: Generating Toolpaths
Start by creating a boundary for the carving. In this case it will be a rectangle equal to my surface size. IMAGE 1, IMAGE 2
I usually create the boundary geometry at 1" above my work piece such that it stands out.
Now that we have surface geometry and a boundary we can start to create tool paths.
Which strategy of machining is up to you, but I have found that a "planar" approach is most efficient. These can take quite some time to carve, so efficiency is important.
BITS: I typically take two passes. Rough and Finish. For the rough pass I choose a ball-nose ½" diameter bit. For the finish pass I have used either a ball-nose ½" or a ball-nose ¼". Itís a catch 22. The ¼" diameter bit can achieve greater detail, but you need a smaller step-over to disguise the toolmarks (aka longer machining time). Surprisingly, the ½" diameter bit will give good detail. It turns out the moonís topography is mostly gentle and hence suitable for a larger bit.
Rough Pass: For hardwoods I have found that a 50% step-over with a 1/8" depth-per-pass configuration works well. I leave a 0.0625" allowance for the finish pass. I use conventional (not climb). I also use ZigZag or two directions of cutting for the rough pass. Some woods prefer to be carved with the grain and others across the grain. This will be up to you to experiment with and decide. IMAGE 4
Finish Pass: Again I will use the ½" ball-nose. No incremental step-down, Conventional cut. With the grain (in my case). 0.04" step-over or smaller. Zig only (one direction only). IMAGE 5
COMPUTER YOUR TOOLPATHS. IMAGE 6
Once everything is set up you can compute the tool paths, simulate, and post-process them.
Simulation. IMAGE 7
Step 6: Examples
Here are some examples and process shots