Intro: Engraving an Image With a CNC Router
Most projects I do involving the CNC router have a 3D result, but the router can be super useful for 2D work too! Here I show a method using Rhino, Grasshopper, and the CNT Motion Systems CNC Router to engrave an image into wood.
This method works by inputting a set of lines or paths (in rhino) that the CNC router will cut/carve along, and using information from your image (in grasshopper) to define the depths of those cuts. Through the choice of your cutting bit, you can therefore achieve different results. A V shaped engraving bit will carve thicker lines the deeper it cuts into the material, so in this tutorial I'm using a fairly typical 60 degree signcutting bit.
Step 1: Choose Your Image
You of course first need to decide what image you'd like to engrave! Here, I'm using a still from a recent video project. The project deals with video as a spatial volume, so I thought it'd be kinda neat to have a physical version of a part of this video.
From trial and error, I've found that higher contrast images will be more legible in the final product. So in photoshop or another image editor of your choice, add a little bit of contrast. As we will see in a few steps, this method samples the brightness of your image to determine the engraving depth, so convert your image to black and white at this stage as well.
Step 2: Get Your Image Into Rhino!
Using the "backgroundbitmap" command, place your image into your rhino file. This is just a visual aid, but I find it helpful for making design decisions. At this point, you'll need to think about the kinds of lines you'd like to engrave. This could be anything from simply parallel lines, to a spiral, to concentric polygons. The only necessity is that there is a constant width between one line and the next.
Here, I used the "arraylinear" command to place 90 horizontal lines across the image, exactly 1/8 inch apart.
Step 3: Send Your Image Information and Lines to Grasshopper
Grasshopper is an incredibly capable program for creating parametric designs in Rhino. If you've ever worked with another block programming language, like Max, this works very much the same way- you can input information or geometry from your rhino file, pass it through a series of "blocks" that each represent a different function or transformation, and output new geometry back into your Rhino file.
My Grasshopper sketch works in the following way:
1. Curves, which you drew in Rhino, are input into the sketch.
2. The number slider defines the number of subdivisions along that curve, aka, the points at which your image will be sampled. Since my lines are pretty short, I'm subdividing about 213 times (the exact number is unimportant).
3. the "Divide" block divides your curves this number of times.
4. The points along which your lines are divided are split into two data streams- one goes into a Point Deconstructor element, which splits apart the XYZ information of every single point.
5. At the same time, the points are being fed into the image sampler. Here, you define the desired dimensions of your image in the Rhino file- use whatever dimensions you want your final product to be. The image sampler will sample at every input point.
5. The output of the image sampler is taken- we only need the new Z information. These values are multiplied by -1.
6. The original X and Y data of every single point is merged with the new Z information of the points. Now, the height of every single point is defined by the brightness of your image.
7. The points are recomposed into lines (polylines).
8. Using the Scale NU block, the new lines are compressed in the Z dimension. A little bit of math is involved here, to find out the correct scaling values- but considering the geometry of the bit you use, you can figure out the maximum cut depth of your paths before your lines start to touch. Because I'm using a 60 degree bit, and my lines in rhino are .125 inches apart, I can cut a maximum of .108 inches into the material. To be a bit more conservative I actually went with an even .100". (original sampled values were from -1 to 1, so the necessary scale factor was .05)
9. Finally, the new geometry is moved to the top of the material stock.
Step 4: Bake Your Resultant Geometry Back Into Your Rhino File
After sending the input geometry through the grasshopper patch, this is what I'm left with. The way the lines are rendered on the screen, you can even begin to see the contours of your image!
At this step, I no longer needed the flat input lines or the other guides I had made in the rhino file, so all the extra geometry can be deleted. We only need the new contoured lines. To simplify the fabrication process, at this stage I also joined all of the lines into one continuous zigzag. This way, instead of the router carving one line and then having to travel all the way back to the start of the next line, a single continuous line can be carved.
Step 5: Make Your Toolpaths and Post Your GCode
To generate the GCode that the router can understand, I'm using Mecsoft's RhinoCAM software. We don't need to get too fancy here- a simple engraving operation will do the trick. I'm using pretty conservative feed rates here, but through experience with the particular bit I'm using, I've found that 90 inches per minute does just great for fine detail in soft wood.
Step 6: Warm-up and Calibrate Your Router
I am using a CNT Motion router with a 4'x4' bed and 11 tool library. This is an incredibly versatile router and I'm really only using a fraction of it's capabilities in this project, but I do have to account for one small flaw in the machine here. The router measures the length of each tool by selecting that tool, and lowering it until it presses into a sensor. Through heavy use, the calibration sensor on this router has actually gained a small divot in its center, meaning a pointy little signcutting bit like I'm using here won't get measured correctly.
Instead, I'm calibrating it manually by jogging the bit to the top of my material, and adjusting my toolpaths accordingly.
Step 7: Seal the Material to the Router Bed and Begin Cutting!
This router uses a vacuum bed to secure the material stock, and since I'm using a relatively small piece of wood, I added scrap materials to the other sections of the bed. This creates a tighter seal and better ensures that the material remains in one place throughout the cut. Once everything was secured, I began the cut and watched the router work its magic!
Step 8: Results
Whether this particular image translated especially well into 3D contours is debatable, but the relationship to the original input is definitely clear, and so I consider this experiment a success! I've photographed the piece here under intense directional lighting in order to reveal the contours better, but in the future I may use this as a block for making prints, or event a mold for thermoplastics.
I've also included some images of past results I've gotten using this approach on the CNC Router- a 30x40 carving on plywood where the cuts were filled with wax, and a 36x36 carving also on plywood.