Cast Cement Mountain Sculptures With a CNC Router

In this instructable, you will learn how to use Google Sketchup and Rhino to obtain specific topographical data of any location on Earth, CNC Mill that topography out of foam to create a waste mold, and cast cement into that mold to get a solid cement sculpture that looks exactly like your topographic source material.

You will need:

Software:
Google Sketchup
Rhinoceros
MasterCAM (or any other CAD-to-CAM software)

Hardware:
a CNC Router table

Materials:
2'' thick insulation foam
Joint Compound
Shellac
Aluminum flashing
Cement (optional: pigment/tint)
Hot Glue
Clay
Paste Wax
Wet/Dry Sandpaper (80 - 800 grit)

Step 1: Select Your Mountains

Open Google Sketchup. Under the 'File' menu select 'Geo Location --> Add Location'. This opens a pop-up window that has a map of the Earth.

Either type in your region's name in the top left corner of the window, or search around for a location with interesting geologic data. Zoom in until a bounding rectangle appears, then continue zooming until you have your desired area of interest within the box. Click 'select region' in the top right corner of the window.

Use the blue pins to refine your bounding box, then click 'Grab' in the top left corner.

Step 2: View and Save Your Mountain File

Once you have selected and imported the topography, it will appear as a flat rectangle. Reveal the topographic data by clicking `File --> Geo Location --> Show Terrain'. The topography of your square should now appear in the viewport.

If you are satisfied with the revealed terrain, save your file (File --> Save As) as a sketchup model (.skp*) to prepare it to be edited in Rhino.

Step 3: Trim/Scale/Prepare Your Mountain in Rhino

Open your saved .skp file in Rhino. It should import as two rectangular meshes; one with the terrain of your mountain, and one empty rectangular plane (a remnant from the sketchup file). Delete the empty plane so that you only have the terrain in your viewport.

Now, prepare your file to be CNC-ed. First, scale it down drastically. You will need to do this several times, as the data imports at actual-scale (mountain size, AKA massive). Continue scaling down the topography uniformly until your square is a size that logically corresponds to your CNC machine table size. (In this example, I chose a 10 inch diameter.)

Once your scale is set, decide what shape you want your sculpture to be. If you want a rectangular extrusion, you don't have to edit anything. (In this example I wanted a circular/cylindrical base for the mountain, so I trimmed the mesh using a circular curve and the MeshTrim command.)

Finally, export your file as an .STL (or whatever file format your CNC software prefers.)

Step 4: CNC Mill the Topography

Run your .STL file through the CAM (computer aided manufacture) software that corresponds to your CNC Mill to generate the tool paths for milling. In this case, I used MasterCAM in conjunction with a 4x8 foot 'Techno LC Series 4896' CNC Mill.

Purchase the proper amount of 2'' thick insulation foam (available at Home Depot/Lowes) to mill out your entire form. Enter the dimensions of your stock material into the CAM software so it generates the tool paths accordingly.

Once you have your toolpaths generated, affix your two inch foam to the spoilboard of the CNC mill, and run the machine. All CAM software functions slightly differently, so this tutorial will not include a step-by-step for generating the toolpaths, nor for operating the CNC machine. Many other tutorials exist for this purpose.

Remember that the machine is milling out the waste mold for your final object, and is not the final object itself. In other words, you will want to mill out the inverse or negative topography of the mountain, so that when cement is poured into the milled-out recess you are left with the positive version of the terrain. The final milled foam form should look like the negative impression of a landscape.

Step 5: Prepare Your Foam Waste Mold

Using a box cutter or hack saw, carefully cut out the topographic sections of the milled forms from the rest of the stock material, and use spray adhesive to glue-up the layers together properly.

Once your layers are all registered and glued up, fill in any gaps / striations / step-over marks from the router bit with joint compound. Once the joint compound dries, sand it down with 220 grit sandpaper until the terrain looks as smooth as you'd like it to.

Clean off any dust remaining from sanding, and paint on 2-3 layers of shellac (or polycrylic, or any other sealing agent) to create a nonporous barrier on top of the foam/joint compound.

Depending on the desired height of your sculpture, create appropriately sized mold walls using aluminum flashing cut to size. Bend the aluminum flashing around the outline of your terrain, and hot glue it into place. Use soft clay to fill in any gaps or seams between the mold walls and the topography.

Finally, once your mold walls are secured in place, brush a layer of paste wax over the entire form (topography and mold walls) in order to ensure the cement wont bond to your mold. Let the wax sit for 2 minutes, then buff it in with a soft, clean microfiber cloth.

Step 6: Cast Your Mountain!

Now that your mold is sealed and waxed, you're ready to mix and pour your cement! (note: you can also cast this form with plaster, resin, or any other casting material, but this tutorial will focus on cement.)

Calculate the proper volume of cement to fill (but not over-fill!) your mold, and following the directions on the bag of your particular cement brand, mix that amount of cement. You can use pigments or paint to tint the cement by dissolving them into the water before adding that water to the cement.

Once you've mixed your cement, strain it using a kitchen strainer to make sure there are no lumps in the mix, and carefully pour the strained mixture into your mold.

Once you've filled your mold, wait for the cement to cure. This can be anywhere from 15 minutes to an hour, depending on your mold size.

Step 7: Demold and Finish

Once your cement is cured, carefully demold the form by turning the foam upside-down and removing the aluminum flashing from the baseboard. You may need to use a putty knife / spatula to help remove the flashing from the foam. Once the flashing is removed, pull gently on the cement until it separates from the foam piece.

Once the cement is separated from the mold, you may notice some joint compound and/or foam is stuck to the cement. This can be easily removed by wet/dry sanding the cement. Depending on how smooth you'd like your final object to be, I reccommend wet/dry sanding anywhere from 80 grit (to remove joint compound, any residual machine marks, and smooth out the entire form) up to 800 grit (to polish).

Once you've sanded off all residual machine marks, let the cast dry out for at least a day, ideally a week, in order to let all residual moisture in the cement evaporate. Once the cast is fully dried (it will looks lighter in color), a final layer of wax can be applied to the cast to seal in the color and protect it from damage. Paint on a layer of wax, let it sit for 2 minutes, then buff it in with a soft, clean microfiber cloth.