The intent of this Instructable is to show how I went about using my CNC machine to carve bas relief animals into logs, as if they were carved into trees. I had the notion first of carving a bear and having it mounted on a tree, appearing to be peeking out. It didn’t happen exactly like that but I did learn a lot about how to carve logs and about the limitations of my CNC machine.

Supplies

& Tools

Slice (slab) of Log (for my CNC, limit of 4 inches thick)
CNC Machine (as much clearance as possible between Z axis carriage and spoil board. Mine is 4")
CNC Dust shoe with enough flexibility to move over the log (optional and somewhat impossible)
Plastic lawn edging and clamps to contain chips that escape the dust shoe (or other approach)
Shop vacuum
Long bits (I used 1/4" diameter end mill and 1/4" diameter ball end)
Strap hinges (for mounting log to spoil board)
Plane (to flatten back and part of front. Electric is preferred.)
Misc. woodworking tools.
Stain and finish as desired.

Step 1: Logs and Prepping

Logs for Carving

The ideal log is a slab of wood sliced or split from a tree round, with the bark or other durable rough surface still in place. Dimensions would be 8 to 10 inches wide, maybe 12 inches long, and 3 to 4 inches deep. My CNC machine Z axis can travel 5 inches (minus bit projection). The spindle carriage clears the spoil board by 4 inches. So the carving depth is limited to 3 inches or so, leaving an inch of uncarved depth. The carving is made on the rough side of the log. One source of material I found was firewood almond. Another source was a friend who had cut Alder logs into slabs and gave me the end cuts. The end cuts had worm trails under the loose bark which gave it a nice texture. I carved mostly animal heads, and some heads required more depth than others. When you find a good candidate log, and you have an idea of how it’s going to fit into the log, you need to prepare the log.

Prepping the Log

The back (bottom) has to be flat so it will sit well on the CNC bed. An electric planer is good for this. And when you’ve decided the final dimensions of your carving, I’ve found it helps to flatten a part of the rounded face of the log so there is wood to carve for all the bumpy pieces. For instance, the animal nose is usually the highest part, but other parts stick up high also and want to start low enough that you'll have wood to carve. Make a flat area the required length and go as wide and deep as you need to include any protruding side features (pokey ears etc.)

Bits

To carve deeply into the log, you need long bits. The longer ones don't seem to be available less than ¼ inch diameter so that's what I used - an end mill for roughing and a ball nose for the finish pass.

Step 2: Model

I borrowed a lot of animal head models from Thingaverse. The owl shown in this Instructable is The Studious Owl by scampbell . I brought it into Cura (3d printing CAM program) and used Cura to re-orient and scale. If there are parts you’re not going to carve (e.g. the owl's book platform) , scale it oversize accordingly. In scaling I changed the depth (Z coordinate) independent of XY, flattening the model so it would fit onto a log and the carved depth is no more than 3 inches or so. There is trial and error here and Cura takes a long time on my PC to rescale a large model. I saved the updated model as an STL file.

Step 3: CAM Software

Software

I imported the STL model into a CAM program called Aspire. As an alternative to using Cura, Aspire can be used to re-orient and scale. This is a fairly complicated and specific process, and this tutorial is not intended as detailed instruction in Aspire (or other Vectric Co. CAM programs). For other CAM programs, the following steps will still apply but possibly with different terminology. But here are the basic steps in generating G-code to carve a log.

Size of Material

Start a new file, and tell it the height, width, and depth of the area to be carved, not the whole log. For carving convenience, I always orient the log so the length is along the X axis, with the animal’s head it to the right. I also set the XY zero position to the center of the work, shifting with an offset to so the Zero point is the highest point (the animal’s nose usually).

Importing the Model

In the Modeling Tab, I use the function that Imports a 3d Model. The model is brought in and superimposed on the material space. At this point I re-orient the model so the head is to the right (rotate around Z axis -90 degrees). Then I go back to the 2D view and resize and orient the model to the part I want to carve is on the material. For the Owl, I didn’t want to carve the book he’s standing on, so that portion was not in the bounds of the "material".

Material Setup

I used the Material Setup pane to add offsets so the animal's nose is the XY Zero point. I pencil an X on this spot on the flattened log. This will become the XY zero point later. This point is also used to set Z zero for both the roughing tool (which doesn’t cut here) and the finish tool (which does cut here).

Roughing Pass

For the roughing pass, cut out to the material boundary. This cuts a flat portion from which the carved animal will protrude as well as roughing out the animal head. Under Roughing Strategy I chose Z Level, with Raster X, so cut marks are going the same as the grain. And I opt for Profile Last, where after each level, it mills around the profile and smooths off the bumps left by the ¼ inch tool. Step-over (how much the bit advances for each pass) is set to 70%. No time wasted here.

Finish Pass

For the finish pass, cut to the model boundary. For Area Machine Strategy I opted for Offset (starting in the center and moving outward in a circular way. The step-over for the ball head ¼” tool was 10%. This makes it take a while but keeps tool marks to a minimum. The tool paths are saved as separate G-code files to a thumb drive, which I take to the basement where the CNC machine lives.

Step 4: Carving

Mounting Log to Table

Although the log is been flattened on the back, the top and bottom cross-cuts are usually irregular. I found the best way to secure the log was to use hinges, screwed to the ends of the log and to the spoil board. This worked great, pulling the log snugly down and preventing movement.

Dust Management

For the log carving, I made an extra wide dust shoe with extra long plastic fringes. But this only catches a percentage of the chips and the roughing pass still made a royal mess. So I came up with the idea of a circle of plastic lawn edging clamped around the work. This seems to work quite well keeping the chips inside the circle. It still requires periodic vacuuming but it doesn’t get all over the shop as it did before.

Roughing

For roughing I make sure the bit extends far enough so the collet doesn’t hit the material when it goes to carve deep. This is why you need long bits. I try to get the dust shoe to be down far enough to catch as many of the chips as possible but not bump into the log when carving deep. I have had to pause the machine and adjust the dust shoe upward as it goes deeper. XY Zero is set to the X mark I made on the top (animal’s nose) and tool Z zero is set at this same location (using the paper clearance method).

Finish Pass

The finish pass is essentially the same, but there are far fewer chips. XY Zero remains as previously set, and Z zero is set at the XY zero location (the x mark should still be uncarved and visible).

Step 5: Displaying the Result

Staining

Depending on the log species, I applied stain to make the carving stand out. For the Alder carvings I dabbed stain on the rough uncarved part of the log to darken it and make the irregularities stand out. I painted dark stain on the flat carved area surrounding the animal. I sprayed clear finish on the carved head to bring out the grain.

Display

I’ve loved carving these things and did rather a large number of them. But what do I do with them all? I came up with the idea of mounting them on trees in a nearby park. I chose locations that were lightly forested so they could be seem from the trail but only if you were looking. So I can visit them whenever I want, and perhaps other people will find them. For mounting I drilled deep ½ inch holes top and bottom leaving about ¾ inch drilled with a smaller hole. A tool extension goes down into the ½ inch hole and drives deck screws to fasten the carvings to the trees. To fill the hole I just find a twig the right size, put it in the hole and break it off. Here are pics of some of my hidden carvings.