3D carving on a CNC: STL models, reliefs and the whole process
A router carves the surface facing up, so 3D work on a CNC means reliefs: a shaped surface, not a free standing object. Every 3D carve is two operations. A flat end mill roughs out the bulk, then a ball nose follows the surface in close passes to finish it.
What a router can and cannot carve
A three axis router moves a tool in X, Y and Z but cannot tilt it or reach underneath anything. So it carves the surface that faces up, and only the parts of it the tool can see from directly above.
This is why almost all CNC 3D work is relief carving. A model that looks like a full sculpture on screen gets flattened into a relief when it is prepared for a router, and models sold for CNC are usually already reliefs.
What the model file contains
- STL is a surface made of triangles. No colour, no units stored reliably, and no true curves: every curve is approximated. Model resolution decides how smooth it can possibly be.
- A relief or heightmap stores a height value for every point in a grid. Some CAM packages work natively in this form, and a greyscale image can be imported as one.
- OBJ and other mesh formats behave like STL for these purposes.
Setting the model up
- Scale it. STL files often arrive at the wrong size or in the wrong units. Set the real physical dimensions you want first.
- Set the Z height. How deep the carve goes into your material. This is often adjustable independently of X and Y, and squashing or stretching the Z changes the character of the relief significantly.
- Check it fits your material. The deepest point of the model has to be above the bottom of your stock, with something left underneath.
- Position it. Where it sits on the stock, and whether it is centred or referenced to a corner.
- Decide whether the machine cuts the background, or whether you carve into a piece that is already the right size.
The two pass structure
| Roughing | Finishing | |
|---|---|---|
| Purpose | Remove the bulk fast | Produce the surface |
| Tool | Flat end mill, the biggest that fits | Ball nose or tapered ball nose |
| Pattern | Steps down in layers, like a contour map | Follows the surface in close parallel passes |
| Stepover | Large, 40 percent or more | Small, set by scallop height |
| Leaves | A stepped approximation plus an allowance | The finished surface |
| Share of total time | Usually the smaller part | Usually the larger part, often by a lot |
Skipping the roughing pass and letting a ball nose do everything is the classic beginner mistake. A ball nose is a finishing tool: it removes material slowly, it is not shaped for heavy cuts, and the carve takes many times longer than it needed to.
The things that decide whether it looks good
- The model. A well made relief looks good even carved coarsely. A bad one looks bad at any stepover.
- Material. Fine detail needs a close grained wood. See hardwoods. Red oak will crumble the detail you spent four hours carving.
- Finishing bit size relative to the smallest detail in the model.
- Stepover, which is the scallop height, which is what you feel with a thumb.
- Sharpness. A dull ball nose burnishes rather than cuts and the surface looks dull and fuzzy.
- Sanding. Some is always needed, and it is part of the process rather than a sign of failure.