3D roughing and machining allowance
Roughing removes everything that is obviously waste with the biggest flat end mill that fits the model, in stepped layers, leaving a small allowance for the finishing pass. The allowance is what makes the finishing pass light enough to cut accurately.
What roughing does
It cuts the model in horizontal layers, like a contour map: at each Z level it clears everything outside the model's cross-section at that height, then steps down. The result is a stepped approximation of the shape, slightly oversize everywhere.
Machining allowance
The allowance, also called stock to leave, is the material roughing deliberately does not remove. It exists for two reasons:
- The finishing pass needs something to cut. A tool cutting zero material chatters and burnishes rather than cutting cleanly.
- The load on the finishing tool stays constant. Without an allowance, the ball nose would alternate between cutting nothing and cutting into a roughing step, which shows up as an uneven surface.
Rule of thumb Something in the region of 0.01 to 0.03 inches is typical in wood. Enough to guarantee an even cut, small enough that the finishing pass stays light. Larger allowances mean the finishing tool is doing roughing work, which is slow and hard on a small ball nose.
Tool choice
Use the biggest flat end mill that can get into the model's major features. Roughing removal rate scales hard with diameter, and roughing is the one place in 3D work where brute removal is exactly what you want.
Do not use a ball nose for roughing. Its round tip removes less material per pass, it is shaped for surface following rather than bulk removal, and on a small diameter it is fragile. Flat end mill for roughing, ball nose for finishing, every time.
Rest machining
A pass with a smaller tool that only cuts where the previous larger tool could not reach: inside corners, narrow valleys, tight detail. CAM works out the leftover regions by comparing the two tool sizes.
It is worth adding when:
- The size gap between your roughing and finishing tools is large.
- The model has deep narrow features full of uncut material.
- Your finishing pass is struggling in specific areas, which shows up as chatter or a dulled ball nose in exactly those spots.
It is not worth adding when the tool sizes are close, or when the model is mostly open and gently shaped. It costs time and it is only paying for itself where the leftover material is real.
Roughing settings
| Setting | Direction | Why |
|---|---|---|
| Tool | Largest flat end mill that fits | Removal rate scales with diameter |
| Depth per pass | As much as the machine holds. See depth per pass. | Fewer layers means less time |
| Stepover | Large, often 40 to 50 percent | The surface does not matter here |
| Allowance | 0.01 to 0.03 inches in wood | Enough for an even finishing cut |
| Strategy | Adaptive if available, otherwise layer clearing | Adaptive keeps the load constant and allows deeper cuts |
| Cut direction | Whichever is faster | Finish quality is not the point of this pass |
Reading a roughed surface
It should look like a terraced hillside: obvious steps, uniform, with an even skin of material over the whole shape. If parts of the model are already at final surface after roughing, your allowance is zero there and the finishing pass will have nothing to cut in those spots. If there are big chunks of untouched waste left, the roughing tool could not reach and you probably want a rest machining pass.