Stepover for CNC pockets and 3D finishing
Stepover is the sideways distance between adjacent passes, usually entered as a percentage of the cutting diameter. Around 40 to 50 percent is the normal clearing range. Smaller means better finish and longer cuts. Larger means heavier engagement and worse chip clearing.
What it is
To clear a wide area, the machine makes a pass, steps sideways, makes another pass, and repeats. Stepover is that sideways distance. CAM usually asks for it as a percentage of the cutting diameter, because the useful number scales with the tool.
It is radial engagement, the counterpart to depth per pass.
What changes as you move it
| Stepover | Cutting time | Tool load | Floor finish | Typical use |
|---|---|---|---|---|
| 10 to 25% | Long | Light | Very good | Hard material, small or fragile bits, finishing |
| 40 to 50% | Balanced | Moderate | Good | Normal pocket clearing. Most CAM defaults. |
| 60 to 80% | Short | Heavy | Acceptable | Fast roughing on a rigid machine |
| 100% | Shortest | Maximum | Poor | Slotting. Not a stepover so much as a warning. |
Why 50 percent is the common default
Below about half the diameter, each pass is cutting with the side of the tool and chips have an open path out. Above it, more of the bit's circumference is buried on every revolution: chips have further to travel to escape, more of the edge is rubbing against a wall it has already cut, and the heat has less open air to leave through. Fifty percent sits just below that transition.
There is a second reason to stay near it. At a light stepover the chip is thinner than the chipload formula says, because the edge is only skimming the material at the start and end of its arc. Go much below 30 percent and a feed that looked right on paper starts to rub. This is called chip thinning, and it is why very light stepovers want a faster feed, not a slower one.
Ridges in the floor
A flat end mill leaves a flat floor, so in theory stepover does not affect a pocket floor at all. In practice it does, for two reasons:
- The machine is not perfectly trammed. If the spindle is tilted even slightly, a flat bit cuts a shallow arc, and every pass leaves a ridge where it meets the next. Wider stepover means taller ridges. See tramming.
- The tool deflects. Under load the bit tips slightly, producing the same effect.
So visible lines in a pocket floor are usually a tramming problem that stepover made visible, not a stepover problem. Reducing the stepover hides the symptom. Tramming the spindle fixes it. See ridges in a pocket floor.
3D finishing is a different calculation
With a ball nose, the tool is round, so adjacent passes always leave a ridge between them regardless of how well the machine is trammed. The height of that ridge is the scallop, and it is set by geometry.
For 3D work, stop thinking in percentages and calculate the scallop height instead. A 10 percent stepover means something completely different on a 1/4 inch ball nose than on a 1/16 inch one. See scallop height.
Adaptive and trochoidal clearing
Modern CAM offers clearing strategies that keep the tool engagement constant rather than the stepover constant, by curving the toolpath instead of running straight lines with corners. They allow a much deeper pass with a light radial engagement, which removes material faster while loading the tool more evenly. If your CAM has one, it is worth learning for large clearing jobs, and the settings look nothing like conventional stepover numbers.