Tool deflection is the bending of a cutting tool under the sideways force of the cut. Even a small deflection shows up as tapered walls, mismatched steps, chatter, and poor finish, because the tool isn’t where the program thinks it is.
What drives it
Deflection increases with cutting force (deeper/wider cuts, higher feed) and, dramatically, with tool length — it scales with roughly the cube of the unsupported length and drops sharply with larger diameter. So a longer, skinnier tool deflects enormously more than a short, fat one.
How to reduce it
- Use the shortest tool possible and minimize stickout.
- Use the largest diameter that fits the feature.
- Lighten the cut — smaller radial/axial engagement, especially on finishing passes.
- Take a spring pass (a repeat pass at the same depth) to clean up deflected material.
- Improve overall rigidity — holder, spindle, and workholding.
Frequently asked questions
What causes tapered walls? Tool deflection — the tool bends away from the cut.
Does diameter matter more than length? Both matter; deflection scales with length cubed and drops sharply with larger diameter.
What’s a spring pass? A repeat finishing pass that removes material left by deflection.