Definition
Tool deflection is the amount a cutting tool bends under the forces of cutting, which increases with tool length and decreases with tool diameter and stiffness. A deflecting tool cuts a slightly different path than programmed, which can leave a feature oversized, undersized, or out of position, especially for long, thin tools reaching into deep features.
Why deep features often need lighter cuts
Deflection risk is why a deep, narrow feature often can't be cut at the same feed and depth of cut as a shallow one with the same diameter tool; a longer, thinner tool needs a lighter cut to stay within acceptable accuracy.
Where deflection risk is implied
Not called out directly; implied by a deep feature with a tight tolerance, where the required tool length-to-diameter ratio raises deflection risk.
Common mistakes
Programming a deep feature with the same aggressive parameters used for a shallow one, without accounting for the longer tool's greater deflection under the same cutting force.
Deflection risk is why a deep, narrow feature often can't be cut at the same feed and depth of cut as a shallow one with the same diameter tool; a longer, thinner tool needs a lighter cut to stay within acceptable accuracy.
See how Blake reviews a partFrequently asked questions
What causes tool deflection?
Cutting forces acting on a tool that isn't perfectly rigid, with longer, thinner tools deflecting more under the same force than short, thick ones.
How is deflection usually managed?
By reducing feed rate or depth of cut for long, thin tools, or by choosing a stiffer tool or holder where the geometry allows it.
Does tool deflection affect surface finish?
Yes, a deflecting tool can leave chatter marks or an inconsistent surface, not just an inaccurate dimension.