GlossaryKind of term: Manufacturing termTool overhang (stick-out)
What is tool overhang?
Tool overhang is the free length of a tool between holder and cutting edge; more overhang makes the tool less rigid and more prone to chatter.
Also called stick-out, it is measured from the face of the holder and usually expressed relative to the diameter, as an L/D ratio: a ⌀10 mm cutter with 40 mm overhang runs at 4 × D. On a lathe the same applies to a boring bar in a bore. How much overhang is needed follows from the depth of the feature, the clearance the holder needs from walls and workholding, and the direction from which the feature is reached.
What does tool overhang depend on?
In a simplified beam model of a clamped solid bar, deflection grows with the cube of the overhang and falls with the fourth power of the diameter. Twice the stick-out gives eight times the deflection at the same force in that model. In reality the fluted cross-section of the cutter, the stiffness of the holder and clamping, and the natural frequency all play a part; with long overhang, chatter is often the limit before static deflection is. Carbide has a Young's modulus roughly three times that of steel, which is why carbide shanks and boring bars deflect less at the same overhang.
How does tool overhang work in practice?
To stay on size and free of chatter, depth of cut, width of cut or feed come down as overhang grows. A deep pocket therefore usually takes more passes and more time than a shallow one with the same volume, and wall finish and tool life can suffer. The usual answers are clamping as short as possible, a slim holder, a cutter with a necked shank, a damped boring bar or holder, or machining the feature from another side if the setup allows. A small internal corner radius between the walls of a deep pocket calls for a small-diameter cutter on a long overhang, and so often decides the time; the radius between wall and floor comes from the cutter's corner radius, not its diameter.
What is tool overhang often confused with?
Overhang is not tool length: a long cutter can be clamped short. Nor is it the length measured on the presetter and entered in the tool table, which is usually taken from the spindle's gauge line, not from the holder face. And overhang is not deflection: overhang is a geometric dimension, deflection the result under cutting force.
Frequently asked questions
Why does a deep pocket take longer than a shallow one?
Because the cutter has to stick out further and is less rigid. To avoid chatter and size errors you reduce depth of cut or feed, so more passes are needed. A small internal corner radius between the walls also calls for a thin, long-necked cutter.
How do you keep tool overhang short?
Clamp the tool as short as the geometry allows, choose a slim holder or a necked cutter, and machine deep areas from another side if the setup permits.
When do you choose a carbide or damped boring bar?
When a steel bar chatters or loses size at the overhang you need. Carbide is stiffer; a damped bar suppresses vibration. The L/D each type handles differs by design and application; the supplier's guide values are a starting point.