Tool overhang ratio calculator estimates relative deflection from a tool’s stickout-to-diameter ratio, using a 3:1 ratio as the sturdy baseline for comparison.
Check how far a tool is stuck out relative to its diameter, and roughly how much more it will deflect compared to a sturdy 3:1 setup.
Ratio = overhang / diameter. Deflection scales with length cubed, so relative deflection = (ratio / 3)^3, using a 3:1 overhang as the sturdy baseline.
How it works
A tool acts like a cantilevered beam, and beam deflection grows with the cube of length, so even a modest increase in stickout can multiply deflection many times over. A 6:1 overhang is not twice as flexible as 3:1, it is roughly eight times as flexible.
FAQ
What overhang ratio is considered safe? Many shops treat 3:1 to 4:1 as comfortable for general milling, with anything past 5:1 to 6:1 needing reduced feeds and depths of cut.
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How it works
Deflection under cutting load scales roughly with the cube of overhang length for a given diameter, so even a modest increase in stickout can produce a large jump in deflection, chatter risk, and poor finish or tolerance. Keeping overhang as short as the job allows, and stepping up tool diameter where possible, is one of the most effective ways to improve rigidity without buying different tooling.
FAQ
What overhang-to-diameter ratio is considered safe? Roughly 3:1 or less is considered a sturdy, low-deflection setup for most general milling and drilling; ratios beyond 5:1 usually call for reduced feeds and speeds, and ratios beyond roughly 8 to 10:1 start to require specialized long-reach tooling and much more conservative parameters.
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