Cutting Mechanics
When a cutting tool engages a workpiece, it does not scrape or plow — it shears the metal along a narrow zone called the shear plane. Understanding this mechanism explains why we choose certain speeds, feeds, and tool geometries, and why cutting generates so much heat.
How Metal Cutting Works
Imagine a wedge being pushed into a block of material. The metal ahead of the tool edge undergoes intense shear deformation along a plane at approximately 20–35° to the cutting direction (the shear angle φ). The material above this plane flows upward along the tool rake face, becoming the chip. This is a plastic deformation process — the chip is permanently deformed, not just displaced.
Key points:
- The tool edge must be sharp enough to initiate shear, not just compress the material.
- The shear zone is extremely narrow — most deformation happens in a thin band.
- The chip is work-hardened as it slides up the rake face.
The Shear Zone
The shear plane is where the bulk of energy is consumed. The shear angle φ depends on the rake angle, friction on the rake face, and the material. A larger shear angle means thinner chips and less energy per unit volume — which is why positive-rake tools cut more efficiently than negative-rake tools.
Cutting Forces
Three orthogonal forces act on the cutting edge. Together they determine machine power, tool deflection, and surface quality.
| Force | Symbol | Direction | Effect |
|---|---|---|---|
| Tangential (main cutting) force | Fc | Opposes cutting speed; vertical in turning | Determines spindle power; causes tool deflection and bend |
| Radial (thrust) force | Fr | Pushes tool away from workpiece; perpendicular to surface | Causes boring bar deflection, chatter, poor surface finish |
| Axial (feed) force | Ff | Opposes feed motion; along tool axis | Feeds the table; usually the smallest of the three |
Typical ratios (steel turning): Fc is the largest. Fr ≈ 0.3–0.5 × Fc. Ff ≈ 0.3–0.4 × Fc. The radial force is why boring bars chatter — it acts perpendicular to the overhang, multiplying deflection with L³.
Specific Cutting Force (kc)
The specific cutting force kc is the cutting force per unit area of undeformed chip. It depends on workpiece material and chip thickness (thinner chips → higher kc due to size effect). Typical values:
- Aluminum: 700–1000 N/mm²
- Low-carbon steel: 2000–2500 N/mm²
- Alloy steel (4140): 3000–3500 N/mm²
- Stainless steel (304): 3500–4500 N/mm²
- Inconel 718: 4000–5000 N/mm²
Try it in the Boring-Bar Deflection Check — it estimates Fc from kc × chip cross-section.
Heat Generation
Almost all cutting energy converts to heat. The heat comes from three sources:
- Shear zone — plastic deformation of the workpiece (≈60% of heat).
- Chip–tool interface — friction as the chip slides up the rake face (≈30%).
- Tool–workpiece flank contact — friction on the clearance face (≈10%).
Heat Distribution
In conventional machining of steel at moderate speeds:
- ≈75% of heat leaves with the chip
- ≈20% enters the tool
- ≈5% enters the workpiece
Why speed matters: as cutting speed increases, chip heat grows, but the chip leaves faster and carries more heat away — so tool temperature initially rises then stabilizes. At very high speeds, however, tool temperature spikes and crater wear accelerates. This is why tool-life curves (Taylor's equation) exist: there is an optimal speed range.
Learn more with the Power & Torque Estimator and Tool Life Planning.