A cutting tool faces high temperature, contact stress, and sliding. It needs: hot hardness, toughness (interrupted cuts), wear resistance, chemical stability/inertness.

Tool Materials (in order of development)

1. Carbon & medium-alloy steels — cheap, easily shaped/sharpened; poor hot hardness → only low speeds.
2. High-speed steels (HSS) — most highly alloyed tool steels; tough, fracture-resistant; good for high positive-rake tools, interrupted cuts, low-stiffness machines.
3. Cast-cobalt alloys — 38–53% Co, 30–33% Cr, 10–20% W; less tough than HSS, impact-sensitive.
4. Carbides (cemented/sintered, 1930s) — tungsten carbide & titanium carbide; high hot hardness, high $E$ and conductivity, low expansion; made by powder metallurgy. Inserts (square = 8 cutting points, triangular = 6) clamp to the shank. Plain = uncoated carbides.
5. Coated tools — lower friction, higher crack/wear resistance → high speeds; tool life up to ~10×. Coatings: TiN, TiC, TiCN, Al₂O₃, 2–15 µm thick; trend toward multiphase.
6. Alumina-based ceramics — fine high-purity Al₂O₃, cold-pressed & sintered; + TiC/ZrO₂ for toughness; very high abrasion resistance & hot hardness.
7. Cubic boron nitride (cBN) — hardest after diamond; a 0.5–1 mm cBN layer bonded to a carbide substrate.
8. Silicon-nitride-based ceramics.
9. Diamond — hardest known; low friction, high wear resistance, sharp edge; for fine finishing of soft nonferrous/abrasive nonmetals; brittle (low rake, careful mounting); not for plain-carbon steels, Ti, Ni, Co alloys (chemical affinity).
10. Whisker-reinforced — e.g. SiC whiskers in silicon-nitride or alumina tools.

Reconditioning: worn tools (especially HSS) are resharpened; recycling matters (W, Co are strategic and expensive).

Cutting Fluids

Functions: reduce friction/wear (↑tool life, finish), reduce forces/energy, cool the zone, wash away chips, prevent corrosion.

• A fluid is mainly a coolant or a lubricant.
• Water cools well but rusts and lubricates poorly.
• Low-speed ops (broaching, tapping) → lubrication matters most; high-speed ops → cooling matters most.
• Consider effects on parts, machine, personnel, environment.

Example — Fluid Shut Off (good lubricant)

Friction at the tool–chip interface rises → shear angle ↓ → shear strain ↑ → chip thickens → BUE likely → shear + friction energy ↑ → total energy ↑ → temperature ↑ (more tool wear) → finish deteriorates → tolerances harder to hold (thermal expansion).

Note: interrupted cutting needs toughness — HSS is the toughest in the lineup, though carbides are improving.