Tool Materials

Requirements for Cutting Tool Materials

1. Hardness - Must be harder than work material (hot hardness important)
2. Toughness - Ability to absorb energy without fracturing
3. Wear Resistance - Resist abrasion and adhesion
4. Hot Hardness - Retain hardness at elevated temperatures
5. Chemical Stability - Resist chemical reactions with work material

Types of Tool Materials

1. High Speed Steel (HSS)

Composition: Iron-based alloy with tungsten (W), chromium (Cr), vanadium (V), and sometimes cobalt (Co)

Properties:

• Good toughness
• Can be hardened to high hardness (Rc 60-65)
• Retains hardness up to 500-600°C
• Easy to fabricate and grind
• Lower cost

Types:

• M-series (Molybdenum type): 6% W, 5% Mo, 4% Cr, 2% V
• T-series (Tungsten type): 18% W, 4% Cr, 1% V

Applications: Drills, taps, reamers, milling cutters, general purpose tools

Cutting Speed: 10-60 m/min

2. Cemented Carbides

Composition: Tungsten carbide (WC) particles bonded with cobalt (Co) binder

Properties:

• Very hard (Rc 70-80)
• High hot hardness (up to 1000°C)
• High wear resistance
• Brittle (low toughness)
• More expensive than HSS

Types:

• Straight WC-Co - For cast iron, non-ferrous metals
• WC-TiC-Co - For steel machining
• WC-TiC-TaC-Co - For difficult-to-machine materials

Grades:

• C-1 to C-4 - Increasing toughness, decreasing hardness (roughing to finishing)
• Higher cobalt % → more toughness, less hardness

Applications: High-speed machining, mass production, CNC operations

Cutting Speed: 100-400 m/min

3. Coated Carbides

Base: Cemented carbide substrate with thin coating (2-10 μm)

Coating Materials:

• TiC (Titanium Carbide) - Wear resistance
• TiN (Titanium Nitride) - Low friction, oxidation resistance
• Al₂O₃ (Aluminum Oxide) - Chemical stability, heat resistance

Advantages:

• Combines hardness of coating with toughness of substrate
• Higher cutting speeds (20-50% more than uncoated)
• Longer tool life (2-10 times)
• Better surface finish

Coating Methods:

• CVD (Chemical Vapor Deposition) - 900-1050°C
• PVD (Physical Vapor Deposition) - 450-600°C

4. Ceramics

Composition: Primarily aluminum oxide (Al₂O₃) or silicon nitride (Si₃N₄)

Properties:

• Extremely hard
• Excellent hot hardness (up to 1200°C)
• Very brittle
• Low thermal conductivity
• Chemically inert

Types:

• Oxide ceramics (Al₂O₃) - White/black ceramics
• Silicon nitride (Si₃N₄) - Better toughness

Applications: High-speed finishing of cast iron and hardened steel

Cutting Speed: 300-2000 m/min

Limitations: Cannot handle interrupted cuts, require rigid setup

5. Cubic Boron Nitride (CBN)

Properties:

• Second hardest material after diamond
• Excellent hot hardness
• Chemically inert to ferrous metals
• Good thermal conductivity

Applications:

• Hardened steels (Rc 45-65)
• Hard cast irons
• Superalloys
• Finishing operations

Cutting Speed: 200-500 m/min

Form: CBN particles bonded to carbide substrate

6. Diamond

Types:

• Single crystal diamond - Natural
• Polycrystalline diamond (PCD) - Synthetic, bonded to carbide

Properties:

• Hardest material
• Excellent wear resistance
• High thermal conductivity
• Chemically reacts with ferrous metals at high temperatures

Applications:

• Non-ferrous metals (aluminum, copper, brass)
• Non-metallic materials (plastics, composites, graphite)
• Ultra-precision machining

Limitations: Cannot machine steel or cast iron (chemical affinity with carbon)

Cutting Speed: 500-3000 m/min

Tool Material Selection Guide

*Diamond loses hardness above 700°C and reacts with ferrous metals