Introduction
Not all materials behave the same under a cutting tool. Factors such as thermal conductivity, work hardening, chemical affinity, ductility, elastic recovery, and abrasive fillers directly dictate cutting forces, tool life, chip evacuation, dimensional accuracy, and overall cycle time.
To help engineers and procurement teams plan machining processes effectively, this guide ranks common metals and engineering plastics across five distinct machinability tiers—from “extremely difficult” to “easiest to machine”—while highlighting the key challenges encountered during CNC milling and turning.
4 Key Factors Determining CNC Machining Difficulty
Before diving into the rankings, it helps to understand the four physical drivers behind material machinability:
- Thermal Conductivity: Materials with poor thermal dissipation trap heat near the cutting edge, causing rapid tool wear and thermal degradation.
- Work-Hardening Tendency: Materials that rapidly harden at the shear zone during cutting complicate subsequent passes and accelerate tool breakdown.
- Chemical Affinity at Elevated Temperatures: At high cutting temperatures, certain materials readily adhere to or diffuse into the tool matrix, causing built-up edge (BUE) and premature chipping.
- Ductility, Elastic Recovery & Abrasiveness: Highly ductile materials form long, stringy chips; elastic materials deflect during cutting; abrasive fillers act like sandpaper on cutting edges.
Reference Baseline: Quenched-and-tempered 45# carbon steel (1045 equivalent) serves as the standard baseline with a relative machinability index of 1.0. Lower numbers indicate increased machining difficulty.
CNC Machining Difficulty Ranking: Five Tiers
Tier 1 — Extremely Difficult to Machine
Relative Machinability Index: <0.15
Process Considerations: Demands advanced ceramic, coated carbide, or CBN tooling, alongside strictly controlled cutting speeds, rigid setups, and high-pressure cooling.
1. Nickel-Based Superalloys
Examples: Inconel 718, Hastelloy grades, GH4169.
Key Challenges: Low thermal conductivity, extreme work hardening, and severe tool adhesion at high temperatures. Cutting edge temperatures often exceed 900°C, drastically shortening conventional tool life. Machining times can be several multiples of 6061 aluminum. Actual performance depends heavily on alloy heat treatment, machine rigidity, tool geometry, and strategy.
2. Titanium Aluminide (TiAl) Intermetallics
Key Challenges: Considerably more challenging than conventional alloys like Ti-6Al-4V. Low thermal conductivity paired with room-temperature brittleness causes frequent edge chipping and high tool wear.
3. Precipitation-Hardening High-Strength Stainless Steels
Examples: Aged 17-4 PH, PH13-8 Mo.
Key Challenges: Hardness levels often reach HRC 40–48. High yield strength generates substantial cutting resistance, while thin-walled components remain prone to elastic deflection and dimensional drift.
Tier 2 — Very Difficult to Machine
Relative Machinability Index:0.15 – 0.30
Applications: Staple materials in aerospace, defense, and medical devices that impose strict demands on machine rigidity, tool selection, and coolant delivery.
1. Titanium Alloys and Commercially Pure Titanium
Examples: TC4 (Ti-6Al-4V), TA2.
Key Challenges: Low thermal conductivity and low elastic modulus lead to localized heat concentration, elastic spring-back, chatter, and tool galling. Generally requires lower cutting speeds and high-pressure through-tool coolant.
2. Hardened Tool Steels
Examples: Cr12MoV, SKD11 (hardened to HRC 55–68).
Key Challenges: Extreme hardness causes severe abrasive wear and rapid tool chipping. Finishing passes typically require CBN inserts, light cuts, and multi-stage machining strategies.
3. Duplex Stainless Steels
Examples: 2205, 2507.
Key Challenges: Combining austenitic and ferritic microstructures, duplex grades exhibit high strength, work hardening, and chip adhesion, making chip breaking difficult compared to standard stainless steels.
Tier 3 — Difficult to Machine
Relative Machinability Index:0.30 – 0.65
Overview: Frequently specified in precision components, requiring focused chip control, optimized toolpaths, and prudent process planning.
1. Austenitic Stainless Steels
Examples: 304, 316 / 316L.
Key Challenges: High ductility, rapid work hardening, and built-up edge formation. Rubbing instead of clean shearing drastically accelerates surface hardening and tool breakdown. Sharp cutting edges and consistent feed rates are critical.
2. Pure Copper
Examples: T2 copper, oxygen-free copper (OFC).
Key Challenges: Extremely ductile, generating long, stringy chips that wrap around tooling. Chip evacuation and material sticking pose severe challenges, particularly in deep-cavity or small-diameter drilling.
3. Fiber-Reinforced Engineering Plastics and Composites
Examples: Glass-filled PA, carbon-filled PEEK, epoxy composites.
Key Challenges: Glass and carbon fibers are highly abrasive, rapidly wearing tool cutting edges. Requires wear-resistant (PCD/diamond-coated) tooling, dust management, and careful control to prevent delamination.
4. High-Strength and High-Carbon Steels
Examples: Heat-treated 65Mn, 85# steel, 40CrNiMo (4340 equivalent).
Key Challenges: Elevated yield strength increases cutting forces. Improper speeds/feeds lead to accelerated flank wear and built-up edge formation.
Tier 4 — Moderate to Relatively Easy
Relative Machinability Index:0.65 – 1.6
1. General-Purpose Carbon & Alloy Steels (e.g., Normalized/Tempered 45# steel, 40Cr, 20# steel)
Standard baseline materials offering predictable cutting behavior and manageable chip formation with modern coated carbide inserts.
2. ray Cast Iron (e.g., HT200, HT250)
Free graphite flakes provide natural lubrication and chip breaking, though abrasive microstructures demand dust collection and wear-resistant inserts.
3. Unfilled Engineering Plastics (e.g., Unfilled PEEK, PA66 Nylon, Polycarbonate)
Low cutting forces, but friction heat can cause melting or gummy edge build-up.
4. 7075-T6 Aluminum Alloy
High strength relative to other aluminum alloys; excels under high-speed machining when paired with polished, sharp tooling for rapid chip evacuation.
Tier 5 — Easy to Machine
Relative Machinability Index:> 1.6
1. Free-Machining Brass (e.g., C3602, leaded brass)
Forms small, crisp chips with minimal tool wear, enabling maximum spindle speeds and feed rates.
2. Standard Aluminum Alloys (e.g., 6061-T6, 6063, 1060)
Exceptional thermal conductivity and low cutting resistance make aluminum the industry standard for high-volume, rapid-cycle precision manufacturing.
3. General-Purpose Engineering Plastics (e.g., POM/Acetal, ABS, PP, PVC)
Minimal cutting resistance; primary focus is heat control to avoid smearing, burring, or clamping deformation.
4. Free-Machining Steels (e.g., Y12, SUM24L)
Alloyed with sulfur or lead to ensure effortless chip breaking and extended tool life.
5. Magnesium Alloys (e.g., AZ31)
Ultra-lightweight with low cutting resistance, though fine chips require strict fire safety protocols.
Quick Comparison Table
| Difficulty Tier | Relative Machinability Index | Representative Materials |
| Tier 1 — Extremely Difficult | < 0.15 | Nickel-based superalloys, TiAl intermetallics, aged PH stainless steels |
| Tier 2 — Very Difficult | 0.15 – 0.30 | Titanium alloys, hardened tool steels (HRC >55), duplex stainless steels |
| Tier 3 — Difficult | 0.30 – 0.65 | 304/316 stainless steel, pure copper, carbon/glass fiber composites, high-strength alloy steels |
| Tier 4 — Moderate / Fair | 0.65 – 1.6 | 45# carbon steel, 40Cr, gray cast iron, unfilled PEEK/PA66, 7075-T6 aluminum |
| Tier 5 — Easy | 1.6 | Free-machining brass, 6061/6063 aluminum, POM/ABS, free-machining steel, magnesium alloys |
Bridging Material Selection & Advanced Manufacturing
Understanding raw material machinability is only the first step in successful engineering. In high-precision manufacturing, process control and specialized engineering capabilities often matter far more than material selection alone.
In demanding sectors like RF, millimeter-wave, and terahertz (THz) hardware, challenges extend beyond basic metal cutting. Engineers must tackle micro-scale tolerances, strict thermal expansion matching, surface finish optimization for low insertion loss, and ultra-high vacuum or hermetic sealing performance.
This is precisely where THZ Precision bridges the gap between challenging engineering specifications and reliable production:
Full-Spectrum Precision Manufacturing: Specialized in custom high-precision fabrication for microwave, millimeter-wave, and terahertz applications. The core portfolio includes custom hermetic cavities, glass-to-metal sealed (GTM) insulators, precision waveguide components (WR-8 down to WR-1.0), and specialized RF feedthroughs.
End-to-End Engineering Support: Whether managing sub-10-9Pa.m3/s helium leak rate requirements or machining tight-tolerance millimeter-wave assemblies, THZ Precision’s engineering team provides full-lifecycle support—from initial Thermal Coefficient of Expansion (CTE) evaluations and prototyping to full-scale volume manufacturing.
Engineering Tip: Prior to full volume production, conduct prototype trials to evaluate thermal expansion, tool deflection, and micro-burr formation under real machining conditions to optimize cycle times and part yields.
The most difficult CNC materials are those combining poor heat dissipation, severe work hardening, high tensile strength, tool affinity, or abrasive microstructures. Nickel superalloys, titanium, hardened steels, and composites require tailored tooling strategies and rigorous thermal management compared to standard aluminum or brass.
By identifying machinability constraints early and partnering with specialized precision manufacturers like THZ Precision, engineering teams can lower cycle times, protect tooling investments, and eliminate costly manufacturing risks.