Machining copper, aluminum, stainless steel - which is more difficult to machine?

In the field of metal processing, copper, aluminum, and stainless steel are the three most widely used materials, but due to differences in physical properties, cutting characteristics, and processing requirements, there are significant differences in their processing difficulty. This article compares the material properties, cutting difficulties, tool selection, process parameters, and other dimensions to analyze the difficulty differences in machining copper, aluminum, and stainless steel.

1. Comparison of material physical properties and basic processing characteristics

Properties	Copper (pure copper as an example)	Aluminum (6061 aluminum alloy as an example)	Stainless Steel (304 austenitic stainless steel as an example)
Hardness (HB)	35~45	95~100	150~180
Thermal Conductivity (W/(m·K))	401 (extremely high)	167 (high)	16.3 (extremely low)
Ductility	Excellent (elongation > 40%)	Good (elongation > 15%)	Moderate (elongation ≈40%, significant work hardening)
Density (g/cm³)	8.96	2.7	7.93
Melting Point (℃)	1083	660	1398

Core differences:

Copper: soft and tough, with extremely strong thermal conductivity, easy to stick to the tool and form built-up edge during cutting, and difficult to control dimensional accuracy.

Aluminum: low density, medium hardness, high thermal conductivity, but small elastic modulus (easy to deform), soft chips and easy to wrap around the tool.

Stainless steel: high hardness, extremely poor thermal conductivity, severe work hardening (hardness can be increased by 50%), large cutting force, and high temperature.

2. Comparison of the difficulties in machining copper, aluminum, and stainless steel

(1) Cutting force and cutting temperature

Copper:

When machining copper, the initial cutting force is small because the material is soft, but due to its good ductility, the chips are easy to be continuous, and the contact length between the tool and the chips is long, resulting in friction heat concentrated near the tool tip. Although high thermal conductivity is conducive to heat dissipation, local high temperature will still aggravate tool adhesion (especially pure copper).

Aluminum:

The cutting force is low (about 1/3 of stainless steel), but due to its low melting point (660℃), the chips are easy to melt and adhere to the tool during high-speed cutting, forming built-up edge and destroying the surface roughness (such as Ra value deteriorating from 0.4μm to 1.6μm).

Stainless steel:

The cutting force is 50%~80% higher than that of copper and aluminum, and the thermal conductivity is poor (only 4% of copper), resulting in more than 80% of the cutting heat being concentrated on the tool. The tool tip temperature can reach 800~1000℃, which aggravates tool wear (for example, the life of carbide tools is only 1/5 of that of copper).

Conclusion: Difficulty in cutting force and temperature control: stainless steel>copper>aluminum.

(2) Tool wear and life

Copper:

The main form of wear is adhesive wear (the tool material has an affinity with copper), especially for high-speed steel tools. The use of carbide or diamond-coated tools can significantly improve (life is increased by 3~5 times).

Aluminum:

The wear is mainly mechanical wear (micro-cutting effect of soft aluminum on the tool), but the repeated shedding of built-up edge will cause the tool surface coating to peel off. Super-hard tools (such as PCD diamond tools) and sharp edge design (front angle +15°~+25°) are required.

Stainless steel:

It faces abrasive wear (carbide hard spots), adhesive wear and diffusion wear (element diffusion at high temperature). The hardened layer causes the tool to bear impact load when cutting in, and the cutting edge is prone to cracking. Ultra-high hardness tools such as ceramics and CBN are required, and the life is extremely short (for example, when processing 304 stainless steel, the life of ceramic tools is only 30 minutes).

Conclusion: Tool adaptation difficulty and life challenge: stainless steel> copper> aluminum.

(3) Chip control and surface quality

Copper:

The chips are ribbon-shaped or spiral-shaped, which are easy to wrap around the tool and workpiece and scratch the machined surface. The chip breaking performance needs to be improved by chip breaker design (such as deep grooves, large rake angles) or adding easy-to-cut elements such as lead and sulfur (such as C36000 brass).

Aluminum:

The chips are soft and sticky. Even if they are broken, they are easy to adhere to the tool, resulting in deterioration of surface roughness. High-speed machining (such as spindle speed>10000rpm) combined with high-pressure air cooling can reduce chip sticking.

Stainless steel:

The chips are short fragments (due to work hardening, the brittleness increases), but the high temperature generated during the cutting process causes the chip and the tool to form a molten welding layer in the contact area, which aggravates the surface scratches. A large curling radius chip breaker and sufficient coolant are required.

Conclusion: Chip control and surface quality difficulty: Stainless steel ≈ copper > aluminum (the surface problem of aluminum mainly comes from built-up edge, and the control is relatively simple).

(4) Processing accuracy and deformation control

Copper:

The thermal expansion coefficient is high (17×10⁻⁶/℃), and the temperature rise during copper processing can easily lead to dimensional deviation (for example, a hole with a diameter of 0.1mm can produce a deviation of 0.017mm with a temperature difference of 10℃). Thin-walled parts need to use vacuum adsorption fixtures to reduce clamping deformation.

Aluminum:

The elastic modulus is low (69GPa, only 1/3 of that of steel). Excessive clamping force will cause deformation of the workpiece, especially thin-walled structures (such as aluminum shells with a thickness of less than 1mm). Low-stress fixtures (such as airbag fixtures) and layered cutting processes are required.

Stainless steel: 

The depth of the hardened layer can reach 0.3~0.5mm. The tool needs to withstand alternating stress during subsequent cutting, resulting in dimensional fluctuations. At the same time, high cutting force can easily cause machine tool vibration, requiring high-rigidity equipment (such as machine tool spindle stiffness>50N/μm).

Conclusion: Difficulty of precision control: stainless steel>copper>aluminum (the deformation of aluminum is mainly due to the low elastic modulus, and the control method is relatively mature).

(5) Process parameter range and equipment requirements

Copper: 

When machining copper, the cutting speed range is wide (50~800m/min). Rough machining can be high-speed cutting to improve efficiency, and fine machining needs to reduce the speed (100~200m/min) to avoid chip sticking. Ordinary three-axis machine tools can meet most needs.

Aluminum: 

Suitable for high-speed machining (optimal speed 500~1500m/min), requiring a high-rigidity spindle (jump <5μm) and a fast feed system (>40m/min). Five-axis machine tools are often used for complex surface machining.

Stainless steel:

The cutting speed is limited (usually ≤100m/min, otherwise the tool life drops sharply), and a high-torque spindle (torque>50N・m) and a strong cooling system (pressure>5MPa) are required. The equipment cost is significantly higher than copper and aluminum processing.

Conclusion: Equipment and process adaptability: aluminum>copper>stainless steel (stainless steel has almost stringent requirements for equipment rigidity, cooling, and tools).

3. Comprehensive processing difficulty ranking of copper, aluminum, and stainless steel

Dimensions	Stainless Steel	Copper	Aluminum
Material Properties	Work hardening, low thermal conductivity, high hardness	High ductility, chip adhesion, thermal expansion	Low stiffness, built-up edge, soft chips
Tool Requirements	Ultra-high hardness (CBN/ceramic), anti-adhesion	Sharp edge, diamond coating	PCD tools, large rake angle design
Equipment Requirements	High rigidity, high torque, strong cooling	Moderate rigidity, chip control	High-speed spindle, precision feed system
Process Complexity	Narrow cutting parameter window, short tool life	Chip adhesion and precision control	Built-up edge and chip entanglement
Overall Difficulty	★★★★★ (extremely difficult)	★★★☆☆ (moderately difficult)	★★☆☆☆ (relatively easy)

Final conclusion:

The most difficult to process: stainless steel, the core challenge is the rapid wear of the tool caused by work hardening, the stability problem caused by ultra-high cutting temperature and extremely low thermal conductivity, and it needs to rely on high-end equipment and special tools, with the highest cost and process difficulty.

Medium difficulty: copper, the main difficulty lies in chip sticking, thermal deformation and dimensional control during precision machining, which can be effectively improved through material optimization (such as the use of easy-to-cut brass) and tool coating.

Relatively easy: aluminum, although there are problems of built-up edge and deformation, due to low cutting force and wide process parameter window, mature high-speed machining technology can achieve efficient and stable production.

4. Comparison of typical processing scenarios

(1) Precision electronic components (such as copper terminals):

Key: dimensional accuracy (±0.01mm) and surface roughness (Ra≤0.4μm)

Difficulty: Pure copper chips cause surface scratches, PCD tool + MQL micro-lubrication is required, and the cutting speed is controlled at 150~200m/min.

(2) Aviation aluminum alloy structural parts (such as 7075-T6 wing ribs):

Key: thin-walled and lightweight (wall thickness ≤0.8mm) and geometric tolerance (flatness ≤0.05mm)

Difficulty: elastic deformation and vibration, five-axis machine tools + vacuum adsorption fixtures are required, high-speed cutting (spindle speed 12000rpm) with emulsion cooling.

(3) Stainless steel valve parts (such as 304 valve body):

Key: deep hole processing (aspect ratio>10) and corrosion resistance

Difficulty: Work hardening causes drill bit breakage, and gun drilling + high-pressure internal cooling (pressure 8MPa) is required. The cutting speed is controlled at 60~80m/min, and the tool is retracted and chip removal is performed every 5mm of processing.

5. Summary: Selection determines difficulty, and process balances cost

The difference in processing difficulty of the three materials is essentially a problem of matching material properties with processing requirements:

If you pursue high thermal conductivity/electrical conductivity, copper is the first choice, but you need to accept the challenges of chip adhesion and precision control when machining copper;

If you pursue lightweight and high-speed processing, aluminum is the best solution, but you need to solve the problems of built-up edge and thin-wall deformation when processing aluminum;

If you pursue high strength and corrosion resistance, stainless steel is irreplaceable, but you need to invest in high-end equipment and tools, and bear high costs and low efficiency.

There is no absolute answer to the difficulty of processing. The key lies in selecting appropriate materials and process solutions based on product requirements (precision, surface, efficiency, cost). With the popularization of super-hard tools (such as diamond, CBN), high-speed machining technology (HSM) and intelligent monitoring systems, the once "difficult-to-machine materials" are gradually becoming controllable, while stainless steel will still be one of the most challenging materials in future high-end manufacturing (such as aerospace, medical devices).