Material Selection Guide for CNC Machining
Choose the right material for your machined parts. Compare strength, machinability, corrosion resistance, and cost across 18+ common engineering materials.
6061-T6 Aluminum
Aluminum • 6061-T6
Most commonly machined aluminum. Excellent all-around properties. Heat treatable. T6 temper is peak strength.
7075-T6 Aluminum
Aluminum • 7075-T6
Highest strength aluminum alloy. More difficult to machine than 6061. Not recommended for welding. Susceptible to stress corrosion cracking.
2024-T3 Aluminum
Aluminum • 2024-T3
High fatigue resistance. Requires cladding or anodizing for corrosion protection. Good for high-cycle fatigue applications.
304 Stainless Steel
Stainless Steel • 304
Most common stainless steel. Work hardens quickly - use sharp tools and maintain feed. Not suitable for chloride environments.
316 Stainless Steel
Stainless Steel • 316
Superior corrosion resistance to 304, especially in chloride environments. Contains molybdenum. More expensive but essential for marine/medical.
17-4 PH Stainless Steel
Stainless Steel • 17-4 PH
Heat treatable stainless with very high strength. Machine in solution treated condition, then heat treat. H900 is highest strength condition.
1018 Carbon Steel
Carbon Steel • 1018
Most common and economical steel. Easy to machine and weld. Requires coating or plating for corrosion protection.
4140 Alloy Steel
Alloy Steel • 4140
Excellent strength and toughness. Can be heat treated to 50+ HRC. Pre-hardened versions available. Requires stress relief after heavy machining.
A2 Tool Steel
Tool Steel • A2
Machine in annealed state, then heat treat. Air hardens - minimal distortion. Good wear resistance. Requires grinding for tight tolerances after hardening.
Ti-6Al-4V (Grade 5)
Titanium • 6Al-4V
Most common titanium alloy. Excellent strength-to-weight ratio. Difficult to machine - low speeds, high feed, flood coolant. Galling tendency with cutters.
CP Titanium (Grade 2)
Titanium • Grade 2
Pure titanium - excellent corrosion resistance. Lower strength than Grade 5. Easier to machine and form. Good for non-structural applications.
360 Brass (Free Cutting)
Brass • C36000
The gold standard for machinability (rated 100). Excellent for high-volume screw machine work. Contains lead - not for food contact.
932 Bronze (SAE 660)
Bronze • C93200
Self-lubricating bearing bronze. Excellent for low-speed, high-load bearings. Can run against hardened steel without lubrication.
C101 Copper (OFC)
Copper • C10100
Highest conductivity copper. Gummy when machining - sharp tools essential. Used where maximum electrical or thermal conductivity required.
Delrin (Acetal/POM)
Plastic • POM
Excellent dimensional stability and low friction. Machines like soft metal. FDA approved. Not for outdoor UV exposure.
PEEK
Plastic • PEEK
Premium engineering plastic. Continuous service to 250°C. Biocompatible for implants. Very expensive but exceptional properties.
Inconel 625
Nickel Alloy • 625
Extreme environment material. Excellent at high temperatures and corrosive conditions. Very difficult to machine - ceramics or CBN tooling recommended.
Inconel 718
Nickel Alloy • 718
Precipitation hardening superalloy. Age hardenable for extreme strength. Machine in solution treated state. Slowest machining of all common materials.
Material Quick Reference
| Material | Strength (MPa) | Density | Machinability | Corrosion | Cost |
|---|---|---|---|---|---|
6061-T6 Aluminum Aluminum | 310 | 2.7 | 90 | Excellent | $ |
7075-T6 Aluminum Aluminum | 572 | 2.81 | 70 | Moderate | $$ |
2024-T3 Aluminum Aluminum | 483 | 2.78 | 70 | Poor | $$ |
304 Stainless Steel Stainless Steel | 515 | 8 | 45 | Excellent | $$$ |
316 Stainless Steel Stainless Steel | 515 | 8 | 40 | Excellent | $$$$ |
17-4 PH Stainless Steel Stainless Steel | 1310 | 7.8 | 50 | Good | $$$$ |
1018 Carbon Steel Carbon Steel | 440 | 7.87 | 78 | Poor | $ |
4140 Alloy Steel Alloy Steel | 655 | 7.85 | 65 | Poor | $$ |
A2 Tool Steel Tool Steel | 1650 | 7.86 | 55 | Poor | $$$ |
Ti-6Al-4V (Grade 5) Titanium | 950 | 4.43 | 25 | Excellent | $$$$$ |
CP Titanium (Grade 2) Titanium | 345 | 4.51 | 35 | Excellent | $$$$ |
360 Brass (Free Cutting) Brass | 385 | 8.5 | 100 | Good | $$ |
932 Bronze (SAE 660) Bronze | 240 | 8.93 | 80 | Good | $$$ |
C101 Copper (OFC) Copper | 220 | 8.94 | 20 | Good | $$$ |
Delrin (Acetal/POM) Plastic | 70 | 1.41 | 85 | Excellent | $$ |
PEEK Plastic | 100 | 1.32 | 70 | Excellent | $$$$$ |
Inconel 625 Nickel Alloy | 827 | 8.44 | 15 | Excellent | $$$$$ |
Inconel 718 Nickel Alloy | 1375 | 8.19 | 12 | Excellent | $$$$$ |
Material Selection Fundamentals
How to Choose the Right Material
Material selection is one of the most critical decisions in part design. The right choice balances functional requirements with manufacturing constraints and cost. Key factors to consider:
- Strength and stiffness needs
- Operating temperature range
- Corrosion and chemical exposure
- Electrical/thermal conductivity
- Weight constraints
- Wear and fatigue life
- Machinability rating
- Available forms (bar, plate, tube)
- Lead time and availability
- Secondary operations needed
- Heat treatment requirements
- Surface finish achievability
Common Material Comparisons
6061-T6 vs 7075-T6 Aluminum
The most common aluminum decision. 6061-T6 is the go-to general purpose alloy—excellent machinability, good weldability, adequate strength, and lowest cost. Choose it for most structural parts, enclosures, and fixtures.
7075-T6 offers 50% higher strength but poorer weldability and corrosion resistance. Use it only when you genuinely need the extra strength—aircraft structures, high-stress components, or weight-critical applications. It costs 50-100% more.
304 vs 316 Stainless Steel
Both are austenitic stainless with excellent corrosion resistance. 304 (18-8) is the workhorse—used for food equipment, architectural, and general industrial.316 contains molybdenum for superior resistance to chlorides and marine environments. Choose 316 for saltwater, chemical processing, medical implants, or pharmaceutical equipment. 316 costs 25-50% more.
When to Use Titanium
Titanium offers the best strength-to-weight ratio of any common metal. Use it when weight savings justify the 5-10x cost premium over steel:
- Aerospace structures (weight = fuel savings)
- Medical implants (biocompatibility)
- Marine applications (corrosion resistance)
- High-performance sports equipment
Be prepared for challenging machining—titanium requires rigid setups, sharp carbide tools, flood coolant, and low speeds.
Unless you have specific requirements, start with these defaults: 6061-T6 aluminum for lightweight parts, 304 stainless for corrosion resistance, 4140 steel for strength, and Delrin for plastic parts. These are always in stock and offer the best value.
Machinability Explained
Machinability ratings indicate how easy a material is to cut. The scale is based on 360 Brass = 100 as the reference. Higher numbers mean faster feeds, longer tool life, and lower costs.
| Rating | Description | Examples | Cost Impact |
|---|---|---|---|
| 80-100 | Excellent | Brass, 6061 Aluminum, Delrin | Baseline |
| 60-79 | Good | 7075 Aluminum, 4140 Steel, PEEK | +10-20% |
| 40-59 | Moderate | 304/316 Stainless, Tool Steels | +30-50% |
| 20-39 | Difficult | Titanium, Copper, Hardened Steel | +75-150% |
| <20 | Very Difficult | Inconel, Hastelloy, Tungsten | +200-400% |
Heat Treatment Considerations
Some materials require heat treatment to achieve their specified properties:
- Aluminum alloys (6061, 7075): Usually purchased in T6 (peak aged) condition. Re-solution treatment may be needed after welding.
- Tool steels (A2, D2, O1): Machine in annealed condition, then heat treat. Expect 0.001"/inch dimensional change. Finish grinding after hardening.
- 4140 Steel: Available pre-hardened (28-32 HRC) or annealed. Pre-hardened saves heat treat cost but machines slower.
- 17-4 PH Stainless: Machine in Condition A, then age harden to H900-H1150 for desired hardness.
Material Certification
For critical applications, you may need certified material with traceability:
- Mill Test Reports (MTR): Chemical composition and mechanical properties from the mill.
- DFARS Compliant: U.S.-origin materials for defense contracts.
- AMS Specs: Aerospace material specifications with full traceability.
- Medical Grade: ASTM F-grade materials with biocompatibility testing.
Find Shops That Work With Your Material
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Frequently Asked Questions
What is the difference between 6061 and 7075 aluminum?
6061-T6 is the most common general-purpose aluminum with good strength (310 MPa), excellent machinability (90/100), good weldability, and excellent corrosion resistance. 7075-T6 is about 50% stronger (572 MPa) but has poorer corrosion resistance, cannot be welded, and costs 50-100% more. Use 6061 unless you specifically need the extra strength.
When should I use 316 stainless instead of 304?
Use 316 stainless steel when your part will be exposed to chlorides (saltwater, coastal environments, swimming pools), strong chemicals, or needs to be implant-grade for medical use. The molybdenum content in 316 provides superior pitting and crevice corrosion resistance. For general indoor or food-grade applications, 304 is usually sufficient and costs 25-50% less.
Why is titanium so expensive to machine?
Titanium is expensive to machine due to its low thermal conductivity (heat stays in the cutting zone), high chemical reactivity with tool materials, tendency to work harden, and springback issues. It requires specialized tooling, rigid setups, low cutting speeds, and flood coolant. Expect 2-4x the machining time compared to aluminum, plus higher tool wear costs.
What is the best plastic for CNC machining?
Delrin (acetal/POM) is the most popular engineering plastic for CNC machining. It has excellent dimensional stability, low friction, good strength, and machines like soft metal. For higher temperatures or chemical resistance, consider PEEK (expensive but exceptional properties) or Ultem. Nylon is good for wear applications but absorbs moisture. PTFE (Teflon) has the lowest friction but is soft and difficult to hold tolerances.
How does material choice affect machining cost?
Material choice significantly impacts total cost through three factors: raw material cost, machinability (cycle time and tool wear), and secondary operations. A difficult-to-machine material like Inconel can cost 5-10x more to machine than aluminum, plus 10x the raw material cost. Always consider total manufactured cost, not just material cost.