Guide to CNC Metal Materials: An Engineer's Perspective

For over 20 years, I’ve turned countless blueprints into precision metal parts. The choice of material is the single most important decision you make before the first tool even touches the stock. Get it right, and you’ll have a part that performs flawlessly. Get it wrong, and you’ll face spiraling costs, delays, and part failure. This guide cuts through the noise and gives you the practical knowledge you need.

Why are metals commonly used materials in CNC machining?

Simply put, metals offer an unmatched combination of properties that make them ideal for subtractive manufacturing like CNC milling and turning.

Excellent Machinability: Many metals cut cleanly, producing predictable chips and allowing for high-speed material removal. This translates to faster cycle times and lower costs.
Superior Strength & Durability: Metals provide high strength-to-weight ratios, toughness, and impact resistance that plastics simply cannot match. This is non-negotiable for structural components, automotive, aerospace, and heavy machinery.
Precision and Stability: Metals are rigid. They resist the cutting forces and heat generated during machining, allowing us to hold extremely tight tolerances, often within a few microns (±0.025mm or better).
Thermal and Electrical Conductivity: Critical for heat sinks, electronic housings, and electrical components. Metals like aluminum and copper excel here.
Wide Range of Finishes: A machined metal part can be left as-milled for a functional prototype or sent for a stunning cosmetic finish like anodizing, plating, or polishing for end-use products.

Common Metals Used in CNC Machining

In my shop, we work with a variety of metals, but a few stand out as the workhorses of the CNC machining industry.

Metal	Key Properties	Common Applications	Machining Tips
Aluminum (e.g., 6061, 7075)	Lightweight, good strength-to-weight ratio, excellent machinability, corrosion resistant.	Aerospace components, consumer electronics, prototypes, automotive parts.	Use high-speed steel (HSS) or carbide tools. High feed rates and speeds are possible. Good chip evacuation is crucial.
Stainless Steel (e.g., 304, 316)	High strength, excellent corrosion resistance, non-reactive.	Medical devices, food processing equipment, marine hardware.	Machinability can be challenging due to its toughness. Use rigid setups, sharp tools, and lower cutting speeds. Work hardening can be an issue.
Carbon Steel (e.g., 1018, 1045)	Strong, durable, cost-effective.	Machine components, gears, shafts, fixtures.	Generally good machinability, especially in low-carbon grades. Can be heat-treated for increased hardness.
Brass (e.g., C360)	Excellent machinability, corrosion resistant, low friction.	Fittings, valves, musical instruments, decorative parts.	One of the easiest metals to machine. Produces fine chips, so good ventilation is important.
Titanium (e.g., Grade 2, Grade 5)	Very high strength-to-weight ratio, excellent corrosion resistance, biocompatible.	Aerospace components, medical implants, high-performance sports equipment.	Difficult to machine due to its toughness and low thermal conductivity. Requires very rigid setups, specialized tooling, and a lot of coolant.

How to Select the Right Metal Material for Machining?

Selecting the right metal is a balancing act between performance requirements, cost, and manufacturability. Here’s my approach:

Define the Part’s Function:

Load-Bearing: Does the part need to withstand significant force? If so, high-strength materials like steel or titanium are necessary.
Environmental Exposure: Will the part be exposed to moisture, chemicals, or extreme temperatures? Stainless steel or certain aluminum alloys are excellent for corrosion resistance.
Weight Constraints: Is weight a critical factor, as in aerospace or automotive applications? Aluminum or titanium are your best bets.

Consider Machinability:

If you need a complex part with intricate features, a highly machinable material like aluminum or brass will be more cost-effective.
Difficult-to-machine materials like titanium or hardened steel will increase machining time and tooling costs.

Budget:

Carbon steel and aluminum are generally the most cost-effective options.
Stainless steel and titanium are premium materials with higher costs.

Post-Processing Requirements:

Does the part need to be welded, anodized, or heat-treated? Ensure the selected metal is compatible with these processes. For example, 6061 aluminum is great for welding and anodizing.

Surface Roughness and Finishing of CNC-Machined Metals

The “as-machined” surface from a CNC mill or lathe has a certain texture, measured in microinches or micrometers (µm). We call this Surface Roughness (Ra).

A standard CNC finish might be around Ra 125 µin (3.2 µm). You can see and feel the toolmarks.
A fine CNC finish achieved with careful toolpath strategies and new tools can reach Ra 32 µin (0.8 µm). It looks and feels smooth.

For many parts, this is sufficient. For others, we need a secondary finishing process.

Finish	Process Description	Best For	Look & Feel
Bead Blasting	Bombarding the surface with fine glass beads.	Aluminum, Steel, Stainless Steel.	Uniform matte, satin texture. Hides tool marks beautifully.
Anodizing (Type II)	Electrochemical process that creates a hard, porous oxide layer, which can be dyed.	Aluminum (primarily).	Matte or glossy. Wide range of colors (black, red, blue, etc.). Increases corrosion and wear resistance.
Hard Anodizing (Type III)	A thicker, harder version of Type II.	Aluminum where extreme surface hardness is needed.	Usually a dark gray/green color. Very rough texture unless sealed.
Passivation	Chemical bath that removes free iron from the surface and enhances the natural chromium oxide layer.	Stainless Steel.	Does not change dimensions or appearance. Maximizes corrosion resistance.
Plating	Electrochemically depositing a thin layer of another metal (e.g., Nickel, Chrome, Zinc).	Steel (for corrosion resistance), Brass/Copper (for solderability).	shiny, reflective Surface. Chrome is very hard and decorative.
Powder Coating	Spraying a dry powder onto the part and baking it to form a thick, durable polymer layer.	Steel, Aluminum.	Thick, durable, any color. Excellent for outdoor equipment.
Polishing	Progressive abrasive steps to create a mirror-like finish.	Stainless Steel, Aluminum, Brass.	Highly reflective, smooth "mirror" finish.

The right finish transforms a machined part into a ready-to-use component. It improves both appearance and performance—adding protection, durability, and environmental suitability.

Density, Weight, and Strength of CNC Metal Parts

Understanding the intrinsic relationship between a metal’s density, the final part weight, and its mechanical strength is fundamental to successful design. This isn’t just academic theory; getting this right dictates performance, cost, and manufacturability. Let’s break down these concepts with practical insights.

The Critical Properties: A Primer

Density (ρ): Mass per unit volume (g/cm³). This defines the weight of your part for a given size. Aluminum is light; steel is heavy; titanium is in between.
Yield Strength (σy): The stress at which the material permanently deforms. This is the practical limit for most designs—the point before failure.
Strength-to-Weight Ratio: Yield Strength / Density. This is the key metric for efficient design. A high value means you get high strength without a weight penalty.

Practical Case Study 1: The UAV Landing Gear Strut

Scenario: A startup designs a heavy-duty unmanned aerial vehicle (UAV). They need a critical landing gear strut that must withstand a 15 kN impact load during landing, with an absolute maximum weight of 800 grams per strut.
The Design Challenge: The initial design in 6061 aluminum met the strength requirement but weighed 950 grams—pushing the total vehicle weight over budget.
Material Analysis:

- AISI 4140 Steel (QT)
  1. Density: 7.85 g/cm³
  2. Yield Strength: 655 MPa
  3. Strength-to-Weight Ratio: ~83 MPa·cm³/g
  4. Verdict: Eliminated. Far too heavy. A strut of equivalent strength would weigh over 2.1 kg.
- 6061-T6 Aluminum
  1. Density: 2.70 g/cm³
  2. Yield Strength: 276 MPa
  3. Strength-to-Weight Ratio: ~102 MPa·cm³/g
  4. Verdict: Failed weight target. The required volume of material to handle the load weighed 950g.
- 7075-T7351 Aluminum
  1. Density: 2.81 g/cm³
  2. Yield Strength: 469 MPa
  3. Strength-to-Weight Ratio: ~167 MPa·cm³/g
  4. Verdict: Viable. Higher strength allows for less material. A redesigned strut weighs 780g.
- Ti-6Al-4V (Grade 5) Titanium
  1. Density: 4.43 g/cm³
  2. Yield Strength: 1100 MPa
  3. Strength-to-Weight Ratio: ~248 MPa·cm³/g
  4. Verdict: Best performance. Offers massive strength reserve and corrosion resistance. The strut could be made even lighter (~650g) but at a significant premium.
The Outcome: The client chose 7075-T7351 Aluminum. It was the most cost-effective material that met both the strength and strict weight budget. Titanium’s extra 130g weight saving did not justify the 5x increase in material and machining cost for this application.

Practical Case Study 2: The High-Performance Hydraulic Manifold

Scenario: A manufacturer of forestry equipment needs a new hydraulic manifold block. It must withstand continuous internal pressures of 350 Bar (35 MPa) with minimal external deformation to prevent seal failure. Weight is not a primary concern; stiffness and pressure integrity are.
The Design Challenge: The initial prototype was machined from 6061 aluminum. Under full pressure testing, the walls deflected slightly, leading to a leak path.
Material Analysis:
- Stiffness is Key: A material’s resistance to bending is its Elastic Modulus (E), not its Yield Strength. Steel is inherently ~3 times stiffer than aluminum (200 GPa vs. ~69 GPa).
- 6061-T6 Aluminum
  1. Elastic Modulus: 69 GPa
  2. Verdict: Insufficient. The calculated deflection under load was 0.2 mm, exceeding the allowable seal compression limit.
- AISI 4140 Steel
  1. Elastic Modulus: 200 GPa
  2. Verdict: Ideal. The same design in steel would deflect less than 0.07 mm, ensuring a perfect seal. The increased weight (~2.8x heavier) was irrelevant for this grounded equipment.
- Bonus Note – Pressure Rating: The thread strength for ports, directly related to the material’s tensile strength, was also higher in steel, allowing for the use of standard fittings with a larger safety margin.
The Outcome: The design was switched to AISI 4140 Steel. The part was heavier and more expensive to machine, but it performed flawlessly under pressure, eliminating field failures and warranty returns.

Key Takeaways for Designers:

Prioritize Your Driver: Is it minimum weight (e.g., aerospace, robotics)? Use Strength-to-Weight ratio. Is it maximum stiffness or fatigue life (e.g., frames, manifolds)? Then absolute strength and modulus matter more.
Stiffness ≠ Strength: A strong aluminum part can still bend more than a weaker steel one. Know which property is critical for your application.
Test Early with Prototypes: Physical testing under load often reveals real-world behavior that calculations alone can miss. Machining a prototype in aluminum is a cheap way to validate a design before committing to expensive materials.

How to Customize Metal Parts?

Customizing metal parts with CNC machining is what we do every day. Here’s the typical workflow:

Design (CAD): The process starts with a 2D or 3D design created in CAD (Computer-Aided Design) software. This is the blueprint for your part. For best results, design with manufacturability in mind (Design for Manufacturing – DFM).
Programming (CAM): The CAD file is then loaded into CAM (Computer-Aided Manufacturing) software. A programmer generates the toolpaths and G-code that will control the CNC machine.
Setup: A block of the selected metal is securely clamped in the CNC machine. The machinist installs the necessary cutting tools and sets the machine’s reference points.
Machining: The CNC machine executes the G-code, precisely cutting away material to create the custom part. This can involve multiple processes, such as milling, turning, and drilling.
Finishing: After machining, the part undergoes any specified finishing processes, such as bead blasting or anodizing.
Inspection: The final part is inspected with precision measuring tools (calipers, micrometers, CMM) to ensure it meets all dimensional tolerances specified in the design.

By following this process, we can create highly complex and precise custom metal parts tailored to your exact specifications.