Choosing the right material is probably one of the most consequential decisions in any machining project. Get it wrong and even perfect programming won’t save the part. Get it right and everything downstream — machining time, surface finish, durability, cost — tends to fall into place more smoothly.
This guide covers the five materials that show up most frequently when producing CNC milled parts. Each has its own personality on the machine, its own strengths, and its own quirks. Understanding these differences helps with specification decisions, supplier conversations, and realistic expectations about what’s achievable.
Comparing Materials for CNC Milled Parts at a Glance
| Matériau | Usinabilité | Strength | Weight | Résistance à la corrosion | Relative Cost |
| Aluminium | Excellent | Moderate | Low | Good | Low-Medium |
| Acier | Moderate | High | High | Variable | Medium |
| Acier inoxydable | Challenging | High | High | Excellent | Medium-High |
| Laiton | Excellent | Moderate | Medium | Good | Medium |
| Titane | Difficult | Very High | Low | Excellent | High |
| Engineering Plastics | Excellent | Low-Moderate | Very Low | Excellent | Variable |
This table simplifies things considerably — real-world alloy selection involves way more nuance — but it gives a useful starting point for comparison.
1. Aluminum: The Go-To Choice for CNC Milled Parts
If there’s a default material in the milling world, aluminum is probably it. Walk into almost any machine shop and aluminum chips will be somewhere on the floor. The reasons are pretty straightforward.
Aluminum machines fast. Like, really fast compared to most alternatives. Spindle speeds can run high, feed rates can push aggressively, and the material just cooperates. Tool life stays reasonable, chips evacuate cleanly, and surface finishes come out looking good without heroic effort.
The most commonly milled aluminum alloys include:
- 6061-T6 — excellent all-around properties, good corrosion resistance, widely available
- 7075-T6 — higher strength, common in aerospace applications
- 2024-T3 — good fatigue resistance, used in structural components
- 5052 — better for forming operations but still mills acceptably
Weight matters too. At roughly one-third the density of steel, aluminum makes sense anywhere mass reduction counts — aerospace, automotive, consumer electronics, drones.
2. Steel: When Strength Cannot Be Compromised
Steel isn’t as friendly to machine as aluminum. It’s harder on tooling, generates more heat, and requires slower parameters. But when CNC milled parts need to handle serious loads, steel often remains the only practical answer.
- 1018 mild steel — easy to machine, good for general-purpose parts
- 4140 alloy steel — higher strength, heat treatable, common for mechanical components
- A36 structural steel — economical, used for fixtures and less critical parts
- 1045 medium carbon steel — balance of strength and machinability
The trade-off is always between machinability and final properties. Softer steels cut easier but can’t handle demanding applications. Harder grades perform better in service but cost more in cycle time and tool wear.
3. Stainless Steel: Corrosion Resistance Meets Durability
Stainless steel occupies its own category because the machining behavior differs substantially from carbon steels. It work-hardens during cutting, meaning the surface gets harder as the tool passes over it. This characteristic makes parameter selection trickier.
- 303 — free-machining grade, best choice when corrosion resistance matters but machining ease is prioritized
- 304 — most common stainless, harder to machine but better corrosion performance
- 316 — superior chemical resistance, common in medical and marine environments
CNC milled parts from stainless steel show up constantly in food processing equipment, medical devices, chemical handling systems, and marine hardware. The material cost and machining difficulty are worth it when the application demands that level of corrosion protection.
4. Brass: Underrated and Exceptionally Machinable
Brass doesn’t get the attention aluminum does, but machinists tend to appreciate working with it. The material cuts beautifully — clean chips, smooth finishes, minimal tool wear. There’s almost something satisfying about how well it behaves.
Typical applications for brass CNC milled parts include:
- Electrical connectors and terminals
- Plumbing valve bodies and fittings
- Decorative hardware and architectural components
- Musical instrument parts
- Low-friction bushings and wear surfaces
C36000 free-cutting brass is the most widely used grade. It’s essentially the benchmark against which other materials’ machinability gets measured. The lead content that makes it machine so well has come under regulatory scrutiny though, pushing some applications toward lead-free alternatives.
5. Engineering Plastics: Lightweight and Versatile
Not every application needs metal. Engineering plastics have carved out a significant niche for CNC milled parts, especially where weight, electrical insulation, or chemical resistance matters more than sheer strength.
- Delrin (acetal) — dimensionally stable, low friction, excellent for gears and bushings
- PEEK — high-performance thermoplastic, withstands extreme temperatures, used in medical and aerospace
- Nylon — tough and wear-resistant, good for mechanical components
- HDPE and UHMW — chemical resistant, common in food processing
- Polycarbonate — impact resistant, optically clear options available
Plastics machine quickly and don’t punish tooling, though they require different approaches than metals. Chip clearance matters more, and some plastics can melt if cutting speeds generate too much heat. Sharp tools and proper feeds prevent most problems.
Factors That Should Guide Material Selection
Picking a material isn’t just about what machines nicely. Several factors deserve consideration:
- Mechanical requirements — strength, hardness, fatigue resistance
- Environmental exposure — temperature extremes, chemicals, moisture
- Weight constraints — critical in aerospace, automotive, portable devices
- Cost sensitivity — material cost plus machining time equals total part cost
- Regulatory compliance — RoHS, FDA approval, industry-specific standards
- Surface finish and aesthetic requirements
Sometimes the answer is obvious. Other times it involves trade-offs that require careful thought. A conversation with an experienced machinist or manufacturing engineer often helps clarify which direction makes the most sense. If you want to know more about cnc milled parts, please read What is CNC Milling? The Complete Guide.
FAQ
Which material is easiest to machine for CNC milled parts?
Aluminum and brass consistently rank as the easiest metals to machine. Among plastics, Delrin and nylon machine very well. C36000 brass actually serves as the machinability benchmark — other materials are often rated as a percentage relative to it.
Can CNC milling handle hardened steel?
Standard CNC milling works best on materials below roughly 45 HRC hardness. Fully hardened steels above that threshold typically require specialized tooling, slower parameters, or alternative processes like electrical discharge machining. Some shops do mill hardened materials, but it’s more demanding and costly.
How does material choice affect the cost of CNC milled parts?
Material affects cost in two ways — raw material price and machining time. Titanium costs more to buy and takes longer to cut, making it expensive on both fronts. Aluminum is affordable and machines fast, keeping costs down. The cheapest raw material isn’t always the cheapest finished part if it’s difficult to machine.
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