Best Metals for CNC Machining: Selection Guide

Best Metals for CNC Machining: Selection Guide

The best metals for CNC machining depend on what the part has to do, but a few dominate practice. Aluminum 6061 is the default for most parts — light, cheap, and highly machinable; aluminum 7075 when you need aerospace-grade strength. Stainless steel 304 covers general corrosion resistance, with 316 for marine and chemical exposure. Carbon and alloy steels (1045, 4140, 4340) give strength at low cost; tool steels (D2, A2, H13) handle dies and molds; brass and copper win on machinability and conductivity; and titanium delivers unmatched strength-to-weight for aerospace and medical work. For specialist needs, magnesium is the lightest structural metal, and nickel alloys (Monel, Inconel) handle extreme heat and highly corrosive environments. The right choice always balances machinability, strength, weight, corrosion resistance, and cost.

Scope note: This guide covers the wrought metal families most commonly CNC milled and turned, and how to choose between them. Castings, superalloys, and engineering plastics are out of scope.

Using the wrong metal for a CNC machined part can easily turn out to be one of the most expensive errors a designer can make. It doesn't happen during the initial stages, but always manifests itself once the parts go into mass production.

A bracket produced using stainless steel instead of aluminum results in higher weight, poorer machinability, and increased cost compared to its alternative. Similarly, the improper choice of steel alloy for a structural part can cause failure due to insufficient resistance to applied loads compared to a properly selected alloy.

Material selection should not be considered a secondary task at the end of product development — it is one of the very first decisions determining the cost, mechanical properties, weight, and manufacture of the part throughout its entire life cycle. This guide lists the major metal families used in precision machining and describes their properties and applications, so you can choose the perfect metal for your CNC machined part.

Why Material Selection Matters in CNC Machining

Before looking at particular materials, it is important to understand what is on the line when choosing between them. The material selected directly impacts:

• Machinability — the ease with which the material is cut, which affects cutting time, tool wear, and machining cost.

• Mechanical strength — whether the material can withstand the loads and stress in service.

• Weight — critical in aerospace, automotive, and anywhere weight really matters.

• Resistance to corrosion and chemicals — whether the material will survive its working environment.

• Finish quality that can be achieved — whether the material takes a good surface finish after machining (see Surface Roughness Ra, Rz & Rms Explained).

• Cost of the part — the raw material price plus the time it takes to machine.

Getting this right up front prevents the expensive redesign that follows when a component is built from the wrong material and the problem is found only after tooling, programming, and the first production batch. Designing the part for manufacturability at the same time helps too — see our Design for Manufacturing (DFM) Guide.

How to Evaluate Metals for CNC Machining

As a general rule, when comparing different metals you should evaluate them against five primary considerations. These make the rest of this guide easy to apply to your specific case.

No single metal outperforms all others across all five categories at once — and that is precisely why selecting the right material is always a balancing act against your specific requirements.

Aluminum — The Material of Choice for Most CNC Applications

Aluminum is, without doubt, the material used most often in CNC machining. It features a great strength-to-weight ratio, excellent machinability, built-in corrosion protection in the form of an oxide layer, and lower cost than most other materials. These properties make it popular across prototyping, structural brackets, and automotive parts.

Aluminum 6061

Aluminum 6061 is probably the most common aluminum alloy in CNC machining. It is a heat-treatable alloy containing mostly aluminum, magnesium, and silicon, combining good machinability, corrosion resistance, and decent strength. It anodizes beautifully, an advantage for producing functional and aesthetically pleasing parts. Its average tensile strength is 310 MPa in the T6 condition, making it appropriate for structural parts, enclosures, fixturing, and mounting brackets.

Aluminum 7075

Aluminum 7075 is the alloy of choice when strength is the priority. Strengthened mainly by zinc, together with magnesium and copper, it is much stronger than 6061 — more than 500 MPa of tensile strength in the T6 temper. Heat-treatable to high hardness, it is the standard aerospace-grade aluminum used in structural parts, tooling, and jigging where strength is required alongside weight reduction. The drawbacks are lower corrosion resistance than 6061 and a higher raw material price.

Aluminum 5083

Aluminum 5083 is a non-heat-treatable magnesium-aluminum alloy with better strength than most non-heat-treated aluminum alloys and good resistance to salt water. It is widely used in marine applications thanks to its excellent saltwater resistance, and it welds very well.

Aluminum 6082

Aluminum 6082 is similar to 6061 in composition and characteristics, with slightly more silicon and manganese. It is popular in Europe due to direct compliance with regional material requirements. Its machinability and strength are very close to those of 6061.

Aluminum 2024

This alloy contains copper and is characterized by high fatigue resistance and strength, at the expense of lower corrosion resistance than 6061 and 7075. It is widely used in the aviation industry for aircraft body parts under cyclic loads.

Aluminum Comparison Table

Stainless Steel — When Corrosion Resistance and Mechanical Properties Are Both Required

If a part must withstand tough conditions — moisture, chemical agents, or frequent cleaning cycles — while also handling mechanical stress, stainless steel is the answer. It machines more slowly and costs more than aluminum, but it delivers the required corrosion resistance and mechanical durability.

Stainless Steel 304

Stainless steel 304 is the most commonly used stainless type for CNC machining. This austenitic grade contains 18% chromium and 8% nickel, providing good general corrosion resistance, decent machinability for a stainless, and satisfactory mechanical properties in most cases. It is non-magnetic in the annealed form and is typically chosen for food-grade equipment, industrial hardware, and components used indoors or in moderate outdoor conditions.

Stainless Steel 316

Stainless steel 316 has everything 304 has but with increased corrosion resistance from molybdenum (about 2–3%). It offers higher resistance to chloride and saltwater corrosion, making it ideal for marine hardware, chemical process equipment, and medical implants. The trade-off is cost — it is 20–30% more expensive than 304 — and it is harder to machine due to its stronger microstructure. A common mistake is choosing 316 simply because it sounds better, without an actual requirement for it.

Stainless Steel 303

Stainless steel 303 is a free-machining austenitic stainless with sulfur added for easy chip formation and machinability. It does not have the corrosion resistance of 304, but it can be machined at a higher rate.

Stainless Steel 17-4 PH

17-4 PH is a precipitation-hardening martensitic stainless containing chromium, nickel, and copper. Unlike 304 and 316, it can be heat treated after machining to significantly increase hardness and strength. It is the choice for stainless components that need both corrosion resistance and high mechanical strength — common in aerospace fittings, valve components, and high-performance fasteners.

Duplex 2205

Duplex 2205 combines austenitic and ferritic microstructures, giving it the highest strength of any commonly used stainless grade alongside excellent corrosion resistance. It is particularly resistant to chloride stress corrosion cracking, suiting offshore equipment and chemical processing components where standard austenitic grades fall short structurally.

Stainless Steel 304L and 316L

The "L" grades are low-carbon versions of 304 and 316. The reduced carbon minimizes carbide precipitation during welding, so 304L and 316L are preferred for welded fabrications and for medical, pharmaceutical, and food applications where weld-zone corrosion resistance matters. Mechanically they are very close to their standard counterparts.

Stainless Steel 410

410 is a martensitic stainless that, unlike austenitic 304/316, can be hardened by heat treatment. It is magnetic, offers moderate corrosion resistance, and is used for valve parts, pump shafts, fasteners, and cutlery where some hardness and wear resistance are needed.

Stainless Steel 420

420 is a higher-carbon martensitic stainless that hardens to a high hardness and takes a fine edge. It is the classic choice for surgical and dental instruments, blades, and plastic injection molds that must resist wear and mild corrosion.

Stainless Steel Comparison Table

Carbon and Alloy Steel — Strength and Low Cost for Structural Parts

CNC machining of carbon and alloy steels offers one of the most favorable strength-to-price ratios in the industry. These steels are frequently used for structural parts, machine components, and tooling where corrosion resistance is not essential.

Steel 1018

1018 is a low-carbon steel that is easy to weld, machine, and use in structural components, shafts, and formings where extreme hardness is not needed. It is relatively soft and easy to machine even on a non-rigid system.

Steel 1045

1045 medium-carbon steel provides a good combination of strength, hardness, and machinability. It is frequently used for shafts, gears, and other mechanical parts that see some wear and stress but do not need the corrosion resistance of stainless steel.

Steel 4140

4140 alloy steel uses chromium and molybdenum as its main alloying metals. It offers toughness, fatigue strength, and can be hardened by heat treatment. It is widely used in shafts, gears, machinery structural components, and automotive and oil & gas industries.

Steel 4340

4340 is a step up from 4140, adding nickel alongside chromium and molybdenum. This improves toughness and fatigue strength, especially for through-hardening of thicker pieces. 4340 steel is used in aircraft gears and crankshafts.

Steel 12L14

This free-cutting steel contains lead to increase machinability and improve chip formation and surface finishing at high speeds. It is used where machining speed and surface finish matter more than strength.

EN-Series Steels (European Designations)

Many production drawings, especially in Europe and Asia, specify steels by their EN (En) designations rather than AISI numbers. These map closely to familiar AISI grades:

If a drawing calls out an AISI grade, the equivalent EN steel above is generally an acceptable substitute, and vice versa — but confirm critical mechanical properties against the specific grade.

Case-Hardening and Spring Steels

Beyond the general structural grades, two specialized steel groups are commonly machined:

• SAE 8620 — a low-carbon nickel-chromium-molybdenum case-hardening steel. It is carburized to give a hard, wear-resistant surface over a tough core, ideal for gears, camshafts, and bushings.

• Spring steels — EN45, EN46, and EN47 — high-carbon silicon-manganese and chromium-vanadium grades engineered for high yield strength and fatigue resistance. They are used for leaf and coil springs, torsion bars, and other components that must flex repeatedly without taking a permanent set.

Steel Comparison Table

Tool Steel — For Hardness, Wear Resistance, and Tooling

Tool steel is developed for extreme hardness and dimensional stability under frequent stress and wear. These properties make it indispensable for cutting tools, molds, and dies.

D2 Tool Steel

High-carbon, high-chromium steel with excellent wear resistance and the ability to retain a sharp edge. Used extensively in blanking dies, shear blades, and punches.

A2 Tool Steel

Combines high toughness and wear resistance with easier heat treatment. Used in cold-work dies, punches, and gauging components.

H13 Tool Steel

A chromium hot-work tool steel with excellent resistance to thermal fatigue and heat hardness. Used in die casting dies, extrusion tooling, and injection mold components.

OHNS (Oil Hardened Non-Shrinking Die Steel)

OHNS (equivalent to AISI O1) is an oil-hardening cold-work tool steel prized for its minimal dimensional change during heat treatment — hence "non-shrinking." That stability makes it a favorite for press tools, dies, punches, gauges, and cutting tools where precise dimensions must survive hardening.

Tool Steel Comparison Table

Brass — The Most Machinable Metal

Of all metals used in CNC machining, brass — particularly Brass C360 — is considered the most machinable. It has a far higher machinability rating than steel, breaks up easily while being cut, and gives an excellent surface finish without much additional processing. The drawbacks: brass is more expensive per pound than aluminum, and about three times denser.

Brass is also specified by European designations — CW614N (CuZn39Pb3) and CW617N (CuZn40Pb2) — both leaded free-machining brasses comparable to the American C360 family. CW614N is common for high-speed machined and precision turned components, while CW617N is suited to hot stamping and forged fittings.

Copper — Where Conductivity Takes Precedence

Copper is chosen for its excellent electrical and thermal conductivity. It is ideal for:

• Electrical contacts

• Busbars

• Heatsinks

• Thermal management parts

Copper is softer than brass and aluminum, and harder to machine to tight tolerances due to gummy behavior during cutting. But when conductivity is the priority, nothing else quite matches it.

Titanium — Superior Strength for Challenging Uses

Titanium provides high strength, corrosion resistance, and biocompatibility, making it ideal for aerospace and medical applications.

Titanium Grade 2

Commercially pure titanium. Highly resistant to corrosion and weldable. Used in chemical processing and marine environments.

Titanium Grade 5 (Ti-6Al-4V)

The most commonly used titanium alloy, containing aluminum and vanadium. Used in:

• Aerospace structures

• Medical implants

• High-performance fasteners

It provides an excellent strength-to-weight ratio and biocompatibility. However, titanium is extremely difficult to machine: it retains strength when heated, conducts heat poorly, and work-hardens quickly. It is usually machined at one-fifth the speed of steel, making it expensive in both raw material and machining cost.

Titanium Comparison Table

Magnesium Alloys — The Lightest Structural Metal

Magnesium is the lightest structural metal — roughly one-third lighter than aluminum — with a good strength-to-weight ratio, making it valuable in aerospace, automotive, and electronics housings where every gram counts. It machines very fast with low cutting forces, but its chips and fine dust are flammable, so it must be machined dry with proper handling and fire precautions.

• AZ31B — a wrought magnesium-aluminum-zinc alloy used for sheet, plate, and extrusions; a good general-purpose, weldable grade.

• AZ80 — a higher-strength wrought alloy, heat-treatable for more demanding structural parts.

• AZ91 — the most widely used cast magnesium alloy, offering excellent castability and good strength, common in die-cast housings and brackets.

Nickel Alloys — For Extreme Heat and Corrosion

When parts must survive extreme temperatures or highly aggressive chemicals, nickel-based alloys take over where stainless steel ends. They are tough, work-harden rapidly, and are among the most difficult metals to machine — but nothing else matches their performance in the harshest environments.

• Monel (nickel-copper) — outstanding resistance to seawater, acids, and alkalis, used in marine hardware, valves, pumps, and chemical processing equipment.

• Inconel (nickel-chromium) — a superalloy that retains its strength and resists oxidation at very high temperatures, used in gas turbines, exhaust systems, and aerospace hot-section components.

Metal Comparison Table — All Families at a Glance

How to Select Your Part's Metal: A Practical Framework

Step 1 — Define the Functional Requirements First

Define what the part must do:

• Mechanical strength

• Wear resistance

• Dimensional stability

• Load-bearing capability

Strong alloys for these needs include 7075 aluminum, 4140/4340 steel, and titanium Grade 5.

Step 2 — Determine the Operating Environment

Consider exposure to:

• Water

• Chemical media

• Salt water

• High temperature

For harsh environments, choose stainless steel 316 or titanium; for extreme heat or highly aggressive chemicals, step up to nickel alloys such as Monel or Inconel. Raw 1018 mild steel without coatings is guaranteed to corrode outdoors.

Step 3 — Evaluate Machinability and Its Effect on Cost

Highly machinable metals include aluminum, brass, and 12L14 steel. Harder metals include titanium, tool steel, and 4340 alloy steel. Harder metals require slower feed rates and higher machining cost.

Step 4 — Match the Material to Production Volume

For prototyping, aluminum 6061 and brass are ideal. For high volume, 303 stainless and 12L14 steel shine, because raw material cost and tool wear become more important as quantities rise.

Step 5 — Avoid Over-Specifying "Just to Be Safe"

Using expensive materials unnecessarily increases cost. Common examples include specifying titanium instead of aluminum 7075, or stainless 316 instead of 304 — adding no extra function, only higher cost.

Material Selection Mistakes to Avoid

• Selecting a metal based on familiarity rather than suitability

• Not considering weight until too late

• Failing to consider plating and finishing compatibility

• Choosing a premium grade unnecessarily

• Ignoring tool wear and cycle time alongside raw material cost

Frequently Asked Questions

What material is most often CNC machined?

By far the most common CNC machining material is aluminum 6061, thanks to its excellent machinability, low cost, and versatility across prototypes and production parts.

What is the best aluminum for CNC machining?

For general-purpose parts, aluminum 6061 is the best all-rounder — machinable, affordable, and easy to anodize. When strength matters more than cost, aluminum 7075 offers a much higher strength-to-weight ratio for aerospace and structural parts.

When should I use stainless steel 316 instead of 304?

Use 316 for saltwater, marine conditions, and harsh chemicals, where its molybdenum content resists chloride corrosion. Otherwise 304 is 20–30% cheaper and machines faster, and it is sufficient for most general and indoor applications.

What is the difference between 4140 and 4340 alloy steel?

Both are chromium-molybdenum alloys, but 4340 also contains nickel, which improves toughness and fatigue resistance — especially in thicker, through-hardened sections. That makes 4340 the choice for aerospace landing gear and other high-demand applications.

Why is titanium so expensive to machine compared to other metals?

Titanium retains its strength when heated, conducts heat poorly, and work-hardens easily. These properties force much slower cutting speeds — roughly one-fifth that of steel — which sharply increases machining time and cost on top of the high raw material price.

Should I choose a higher-grade material just to be safe?

No. Specifying a higher grade without an actual functional requirement usually adds cost without adding value — for example, titanium where 7075 aluminum would do, or 316 where 304 is sufficient. Match the material to the real requirement instead.

Conclusion

There is no single "best" metal for CNC machining — the right choice depends on balancing strength, weight, corrosion resistance, cost, and manufacturability. As a quick reference:

• Aluminum 6061 — best general purpose

• Aluminum 7075 — high-strength applications

• Aluminum 5083 — marine applications

• Stainless steel — high corrosion resistance

• Carbon & alloy steel — maximum strength at lower cost

• Tool steel — dies and molds

• Brass & copper — machining efficiency and conductivity

• Titanium — extreme aerospace and medical conditions

• Magnesium — the lightest structural parts

• Nickel alloys (Monel, Inconel) — extreme heat and corrosion

The engineers and buyers who get this right are not the ones who memorize alloys — they are the ones who understand what the component actually needs to do before selecting any metal. Once that is clear, the answer is usually clear too.