What is material selection?

Broadly speaking, the material selection process for your CNC machining project should follow these steps:

Step 1: Determine material requirements

Evaluate the material's functionality, electrical properties, strength, and hardness to ensure it fits your project needs. Additionally, consider the environment where the part will be used and the conditions it will face.

Step 2: Create a list of material options

Organize all the materials that meet your requirements, including your design specifications.

Step 3: Choose the most appropriate material

Select the material that meets most of your needs. Sometimes, you may need to prioritize a material with better machinability over a cheaper option to maintain part quality.

Design and regulatory requirements are the basis of your material selection process, but they can leave you with several material options to choose from. To make the best choice, consider the material's properties, machining needs, finishing requirements, and procurement factors.

By thoroughly evaluating these factors, you can identify the most cost-effective material that guarantees high-quality machining and reduces unnecessary delays in your operations.

Material properties determine both whether a machined part will survive in service and how reliably, quickly, and economically it can be produced. The right choice is therefore a trade-off among functional performance, machining behavior, dimensional control, finishing requirements, supply, and cost—not simply selecting the strongest alloy.

Start with service requirements

Translate the part’s job into measurable property requirements before choosing a grade.

• Need strength and stiffness? Prioritize yield strength
• Need wear resistance? Higher hardness improves resistance to abrasion
• Need impact, shock loading, and crack resistance? Prioritize toughness
• Need fracture resistance? Focus on ductility
• Need fatigue resistance? Prioritize fatigue strength, notch sensitivity, surface condition, and corrosion resistance
• Need lightweight components? Focus on strength-to-weight ratios

Account for environment

A material can meet its room-temperature strength requirement yet fail prematurely in its actual environment.

• Moisture or chemicals? Corrosion resistance
• High temperatures? Thermal stability, oxidation resistance, creep resistance
• Sliding or abrasive contact? Hardness, wear resistance, lubricity
• Electrical or thermal? Electrical and/or thermal conductivity

Corrosion resistance should be evaluated for the specific exposure—chlorides, acids, alkaline wash, salt spray, fuel, or galvanic coupling

Consider machinability

“Machinability” is not one intrinsic number. In practice, it reflects cutting forces, chip control, surface finish, attainable tool life, required tooling, cycle time, and the risk of distortion.

• Hardened or carbide-rich alloys demand more robust cutters, lower practical material-removal rates, and more frequent tool changes.
• Austenitic stainless steels, titanium alloys, and nickel alloys can harden beneath the cut. Light rubbing, dwell, poor edge condition, or insufficient feed can quickly damage tools and compromise the next pass.
• Low-conductivity alloys retain heat near the cutting edge. Stainless steel, titanium, and nickel alloys are more likely to concentrate heat at the tool–chip interface, raising tool wear and affecting dimensional control. High-conductivity materials shed heat more readily.
• Free-machining brasses commonly produce short, manageable chips; ductile aluminums can form long chips or built-up edge; gummy materials can create burrs and poor finish. Chip behavior influences unattended machining, deep-pocketing, drilling, and part handling.
• Titanium, austenitic stainless, and aluminum can adhere to tools under unfavorable conditions. Tool substrate/coating, sharp geometry, coolant delivery, speed, and feed need to be chosen around the specific grade.
• Annealed, cold-worked, precipitation-hardened, normalized, quenched-and-tempered, and stress-relieved conditions can behave very differently despite having the same alloy designation.

A higher-machinability material generally requires less cutting force, gives better surface finish, and extends tool life; low-machinability choices tend to increase cycle time, tool cost, and process risk. 

As it relates to your project, here are the key factors you need to consider when selecting the appropriate material.

Part Application

Choosing the right material for CNC machining is crucial, and different applications require different materials.

The material's physical properties, like tensile strength, strength-to-weight ratio, crack resistance, rigidity, or flexibility, depend on its application. Heavier materials can handle more stress, but for weight-sensitive uses, choose lighter materials with a strong strength-to-weight ratio.

For example, aerospace parts must be lightweight, unlike parts for structural support. Aluminum 3.3211 is often used in aerospace because it has a strong strength-to-weight ratio.

Machining Conditions

The final application of your part will guide many of your material choices, but first, the raw material must undergo the machining process.

Machining rates, also known as speeds and feeds, determine how fast material is removed. Each material has specific machinability traits, like hardness and melting point, that affect how quickly it can be processed, how heat is handled, and how chips are cleared.

The material you choose impacts how quickly your cutting tools wear out. Some materials, like high-temperature alloys, can cause more wear, requiring special tools and increasing costs.

Finishing Requirements

Don't be fooled by the word "finishing;" you need to carefully consider your parts' finish early in the design process, not as an afterthought. This helps avoid compatibility issues between your chosen material and the necessary surface process.

If your project requires a specific surface finish, choose a material that can be easily machined to meet those needs. Materials like aluminum and plastics can achieve a smooth finish effortlessly, while others, like cast iron, may require additional steps to reach the desired quality.

Check if the plating type matches the base material. For example, if nickel plating is needed, aluminum might not be the best choice. Instead, use stainless steel, which works well with nickel plating, ensuring you select the right material.

While design and industry regulations are crucial, consider these procurement factors when selecting a material for your machined component, which can help reduce unnecessary costs and affect your lead times.

Raw Material Cost

The cost of materials is a key factor in procurement. High-end aerospace alloys can be expensive, but more affordable options like plastics are available.

Machining Cost

The machining expenses for each material should be taken into account as well. Certain materials may demand additional time and labor to machine effectively, thus affecting the overall cost of machining.

Availability

When selecting materials, consider their availability. Some materials might be hard to find or take longer to get, which could delay your project. Make sure the material you choose is available within your required timeframe.

Lead Time

Consider the project's timeline as well, as the lead time for certain materials can significantly affect your material selection process. Some materials may have longer lead times than others, potentially causing delays in your overall project schedule.

Choosing a material with more advanced properties than needed for your application can unnecessarily raise the cost of your CNC machined parts.

To minimize cost, select the material with the lowest cost that has properties that fulfill your design requirements.

Here is a table that summarizes how the cost of a part varies based on some of the most common materials.

What are the most common materials used in CNC machining?

CNC machining commonly uses materials like metals (aluminum, steel, stainless steel, brass, titanium), plastics (ABS, polycarbonate, nylon), and composites.

Should stock form and condition be specified?

Yes! Determine whether bar, plate, billet, forging, extrusion, casting, or near-net preform is best—and specify condition such as annealed, solution-treated, cold-worked, stress-relieved, or aged. The same nominal alloy can machine and perform very differently by condition.

Do part features like internal corners, deep pockets, thin walls, and threads drive a different material choice?

Typically, yes. Difficult geometry can make a nominally low-cost material expensive due to long cycle times, specialty cutters, additional setups, burr control, inspection burden, or scrap risk. Avoiding inaccessible features and sharp internal corners improves manufacturability.

What material properties should be defined first?

For most components, identify the minimum requirements for yield and ultimate tensile strength, elastic modulus, hardness, wear resistance, galling resistance, toughness, impact resistance, fatigue strength or cyclic-life requirement, density or mass limit, operating-temperature range, corrosion and chemical resistance, thermal and electrical conductivity, thermal expansion and creep resistance.

How do I select material for corrosion resistance?

Start by documenting the actual environment instead of simply specifying “corrosion resistant.” Note exposure to humidity, freshwater, salt spray, chlorides, seawater, acids, bases, process fluids, solvents, road salt, and cleaning chemicals. Then identify the relevant corrosion mechanisms—general, pitting, crevice, galvanic, stress cracking, chemical attack, or high-temperature oxidation. Finally, consider adjacent materials, fasteners, coatings, drainage, trapped moisture, and electrical isolation. Material selection should account for the entire assembly, not just the part itself.

How do I determine whether fatigue matters?

Fatigue matters when the component experiences repeated or fluctuating stress, vibration, pressure cycling, rotation, thermal cycling, or repeated assembly loads. Define load spectrum, number of expected cycles, stress concentration locations, surface finish requirement, mean stress/preload, if failure is safety- or mission-critical, and corrosion environment. Surface condition, notches, threads, corrosion, and residual stress can substantially reduce practical fatigue performance relative to smooth-specimen material data.