Material selection is one of the highest-leverage decisions in a CNC project. It affects strength, weight, corrosion resistance, how the part machines, what finishes are possible and — directly — cost. This guide gives you a framework for choosing, then a quick reference for the common machinable materials.
Start With the Function, Not the Catalog
Before naming a grade, write down what the part must do in service. The answers point to the properties that matter:
- Load — does it carry force, hold a thread, or just locate something?
- Environment — indoor, outdoor, washed, salty, hot, lubricated?
- Weight — is mass or inertia a concern?
- Conductivity — must it carry electrical current or move heat?
- Wear — does it slide, rotate or mate against another surface?
- Appearance — visible part, or hidden function?
Once those are clear, material choice becomes a trade-off rather than a guess.
The Properties That Usually Decide It
Strength and Stiffness
Steels and many alloys give high strength for loaded parts. Aluminum is strong enough for most non-critical structures but lighter. If the part must not deflect, stiffness (related to modulus) and wall thickness matter as much as the grade.
Corrosion Resistance
Stainless steel, anodized aluminum and certain platings survive harsh environments. Plain carbon steel rusts without protection, so it needs finishing if exposed.
Machinability
Some materials cut faster, hold better surface finish and wear tools less. Free-machining brass and 6061 aluminum are easy; hardened steels and exotic alloys are slower and more expensive to machine. Machinability quietly drives cycle time and cost.
Weight
Aluminum is the default when mass matters (roughly one-third the density of steel). Titanium is strong and light but costly; plastics are lightest but weak.
Thermal and Electrical Behavior
Copper and aluminum conduct both heat and electricity; plastics insulate. This decides enclosures, heat paths and connector bodies.
Common Machinable Materials
Aluminum (e.g., 6061-T6, 7075-T6)
Light, corrosion resistant, easy to machine and good looking anodized. 6061 is the general-purpose choice; 7075 is stronger for stressed parts. Best for housings, brackets, robotics links and visible components. More on aluminum.
Stainless Steel (e.g., 304, 316, 17-4 PH)
Corrosion resistant and strong. 304/316 suit most environments; 316 adds seawater resistance; 17-4 PH can be precipitation-hardened for higher strength. Tougher to machine than aluminum, so slightly higher cost. More on stainless.
Carbon and Alloy Steel
Highest strength per dollar and excellent for wear. Needs protection (plating, coating) if exposed to corrosion. Grade follows your drawing's required mechanical properties. More on steel.
Brass and Copper
Free-machining, conductive and corrosion resistant. Brass (e.g., C360) is ideal for connectors, fittings and pins; copper (e.g., C110) for thermal and electrical parts. More on brass.
Engineering Plastics (POM, PA, PEEK)
Light, non-conductive, low-friction and quiet. POM for precise, stable parts; PA (nylon) for toughness; PEEK for high temperature and chemical resistance. Offered only where the factory can actually machine them. More on plastics.
How Material Affects Cost
Three mechanisms:
- Raw stock price — aluminum and steel are cheap; titanium, specialty alloys and PEEK are not.
- Machinability — hard or gummy materials need slower feeds, more tool changes and more time.
- Finishing compatibility — some finishes only suit some materials, which can force a more expensive base metal.
Choosing the cheapest adequate material — rather than the "best" material on paper — is often the biggest single saving available.
Practical Takeaway
Pick the material from the function, then down-select to the cheapest grade that meets it. If you are unsure, tell your supplier the in-service requirements and let them suggest a grade and confirm availability. Our materials page lists the families we commonly machine.