CNC Machining Materials Selection Guide: Metals and Engineering Plastics
Compare CNC machining materials by strength, weight, machinability, corrosion resistance, temperature, dimensional stability, finish options, cost, and inspection risk.
Material selection is one of the highest-leverage decisions in CNC machining. The alloy or polymer affects cycle time, achievable tolerance, distortion, surface finish, inspection method, and total landed cost. This guide gives engineers and buyers a practical framework for selecting materials before requesting a quote.
1. Start with functional requirements
Do not select a material from strength alone. Define load case, stiffness, mass, operating temperature, corrosion exposure, electrical or thermal requirements, wear, appearance, regulatory constraints, production quantity, and inspection needs. Separate mandatory requirements from preferences.
| Decision factor | Question to answer | Typical machining impact |
|---|---|---|
| Strength and stiffness | Static, fatigue, impact, or deflection controlled? | May require high-strength alloy, heat treatment, or larger section |
| Mass | Is weight a system-level constraint? | Aluminum and titanium reduce mass; titanium raises cycle time |
| Environment | Humidity, salt, chemicals, vacuum, or food contact? | Changes alloy, coating, passivation, and traceability needs |
| Temperature | What is the operating and inspection temperature? | Thermal expansion can consume tolerance budget |
| Quantity | Prototype, bridge, or repeat production? | Influences stock form, setup strategy, and tooling |
2. Material-family comparison
| Family | Main advantages | Main risks | Common applications |
|---|---|---|---|
| Aluminum | Low density, fast machining, good finish options | Lower modulus, thermal expansion, distortion in thin walls | Housings, brackets, fixtures, aerospace structures |
| Carbon/alloy steel | Strength, stiffness, wear resistance, low raw-material cost | Corrosion, heat-treatment movement, slower cutting | Shafts, tooling, gears, machine components |
| Stainless steel | Corrosion resistance, strength, hygienic surfaces | Work hardening, heat generation, burr control | Medical, food, marine, chemical equipment |
| Brass/copper | Conductivity, corrosion resistance, excellent machinability for many brasses | Copper can be gummy; alloy compliance must be checked | Electrical parts, fittings, valves, heat-transfer components |
| Titanium | High specific strength, corrosion resistance, biocompatibility | High cost, low thermal conductivity, tool wear | Aerospace, medical, high-performance equipment |
| Engineering plastics | Low mass, insulation, chemical resistance | Creep, moisture absorption, thermal movement, clamping distortion | Insulators, guides, seals, low-friction components |
3. 6061-T6 vs 7075-T6 aluminum
| Property | 6061-T6 | 7075-T6 | Design implication |
|---|---|---|---|
| Typical yield strength | about 276 MPa | about 503 MPa | 7075 supports higher load at similar geometry |
| Density | about 2.70 g/cm³ | about 2.81 g/cm³ | Weight difference is usually secondary |
| Corrosion resistance | Good | Lower than 6061 | 7075 often needs careful environmental review |
| Weldability | Generally good | Generally poor | 6061 is normally safer for welded assemblies |
| Machinability | Good | Very good | Both are production-friendly with correct tooling |
| Finish options | Anodizing, conversion coating, paint | Anodizing and protective finishes | Color consistency must be qualified by alloy and lot |
Choose 6061 when corrosion resistance, weldability, availability, finishing, and cost dominate. Choose 7075 when strength-to-weight performance justifies higher material cost and stricter corrosion control.
4. Carbon and alloy steels
Low-carbon steels such as 1018 are economical and weldable but are not selected for high wear resistance. Medium-carbon and alloy steels such as 1045, 4140, and 4340 provide higher strength and heat-treatment options. Specify material condition clearly: annealed, normalized, pre-hardened, quenched and tempered, or case hardened.
Heat treatment can move features. For tight bores, bearing seats, or datums, plan machining allowance before treatment and finish machining or grinding afterward. Define hardness range, case depth, decarburization limits, and whether dimensions apply before or after treatment.
5. Stainless steel grades
| Grade | Why it is selected | Machining concern |
|---|---|---|
| 303 | Improved machinability for general components | Lower corrosion performance than 304 in some environments |
| 304 | General corrosion resistance and availability | Work hardening and built-up edge |
| 316/316L | Better chloride and chemical resistance | Higher cutting load and careful heat control |
| 17-4PH | High strength with precipitation hardening | Condition must be stated; heat treatment affects dimensions |
Passivation is not a substitute for selecting the correct alloy. State the required standard, surface condition, and whether free-iron testing or certificates are needed.
6. Brass, copper, and titanium
Free-machining brass can produce excellent finishes and short cycles, while lead-free grades may behave differently. Copper offers outstanding conductivity but can smear, burr, and retain heat. Titanium Grade 5 combines strength and low density, yet its low thermal conductivity concentrates heat at the cutting edge. These differences affect tooling, chip control, and quote assumptions.
7. Engineering plastics
| Material | Useful characteristics | Key dimensional risk |
|---|---|---|
| Delrin/POM | Low friction, good machinability, low moisture absorption | Thermal movement and stress relaxation |
| Nylon | Toughness and wear resistance | Moisture absorption changes dimensions |
| PEEK | Temperature, chemical, and fatigue performance | High material cost; heat and residual-stress control |
| PTFE | Chemical resistance and very low friction | Creep, softness, and difficult clamping |
| Polycarbonate | Impact strength and transparency | Stress cracking and cosmetic handling |
Plastic tolerances should reflect the service temperature and conditioning state. Avoid transferring metal tolerances directly to polymers without a functional reason.
8. Thermal expansion calculation
Linear expansion can be estimated with ΔL = α × L × ΔT. For a 100 mm feature measured across a 30°C temperature change:
- Aluminum, α ≈ 23 µm/m·°C: ΔL ≈ 0.069 mm.
- Steel, α ≈ 12 µm/m·°C: ΔL ≈ 0.036 mm.
- Relative movement between the two parts is approximately 0.033 mm.
If the assembly clearance is only 0.02 mm, temperature alone can reverse the intended fit. Drawings should state the reference temperature when thermal movement is significant; dimensional inspection is commonly referenced to 20°C.
9. Surface-finish compatibility
| Material | Common finish options | Critical note |
|---|---|---|
| Aluminum | Anodize, hard anodize, conversion coat, paint | Coating thickness affects bores, threads, and fits |
| Carbon steel | Black oxide, zinc, nickel, phosphate, paint | Masking and corrosion targets must be defined |
| Stainless steel | Passivation, electropolish, bead blast | Specify cosmetic direction and contamination control |
| Titanium | Passivation, anodizing, blasting | Color anodizing is not a substitute for dimensional control |
| Plastics | Machined, polish, vapor polish for suitable polymers | Finishing can alter stress and dimensions |
10. Weighted material-selection example
For a lightweight outdoor instrument housing, assume weights of corrosion 30%, stiffness 25%, machinability 20%, finish 15%, and cost 10%. Score each candidate from 1 to 5 and multiply by the weight. An example comparison might give 6061-T6 a weighted score of 4.55, 7075-T6 3.75, and 316 stainless 3.40. The result does not replace engineering validation, but it exposes why one material fits the full requirement better than a single-property choice.
11. RFQ checklist
- Material designation, temper or condition, and approved equivalents
- Required mill certificates, country of origin, or lot traceability
- 3D CAD plus controlled 2D drawing and revision
- Critical dimensions, datums, fits, GD&T, and surface roughness
- Heat treatment, hardness, coating, masking, and post-finish dimensions
- Prototype and production quantities with annual demand
- Inspection report, sampling level, CMM needs, and special gauges
- Packaging, cleanliness, corrosion protection, and delivery destination
Frequently asked questions
Which metal is easiest to CNC machine?
Many free-machining brasses and aluminum alloys machine efficiently, but the best choice depends on function, finish, compliance, and availability—not cutting speed alone.
Can I specify an equivalent material?
Yes, when functional requirements and acceptable standards are clear. Approval should consider chemistry, mechanical properties, heat-treatment condition, corrosion behavior, and certification.
Why are plastic tolerances often wider?
Polymers have lower stiffness, higher thermal expansion, creep, and sometimes moisture absorption. Tolerances should be tied to function and inspection conditions.
What information reduces quotation risk?
A revision-controlled CAD model and drawing, exact material condition, critical tolerances, finish specification, quantity, and inspection requirements let the supplier build a reliable process plan.
Need a material and manufacturability review?
Send Zync Precision your CAD files, drawings, quantities, target material, finish, and inspection requirements. Our engineering team can review machining risk, tolerance strategy, and practical alternatives before quotation.