Key takeaways
- Choose on load, environment and temperature first. Machinability decides the price; the other three decide whether the part works.
- Aluminum 6061-T6 is the correct default: nominal density 2.70 g/cm³, tensile strength 310 MPa, thermal conductivity 167 W/m·K.
- Stainless steel 316L is specified over 304 for one reason — its molybdenum content resists chloride pitting in seawater, de-icing salt and body contact.
- Ti-6Al-4V carries a nominal 950 MPa tensile strength at 4.43 g/cm³ against 7.87 g/cm³ for carbon steel, which is why it survives despite being the slowest common metal to cut.
- MW+ stocks 70+ grades and holds general machining to ±0.01mm under ISO 2768-m, ±0.005mm on precision features and ±0.001mm at the floor — but the achievable figure moves with the material.
- Published property values are nominal. Where a property is load-bearing, work from the mill certificate, not a handbook table.
Material selection is the earliest decision on a machined part and it constrains everything after it. This guide is for the engineer turning a functional requirement into a grade designation on a drawing: what each property buys you, which grade covers which application, and where the obvious answer is wrong.
- How do you choose a material for a CNC machined part?
- Material property reference: nominal values by grade
- Aluminum grades: 6061-T6, 7075-T6 and 5052-H32 compared
- When is stainless steel worth the extra machining time?
- Titanium, copper alloys and engineering plastics
- How does material choice change tolerance and surface finish?
- Which material should you specify for your application?
- When the obvious material is the wrong choice
- How to specify material on the drawing and the RFQ
- Frequently asked questions
How do you choose a material for a CNC machined part?
Choose a CNC machining material by working through four requirements in fixed order: the mechanical load the part carries, the environment it sits in, the temperature range it operates across, and the dimensional stability the tolerance demands. Only then should machinability and price narrow the shortlist. Reversing that order produces a cheap part that fails in service.
The four requirements usually leave two or three candidates rather than one. That shortlist lets cycle time and tool wear break the tie without compromising function.
1. Mechanical load
Establish whether the part is loaded statically, cyclically or in impact. Static loading is answered by yield strength. Cyclic loading is answered by fatigue behavior, and here aluminum and steel differ in kind: steels show a fatigue limit below which they survive indefinitely, while aluminum alloys do not, so an aluminum part under continuous cyclic load has a finite life at any stress.
2. Environment and corrosion
Name the corrosive agent, not the word “outdoors”. Aluminum forms a self-passivating oxide that handles ordinary atmospheric exposure but not sustained chloride immersion. Stainless steel 316L handles chlorides because of its molybdenum content; stainless steel 304 does not, and specifying 304 for a marine fitting is one of the most common material errors that reaches a machine shop.
3. Temperature and thermal path
Two questions live here. Service temperature: aluminum alloys lose a meaningful fraction of room-temperature strength well below 200 °C, while austenitic stainless steels retain useful strength far higher. Heat transfer: if the part has to move heat, thermal conductivity governs, and that ranking is not the strength ranking — copper C101 conducts roughly twice as well as 6061 aluminum and over twenty times as well as 304 stainless.
4. Dimensional stability
Every wrought bar and plate carries residual stress from rolling, extrusion or heat treatment. Machining removes material asymmetrically, releases that stress, and the part moves. On thin or long parts the movement is often larger than the tolerance band, which is why stress relief between roughing and finishing is a real process step. Tool steels, pre-hardened alloy steels and thick aluminum plate are the usual offenders — say so when you request a CNC machining quote.
Material property reference: nominal values by grade
This table lets you eliminate grades before you draw anything. All values are nominal published figures for the stated temper or condition. Independent property data can be checked on MatWeb, and the chemistry and mechanical limits themselves are set by the governing specification: ASTM B209 and ASTM B211 for wrought aluminum, ASTM A276 for stainless bar, ASTM A564 for precipitation-hardening grades, ASTM B348 for titanium bar and ASTM B16 for free-cutting brass rod.
| Material grade | Density (g/cm³) | Tensile strength (MPa, nominal) | Thermal conductivity (W/m·K) | Relative machinability | Primary reason to specify it |
|---|---|---|---|---|---|
| Aluminum 6061-T6 | 2.70 | 310 | 167 | Excellent | Best balance of strength, weight and cycle time |
| Aluminum 7075-T6 | 2.81 | 572 | 130 | Good | Highest strength-to-weight in a common aluminum |
| Aluminum 5052-H32 | 2.68 | 228 | 138 | Fair (gummy) | Marine atmospheric resistance and formability |
| Stainless steel 304 | 8.00 | 515 | 16.2 | Fair | General corrosion resistance and food contact |
| Stainless steel 316L | 8.00 | 485 | 16.3 | Fair | Chloride and body-fluid resistance |
| Stainless steel 17-4 PH | 7.80 | 1310 (H900, ASTM A564 minimum) | 17.9 | Fair to poor | Corrosion resistance at near-tool-steel strength |
| Carbon steel 1018 | 7.87 | 440 (cold drawn) | 51.9 | Good | Lowest-cost structural metal; case-hardenable |
| Alloy steel 4140 | 7.85 | 655 (annealed) | 42.6 | Fair | Through-hardening shafts, gears and tooling |
| Titanium Ti-6Al-4V | 4.43 | 950 | 6.7 | Poor | Strength-to-weight plus biocompatibility |
| Brass C360 | 8.50 | 385 | 115 | Excellent | Free-machining fittings, valves and connector bodies |
| Copper C101 | 8.94 | 220 (annealed) | 391 | Fair (gummy) | Electrical and thermal conduction |
| Acetal (POM-C) | 1.41 | 70 | 0.31 | Excellent | Low-friction bearings, gears, insulators |
| PEEK | 1.30 | 100 | 0.25 | Good | High-temperature, chemically resistant polymer |
Read density and tensile strength together: that ratio is specific strength, the number that matters for anything that moves. On that basis Ti-6Al-4V and 7075-T6 both beat 4140 steel, and 6061-T6 is close to 1018 at a third of the mass.
Worked example: sizing the same part in five grades
Take a tie bar carrying a static tensile load of 20 kN with a design factor of 2 against ultimate strength. The required cross-section is A = F × n ÷ UTS, and the mass of a 100 mm length is m = A × 100 mm × density. Working that through with the nominal values above:
| Grade | UTS (MPa) | Area A = 40,000 N ÷ UTS | Mass per 100 mm | Against 6061-T6 |
|---|---|---|---|---|
| Aluminum 6061-T6 | 310 | 129.0 mm² | 34.8 g | baseline |
| Aluminum 7075-T6 | 572 | 69.9 mm² | 19.6 g | −44% |
| Titanium Ti-6Al-4V | 950 | 42.1 mm² | 18.7 g | −46% |
| Alloy steel 4140 | 655 | 61.1 mm² | 48.0 g | +38% |
| Stainless steel 304 | 515 | 77.7 mm² | 62.1 g | +78% |
Titanium buys almost nothing over 7075-T6 on mass in this load case while costing far more to cut, which is why the aerospace answer is so often aluminum. Stainless is nearly twice the mass of the aluminum baseline, so specifying it for corrosion reasons on a moving part carries a weight consequence that should be stated rather than discovered.
Aluminum grades: 6061-T6, 7075-T6 and 5052-H32 compared
Aluminum is the most-specified family in CNC milling services work, and most of the value sits in choosing between three grades. The cost consequences of the aluminum-versus-stainless decision are set out in our aluminum vs stainless steel comparison.
6061-T6 — the correct default
Aluminum 6061-T6 is a magnesium-silicon alloy that machines cleanly, welds, anodizes evenly and holds tight tolerances without unusual process control. Its nominal 310 MPa tensile strength covers most brackets, housings, manifolds and fixtures, and 167 W/m·K makes it a competent heat spreader without moving to copper.
7075-T6 — strength, with three trade-offs
Aluminum 7075-T6 is a zinc-alloyed grade with a nominal tensile strength of 572 MPa, roughly 85% higher than 6061-T6 at 4% more mass. It is not readily weldable, it is more susceptible to stress-corrosion cracking, and it costs more per kilogram. Use it where a weight target and a strength target conflict.
5052-H32 — corrosion resistance, not machining
Aluminum 5052-H32 is chosen for marine atmospheric resistance and sheet formability. It is the weakest of the three and tears rather than chipping cleanly. Specify 5052 for formed and welded enclosures; for a machined-from-solid part with the same corrosion requirement, 6061-T6 with a hard anodize is usually better.
When is stainless steel worth the extra machining time?
Stainless steel is worth the extra machining time when the part faces sustained corrosion, needs a cleanable surface for food or medical contact, must survive above roughly 200 °C, or has to resist wear at a sliding interface. Those four conditions cannot be bought back with a coating on aluminum. Outside them, stainless usually adds cost without adding function.
Austenitic stainless steels work-harden as they are cut, which is what makes them slow. The cutting edge must stay engaged and loaded; light, hesitant passes glaze the surface and destroy the tool faster than aggressive ones. Where corrosion resistance is not the driver, carbon and alloy steels are often better: 1018 is the cheapest metal here and case-hardens well, while 4140 through-hardens and is the standard answer for shafts and gear blanks produced on CNC turning services.
Titanium, copper alloys and engineering plastics
Titanium Ti-6Al-4V
Ti-6Al-4V is an alpha-beta titanium alloy combining a nominal 950 MPa tensile strength with a density of 4.43 g/cm³ and near-complete inertness in body fluids and seawater. Its nominal thermal conductivity of 6.7 W/m·K is the machining problem: heat generated at the cutting edge stays there instead of leaving in the chip, so tool life is short and coolant delivery matters more than in any other common metal. The factors behind that are broken down in our guide to titanium CNC machining cost.
Copper and brass
Copper C101 is specified when conduction is the function — busbars, heat sinks, EDM electrodes. Its nominal 391 W/m·K has no substitute among machinable metals, but it is soft and gummy and will not hold a fine edge. Brass C360 is the opposite, and is why valve bodies, fittings and machined connector pins are so often brass.
Engineering plastics
Acetal is the low-friction, dimensionally stable choice for bearings, gears and insulating parts at a nominal density of 1.41 g/cm³. PEEK costs substantially more and earns it in high-temperature, chemically aggressive or repeatedly sterilized service. Both machine quickly, but both move with temperature and humidity far more than metal, so tolerance expectations have to be set accordingly.
How does material choice change tolerance and surface finish?
Material choice changes achievable tolerance and surface finish because it changes cutting forces, heat behavior and how much the part relaxes after cutting. MW+ holds general machining to ±0.01mm under ISO 2768-m, ±0.005mm on precision features and ±0.001mm at the tolerance floor, but reaching the tighter figures takes more passes and tighter thermal control in a hard-to-cut grade than in 6061-T6.
Surface roughness is defined and measured under ISO 21920-2:2021, which supersedes the withdrawn ISO 4287. Across MW+ materials the as-machined baseline is Ra 3.2µm, fine machining reaches Ra 0.4µm and polishing reaches Ra 0.1µm. Which of those a grade reaches without a secondary operation depends on how it forms a chip.
| Material grade | Chip behavior at the cutting edge | Consequence for tolerance | Route to Ra 0.4µm or better |
|---|---|---|---|
| Aluminum 6061-T6 | Clean short chips; heat leaves in the chip | Tight tolerances in fewer passes | Fine machining, no secondary operation |
| Brass C360 | Free-breaking chips, low cutting force | Very stable on small turned features | Fine turning direct from the machine |
| Stainless steel 304 / 316L | Work-hardens ahead of the tool; stringy chips | Needs constant engagement; more thermal drift | Fine machining plus grinding or polishing |
| Alloy steel 4140 | Predictable but abrasive; releases residual stress | Stress relief needed mid-process | Grinding after hardening |
| Titanium Ti-6Al-4V | Heat concentrates at the edge; springback at thin walls | Slow passes, frequent tool change | Fine machining plus polishing |
| Copper C101 / Acetal | Gummy; smears rather than shears | Sharp uncoated tooling, high speed | Diamond-tool finishing or polishing |
Where a feature is small and tight, the process matters more than the grade: Swiss machining supports the stock at the cut, and micro-machining reaches features conventional milling cannot.
Which material should you specify for your application?
Specify the grade that satisfies the dominant requirement most cheaply. This table maps common applications to a first choice and a named alternative, with the reason attached so you can check the logic against your own part.
| Application | Dominant requirement | First choice | Alternative | Why |
|---|---|---|---|---|
| Structural bracket, indoor equipment | Stiffness at low cost | Aluminum 6061-T6 | Carbon steel 1018 | 6061-T6 needs no coating; 1018 is cheaper if mass is irrelevant |
| Airframe or drone structure | Strength-to-weight | Aluminum 7075-T6 | Ti-6Al-4V | 7075-T6 gives most of the specific strength at far lower cycle time |
| Surgical instrument or implant contact | Biocompatibility, sterilization | Stainless steel 316L | Ti-6Al-4V | Both inert and autoclavable; titanium where mass matters |
| Marine or offshore fitting | Chloride corrosion resistance | Stainless steel 316L | Nickel-copper alloy | Molybdenum resists pitting where 304 fails |
| Drive shaft, gear blank, spindle | Wear and fatigue resistance | Alloy steel 4140 | Stainless steel 17-4 PH | 4140 through-hardens; 17-4 PH adds corrosion resistance |
| Heat sink or thermal interface | Thermal conduction | Copper C101 | Aluminum 6061-T6 | Copper conducts roughly 2.3× better; aluminum wins on mass and cost |
| Valve body, fitting, connector pin | High-volume turned features | Brass C360 | Stainless steel 303 | Brass cuts fastest of any common metal |
| Bearing, bushing, insulating part | Low friction, no lubrication | Acetal (POM-C) | PEEK | Acetal is self-lubricating; PEEK for heat or chemicals |
| Food or beverage contact | Cleanability and hygiene | Stainless steel 304 | Stainless steel 316L | 304 is standard; 316L where cleaning agents contain chlorides |
When the obvious material is the wrong choice
The defaults above are wrong often enough to list explicitly. Each row is a case where the first grade a reasonable engineer reaches for causes a problem later.
| Situation | The obvious choice | Why it is wrong here | Specify instead |
|---|---|---|---|
| Continuous cyclic load, long design life | Aluminum 6061-T6 | Aluminum alloys have no fatigue limit, so life is finite at any stress | A steel grade sized on the fatigue limit |
| Welded assembly needing high strength | Aluminum 7075-T6 | Not readily weldable; loses temper in the heat-affected zone | 6061-T6, or bolt rather than weld |
| Seawater or de-icing salt exposure | Stainless steel 304 | 304 pits in chlorides and looks fine until it perforates | Stainless steel 316L |
| Thin-walled part from thick plate | Any grade, unrelieved | Released residual stress moves the part past the tolerance band | Same grade with stress relief mid-process |
| Conductive part needing sharp detail | Copper C101 | Copper smears and will not hold fine features | Brass C360, or copper finished by sinker EDM |
| Polymer part, tight bore, humid environment | Acetal at metal tolerances | Polymers move with temperature and moisture uptake | PEEK, or open the tolerance and state the measuring condition |
How to specify material on the drawing and the RFQ
A callout that says “aluminum” or “stainless” is not a specification, and a supplier who quotes it is guessing. State grade and condition together — 6061-T6, not 6061; 316L, not 316 — because the temper or heat-treat condition changes mechanical properties more than the alloy family does.
- Grade and condition, including temper or heat-treat state.
- Governing specification, so the mill certificate can be checked against something.
- Substitution rule — whether an equivalent grade is acceptable. If it is not, say so on the drawing.
- Surface treatment: anodize, passivation or plating change dimensions and must sit explicitly inside or outside the tolerance.
- Documentation. Material certificates ship with every MW+ order alongside a certificate of conformance and a CMM inspection report. First article inspection to AS9102 and PPAP Level 3 are available on request, quoted per programme.
MW+ works from STEP, IGES, DXF, DWG, SolidWorks and PDF files and returns a quotation within 24 hours. If two grades both satisfy your requirement, send both on the RFQ — the machining difference is usually larger than the raw material difference. Inspection practice is set out under CNC machining quality control.
Frequently asked questions
Why is the same part quoted so much higher in stainless steel than in aluminum?
A stainless steel part is quoted higher than the same geometry in aluminum because cutting speed is lower, tool life is shorter, and the material work-hardens as it is cut, forcing heavier and more conservative passes. Raw material price is the smaller part of the gap; machine time and tooling consumption are the larger part, and both scale with how much metal has to be removed. Which levers actually move that number is covered in our guide to reducing CNC material cost.
Can I change material after the first article has been approved?
Changing material after first article approval invalidates the approval, because the dimensional results came from a specific grade with specific stress and thermal behavior. A new first article inspection is required, and on regulated programmes the change also affects the material certificates on file. Treat a material change as a revision to the part, not a sourcing substitution.
Should I let my supplier choose an equivalent grade to save cost?
Allow a substitution only where you have stated which properties are load-bearing. An equivalent grade matched on tensile strength may differ in corrosion resistance, weldability or thermal expansion, and those differences do not appear in an incoming inspection. If the drawing is silent on substitutions, ask each supplier what they actually quoted before comparing prices.
Does material selection change lead time as well as price?
Material selection changes lead time mainly through availability rather than machining speed. Common grades held in stock move straight into production, while specialty alloys and unusual bar sizes must be procured first. MW+ quotes within 24 hours and offers 48-hour express prototypes, 3–5 business days for standard prototypes and 10–15 business days for volume production, with material availability the usual variable on unusual grades.
What material should I use for a prototype I will later produce in steel?
Prototype in the production material whenever the prototype has to prove function. Aluminum is a good form-and-fit stand-in for a steel part but a poor functional one, because stiffness, fatigue behavior, wear and thermal expansion all differ. If cost forces an aluminum stand-in, order it through CNC prototyping and be explicit about which tests it can and cannot validate.
Why does my part come back distorted when the machining measured in tolerance?
A part that measures in tolerance at the machine and distorts afterwards has released residual stress from the raw stock. Rolling, extrusion and heat treatment leave bar and plate internally stressed, and removing material asymmetrically lets it relax. The fix is a stress-relief step between roughing and finishing, agreed at quotation, not a tighter machining tolerance.
How do quality certifications affect which materials a supplier can offer?
Quality system certifications govern traceability and process control rather than the materials themselves, but they determine whether the paperwork behind a grade is auditable. MW+ operates under ISO 9001:2015, AS9100D, ISO 13485, IATF 16949 and NADCAP, which is what allows aerospace and medical grades to be supplied with full material traceability.
If you have a requirement and no settled grade, send the STEP file with the load, environment and temperature stated. MW+ returns a material recommendation and a quotation together, so the cost of each candidate is visible before the decision. Start at CNC machining services.
For grades beyond the thirteen tabulated here, including less common alloys and polymers, see our guide to 30 CNC machining materials.
