6061 or 7075 aluminium — which should I choose?

6061 when you need weldability, corrosion resistance and the lowest cost. 7075 when you need strength close to mild steel at a third of the weight — but it does not weld, and it costs more per kilogram and per hour.

Why is stainless steel more expensive to machine than aluminium?

Work hardening, low thermal conductivity and stringy chips. Cutting heat stays at the tool edge instead of leaving with the chip, so tool life drops and cutting speeds must be reduced.

Is titanium worth the cost?

Only when you need its strength-to-weight ratio, corrosion resistance or biocompatibility. Titanium conducts heat poorly, so tool wear — not machine time — dominates the price.

Do you machine lead-free brass?

Yes. CW510L and similar lead-free grades are available, with longer cycle times than CW614N because they chip less freely.

Resources · Materials

CNC Milling Metal Parts — How to Choose the Right Metal

Q: What is the easiest metal to CNC mill?

Brass (C360) and aluminium 6061. Brass is the reference point at 100% machinability in most rating tables; 6061 cuts almost as freely with sharp tools and generous coolant, so it sits close to brass rather than far above it.

Most milling quotes are decided before the drawing reaches the shop: by the material written in the title block. This guide sets out what machinability actually means, compares the ten metals we mill most, and shows which part of the cost each one moves.

What "machinability" actually means

Machinability is a relative index of how easily a metal can be cut — expressed against free-cutting brass C360, which is defined as 100%. It combines cutting speed, tool life and surface finish: a grade rated 50% takes roughly twice the time of brass for the same feature. The index is a planning tool, not a physical constant; heat treatment, hardness and the tool grade all shift the number.

What the index does not capture is why a metal is slow. Aluminium and stainless are both "harder than brass", but for opposite reasons: aluminium cuts fast and then sticks to the tool, while stainless refuses to conduct heat away and hardens under the cut. Those are different problems, and they are solved differently.

Metal comparison table

The relative cost column is a multiplier against 6061-T6 aluminium, for the same part geometry and quantity. It is not a price list — it compares materials, not suppliers.

Material	Machinability	Typical tolerance	Finish (Ra)	Relative cost	Best for
Brass C360	Excellent (100% index)	±0.01 mm	0.8 µm	1.2×	Fittings, connectors, small high-volume parts
Aluminium 6061-T6	Excellent	±0.01 mm	0.8 µm	1.0× (baseline)	Brackets, housings, fixtures, anything welded
Aluminium 7075-T6	Good	±0.01 mm	0.8 µm	1.5–1.8×	High-strength structural parts, aerospace jigs
Stainless 303	Good (free-cutting)	±0.01 mm	1.6 µm	2.0–2.5×	Shafts, bushings, high-volume turned parts
Stainless 304	Moderate	±0.01 mm	1.6 µm	2.2–2.8×	General corrosion resistance, food equipment
Stainless 316L	Moderate–poor	±0.01 mm	1.6 µm	3.0–4.0×	Marine, medical, chemical, chloride exposure
Stainless 17-4PH	Moderate (pre-hardened: poor)	±0.01 mm	1.6 µm	3.5–5.0×	High-strength shafts, valve stems, aerospace
Copper C110	Moderate (gummy)	±0.02 mm	1.6 µm	3.0–4.0×	Busbars, heat spreaders, RF components
Titanium Ti-6Al-4V	Poor	±0.01 mm	1.6 µm	6–10×	Aerospace, medical implants, weight-critical parts
Mild steel 1018	Good	±0.01 mm	1.6 µm	1.1–1.3×	Structural plates, weldments, painted parts

Typical tolerance figures are routine capability; tightening below them — for example to ±0.005 mm — is reviewed per part before it is quoted.

Two patterns follow from the table. First, cost scales with how hard the material is to cut, not with how expensive the material is to buy — 316L and 17-4PH cost more in machine time than in stock. Second, tolerance is driven by process rather than by material: ±0.01 mm is achievable in all of them except the copper alloys, where ±0.02 mm is the practical figure, and tightening below that is a separate decision from choosing the metal.

Aluminium: 6061 or 7075?

6061-T6 — the general-purpose default

Weldable, corrosion resistant, cheap, available in every thickness and easy to anodize. If a drawing says "aluminium" with no grade, 6061-T6 is almost always what was meant. Its weakness is strength: about 276 MPa yield, which is fine for brackets and housings and not enough for a loaded structural member.

7075-T6 — strength at a price

Roughly 503 MPa yield, close to mild steel at a third of the density. It machines well but is not weldable, has lower corrosion resistance (zinc is the main alloying element) and costs more per kilogram. Choose it when the part is strength-limited and weight-critical — never to "be safe".

The decision rule: if the part will be welded, 6061. If the part will be thin and loaded, 7075. If neither applies, 6061 and put the saved money into the surface finish.

Stainless steel: 303, 304 or 316L?

303 — sulphur added for chip breaking, the fastest-cutting stainless. Use it for high-volume turned parts. Do not weld it, and do not use it in marine or chloride service.
304 — the general austenitic grade. Weldable, formable, moderate machinability. The default where corrosion resistance is needed but the environment is not aggressive.
316L — molybdenum added for chloride resistance; medical, marine and chemical service. Work-hardens noticeably more than 304.
17-4PH — precipitation hardening, roughly 1,000 MPa after aging. Usually machined in the solution-treated condition and aged afterwards, which means the final dimensions are set before the last heat treatment.

Stainless costs more than aluminium for three reasons that compound: it work-hardens under the cut, it conducts heat poorly so the tool edge runs hot, and its chips are tough and stringy. All three push you toward lower cutting speeds and shorter tool life.

Brass and copper: the easiest, and the trickiest

Brass is the most forgiving metal in the shop: low cutting force, short chips, long tool life. Its real constraint is regulatory rather than technical — leaded free-cutting grades are under pressure from RoHS, and lead-free alternatives (CW510L and similar) cut noticeably slower, with the penalty depending on the grade.

Copper is the mirror image. It is soft enough to be easy and sticky enough to be difficult: it forms a built-up edge on the tool, the chips weld to the flutes, and continuous chips wrap around the tool and block coolant. Copper needs sharp, polished flutes, high rake angles and generous coolant — parameters that look aggressive on paper and work in practice. See brass fittings for the parts we run in copper alloys.

Titanium: when it's worth the cost

Titanium conducts heat roughly twenty times worse than aluminium, so the heat of cutting stays in the tool edge instead of leaving with the chip. Tool life falls, cutting speed falls with it, and the price is dominated by tooling rather than by stock. Choose titanium only when you need something it uniquely provides: strength-to-weight near 7075 at higher temperature, chloride and seawater resistance, or biocompatibility.

How material choice drives cost

Stock — priced by the blank size you must start from, not the finished weight.
Cycle time — set by cutting speed and by how many passes the material tolerates.
Tool wear — the hidden multiplier in stainless and titanium; it shows up as both insert cost and downtime.
Finishing — anodizing works on aluminium, passivation on stainless, plating on brass; the wrong pairing costs a rework cycle. Our surface finishing guide covers the combinations.
Inspection — hard materials with tight tolerances need more frequent in-process checks.

Machines, tolerances and inspection equipment are listed on the capabilities page; the milled part families are on CNC milled parts.

Frequently asked questions

What is the easiest metal to CNC mill?

Brass, then 6061 aluminium. Both cut fast with long tool life; brass gives the better finish, aluminium the lower cost.

6061 or 7075?

6061 for weldability, corrosion resistance and cost; 7075 when strength per kilogram decides the design.

Why is stainless more expensive to machine?

Work hardening, poor heat conduction and stringy chips — all of which force slower cutting and shorter tool life.

Is titanium worth it?

Only for its specific properties. On cost alone it never wins; on weight or corrosion it sometimes is the only option.