CNC Turning Aluminum, Stainless Steel and Brass: What Changes Between Them

The three materials fail in different ways, so they cannot share one program. Aluminum 6061-T6 turns at 300–500 m/min but builds up on the edge and moves with temperature; 304 stainless at 100–150 m/min work-hardens if the tool rubs; C3604 brass at 200–400 m/min cuts freely but contains lead. We run all three on 70 Tsugami Swiss-type lathes and 50 CNC lathes, holding ±0.005 mm on critical diameters.

A drawn part does not say which material it is for nothing. Aluminum, stainless and brass differ in cutting speed by a factor of four, in the way they fail by more than that, and in how much the finished size moves when the shop warms up by ten degrees. A shop that runs one program across all three is either very conservative on two of them or scrapping on one.

What follows is the working knowledge behind a turning quote: what each material does at the cutting edge, which parameters start a trial, where the tolerance risk sits, and which questions we ask before quoting a turned part.

1. Three materials, three different failure modes

Before parameters, understand what each material does wrong. Most scrapped turned parts are not dimensionally wrong at the machine — they are wrong because the process fought the material's specific behaviour.

2. Cutting parameters by material

The table below gives starting values for carbide tooling on a rigid machine with through-tool or high-pressure coolant. Treat them as the first trial, not the final program: the correct value depends on the bar stock condition, the insert geometry actually on the shelf and how the part is held.

Two footnotes matter more than the numbers. First, feed and depth interact with the nose radius: a depth of cut below roughly half the nose radius pushes the cut into the previous pass's work-hardened layer, which is the single most common cause of poor stainless tool life. Second, coolant is not optional on stainless and not always desirable on brass — brass chips break short and a heavy flood can recirculate fines.

3. Aluminum 6061-T6 and 7075: built-up edge and thermal drift

Aluminum cuts easily and that is exactly the trap. At low surface speed the chip welds to the tool edge, forming a built-up edge that grows, breaks off, and takes a piece of the tool with it — the finished surface shows it as a torn, matte patch between otherwise bright cuts. The cure is speed and edge geometry: raise the surface speed, use a polished flute with a high rake angle, and keep the tool path continuous so the cut never dwells.

7075-T6 is a different proposition. Its zinc content raises strength and abrasion, so tool life drops and surface speed should come down to roughly 200–350 m/min. In exchange it holds thin walls and sharp corners that 6061 deflects away from. For a housing with a 1 mm wall, 7075 is sometimes the cheaper choice even at a higher material price, because it survives the fixture.

The third aluminum problem is thermal, and it is the one that decides whether ±0.005 mm is real. Aluminum expands about 23.6 µm per metre per kelvin. On a 20 mm diameter, a 10 °C swing moves the size by roughly 0.005 mm — the whole tolerance band. So the measurement has to happen in a stable room, on a part that has cooled, and the CMM report has to state the condition. A number taken at the machine, on a warm part, is not a tolerance result.

4. Stainless 303, 304 and 316: work hardening and the 303 shortcut

Austenitic stainless work-hardens whenever the tool rubs instead of shearing. Once a hardened layer forms, the next pass cuts through hardened metal, which dulls the insert faster, which rubs more — the loop closes quickly. Three rules break it: depth of cut above half the nose radius so the tool stays under the hardened skin, feed high enough to cut rather than rub, and a tool change at the first sign of flank wear rather than at the first sign of a bad surface.

Surface speed separates the grades. 303 with its sulfur addition runs around 120–180 m/min and breaks chips cleanly. 304 runs 100–150 m/min and tends to produce stringy chips that need a pecking cycle or a chipbreaker matched to the feed. 316, with molybdenum for chloride resistance, runs 80–130 m/min and is the most prone to smearing and built-up edge of the three.

303 is the practical shortcut and should be chosen deliberately. It can cut cycle time by 30–40% on small parts because the chip breaks on its own. It is also slightly less corrosion resistant and not reliably weldable. Use it for internal parts that will not see chlorides and will not be welded; keep 304 or 316 where corrosion resistance or a weld is functional, not cosmetic.

5. Brass C3604: free machining, and the lead question

C3604 is the material that makes turning look easy: 200–400 m/min, light feeds, short chips, Ra 0.4–0.8 µm straight off the tool, and tool life measured in thousands of parts rather than hundreds. Its edge geometry is unusual — a near-zero or slightly negative rake with a generous clearance angle, because a sharp positive edge digs into brass instead of shearing it.

The complication is lead. C3604 carries roughly 2.5–3.5% lead to achieve that machinability, and it is covered by a copper-alloy exemption under RoHS rather than by being lead-free. That exemption has been narrowing for years. For parts entering regulated markets — drinking water, toys, electronics with extended scope — we quote lead-free free-cutting brass instead. Machinability is lower, so the cycle time rises and the price with it; plan that difference into the comparison rather than discovering it at the compliance review.

One more brass-specific point: brass is often specified as "brass" with no grade. That is not a specification. C3604, C27450 and C69300 machine differently and price differently, and a quote written against an unnamed brass will be renegotiated the moment the material certificate arrives.

6. How ±0.005 mm is held across all three

Tolerance on a turned part is a system, not a parameter. Four things have to be true at once.

The work has to be supported at the cut. On long slender parts this means a guide bushing on a sliding-headstock machine, which is why 70 of our lathes are Tsugami Swiss-type machines covering Ø2–20 mm. On chuck work it means jaws cut to the part, not soft jaws reused from the last job.

The back side has to finish in the same setup. A sub-spindle removes the re-chuck error, and on a part with a coaxial bore that error is usually larger than the tolerance itself. If a supplier plans a second operation, ask what the stack-up is and how it is measured.

The measurement has to be independent of the machine. We verify critical diameters on a CMM (Leyden) with 8 in-house inspection instruments, and the first-article report carries measured values rather than pass/fail stamps. A stamp tells you the part passed; a value tells you how much margin is left for the next 20,000 pieces.

The thermal state has to be defined. Aluminum moves 23.6 µm/m·K, brass about 19–21, stainless 16–17. The same nominal part measured at 18 °C and at 28 °C is not the same part. Our inspection room is temperature-stabilised and the report states the condition.

7. Batch size, changeover and what switching material really costs

Material changeover is not free, and it is the reason a small brass order can price above a larger aluminum one. Between materials a line needs tooling changes, coolant evaluation if the families are mixed, chip-conveyor cleaning, and a fresh first article. On Swiss-type machines the guide bushing and the bar feeder settings change too.

Practically, this means: batch by material. If a program has aluminum brackets, stainless pins and brass fittings, quoting them as one lot is usually more expensive than quoting three lots scheduled back to back. We accept a single piece as a minimum order with tiered pricing at volume, so a design can be validated on the same line that will run production — but the tier table is where material batching shows up as a real discount.

8. What to send with an RFQ for a turned part

Six items remove almost all of the back-and-forth. Send them together and the quote comes back in one pass.

Frequently asked questions

What cutting speed should be used for 6061-T6 aluminum turning?

As a starting point for carbide tooling: 300–500 m/min surface speed, 0.1–0.3 mm/rev feed and 0.5–3 mm depth of cut. 7075-T6 runs lower, around 200–350 m/min, because its higher strength and alloy content accelerate flank wear. These are starting values for a trial, not a fixed recipe.

Why does 304 stainless work-harden during turning and how is it avoided?

304 austenitic stainless work-hardens whenever the tool rubs instead of cutting. The cure is mechanical, not chemical: keep the depth of cut above roughly half the tool nose radius, keep the feed high enough to stay under the hardened layer, never let the tool dwell, and change the insert before it dulls rather than after.

Is 303 stainless a legitimate substitute for 304?

Mechanically no, machinability yes. 303 adds sulfur or selenium for chip breaking, which can cut cycle time by 30–40% on small parts. It is also slightly less corrosion resistant and not always weldable. Use 303 for internal, non-welded, non-marine parts; keep 304 where corrosion resistance or welding is functional.

Does C3604 brass comply with RoHS?

C3604 contains roughly 2.5–3.5% lead for machinability, which is covered by a copper-alloy exemption under RoHS rather than by being lead-free. That exemption has been narrowing. For regulated markets we quote lead-free free-cutting brass instead — the machinability is lower, so plan the cost difference into the comparison.

How does temperature affect a ±0.005 mm tolerance on aluminum?

Aluminum expands about 23.6 µm per metre per kelvin. On a 20 mm aluminum diameter, a 10 °C swing moves the size by roughly 0.005 mm — the entire tolerance. That is why aluminum precision work needs a temperature-stabilised measuring room and why a part measured straight off the machine reads differently an hour later.

Which parts go on a Swiss-type lathe and which on a CNC lathe?

Swiss-type sliding-headstock machines take long, slender parts where the length-to-diameter ratio would let a chuck lathe deflect the work — typically Ø2–20 mm pins, connectors, fittings and shafts. Chuck lathes take larger diameters and short parts where rigidity comes from the chuck. We run 70 Tsugami Swiss-type lathes and 50 CNC lathes for exactly this split.

What surface finish is realistic in production across the three materials?

As produced: aluminum typically Ra 0.8–1.6 µm and down to 0.4 µm with a wiper insert; brass 0.4–0.8 µm; stainless 0.8–1.6 µm, with 0.4 µm achievable on 303 but harder on 316. Finer than that is usually a finishing operation, not a turning parameter, and should be priced separately.

What information changes the quote most on a turned part?

Four items: the material and temper, the bar diameter available (not just the finished diameter), which dimensions carry ±0.005 mm, and the batch size. A part that looks identical on a drawing can differ 40% in price depending on whether the bar has to be special-ordered and how many features need CMM inspection.

Shenzhen Changhexin Precision Hardware Co., Ltd. (CHX Metal) · Shajing Street, Bao'an District, Shenzhen, Guangdong, China · ISO 9001:2015 (NOA198428) · ISO 14001:2015 (NOA2100184)
Parameter ranges are starting points for machining trials, not a commitment to any specific surface finish on any specific drawing.