Chinese CNC brass parts: alloys, tolerances and how to buy them

Most brass parts that come out of China are turned from one of three families: free-cutting brass such as C36000, hot-forging grades such as C27400 and C28000, and lead-free alternatives. C36000 contains roughly 3% lead, which acts as a chip breaker. It machines faster than any other copper alloy, holds a fine surface, and is the default for connectors, fittings, valve bodies and instrument parts. If your drawing does not name an alloy and the part is not for drinking water, C36000 is usually the right starting point.

C27400 and C28000 sit in the forging and hot-stamping family. They deform well, resist dezincification better than C36000, and are common for hardware, nuts and architectural fittings. They machine acceptably but produce stringier chips than free-cutting brass, so cycle times run longer and chip control needs more attention on a lathe.

Lead-free grades are where the cost changes. If a part touches potable water or falls under a food-contact or medical rule, you cannot use C36000. The usual substitutes are bismuth or silicon brasses, and they behave differently: bismuth brass chips break less cleanly, so feeds and speeds drop, tool wear rises and surface finish is harder to hold. Expect a different price and a different cycle time, not the same part with a swapped label.

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  C36000Best machinability. Not for potable water or food contact.
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  C27400 / C28000Better corrosion resistance, suited to forging and hardware.
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  Lead-free brassRequired for water and medical work. Slower to machine.
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  Beryllium copperUse when you need spring properties or higher conductivity.

Brass parts are mostly turned. A CNC lathe with live tooling or a mill-turn center can produce a threaded fitting with cross-holes and a milled flat in one setup, which matters because every extra setup adds a datum error. For a part like a valve body, the sequence is typically bar feed, face and center, turn the outside profile, drill and bore the through port, then cross-drill the side ports on the live tooling station.

Milled brass parts are less common but not rare. Thin-walled housings, busbars and RF shielding covers are milled from plate or from a sawn billet. Brass cuts easily, so the risk is not the cutting force. The risk is clamping distortion on thin walls and burrs on the exit edge. A 0.8 mm wall on a 40 mm brass housing will deflect under a standard vise unless the shop uses soft jaws, a fixture plate or a filled pocket.

After machining, brass usually gets at least one finishing step. Tumbling or vibratory finishing removes the fine burr left by the turning insert. Bead blasting gives a uniform matte look and hides small tool marks. Polishing brings the surface to a mirror finish for decorative or optical parts. Plating comes last: electroless nickel for wear resistance, silver or gold for conductivity and solderability. Brass also tarnishes, so parts that will sit in storage need a lacquer, a plating or a sealed bag.

Marking is often overlooked on small brass parts. Laser marking works well on brass and reads clearly, but the minimum character height is 1.5 mm. If your part number or lot code is smaller than that, it will not be legible, and the shop should tell you before the run starts rather than after.

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  One setup if possibleEvery extra setup adds a datum shift.
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  Thin wallsUnder about 1 mm, plan a fixture, not a vise.
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  Deburr earlyTumbling before plating keeps edges clean.

Brass is stable and thermally forgiving, which is why tight tolerances are realistic. On a turned diameter, ±0.005 mm is achievable on a well-maintained lathe with temperature-controlled coolant, and it is the number we quote when a drawing calls for it. That is not the same as saying every dimension should be that tight. A drawing full of ±0.005 mm callouts on non-functional surfaces raises inspection time and cost for no benefit.

The dimensions that deserve tight tolerance are the ones that mate: a bearing bore, a shaft journal, a pin hole, a sealing face. Lengths, chamfers, reliefs and radii usually need ±0.1 mm or looser. If you mark only the critical dimensions, the machinist knows where to spend the time and the inspector knows where to look.

Surface finish follows the same logic. A turned brass surface comes off the machine at around Ra 1.6–3.2 μm. That is fine for most fittings. If the part is a sealing face or a sliding surface, you need Ra 0.8–1.6 μm, which usually means a finishing pass with a fresh insert and a slower feed. For optical or vacuum parts, Ra 0.2–0.8 μm is possible but requires polishing after turning.

Inspection is where a tolerance claim is either proven or not. Every order should ship with a dimensional report for the critical features, and the shop should be able to show raw material certification, in-process checks and a final inspection record. If a supplier cannot produce a material cert for the brass bar, the grade on the drawing is only a suggestion.

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  Tight where it matesBores, journals and sealing faces earn ±0.005 mm.
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  Loose where it does notChamfers and reliefs can take ±0.1 mm.
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  Ask for the reportDimensional results plus material certification.

The first thing to check is not the machine list. It is whether the shop can tell you which alloy it will buy and from which mill. Brass bar stock varies in lead content and in residual stress, and a shop that orders by grade name without a mill certificate will eventually ship parts that machine differently from the first batch.

The second thing is DFM feedback. A useful supplier reads your drawing and comes back with specific notes: this wall is too thin for the chuck, this thread is too close to a shoulder for the tool to clear, this tolerance will need a second operation. That feedback should arrive before the quote, not after the first article fails. We return a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours once the drawing is locked.

The third thing is confidentiality. Brass parts are often proprietary, and drawings carry dimensions that matter to your product. Uploads should be handled as confidential by default, and a signed NDA should be available on request without a negotiation.

Finally, look at how the shop handles volume change. A supplier that can run one prototype and also run 10,000 pieces without retooling the process is easier to work with than one that farms out the high-volume run. Our own capacity sits around 127 high-precision CNC machines across three plants in Dongguan and Singapore, including 16 simultaneous 5-axis centers and 16 mill-turn centers, with a maximum processing size of 4,000 mm. Most brass work is far smaller than that, which means the machines are not the constraint.

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  Ask for the mill certAlloy grade and heat number on paper.
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  Expect DFM notesSpecific, not generic, before the quote.
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  Check the volume rangeOne part and 10,000 parts on one process.