Brass and Copper Machining Guide: Conductivity Comes First

Brass and Copper Machining Guide: Conductivity Comes First Brass and copper are chosen for what they do — conduct electricity and heat, resist corrosion, machine beautifully — not for strength. Here is how to pick between them. The grades that matter MaterialWhat it is forMachinability Copper C110Maximum electrical and thermal conductivityFair — soft and gummy Brass C360Free-machining fittings, connectors, bushingsExcellent — the benchmark BronzeBearings, wear surfaces, marine hardwareGood C360 brass is the easiest metal to machine in the shop — it cuts fast and clean. Pure copper is the opposite: soft, sticky and prone to tearing unless the tooling and feeds are right. Grade reference: brass, copper, and bronze side by side Machinability here is quoted on the industry scale where C360 free-cutting brass equals 100. Conductivity is percent IACS, the standard where annealed pure copper is taken as 100. Both numbers come from published alloy data; use the ASM Handbook and MatWeb when you need the full property sheet for a specific temper. AlloyFamilyKey additionMachinabilityConductivityTypical job C360 / C3604Free-cutting brass≈3% lead100≈26% IACSFittings, connectors, valve bodies C3771Forging brass≈2% leadGood≈27% IACSForged plumbing, machined fittings C27450 / ecobrassLead-free brassSilicon (ecobrass)≈85≈25% IACSPotable-water and RoHS parts C110ETP copper99.90% Cu≈20≈100% IACSBusbars, contacts, heat sinks C101OFE copper99.99% Cu≈20≈101% IACSVacuum, RF, high-purity work C122DHP copper0.02% P≈20≈85% IACSBrazed assemblies, plumbing Bronze (C932)Bearing bronzeTin + lead≈70≈12% IACSBearings, wear surfaces, marine The pattern to remember: lead makes brass cut, purity makes copper conduct, and tin makes bronze wear. A C360 fitting machines in seconds where a C110 copper part of the same geometry fights the tool for minutes. If you do not need conductivity, brass is almost always the cheaper, faster, straighter part. Physical properties that shape the part Conductivity gets the attention, but three other numbers decide whether a design survives the assembly line and the field: density, thermal expansion, and strength in the temper you actually buy. Copper is the heaviest and, in its annealed state, the softest of the three. Brass is stronger as supplied and expands more with heat. Bronze sits between on weight and expansion but wins on wear. AlloyDensityThermal expansionMeltingTensile (annealed → hard) Copper C110≈8.9 g/cm³≈17 ppm/°C1085 °C≈220 MPa → ≈390 MPa cold-worked Brass C360≈8.5 g/cm³≈20 ppm/°C≈900–940 °C≈340–470 MPa by temper Bronze C932≈8.9 g/cm³≈18 ppm/°C≈850–1000 °C≈240–310 MPa The strength row matters most for pure copper. Annealed copper is soft, and the only way to strengthen it is cold work, so a structural copper part should call out a harder temper — H02 or H04 — or accept a larger section. Brass needs no such caution; it arrives at 340 MPa and up straight from the bar, which is why a threaded brass fitting survives handling that would dent an annealed copper one. Thermal expansion is the trap for press fits and plated assemblies: brass at about 20 ppm/°C grows roughly 15% more than steel, so a brass bushing pressed into a steel housing loosens as the pair heats. Copper at about 17 ppm/°C is gentler and closer to aluminum. Density is the quiet cost driver — copper at roughly 8.9 g/cm³ makes a solid heat sink heavy, and brass at about 8.5 g/cm³ trims a little weight only where conductivity is not the point. Lead-free compliance and end-market rules C3604's lead is a feature, not a flaw, until the part touches a person. EU RoHS caps lead in most electrical and electronic equipment, and drinking-water rules across the EU, China, and North America restrict lead in wetted components. In those markets C3604 is off the table and the job moves to a lead-free grade. C27450 is a lead-free yellow brass that machines cleanly for straightforward fittings. Ecobrass — a silicon-bearing lead-free brass — is the closest drop-in for C3604: it cuts at roughly 85 on the machinability scale, threads cleanly, and plates like standard brass at a modest price premium. The cost is a slightly longer cycle and a different chip, not capability. State the market and the regulation on the print, and we will grade and finish to match; the tolerance and finish outcome stay the same. Brass and copper grades that actually run Beyond the three headline families, the alloys we run most are leaded free-cutting brass C3604 and forging brass C3771, electrolytic tough-pitch copper C110, oxygen-free C101 for high-purity conductivity, and deoxidized C122 for brazed assemblies. C3604 carries about 3% lead and that is exactly why it cuts clean at speed; C101 and C110 have none, so they work-harden and drag under the tool. When to choose brass vs copper Electrical contact, busbar, heat sink → copper (C110). Nothing conducts better except silver. Connector, fitting, threaded part, bearing → brass (C360). It machines to a fine finish and resists corrosion. Wear surface under load → bronze. It handles sliding contact where brass would gall. Conductivity and thermal use-cases Copper is the reference conductor. C110 conducts at roughly 100% IACS and C101 at about 101% IACS, so the two are effectively interchangeable for any current-carrying part where oxygen content does not matter. Brass drops to around 26–28% IACS, and bearing bronze sits near 12% IACS — still conductive, but no longer a low-resistance path. The thermal numbers follow the same ranking: copper moves heat at about 390 W/m·K, brass at about 115 W/m·K, and bronze lower still. That gap is what decides the application. Choose copper when the part is the conductor: busbars, switch contacts, ground straps, motor terminals, resistance-welding electrodes, EDM electrodes, and heat sinks or cold plates that must pull heat out of a device. Choose brass when the part needs strength and machinability first and only incidental conductivity — a spring contact that must hold its shape, a threaded terminal post, a connector shell. Bronze earns its place where current is not the point but sliding wear is: a bushing that carries a rotating shaft, a marine fitting that must survive salt water. Tooling and coolant: why copper tears and brass sings Brass wants sharp, positively-raked carbide or HSS at high speed and light feed; it chips into a tidy curl and often runs dry. Copper is the opposite: too much speed makes it grab and tear, so we slow the spindle, feed harder, and flood coolant or use minimum-quantity lube to flush the gummy swarf and stop built-up edge. A fresh edge beats any trick in the shop. Machinability and tooling: chip control wins the job Machinability is not one number — it is the sum of cutting force, chip form, tool life, and surface finish. Brass scores well on all four, which is why C360 is the shop benchmark. Its lead content acts as a chip breaker: the material shears into short, brittle curls that clear the cut instead of wrapping the tool. Cutting forces stay low, so the part does not deflect, thin walls hold their size, and tool edges last. Copper fails on every axis at once. It is soft enough to smear instead of shear, it work-hardens the instant an edge rubs, and it welds to the tool as built-up edge, which then tears the surface. The countermeasures are specific. Run polished, high-positive-rake inserts or sharp HSS with a keen edge and a light hone; a dull edge rubs, and rubbing is what hardens copper. Use chip breakers and keep the cut deep enough to stay ahead of the hardened skin. Lower spindle speed and raise feed so the chip is thick enough to break. Flood coolant or minimum-quantity lubrication to carry the gummy swarf away. On brass, the same discipline in reverse: high speed, light feed, and often no coolant at all, because the chip already breaks and the heat leaves with it. Get chip control right and both metals become predictable; get it wrong and copper wraps the chuck in one pass. Finishing brass and copper Both metals tarnish without protection. The common finishes are plating (nickel, chrome, tin), clear coating to preserve the bright finish, and brushing or polishing for a cosmetic grain. See the full finish list. Plating and clear coats that stop tarnish Copper and brass both oxidize, so an unprotected part dulls within weeks. For function and salt-spray life we spec nickel or tin; for solderability, tin or silver. A thin clear lacquer or e-coat keeps the bright look without changing dimensions. Choose the finish from the environment, and confirm the spec before plating. Corrosion, tarnish, and how each finish answers it The three families corrode differently, so the finish has to match the failure mode. Copper forms a green patina in air — the carbonate layer actually protects the base metal and is harmless electrically for many applications, but it is ugly on a visible part and raises contact resistance if you let it grow. Brass resists atmospheric corrosion well but can dezincify in aggressive water, where the zinc leaches out and leaves a weak, porous copper sponge. Bronze, with its tin, is the most corrosion-resistant of the three and the standard for salt water and marine hardware. Galling is the other failure to design around. Brass and copper are both prone to cold-welding under sliding contact, so a threaded brass part can seize against a mating surface. Bronze is the fix when two surfaces must slide or thread against each other under load. Finish selection follows from these behaviors. Nickel plating gives corrosion resistance, hardness, and a solderable surface, and it is the default when a copper or brass part must survive salt spray. Tin plating is the RoHS-friendly choice for solderability and contact surfaces. Silver preserves maximum conductivity on contacts. Chrome is decorative. Passivation on copper alloys means a thin chemical conversion coating — a chromate or benzotriazole treatment — that locks in the bright surface and stops tarnish without adding measurable thickness. Clear lacquer or e-coat seals the cosmetic look for parts that will be seen. Confirm the spec and the salt-spray hours before plating, because each option changes conductivity, solderability, and dimension differently. DFM for brass and copper parts Design for manufacturability changes sharply between the two families because brass is stiff and free-cutting while copper is soft and sticky. Thin walls: brass holds unsupported walls down to roughly 0.5 mm and flat features without bowing; copper deflects under the tool, so keep copper walls thicker, add ribs, or accept a lighter cut and a longer cycle. Threads: brass cuts crisp threads, so small pitches and fine finishes come easily; copper threads tear and gall, so spec a coarser pitch, a sharper tool, and consider thread forming only on brass, not copper. Tolerances: both hit ±0.005 mm on standard mill and turn work, but copper is more temperature-sensitive, so a tight bore can move as the part warms during machining — hold the part in the machine until it is stable, and let grinding or wire EDM carry the ±0.002 mm bands. Three rules carry across both metals. Keep internal corners radiused so a sharp cutter can reach them; a zero-radius pocket means a second EDM op. Watch burrs — brass burrs lightly, copper throws a heavy rolled burr that must be deburred, so avoid sharp edge breaks in copper wherever the print allows. And remember that deep, narrow pockets in copper are the worst case for chip evacuation; add a relief or open the feature up rather than fight stringy swarf at the bottom of a blind hole. The buyer's call: cost, conductivity and lead-free rules Brass is the cheaper stock and the faster job, so it wins on fittings, valves and decorative hardware. Copper costs more and machines slower, but nothing else conducts. Watch the lead: C3604 is leaded and fine for most markets, yet EU RoHS and drinking-water rules push lead-free variants where the part touches people. Tell us the end market and we will grade and finish to match it. Cost and sourcing: copper moves with the market Copper is a globally traded metal, and its price swings on the London Metal Exchange with industrial demand, supply disruptions, and currency moves. That volatility flows straight into a copper part: the raw bar can shift meaningfully between quote and order, and a heavy copper component is mostly raw-material cost. Brass smooths that out. It is roughly a third to forty percent zinc, and zinc trades lower and steadier than copper, so brass stock is cheaper per kilogram and less jumpy. The machining cost stacks on top: a C360 part runs in a fraction of the time of the same part in C110, so the labor savings multiply the material savings. Two more sourcing levers matter. Standardize on a grade you can buy off the shelf — C3604 and C110 bar are stocked everywhere, while a niche bronze or an oxygen-free C101 may carry a longer lead and a minimum order. And price in scrap: both copper and brass have strong recycling value, so a high-volume part returns a real credit on the chips, which is worth asking your shop to account for. When the end market allows lead, C3604 is the cheapest fully-machined path; when it does not, ecobrass or C27450 buys compliance at a small premium. Workholding and sequence notes for soft metals Soft, gummy metal is as hard to hold as it is to cut. Copper and brass mark under hard jaws, so we run soft jaws, pie jaws, or collets on turned parts and keep clamping pressure low enough not to imprint the finish. Copper's high thermal expansion means a part that mics in spec while warm can drift a few microns as it cools, so tight bores get finished after the part has stabilized rather than on the first pass. Sequence matters the same way: roughing copper hardens the skin, so we rough and finish in separate passes with a fresh edge on the finish cut, then deburr the rolled edge by hand or with a light brush. Brass needs none of this care — it holds dimension, stays put in the chuck, and deburrs in a single pass. How the shop runs copper and brass Our floor runs 80+ CNC machines — Brother SPEEDIO and Mazak VCE, QT-COMPACT and INTEGREX turning centers — across 3-, 4- and 5-axis work, holding ±0.005 mm. Every incoming bar is checked with a handheld XRF analyzer before cutting, and final sizes go to a Hexagon CMM. MOQ is one piece; lead times run 3 / 7 / 30 days by complexity. 80+CNC machines on the floor ±0.005 mmStandard turning & milling tolerance XRFAlloy verified on every incoming bar Cpk ≥1.67On critical features Nex-G capability for brass and copper Nex-G machines copper and brass in-house at a 6,800 m² plant in Dongguan Hengli, running 100+ staff on a floor we have operated since 2006 (Zhuohang). The machine list is purpose-built for these two metals: 80+ CNC machines spanning Mazak and Brother machining centers, TSUGAMI Swiss-type lathes for small turned parts, and Sodick wire EDM for the features a cutter cannot reach. Standard milling and turning hold ±0.005 mm; grinding and wire EDM hold ±0.002 mm. On critical features we run statistical process control to Cpk ≥1.67, and our standard process target is Cpk ≥1.33. Material integrity is enforced at the door. Every incoming bar is verified with a handheld XRF analyzer before it reaches a machine, so a mislabeled alloy is caught as raw stock, not as scrap at final inspection. Final dimensions go to a Hexagon CMM, and the plant holds ISO 9001, IATF 16949 (design excluded under clause 8.3), and ISO 14001, with our URS audit current through 2027. MOQ is one piece, and lead times run 3 / 7 / 30 days by complexity — so a single prototype and a volume run come off the same tooling path. Frequently asked questions Can you hold tight tolerances on copper?Yes. Standard milling and turning hold ±0.005 mm; tighter bands come from 4-/5-axis setups and grinding (±0.002 mm). Copper's softness demands sharp tooling, but the numbers match brass. Which brass grade should I specify?For most parts C3604 (leaded free-cutting) is the default — fast and easy to plate. Use C3771 when forging, and a lead-free variant where RoHS or potable-water rules apply. Do you verify the alloy?Every incoming bar is screened with an XRF analyzer before machining, and we keep mill certificates on file. That catches a near-miss alloy before it becomes scrap. What is the difference between C110 and C101 copper?Both conduct at about 100% IACS. C110 is electrolytic tough-pitch copper with a trace of oxygen; C101 is oxygen-free, so it is specified for vacuum, hydrogen, and high-purity applications. For a busbar or contact, C110 is cheaper and equivalent. How do I keep a brass part from tarnishing?Nickel or tin plating for function and salt-spray life, silver for conductivity, or a clear lacquer, e-coat, or chemical passivation to keep the bright look. The right choice depends on the environment and whether the surface must solder. Do you offer lead-free brass?Yes. We run lead-free grades such as C27450 and ecobrass where RoHS, potable-water, or customer policy requires it. They machine slightly slower than C3604 but hit the same tolerances. What is your minimum order quantity?One piece. Prototypes and full production run through the same machines and the same quality checks, so a single part costs no more than it should to make. How long does a copper or brass order take?Lead times run 3 / 7 / 30 days by complexity. Simple turned parts ship in about three days; milled and multi-setup parts land in the middle; tight-tolerance, plated, or 5-axis work takes the full cycle. If the part carries current or sheds heat, brass and copper earn their place where aluminum and steel cannot — the material choice is a thermal and electrical decision as much as a mechanical one. One warning for high-volume buyers: copper and its alloys price off the commodity market, so a quote can move between order and reorder. We lock pricing at the quote and flag the metal-market risk up front rather than surprising you later. Machining copper or brass parts?Send the drawing — we will confirm the grade and the finish that keeps it from tarnishing.Request a quote Related articlesAluminum Machining Guide: Grades and FinishesStainless Steel Machining GuideCNC Milling vs Turning