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Copper Machining Processes
How copper is milled, turned, drilled, tapped and EDM cut, what its softness and conductivity do to each operation, and what tolerance, finish and plating are realistic. Written so the drawing you send matches how the part will actually be made.
Three properties set every process decision
Copper is not hard to cut because it is strong. It is hard to cut because it gives way. Pure C11000 is rated about 20 for machinability against free-cutting brass at 100, in our copper grade reference.
- Ductile and gummyThe chip stretches into a continuous ribbon instead of breaking. It wraps the tool, nests on a lathe and leaves a rolled burr on every exit edge.
- ConductiveAbout 390 W/m·K for pure copper. Heat goes into the part and the holder instead of leaving with the chip, and copper grows about 17 µm per metre per degree while it is cut.
- AdhesiveCopper welds to the cutting edge under pressure and forms a built-up edge. The finish turns streaky and the burrs get heavier with every pass.
Process against part feature
Which operation makes which copper feature, the tolerance it typically holds, and the copper specific point the shop has to manage. Typical values for capable shops, never a promise. The shop quoting your geometry confirms what it can hold.
| Process | Typical copper parts | Typical tolerance | What copper changes |
|---|---|---|---|
| CNC milling, 3 axis | Busbars, heat sink bases, cold plates, flanges, electrode blanks | ±0.025 mm on machined features, normal care | Long stringy chips in pockets. Climb milling and high pressure coolant keep them out of the cut. |
| CNC milling, 4 and 5 axis | RF and waveguide bodies, manifolds, EDM electrodes with undercuts | ±0.025 mm, ±0.01 mm on one or two features with a finishing pass | Fewer setups means fewer clamping marks on a soft part. Worth it on complex geometry. |
| CNC turning and Swiss turning | Pins, contacts, terminals, welding tips, vacuum feedthrough parts | ±0.025 mm on diameters, tighter on free-machining grades | Pure copper builds a chip nest on a lathe. Free-machining grades are the usual choice for turned volume. |
| Drilling and reaming | Bolt holes, coolant passages, cross holes | Reamed holes typically H7 | Drills grab in copper. Polished flutes, peck cycles and modified point geometry prevent breakage. |
| Tapping and thread forming | Terminal threads, mounting threads | 6H thread class typical | Cut taps tear pure copper threads. Forming taps suit ductile copper and leave no chip. |
| Wire EDM | Thin slots, sharp inside corners, comb and fin profiles | ±0.005 to ±0.01 mm typical | No cutting force and no burr. Slower than in steel because copper conducts the spark heat away. |
| Sinker EDM (copper as electrode) | Copper electrodes that burn cavities into tool steel | Set by the electrode and the overburn | The electrode is itself a precision copper part. C10100 is the usual grade. |
| Diamond turning and lapping | Mirror faces, sealing faces, thermal contact faces | Flatness and finish driven, not size | Separate specialist operations. Specify only on the faces that need them. |
Milling and turning: tooling that works
None of this needs a special machine. It needs the right tool, the right feed and enough coolant. These are the decisions a shop that cuts copper regularly has already made. Operation by operation detail is in how copper behaves in milling, turning, drilling and tapping.
- Tool materialSharp uncoated or polished carbide for most work. PCD for long finishing runs. Coatings developed for steel often promote built-up edge on copper.
- GeometryHigh positive rake, a keen edge and a polished flute. A honed edge that suits steel rubs copper instead of shearing it.
- Speed and feedHigh surface speed with a real feed per tooth. Backing off the feed to be gentle causes rubbing, smearing and heavier burrs.
- CoolantFlood or high pressure emulsion to carry heat away and flush the long chip. Avoid active sulphur cutting oils, which stain copper.
- ToolpathClimb milling, single direction finishing passes and programmed chamfers so the tool enters the material rather than dragging across it.
- WorkholdingSoft jaws, sleeves, vacuum or glue fixtures and clamping that spreads the load. Copper marks and springs where steel would not.
- TemperHalf hard H02 or hard H04 cuts cleaner than annealed O60. Machine in the harder temper when the part is not formed afterwards.
Drilling, tapping and threads
Drills grab in pure copper. The flute loads with a soft chip, torque spikes and small drills break. Shops use polished flute carbide drills, peck cycles and plenty of coolant through the tool where the machine allows it.
Tapping is where pure copper parts get scrapped, because it happens at the end of the cycle. A cut tap loads up and tears the thread flanks. Forming taps suit ductile copper: they displace the material instead of cutting it, leave no chip and give a stronger thread. They need a larger pre-drill and raise a small lip at the hole entry, so a countersink on the drawing helps.
EDM: cutting copper and cutting with copper
Wire EDM of copper parts. Electrical discharge machining removes metal by spark erosion, so there is no cutting force and no mechanical burr. That makes it the right process for thin slots, comb and fin profiles, and sharp inside corners on copper. Copper's conductivity carries spark heat away from the gap, so the cut is typically slower than in tool steel and priced by length times thickness.
Copper as the electrode. In sinker EDM a machined copper electrode burns its shape into hardened steel. Oxygen free C10100 is the usual grade because its uniform structure gives consistent burns. The electrode is a precision copper part with every milling problem above, which is why graphite competes with it. The trade off is in copper against graphite EDM electrodes and on the EDM electrode page.
Burr control
On thin copper parts the deburring can take longer than the cutting. Control starts on the drawing, not at the bench. More in deburring copper parts.
- Design it outChamfers and radii cut on the machine as real features. The cheapest burr is the one that never forms.
- Choose the gradeFree-machining C14500 and C14700 burr far less than C11000 and C10100 because the chip breaks.
- Keep the tool sharpA dull or built-up edge pushes material instead of cutting it. Tool life on copper is managed for burr, not for wear.
- Plan cross holesThe burr forms inside, where hands cannot reach. Drill order, back chamfer tools or a secondary process deal with it, and each costs.
- Finish by processHand deburring, vibratory or tumble finishing for small parts, electropolishing for vacuum and clean parts. Each changes the edge differently.
Tolerances and surface finish
Heat and clamping move a soft metal, so copper tolerances are set by temperature and fixturing as much as by the machine. Free-machining grades hold size more easily because they cut with less force and less heat. Detail in copper machining tolerances.
| Feature | Pure copper C10100, C11000 | Free-machining C14500, C14700 |
|---|---|---|
| General dimensions | ±0.1 mm comfortably | ±0.05 mm comfortably |
| Machined features, normal care | ±0.025 mm | ±0.015 mm |
| Single critical feature, finishing pass | ±0.01 mm achievable | ±0.01 mm achievable |
| Tighter than ±0.01 mm | Specialist, temperature controlled | Specialist, temperature controlled |
| Surface condition | Typical roughness |
|---|---|
| As machined, general milling and turning | Ra 1.6 µm |
| Dedicated finishing pass, sharp tooling | Ra 0.8 µm |
| Fine finishing, controlled setup | Ra 0.4 µm |
| Polished, lapped or diamond turned | Below Ra 0.2 µm, a separate operation |
Typical expectations for machined copper, not guarantees. Finish is per surface: Ra 0.8 on the whole part when one face needs it is one of the most common ways a copper quote gets inflated. See copper surface finish.
Plating, passivation and anti-tarnish
Bare copper oxidises in air and turns brown. That is not a defect, and no shop can stop it permanently on a bare surface. Decide the finish before the first shipment. "Passivate" in the stainless sense does not apply to copper, so write what the surface has to do.
| Finish | Common specification | Typical use | Note |
|---|---|---|---|
| Tin | ASTM B545 | Busbars and low voltage joints, indoor service | Soft, deforms into the mating face. Kept to moderate service temperatures. |
| Nickel | ASTM B689 electroplated, ASTM B733 electroless | Hot joints, battery connections, barrier layer under silver or gold | Hard and wear resistant. Higher contact resistance on its own. |
| Silver over nickel | ASTM B700 | High current switching and disconnect contacts | Lowest contact resistance. Its tarnish still conducts. |
| Gold over nickel | ASTM B488 | Low level signal contacts, RF parts | Thin and expensive. Nickel stops copper diffusing into the gold. |
| Anti-tarnish treatment or clear lacquer | Name the product or the requirement | Bare copper that must arrive bright | Benzotriazole type inhibitors are common. Confirm it is compatible with later brazing or soldering. |
| Electropolishing | State material removal allowed | Vacuum parts, clean surfaces, micro burr removal | Removes material, so the tolerance has to allow for it. |
Plating copies the surface beneath it, so contact pads are finished before they are plated. State each layer thickness, the plated and masked areas, and whether dimensions apply before or after plating. Busbar specifics are in tin, nickel or silver plating on a busbar.
Design for manufacture checklist
Each line removes a decision a shop would otherwise make for you, and makes the quotes you receive comparable. The RFQ builder writes the material lines for you.
- Write the UNS number and temper, and a conductivity minimum if you are open to a free-machining grade
- Put tight tolerances on the two or three features that need them, not in the title block
- Draw edge breaks as dimensions, for example 0.3 to 0.5 mm, rather than a general deburr note
- Keep walls above about 1 mm where you can. Thin copper walls deflect under the cutter and the clamp
- Give inside corners a radius a standard end mill can cut, or mark the ones that need wire EDM
- Say whether dimensions apply before or after plating, and mask what must stay bare
- Name the faces that carry current, heat or a seal, and put Ra only on those
- State how the part must arrive: bright, lacquered, plated, or bare in a sealed bag
Questions about copper processes
Something else? Email . Terms are defined in the glossary.
Why is pure copper harder to machine than brass when it is softer?
Because softness is the problem. Pure copper is ductile, so the chip stretches instead of breaking, and it is adhesive, so it welds to the cutting edge. Brass C36000 breaks its chip and is the machinability reference at 100. C11000 is rated about 20 on the same scale.
Which copper grade should I choose if the part is mostly machining?
If the conductivity requirement has some headroom, a free-machining grade. C14500 tellurium copper and C14700 sulphur copper are rated about 85 and keep well above 90 percent IACS. They break their chip, burr less and tap cleanly, which shortens the cycle and the deburring on every piece.
Can copper be wire EDM cut?
Yes. Copper conducts, so wire EDM works on it, but its high thermal conductivity carries spark energy away and the cut is typically slower than in tool steel. Wire EDM makes sense for thin slots, sharp inside corners and burr free profiles that a cutter cannot reach.
Is "passivate" a valid finish callout for copper?
Not in the stainless steel sense. Passivation standards such as ASTM A967 are written for stainless steel. For copper, write what you want the surface to do: an anti-tarnish treatment, a clear lacquer, a plating stack with thickness, or bare copper in a sealed bag with a corrosion inhibitor.
Know the process. Now get it quoted.
Send the drawing and we match it with up to 3 shops that cut copper regularly and run the processes your part needs. We are a matching service, not a machine shop. Reply within 24 business hours.