What Is Copper? C101 vs C110, Properties, CNC Machining and Conductivity

Copper is a high conductivity nonferrous metal used for busbars, electrical contacts, terminals, heat spreaders, electrodes, and other parts that must carry current or move heat. For machined parts, the useful question is not only what is copper, but which copper grade and condition should be specified. C10100, C10200, and C11000 differ in oxygen condition, chemistry limits, electrical acceptance, temper, product form, availability, and cost. ASTM B187/B187M-26 covers selected copper bar, copper rod, copper bus bar, and shapes, but it does not define CNC cutting parameters, final copper surface finish, or component thermal resistance. This guide connects copper properties to material selection, copper CNC machining, inspection, and purchasing decisions.

1. What Is Copper?

Copper metal has the chemical symbol Cu and atomic number 29. In manufacturing, the most important copper characteristics are high electrical and thermal conductivity, ductility, corrosion resistance, and formability. Those properties of copper explain its use in a copper conductor, copper electrode, copper terminal, copper connector, copper contacts, and copper electrical contacts.

The main copper grades and types of copper in this article are C10100 copper, C10200 copper, and C11000 copper. C10100 is OFE copper, or oxygen free electronic copper. C10200 is oxygen free copper. C11000 is ETP copper, or electrolytic tough pitch copper. Commercial phrases such as OFHC copper are not precise enough for a purchase order by themselves, so specify the UNS grade and applicable standard.

When copper material is being compared with aluminum, brass, stainless steel, or titanium, the CNC machining materials guide provides broader material selection context.

2. Copper Properties That Matter in Manufacturing

Copper physical properties, copper mechanical properties, and copper material properties are useful only when the source and condition are clear. The Copper Development Association lists C10100 and C11000 at about 8.9 g/cm³ density, about 101% IACS electrical conductivity at 68°F, about 391 W/(m·K) thermal conductivity at 68°F, and a machinability rating of 20. CDA labels these values as general engineering approximations, not purchasing specifications.

UNS	Type	ASTM chemistry / oxygen basis	Density g/cm³	CDA %IACS	CDA k W/(m·K)	Decision use
C10100	OFE	Cu excl. Ag ≥99.994%; O ≤5 ppm	8.94	101	~391	Use when low oxygen and residual element control are functional requirements.
C10200	OF	Cu ≥99.95%; O ≤0.0010%	8.94	101	~391	Use when oxygen free copper is required but OFE residual limits are unnecessary.
C11000	ETP	Cu incl. Ag ≥99.90%	8.91	101	~391	Common choice for busbars, terminals, connectors, and general conductor service.

The copper melting point is about 1,981°F liquidus for C10100 and C11000, while C11000 has a 1,949°F solidus in CDA data. The melting point of copper matters for thermal processes, not CNC feeds and speeds. Copper density, or the density of copper, matters for mass and inertia. Copper hardness, hardness of copper, copper yield strength, tensile strength of copper, and copper tensile strength depend strongly on temper, product form, and size. ASTM B187/B187M-26 therefore ties mechanical requirements to conditions such as O60, H02, and H04 instead of assigning one strength number to an alloy name.

3. C101 vs C110, with C102 in Between

C101 vs C110 is a real procurement decision when oxygen condition, joining route, conductivity, or stock availability matters. C101 copper is OFE copper with Cu excluding Ag at 99.994% minimum by difference and oxygen at 5 ppm maximum under ASTM B187/B187M-26. C102 copper is OF copper with Cu at 99.95% minimum and oxygen at 0.0010% maximum. C110 copper is ETP copper with Cu including Ag at 99.90% minimum. C110 vs C101 should not be decided by purity alone.

Decision condition	Grade direction	Engineering meaning
Low oxygen or oxygen-sensitive joining is explicitly required	C10100 / C10200	Select the exact UNS grade from the oxygen and residual-element requirement.
Standard high-conductivity busbar or bolted connector service	C11000	Use when ETP copper meets electrical and joining requirements and no OF condition is needed.
Oxygen-free condition needed, but C101 residual limits are not specified	C10200	Avoid automatically buying the tighter C10100 specification.
Grade choice driven only by “highest purity”	Do not decide this way	Check function, MTR requirements, stock form, availability, joining route, and cost.

In the EPOC production case, the selected stock was C11000 H02 because the air service bolted connector needed approximately 100% IACS class conductivity and readily available busbar stock, while the quoted C10100 OFE alternative had no measured functional benefit for that application.

4. Copper Conductivity, Resistivity, and Thermal Design

Copper conductivity, or the electrical conductivity of copper, is usually the reason a current carrying part stays copper. ASTM B187/B187M-26 controls electrical resistivity for applicable conductor products at 20°C. Its Appendix X1 relates 100% IACS conductivity to 0.15328 Ω·g/m² at 20°C. This is more precise than writing only “high copper electrical conductivity” on a drawing.

ASTM condition	C10100	C10200 / C11000 group	Decision meaning
O60 soft anneal rod/bar	0.15176 Ω·g/m² max	0.15328 Ω·g/m² max	Use the table that matches alloy group and condition.
Selected H04 rod small-size group	0.15585 Ω·g/m² max	0.15737 Ω·g/m² max	Do not copy O60 electrical limits into a hard temper.
Selected H04 larger rod / H02 bar conditions	0.15425 Ω·g/m² max	0.15577 Ω·g/m² max	Product form, size, and temper remain part of acceptance.

For engineers asking is copper conductive, is copper a good conductor, what is the electrical conductivity of copper, or why is copper conductive, the useful answer is that bulk conductivity is only one part of the finished electrical joint. Contact area, flatness, plating, contamination, and clamp load still affect connection resistance. The same distinction applies to copper resistivity and resistivity of copper values taken from a handbook.

The thermal conductivity of copper, sometimes searched as copper heat conductivity or what is the thermal conductivity of copper, is also high. CDA lists about 391 W/(m·K) for C10100 and C11000 near room temperature. Copper thermal conductivity is an input to a thermal model, not a finished copper heat sink rating. A copper heatsink, copper heat spreader, copper cooling plate, or other copper heat transfer component also depends on section thickness, path length, interfaces, airflow or coolant, and heat rejection area.

This is also why the question why is copper used for heat sinks has no one-number answer. For designs comparing copper with aluminum, the aluminum heat sink material selection guide covers the conductivity, mass, geometry, and process tradeoff.

5. Copper CNC Machining and Machinability

The machinability of copper is driven less by hardness than by ductility and chip behavior, which is why copper machining needs a process-specific plan. Pure copper can produce long chips, built up material, smear, and copper burrs. Copper machinability also changes with grade, temper, tool geometry, coolant, feature shape, and workholding. That is why how to machine copper cannot be answered with one universal speed and feed.

In the EPOC C11000 H02 busbar case, copper milling used a polished three flute carbide rougher, directed flood coolant, and air blast for the long slot. The pilot run showed adhered copper on the cutter, slot smear, burrs of 0.07 to 0.09 mm, and flatness up to 0.118 mm against a 0.08 mm limit. The correction changed flute geometry, chip evacuation, clamp position, final face stock from 0.35 to 0.20 mm, and the deburring sequence. The next 240 parts held flatness to 0.061 mm maximum, mean contact pad roughness to Ra 0.79 µm, and slot exit burr to 0.025 mm maximum. Rework fell from 11.7% to 1.3%.

Metric	Pilot	Corrected 240 pcs	Decision meaning
Pilot flatness	0.118 mm worst	0.061 mm max	Changed support, clamp location, and final stock removal.
Contact pad roughness	Ra 1.42 µm on affected parts	Ra 0.79 µm mean	Improved chip evacuation and finish strategy.
Slot exit burr	0.07 to 0.09 mm	≤0.025 mm	Added in-machine chamfer plus controlled brushing.
Rework	11.7%	1.3%	Process correction addressed multiple causes, not one feed-rate setting.

Copper turning, turning copper, milling copper, copper cutting, CNC copper, and CNC machining copper all need the same discipline: validate the process on the actual grade and geometry. ASTM B187 does not specify feeds, speeds, cutter material, flute count, coolant strategy, or a machinability ranking.

The CNC DFM guidelines for copper features are useful before release because thin webs, deep holes, datum strategy, and clamp support often control the result as much as the tool.

When a drawing is ready for process review, the copper CNC machining capabilities page shows the available CNC route and DFM context.

6. Copper Surface Finish and Inspection

Copper surface finish is functional on contact pads, mating faces, plated areas, and sealing surfaces, whether the final state is as-machined, plated, or polished copper. In the production case, mating faces required Ra 1.6 µm maximum and bolted contact pads Ra 0.8 µm maximum. Released parts measured Ra 0.72 to 0.86 µm on contact pads using a Mitutoyo SJ-210 with a 0.8 mm cutoff and a 4.0 mm evaluation length. Flatness and hole position were verified separately on a CMM.

A low Ra value does not replace copper deburring or visual acceptance. A part can pass copper surface roughness and still carry a raised burr, smear, dent, or embedded abrasive. Copper polishing, copper finishing, and any copper surface treatment also need protection for functional electrical surfaces.

The surface roughness Ra chart for CNC parts helps separate Ra from process capability, while functional faces should still be tied to drawing and inspection requirements.

When plating, polishing, or another post-machining operation is required, the copper surface finishing options should be reviewed with masking, coating build, and electrical contact requirements.

7. Copper Stock, Temper, and ASTM B187 Procurement

ASTM B187 copper procurement should start with the complete copper specification, not the grade name alone; a generic copper standard reference is not enough. A useful copper material specification identifies the standard, UNS grade, copper temper, product form, dimensions, electrical requirement when applicable, MTR, and traceability.

The standard covers defined forms such as copper bar, copper rod, copper busbar, copper bus bar, copper flat bar, copper round bar, and shapes within its scope. It is not a general specification for copper sheet, copper plate, copper billet, tube, castings, or finished machined copper parts.

RFQ field	Example	Why it matters
Standard	ASTM B187/B187M-26	Confirms the applicable raw-product specification.
UNS grade	C11000	Separates ETP from C10100 OFE, C10200 OF, and other coppers.
Temper	H02	Defines material condition used for mechanical and electrical requirements.
Product form	Rectangular C110 copper bar	Links the order to the correct product-form requirements.
Dimensions	8.0 × 45.0 mm stock in the case	Defines incoming geometry and machining allowance.
Electrical requirement	Applicable resistivity or conductivity at 20°C	Make electrical acceptance measurable when it is a CTQ.
Documentation	MTR plus heat/lot traceability	Keeps the certificate tied to the production material.

For example, a C110 copper specification can read: ASTM B187/B187M-26, UNS C11000, H02, rectangular C110 copper bar, dimensions per purchase order, electrical requirement per the applicable table, MTR, and heat traceability required. A C101 copper specification or C101 copper bar order should be written with the same level of detail.

Incoming stock requirements and finished part requirements are different. The engineering drawing tolerances for CNC parts should control dimensions, GD&T, Ra, burrs, coating condition, and inspection after machining.

8. What the Production Case Proved

The anonymized production part was a C11000 H02 power distribution copper connector, one of the machined copper components used in industrial automation and power electronics. The 1,200 part lot required 145.0 ±0.10 mm length, 110.0 ±0.05 mm terminal hole pitch, 0.08 mm maximum mating face flatness, and position Ø0.10 mm at MMC. Conductivity measured 100.1 to 100.4% IACS at 20°C on cleaned flat areas before plating.

The released process produced 98.5% first pass yield and 99.8% accepted yield after permitted rework. Cycle time fell from 8.6 to 7.4 minutes per part after the chip evacuation and fixturing correction. These are internal project results, not general C110 capability claims.

The purchase decision also stayed narrow: for that order, C11000 H02 was quoted 13.8% below the same size C10100 OFE alternative. That percentage is not a market rule. It only shows why a higher purity grade should not be purchased when its oxygen condition provides no measured benefit to the assembly.

9. Copper vs Brass, Bronze, and Aluminum

Copper vs brass, copper vs bronze, and copper vs aluminum are different decisions. C36000 free cutting brass has a CDA machinability rating of 100 but only 26% IACS electrical conductivity, so it suits threaded and screw machine parts better than high current busbars. C93200 bearing bronze has a CDA machinability rating of 70 and about 12% IACS, which fits bearing and wear applications rather than conductor duty. Aluminum is far lighter, with Aluminum Association product data showing about 2.66 to 2.84 g/cm³ density and typical thermal conductivity of 113 to 234 W/(m·K) across covered products.

Material direction	Use it when	Boundary / verified reference
C101/C102/C110 copper	High electrical or thermal conductivity in compact volume	Machinability is relatively low for unalloyed copper; cost and oxygen condition matter.
C36000 free-cutting brass	Threads, fittings, screw-machine parts, machining productivity	CDA: machinability 100, electrical conductivity 26% IACS.
C93200 bearing bronze	Bushings, bearings, sliding wear	CDA: machinability 70, electrical conductivity about 12% IACS.
Aluminum products	Weight-sensitive conductors or heat rejection structures	AA product data: density 2.66 to 2.84 g/cm³; thermal conductivity 113 to 234 W/(m·K), depending on alloy/product.

Use high conductivity copper when compact electrical or thermal performance dominates. Use brass when machining productivity and threaded geometry dominate. Use a suitable bronze when sliding wear dominates. Evaluate aluminum when mass, extrusion, or large heat rejection area matters more than copper’s higher conductivity.

10. Engineering FAQ

10.1 C101 or C110 copper, which should I specify?

Choose the grade from the oxygen, joining, electrical, and procurement requirement. C101 is justified when OFE chemistry is functional. C110 is often the practical choice for standard conductive busbar and connector work.

10.2 Is copper machinable, and can copper be CNC machined without gummy chips?

Yes, but the process needs sharp tooling, chip evacuation, suitable coolant, and workholding validated on the actual grade and geometry. Do not copy generic feeds and speeds into production.

10.3 How do I keep a deep hole in copper from walking?

Treat alignment, drill length, chip evacuation, and intermediate sizing as process variables. The method depends on diameter, depth, machine, and tolerance, so a single universal drill cycle is not defensible.

10.4 How can I improve the finish on machined copper?

Control cutting edge condition, smear, burrs, and the measurement method before adding a polishing process. Polishing can improve appearance, but it should not be used to hide an unstable cutting process on a functional contact surface.

10.5 What is oxygen free copper?

Oxygen free copper is a family term, not one procurement grade. C10100 is OFE and C10200 is OF under ASTM B187/B187M-26, so the UNS number should appear on the RFQ when the oxygen condition matters.