Copper for sheet metal parts: grade, thickness, formability

Copper for sheet metal parts is usually specified for one property: resistance. A busbar, a contact plate, a gasket or a shielding can is bought because it conducts, and every other decision — grade, temper, thickness, edge finish — exists to protect that conductivity through forming, joining and service.

The grade choice is narrower than it looks. Two grades cover almost everything a fabricator sees, and the difference between them is not conductivity but oxygen: 0.02–0.04% in C11000 ETP, below 0.001% in C10200. That difference decides whether a brazed joint cracks in service, and it is invisible on the drawing unless somebody writes it down.

Which copper grade does the part need?

C11000 ETP for busbar, gaskets and general sheet work; C10200 oxygen-free or C12200 phosphorised deoxidised copper when the part is brazed, welded or annealed in a reducing atmosphere.

The mechanism is hydrogen embrittlement. At brazing temperature, hydrogen from the flame or the furnace atmosphere diffuses into the copper and reacts with the oxygen held in the grain boundaries to form steam. The steam cannot escape, the boundary opens, and the joint fails along a plane that looks like a brittle fracture with no deformation. The part passes a visual inspection and breaks under a torque wrench.

C12200 takes a different route to the same safety. It is deoxidised with phosphorus, which removes the oxygen, and it brazes cleanly; the cost is conductivity, which falls to about 85% IACS because the phosphorus that remains in solid solution scatters electrons. For a busbar sized on loss, that is 16% less current at the same section — a real derating, not a rounding error.

The third grade that turns up on drawings is C14500, tellurium copper. It machines like brass, holds 90–95% IACS, and belongs in turned contacts rather than sheet. It is not a substitute for C11000 in a formed part.

Thickness availability is the other constraint on the drawing. Sheet copper is normally stocked from 0.5 mm to 3 mm in a forming temper; above that the material is plate, supplied mill finish and flat, and it forms badly. A 4 mm copper cover that only has to close or shield a box is a plate part with a plate part's flatness and handling, while the same job is often done in 1.5 mm sheet with a formed edge. Check the section against the duty before the thickness is fixed.

Copper for sheet metal parts: grades, conductivity and thickness

The table covers the grades a fabricator will actually be asked to quote. The last column is the route at Ganesh, which is cutting and forming rather than rod machining or heat treatment.

The row that gets skipped is C12200. A designer who needs a brazed water-cooled plate and specifies it for corrosion resistance, then sizes the conductor from a C11000 table, ends up with a part that runs 15% hotter than the calculation. Either derate the current or upsize the section; the drawing should say which. The hole and slot rules that go with a laser-cut copper blank are covered in design rules for laser-cut copper sheet.

Thickness, mass and the bend allowance

Copper at 8.94 g/cm³ is 14% denser than steel and 6.6% denser than brass, so mass per square metre is a useful first check. At 1 mm it is 8.94 kg/m², at 2 mm 17.9 kg/m² and at 3 mm 26.8 kg/m². A 1,000 × 500 mm busbar cover in 2 mm copper weighs 8.9 kg, which is a two-person lift and a packing decision before it is an engineering one.

The bend allowance follows the same geometry as any metal, with copper's own K-factor. For annealed C11000 the neutral axis sits at about K = 0.44t, so:

• BA = θ × π/180 × (R + K·t)
• BD = 2 × (R + t) × tan(θ/2) − BA

For a 1.5 mm C11000 O60 part with a 1.5 mm inside radius and a 90° bend: BA = 1.571 × (1.5 + 0.66) = 3.39 mm. The setback is 3.00 mm, so the bend deduction is 6.00 − 3.39 = 2.61 mm. Two outside dimensions of 80 mm therefore need a flat blank of 157.4 mm.

For half-hard C11000 use K = 0.48 and expect 3–5° of springback instead of 1–2°. That is still less than steel at the same yield, because the modulus is lower at 117 GPa, but it is enough to make a three-bend part drift.

Copper also galles. The soft surface picks up on a hardened die shoulder and leaves a smear that grows with every stroke, so tooling for copper is polished, sometimes chrome-plated, and the V opening is wider than a steel programme would use — around 8t rather than 6t. A shop that runs copper on the steel tooling will produce good parts for an hour and marked parts by the afternoon.

Why copper does not cut or weld like steel

At 1,070 nm, polished copper reflects about 95% of a fibre laser beam. Mild steel absorbs 60–70% at the same wavelength, which is why a 12 kW laser that cuts 20 mm steel will not cut 20 mm copper at the same speed: the process depends on getting enough energy into the surface to start absorption, and on keeping the melt flowing once it starts.

In practice the cut runs with nitrogen assist, high power density and a small nozzle standoff, and the edge comes out clean on thin sheet with a small heat-affected zone. Thicker plate cuts, but slowly, and the economics move from metres per minute to seconds per part, which is why the practical limits are set out in laser cutting copper and brass. That is also why laser-cut copper is quoted per part rather than against a cutting chart.

Welding is harder still. Copper conducts heat at about 390 W/m·K, roughly eight times steel, so the heat put into a joint leaves through the parent metal before the pool forms. TIG welding works on thin sheet with DCEN and a deoxidised filler, and thicker sections need preheat in the 200–500 °C range, which is a controlled operation rather than a bench one. For busbar, brazing or a bolted lap joint is the more reliable production answer, and the joint design should be chosen before the material is ordered.

What a copper busbar drawing has to carry

A busbar is a copper part that is designed by an electrical engineer and manufactured as sheet metal, and the two drawings rarely agree. Four fields close the gap.

Current density. Natural-convection busbar in air is usually sized between 1.5 and 2 A/mm² of section, which is why the 50 × 10 mm bar at 1,000 A sits at the top of the range. The temperature rise that goes with it — typically 30–50 K above ambient — is the switchgear builder's calculation, not the fabricator's, but the bar has to be wide enough to make it possible.

Bend relief. A bend across a busbar needs a relief slot or a change of section at each end, or the corners tear when the bar is formed. The relief is 1–1.5 mm wide and at least as deep as the bend radius, and it is cheaper to put on the drawing than to cut at the bench.

Hole edge distance. A bolt hole needs at least 2× its diameter of material to the nearest edge, and more on the current path. An M10 hole 12 mm from a cut edge is a stress riser and a hot spot, because the current crowds into what is left.

Thermal movement. Copper expands at about 17 µm/m·K, so a 1 m bar rises 0.85 mm over a 50 K temperature swing. A bar clamped at both ends with no slotted hole will bow or load the insulator; one end of the run usually needs a slotted fixing.

A bolted lap joint also needs a specified torque — an M10 grade 8.8 bolt at 40–45 N·m dry, with a washer that keeps the clamp load through thermal cycling. Under-torqued joints heat up, and a hot joint oxidises, and an oxidised joint runs hotter still.

What Ganesh runs for copper parts

Copper arrives as sheet or plate, is nested and cut on the 12 kW fibre laser with a 40 ft × 16 ft bed, formed on the 200 T, 5-axis CNC press brake with a 4,450 mm bed and a Delem control, and where the volume justifies it, blanked or drawn in tooling built in our own tool room. The custom part range is 0.8–40 mm, and the laser handles copper, brass, stainless, aluminium and CRCA on the same floor.

The forming side has its own constraints. A 4,450 mm bed lets a full-length busbar cover or a long copper shield form in one piece, which removes a joint and a rivet line. The 200 T capacity is more than a 3 mm copper part needs, so the limit on copper is not tonnage; it is flatness and handling, because a soft 2 mm copper panel marks against any unpolished tooling.

We have fabricated copper and brass components alongside steel fabrications for customers including Mahindra and Godrej & Boyce since 1992, from five units in Mumbai (Vasai) and Nagpur, under ISO 9001:2015 and ISO 14001:2015. The copper and brass route is described on the copper, brass and aluminium fabrication page.

What to put on the copper drawing

• UNS grade and oxygen requirement, for example C11000 or C10200, not "copper sheet".
• Temper, O60 or H02, with the temper code from ASTM B152.
• Conductivity, if the part is sized on resistance: state % IACS minimum.
• Thickness with tolerance, because a 5% thickness change is a 5% resistance change.
• Inside bend radius and grain direction, since copper bends best across the grain.
• Joining method and filler, with the brazing atmosphere stated if the joint is furnace brazed.
• Annealing between operations, if the part has more than one severe form.
• Surface: bare, tinned, silver-plated or coated, and which faces must stay conductive.
• Flatness, which is the tolerance that most often fails on a large copper panel.

The DXF and RFQ checklist covers the file side, and the materials hub carries the brass, stainless and aluminium equivalents. For a part sized on current, request a quote with the amps and the allowable temperature rise, and the section can be checked before the bar is ordered.

Where this route stops

We cut and form copper, and we coat it on the same pretreatment and powder line as steel where the part needs protection. We do not tin-plate, silver-plate or apply busbar insulation in-house, and we do not run a hydrogen annealing furnace — so a C10200 part that needs a full anneal between draws is a bought-in heat treatment with its own lead time.

Beryllium copper is outside our route. It machines and forms like a different family of material, and the dust controls it needs are not something a general sheet metal shop should improvise. If the part is a spring contact, phosphor bronze or a stainless spring is usually the practical answer.

The last limit is thickness. Copper plate above about 10 mm is a slow laser cut and a heavy manual form; where the design needs that section, a machined or extruded bar is usually cheaper than a plate part, and the conversation belongs at the drawing stage.