Copper thickness tolerance and what it does to a formed part

A 2 mm copper channel arrives from the mill at 2.10 mm, which is inside the ASTM B152 band and inside the mill certificate. Four bends later the part is 0.54 mm over its outside length and the assembly jig will not close. Nothing was made badly; the flat pattern was calculated on a thickness the coil did not have.

Copper thickness tolerance is the least discussed and most consequential material tolerance on a formed part. It feeds the bend deduction directly, it changes the mass that the part is bought and sold by, and it changes the fit where two parts stack. On 2 mm strip the band is ±0.10 mm, roughly 5% of nominal, which is the same order as the forming tolerance most drawings allocate.

The sections below give the band by thickness, work the arithmetic through a bend, and set out what belongs on the drawing. The surrounding grade decisions are in the materials and grades hub.

How much copper thickness tolerance does strip carry?

Copper sheet in the 1.63–3.15 mm range carries ±0.10 mm at widths up to 610 mm and ±0.13 mm from 610 to 1,220 mm under ASTM B152. Thinner strip is proportionally worse: at 0.8 mm the band is ±0.076 mm, which is 9.5% of nominal, and the percentage falls as thickness rises.

Two things drive the width split. A rolling mill controls thickness through the roll gap, and the gap is less uniform across a wide sheet than across a narrow one, so the standard allows more variation above 610 mm. Copper is also sold by weight, so a mill has less commercial reason to hold the middle of the band than a mill supplying a machined billet.

The bend deduction column is the number that matters on a drawing. It is calculated as (2 − πK/2) × Δt for a 90° bend, which with K = 0.42 gives 1.34 × Δt. A tolerance of ±0.10 mm therefore moves the deduction by ±0.134 mm, and a four-bend box accumulates four of those shifts in the same direction when the whole coil sits at one end of the band.

Where the band lands in the finished part

Bend deduction and flat pattern

Take 2 mm C11000 half-hard, a 1 t inside radius of 2 mm and K = 0.42. The bend allowance is (π/2)(R + Kt) = 1.5708 × (2 + 0.84) = 4.462 mm. The bend deduction is 2(R + t) − BA = 8.000 − 4.462 = 3.538 mm.

Now run the same part on a 2.10 mm coil. The allowance becomes 1.5708 × (2 + 0.882) = 4.529 mm and the deduction becomes 8.204 − 4.529 = 3.675 mm. The blank was cut for 3.538 mm, so each bend consumes 0.137 mm more than planned.

A 300 mm outside-length channel with two bends finishes 0.274 mm over. A 500 × 400 mm four-bend box finishes 0.548 mm over on the dimension that crosses all four bends. Against a ±0.5 mm formed tolerance, the thickness band alone has used the whole allowance.

The error has a direction, and the direction matters. A coil at 1.90 mm makes the same part 0.274 mm short. A long part can be dressed or re-bent; a short part cannot be made longer without welding, so the thin end of the mill band is the more expensive side to discover at first article.

Outside dimensions and stack-up

The outside dimension of a flange carries one thickness as well as the deduction. A 50 mm outside flange formed from a 2.10 mm strip against a 2.00 mm flat pattern finishes about 0.10 mm over at the flange face and 0.14 mm over at the far edge. Individually neither is alarming; in a stack of four flanges the errors add.

Stacking is where the band does real damage. Two 5 mm copper bars in a 10.2 mm slot have 0.2 mm of nominal clearance. At the top of the B152 band each bar is 5.102 mm, so the pair measures 10.204 mm and the joint is an interference fit. The drawing was correct and the material was conforming; the clearance was sized on nominal thickness instead of maximum material.

Mass and current

Copper is bought by weight and many copper parts are sold by weight, so the thickness band is a commercial number as well as a dimensional one. A 400 × 600 mm part in 2 mm copper has an area of 0.24 m² and weighs 4.30 kg at nominal, 4.52 kg at the top of the band and 4.08 kg at the bottom. The 0.44 kg spread across the band is 5.1% of the part, and on a 500 kg order it is 25 kg of metal.

For a current-carrying part the band changes resistance. A 50 × 5 mm bar has a 250 mm² section. At 4.898 mm it has 244.9 mm², 2% less, and at 5.102 mm it has 255.1 mm², 2% more. A bar designed at 1.6 A/mm² for 400 A therefore runs between 392 A and 408 A at the same temperature rise. Two per cent is inside most design margins, but it is not inside a margin that was set on nominal thickness with no allowance for the mill band.

Copper is not unusual in carrying this band. A 2 mm CRCA or 5052 aluminium sheet carries a thickness tolerance of the same order under its own material standard, and neither is rolled to a machined tolerance. What makes copper different is the consequence: a steel panel that is 5% heavy is a heavier panel, while a copper bar that is 5% heavy is a heavier conductor with a different resistance and a heavier invoice. The tolerance is the same physical fact; the exposure is larger.

What happens at the bend itself

Bending thins the section at the bend. On copper at a 1 t inside radius the thickness at the crown drops by roughly 5–10%, so a 2 mm strip measures 1.80–1.90 mm through the bend. That is normal and it is not covered by the mill tolerance, because B152 describes the flat strip, not the formed part.

The thinning matters in three places: a thread rolled or tapped through a bend, a bearing surface formed on the outside of a radius, and a bend at minimum radius on strip from the thick end of the band. The last case is where cracking starts, because the outer fibre strain is t × 0.5 / R and a thicker strip strains the outer fibre more for the same radius.

Springback also moves with thickness, though less than the deduction does. Copper half-hard recovers 2–4° after a 90° bend at a 1 t radius, against 8–12° for 304 stainless. A 5% thickness change moves that recovery by a fraction of a degree, which the ram corrects; the 0.137 mm deduction shift it does not correct.

Bending force moves faster than thickness because it scales with t² at a fixed V opening. A 16 mm V-die suits 2 mm copper at the usual 8 t rule. The same part on a 2.10 mm strip needs 10% more force, which is inside the capacity of the 200 T brake but is worth knowing when the job is set at 95% of a smaller press. The practical rule is to set the tooling from the measured thickness of the coil, not from nominal, and to record which V opening was used on the setup sheet.

Where the standards stop

ASTM B152 describes the material as delivered. It sets composition, temper, mechanical properties and dimensional tolerance, including thickness measured at 20 °C at the points the standard specifies. It does not describe the formed part, it does not control thickness after bending, and it does not tell you what the drawing tolerance should be.

ISO 2768-1 is the other standard that gets cited here, usually in a title block. It covers general tolerances for linear and angular dimensions that carry no individual tolerance, and it applies to the dimensions of the part. It says nothing about the thickness of the strip you bought. A title block of ISO 2768-m does not give the coil a thickness tolerance, and it does not make a formed dimension hold ±0.3 mm when the material band is ±0.10 mm and the process stack is wider still.

That gap is why copper drawings need two statements: one for the material, one for the finished feature. The bending tolerances and inspection guide covers how the formed side is measured, and the K-factor guide covers the assumption that sits between the two.

How to write the thickness requirement

1. Name the alloy, temper and standard edition, for example C11000 H04 to ASTM B152/B152M-24. Temper changes yield and springback as much as thickness does.
2. State nominal thickness and its tolerance, and the width band it applies to. A note of 2.0 ±0.10 mm is only true up to 610 mm wide.
3. Say whether the tolerance is on the material or the formed feature. If it is the feature, give the datum and the gauge.
4. Give the bend deduction basis or accept the supplier setting it from first-off. State the inside radius and whether K is fixed by the tooling or by the material.
5. Size clearance on maximum material, especially where two copper parts stack or where a copper bar enters a slot in a steel part.
6. Say whether plating counts toward the tolerance. Tin at 8 µm per side adds 0.016 mm, which is small against the mill band but not against a ±0.02 mm callout.

What Ganesh checks on a copper job

The bend deduction is set from the measured thickness of the coil rather than the nominal, and the first article confirms it before the batch runs.

Copper sheet is cut on the 12 kW fibre laser with a 40 ft × 16 ft bed, alongside two 6 kW and two 3 kW machines, and formed on the 200 T, 5-axis brake with a 4,450 mm bed and Delem control. The custom part range runs from 0.8 mm to 40 mm, with ±0.5 mm typical on formed parts and tighter features agreed on the drawing. Repeat parts run with first article inspection and PPAP where the customer asks, under ISO 9001:2015 and ISO 14001:2015. The bending capability sets out the bed and tonnage envelope.

When a tighter tolerance is the wrong answer

If a copper part needs ±0.02 mm on a formed feature, a tighter general tolerance will not deliver it. The route is to order rolled strip to a tighter purchase-order tolerance, which costs more and moves the delivery date, or to form oversize and coin or machine the controlled face. Copper coins easily, so a sizing station on a 20 T press can hold a pad to ±0.05 mm without a special coil.

The same logic applies to a hole pattern that must line up after bending. Drill or pierce the pattern after forming from the fixture, and the thickness band stops mattering to the hole position. The pressed copper and brass components guide covers the die clearance and burr side when the part is coined rather than bent.

Send the flat pattern with the alloy, temper and coil width to our engineering team. We will return the bend table against the actual strip band and flag the dimensions where the tolerance and the material are arguing with each other.