Specifying flatness, straightness and twist after processing

A drawing that specifies "bar to be flat" defines nothing. The word covers three geometrically distinct characteristics with different causes, different measurement methods, and different achievable values. Incoming inspection disputes typically arise from ambiguity about which characteristic was intended.

Flatness is a property of a surface: every point must lie between two parallel planes separated by the tolerance value. It does not define the direction of the error.

Straightness is a property of a line, an edge, or the axis of the bar. On rectangular busbar it splits into two. Bow is curvature in the thickness direction, causing the bar to rock when laid on its face. Camber is curvature in the width direction, making the bar appear banana-shaped when laid flat. A bar can exhibit significant bow with no camber, or vice versa.

Twist is rotation of the cross-section about the longitudinal axis, so the two ends are not coplanar. It is the only characteristic of the three that is not visible by laying the bar on a table and looking along it, and it is most often discovered at assembly.

None of these properties is meaningful without a reference length. "Camber 2 mm" is not a specification. "Camber 2 mm maximum over any 1000 mm" is a specification.

What the mill already gave you

Before considering process effects, the material’s incoming condition must be assessed. ASTM B187/B187M, the specification for copper bus bar, rod and shapes, defines straightness in Table 10 as a maximum depth of arc measured over a stated length:

• Bar generally: 1/4 in [6 mm] over 60 in [1500 mm]
• Half-hard rectangular bar, 3 to 15 mm thick and 50 to 150 mm wide: 1/8 in [3 mm] over 96 in [2400 mm]
• Rod: 1/2 in [13 mm] over 120 in [3000 mm]
• Shapes: 1/2 in [13 mm] over 72 in [1800 mm]

The measurement method is as important as the value. The standard requires placing the bar on a level table so the deviation from straightness is horizontal, and measuring the maximum depth of arc with a steel scale and straightedge to the nearest 1/32 in [0.8 mm]. Horizontal measurement eliminates gravity effects. A 3 m length of 100 × 10 mm copper weighs 27 kg and will sag several millimetres under its own weight if measured vertically at the ends; this sag is not a property of the bar.

Three millimetres of camber over 2.4 m as supplied is a significant amount, exceeding most assembler expectations. If the finished-part tolerance is tighter than the incoming tolerance, the process must remove error rather than preserve it, and this must be designed in.

How punching produces bow and camber

Punching does not remove material neutrally. It creates a residual stress field around each hole, and the sum of these fields distorts the bar.

Two mechanisms are active. Around the hole itself, material is drawn inward and downward as the punch enters, placing the entry-side region into compression, while material is pushed out through the exit face during fracture, placing that region into tension. When the tensile component dominates, the bar bows. This effect is local: a heavily punched region will bow while an unpunched region of the same bar remains flat, which is why bow often appears as a kink rather than a smooth arc.

The second mechanism is the stroke. As the punch drives downward, the bar deflects into the die opening. The top surface goes into compression and the bottom into tension, and the plastic component of this deformation does not recover.

Four factors worsen these effects, and all are controllable. Hole density sets the accumulated stress and comes from the design. Thickness increases rollover and force, and force is what deforms surrounding material. Tool condition is the most common practical factor: a dull punch requires materially more force than a sharp one and produces more rollover, which typically explains a bar that suddenly starts coming off the machine bowed when it never used to. Clearance, when tight, raises force and increases bow. This last variable also drives tool life and is discussed in punch-to-die clearance for copper and aluminium busbar.

Camber, distinct from bow, originates from asymmetry in the width direction. A hole pattern along one edge of a wide bar places the stressed zone off the neutral axis, causing the bar to curve in plan. Patterns symmetrical about the centreline largely avoid this, which supports the standard hole positions in DIN 43673-1.

Beyond sharp tools and correct clearance, two countermeasures apply. Control stripper or presser-foot pressure: it holds the bar flat during the cut, but on soft copper excessive pressure will itself bow the bar, and reducing it is sometimes the solution. Sequence the holes to distribute the work rather than completing one end before starting the other, which the CNC can accomplish if the nesting is configured accordingly.

How shearing produces twist

A guillotine shear does not cut across the whole width simultaneously. The upper blade is set at a rake angle so the cut progresses from one side to the other, keeping the force manageable. This progression applies a couple to the offcut, causing it to rotate.

Rake angle is the dominant variable. A larger rake reduces cutting force and increases twist; a smaller rake has the opposite effect. Where the rake is adjustable, the correct setting is the smallest angle that still makes the cut. Using the same angle for 6 mm and 16 mm bar guarantees excess twist on the thinner material.

Aspect ratio determines whether the twist is visible. Shear manufacturers commonly cite a minimum width of 10 to 15 times material thickness before a cut strip will lie flat. Busbar cropped to length is wide relative to its thickness, so the twist appears as a small wind at the sheared end rather than a corkscrew. On narrow bar, such as 20 × 10 mm, the ratio is 2

and the twist is pronounced.

Two other shear faults cause distortion rather than twist. A blade gap set too wide allows the material to roll and fold over the lower blade before fracturing, producing a large burr and a bowed part. Dull blades crush before they cut, driving stress into the piece and bowing it downward after separation. Both are diagnosed by examining the cut face for excessive rollover and a short burnish band. The shearing process overview covers the geometry.

How bending distorts

Bending is intentional deformation. The unintentional effect occurs around the bend.

On a flatwise bend, the outer fibre stretches and the inner fibre compresses. Because these strains are not equal, the flange acquires a slight camber and the section becomes marginally hollow across its width near the bend. Springback compounds this: the bar relaxes after the punch retracts, and uncompensated springback leaves the included angle incorrect and the flange proud.

The specific failure to monitor is a hole close to a bend tangent. Material around the hole is drawn into the bend, the hole goes oval, and its position moves. Clearance holes should be kept at least two thicknesses away from the tangent, and where the design forces a closer position, the hole should be punched after bending rather than before.

Measuring it properly

Support the bar at three points on a flat reference surface, such as a granite surface plate or a machine table verified with a straightedge, rather than along its entire length on an unknown surface.

For flatness of a face, lay a straightedge across the surface in several orientations and read the gap with feeler gauges, or sweep a dial indicator on a height gauge. Record the worst reading and the length over which it was taken.

For bow and camber, use the ASTM B187 method: bar on a level table with the deviation from straightness lying horizontally, straightedge along the edge under test, maximum depth of arc read with feeler gauges. Record which of the two directions was measured. Reporting "bow" when camber was measured is the most common inspection error on busbar.

For twist, clamp one end flat on the surface plate and measure the gap under each of the two corners at the free end. Twist is the difference between those gaps divided by the bar width, reported as an angle or as millimetres per metre. State the length: 0.5 mm of twist over 300 mm and 0.5 mm over 3000 mm are not comparable.

Where any of these properties is subject to an acceptance test, the reference length and support condition belong in the specification alongside the numerical value.

What is realistically achievable

The figures below are indicative planning values, not guarantees, and they assume sharp tooling in good condition. Machine specifications quoted are published values.

Angle accuracy in bending is a separate specification and should not be confused with flatness. A servo bending machine holding ±0.1° on the bend angle provides no information on whether the flange is flat, and both parameters require specification if both are critical.

Where the stack-up bites

Consider a three-phase stacked run: four bars of 100 × 10 mm copper on insulated supports, 2 m between joints, bolted at each end to the adjacent section.

Each bar possesses some camber, and the direction is random from bar to bar. At the ASTM limit, this is approximately 2.5 mm over 2 m. Two bars with opposing camber leave a gap of several millimetres at the joint face. The bolts will close it, because copper is compliant and an M12 bolt can easily bend a 10 mm bar. The result is that the joint now carries a bending preload, contact pressure across the face is no longer uniform, and the effective contact area is smaller than the drawn area.

This is a temperature-rise problem, not a cosmetic one. Joint resistance is determined by real contact area, and under IEC 61439 the limits are 105 K rise for bare copper busbars and 70 K at terminals intended for connection of external conductors. A joint bolted through a bowed bar starts life with less contact area than the design assumes and loses more through thermal cycling. If the design is already close to its ampacity limit at the joint, this margin was not available.

Hole position stacks on top. A cambered bar carries its hole pattern on a curve, so even a perfectly punched pattern is displaced relative to a straight mating bar. The bolts pass through, because clearance holes are generous. The joint is pre-stressed. Thickness tolerance stacks in the third direction: four bars per phase in a laminated stack, each at the low end of tolerance, changes the total stack height and therefore the clamping bolt length and spacer heights. This calculation should be performed once per standard configuration, not on site.

When straightening is warranted

Straightening is warranted when residual bow or twist exceeds what the assembly can absorb without pre-stressing a joint, or when joint face flatness is itself out of specification. It is not performed to make bars look better in the rack.

The order of operations is the part most often misapplied. Level the stock before punching. Levelling a punched bar redistributes stress around holes that already exist, can distort holes near the bar edge, and on hard-temper material can leave a new residual stress field that reappears after the next thermal cycle. Roller levelling redistributes stress; it does not eliminate it.

This makes straightening a last resort in a rational process route. Start with levelled stock, keep punch tooling sharp and correctly clearanced on the punching and shearing machine, sequence the hole pattern to distribute the work, reduce the rake angle on the shear where the machine allows, and compensate springback on the bending station so the flange lands as drawn. Bars produced this way rarely need straightening.

Where it is required, specify it as a process step with its own acceptance measurement, rather than leaving it to a mallet on the assembly bench, where much of it currently occurs and is unrecorded.