Key takeaways
- Flatwise bending turns the bar about an axis across its width, so the outside fibre sits at half the thickness.
- Edgewise bending turns the bar about an axis through its thickness, so the outside fibre sits at half the width and the force and radius both rise.
- Offset and twist are alignment features: they add tooling, springback and inspection without changing the current path much.
- A bend drawing needs the type, inside radius, angle, direction, leg lengths and a datum for each bend.
Four bend families, one bar
Most busbar bends fall into four families. Flatwise and edgewise bends change the direction of the run in one plane. Offset bends shift the run sideways or vertically without turning it. Twist bends rotate the section so that a bar can meet a terminal or clear another part.
The families are not interchangeable. The same bar, the same angle and the same inside radius can be easy in flatwise bending and difficult in edgewise bending, because the material at the outside of the bend is a different distance from the neutral axis. That distance sets the strain, the springback and the tooling force.
Flatwise bending: the easy way
In a flatwise bend the bar turns about an axis across its width, so the wide face stays in the plane of the bend and the outside fibre is at half the thickness. For a 10 mm thick bar, that is 5 mm from the neutral axis, which keeps the strain low and allows a tight radius.
The tooling is usually a press brake, a ram bender or a pair of form tools with flat dies. The bar is supported across its width, and the inside radius is set by the punch or the former. Because the bend is made about the easy axis, the force is lower and the springback is easier to manage than in edgewise bending.
Flatwise bends are common in panel runs where the bar has to turn a corner and stay flat against the mounting plate. The main risks are a radius that is too tight for the temper, marks from the dies and a leg that twists because the bar was not held square during the stroke.
Edgewise bending: the hard way
In an edgewise bend the bar turns about an axis through its thickness, so the wide face rotates through the bend and the outside fibre is at half the width. For a 100 mm wide bar, that is 50 mm from the neutral axis. The strain for a given radius is ten times the flatwise value on a 10 mm thick bar, which is why edgewise bends need a much larger radius or a softer temper.
The tooling has to support the wide face and resist a larger force. A horizontal ram bender with side support is common. Without support, the compression side can wrinkle and the width can grow, and the bar can twist out of plane before the bend is complete.
Edgewise bends are used when the run has to change level in a tight space or when the bar has to meet a terminal at right angles to the mounting plane. They are a geometry solution, not a current solution. The current path is the same; the difference is the stress, the tooling and the space needed around the bend.
Offset bends: moving the run without turning it
An offset, sometimes called a joggle, uses two opposite bends close together to shift the bar sideways or vertically. The run stays parallel to its original direction, but its centre line moves. Offsets are used to clear a fixing, to step around another bar or to bring a joint face into line.
The two bends interact. The distance between them sets the offset, and the leg between them has to be long enough for the tooling to clear the first bend. If the offset is short and the bar is thick, the tools may not fit, or the second bend may pull the first one out of angle.
Springback is the main quality risk. Each bend springs back, and the two errors can add or cancel depending on direction. The drawing should give the offset dimension and the angle, and the first article should check both the offset and the parallelism of the two legs, not just the angle of each bend.
Twist: rotating the section
A twist rotates the bar about its long axis so that the wide face changes orientation. It is used to meet a terminal that sits at an angle, to clear a component or to transition between a flat run and an edgewise connection. The twist itself does not change the direction of the run, but it changes the orientation of everything downstream.
Twisting puts shear across the section and leaves residual stress in the bar. The angle has to be specified with a tolerance and a gauge length, because a twist measured over 100 mm and a twist measured over a metre are different calls. The transition length also matters: a short twist concentrates the strain, while a long twist is gentler but takes more space.
A twisted bar is harder to inspect than a bent bar. A simple angle gauge on the end face may be enough for a short transition, but a long bar needs a fixture that holds the datum end and measures the rotation at the other. If the twist feeds a punched hole pattern, check the hole orientation after twisting, not before.
Tooling, bend allowance and springback
The bend allowance calculation is the same in principle for all four families, but the inputs differ. The inside radius, the thickness and the K factor set the allowance; the bend angle sets the arc length. For edgewise bends, the relevant thickness in the strain calculation is the width, so the allowance and the springback estimate have to use the correct dimension.
Springback rises with the yield strength and the bend radius, and it falls with the bar thickness. A hard temper bar bent edgewise combines a high yield strength with a large outside fibre distance, so it springs back more than the same bar bent flatwise. The tool has to be overbent, or the part has to be re-struck, to land on the angle.
The table below compares the four families on the features that decide the process. Use it to choose the bend type before the detail drawing is fixed, not after the tooling has been ordered.
| Bend type | Bend axis | Outside fibre | Main tooling | Main risk |
|---|---|---|---|---|
| Flatwise | Across the width | Half the thickness | Press brake or ram with flat dies | Tight radius and die marks |
| Edgewise | Through the thickness | Half the width | Horizontal bender with side support | Wrinkling, width growth and high force |
| Offset | Two opposite bends | Depends on direction | Joggle tools or two separate hits | Offset dimension and leg parallelism |
| Twist | Along the length | Shear across the section | Twist head or fixture | Angle over gauge length and residual stress |
The outside fibre distance is the value used in the strain estimate. Confirm the tooling and the springback with a trial bend.
What the drawing and the first article should show
A bend drawing should state the bend type, the inside radius, the angle, the direction and the leg lengths to a defined datum. If the part has more than one bend, number them and give the order if the order affects the result. An offset needs the offset dimension and the distance between the two bends; a twist needs the angle and the gauge length.
The first article should check the features that the process can move: the bend angle, the inside radius, the leg lengths, the offset and the twist. Check the hole pattern after forming, because a bend or a twist can move a hole that was correct on the flat blank. Record the springback observed on the trial so the next batch starts with the right compensation.
- Bend type and number on the drawing, with a direction arrow.
- Inside radius and angle with a tolerance, not a reference only.
- Leg lengths measured from a stated datum, not from each other.
- Offset dimension and the gap between the two bends.
- Twist angle and the gauge length over which it is measured.
- Hole positions checked after forming, especially near a bend or twist.
Frequently asked questions
What is the difference between flatwise and edgewise busbar bending?
Flatwise bends about an axis across the width, so the outside fibre is at half the thickness and the bend is relatively easy. Edgewise bends about an axis through the thickness, so the outside fibre is at half the width, which raises the strain, the force and the minimum radius.
Why does an edgewise bend need a larger radius?
The outside fibre is further from the neutral axis, so the strain at the surface is higher for the same radius. A larger radius or a softer temper keeps the strain within what the material can take without cracking or wrinkling.
Can a twist replace an offset bend?
They do different jobs. An offset moves the run sideways while keeping its direction; a twist rotates the section. A twist can solve an orientation problem, but it will not move the centre line.
How is twist measured on a busbar?
Hold the datum end in a fixture and measure the rotation of the other end over a stated gauge length. The drawing should give both the angle and the length, because the same angular error over a longer length is a different part.
Sources and further reading
- IEC 61439-1, Low-voltage switchgear and controlgear assemblies
- Copper Development Association, Copper for Busbars
- Qumek busbar forming and first article inspection notes
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