Minimum bend radius for copper busbars

What sets the limit

A bend radius is a strain limit in disguise. When a bar bends, the material at the outside of the bend stretches and the material at the inside compresses. The strain depends on how far the outside fibre sits from the neutral axis and how tight the radius is. A smaller radius means more strain, and once the strain passes what the material can take, the outside edge cracks or the inside edge wrinkles.

For a rectangular bar, the distance from the neutral axis to the outside fibre is half the thickness in a flatwise bend and half the width in an edgewise bend. That single difference explains most of the confusion around busbar radii. A 100 x 10 mm bar can take a tight flatwise bend because the outside fibre is only 5 mm from the neutral axis. The same bar bent edgewise has its outside fibre 50 mm away, so the radius has to be much larger.

Temper changes the number

Soft or annealed copper is the most forgiving. It yields before it cracks, so a flatwise radius of about half the thickness is often enough for a gentle bend. Half hard copper, which is common for panel bars because it holds its shape, needs about one thickness. Hard copper needs about two thicknesses and may still need a stress relief after bending.

The multipliers are a starting point, not a guarantee. The exact limit depends on the alloy, the grain direction, the edge condition and the bend line relative to the rolling direction. A sheared edge has micro-cracks that can grow into a bend crack, while a deburred or rounded edge behaves better. A bend line across the rolling direction is usually more tolerant than one along it.

An illustrative radius table

The table below gives starting multipliers for copper busbar. Flatwise radii are expressed as a multiple of thickness, because thickness sets the outside fibre distance. Edgewise radii are expressed as a multiple of width, because width sets the outside fibre distance in that direction. The values are illustrative and should be confirmed with a trial bend on the actual material.

A worked example on a 100 x 10 mm bar

Take a 100 mm wide, 10 mm thick copper bar in half hard temper. A flatwise bend uses the thickness, so the starting inside radius is 1.0 x 10 mm, or 10 mm. An edgewise bend uses the width, so the starting inside radius is 1.0 x 100 mm, or 100 mm. The edgewise radius is ten times the flatwise radius even though the bar and the temper are the same.

If the design calls for a 40 mm inside radius in the edgewise direction, the table says the bend is outside the starting limit for half hard material. The options are to soften the bar, to increase the radius, to change the bend direction or to accept a stress relief step after forming. Each option has a cost, and the drawing should show which one was chosen.

For a hard temper bar, the flatwise starting radius rises to 20 mm and the edgewise radius to 150 mm. That is a large space claim. If the route needs a tight edgewise corner, the design may be asking for soft material, a different joint or a fabricated corner rather than a bend.

Edge condition and grain direction

The outside edge of a bent bar is where a crack starts. A sheared edge has a rough, work-hardened zone and sometimes a burr. If that edge is on the outside of a tight bend, it can open up. Deburring helps, and a small edge radius is better than a sharp corner. For a critical bend, ask for the edge condition to be stated on the drawing.

Grain direction is set by the rolling process. A bend line across the grain behaves differently from one along the grain. In a wide bar, the two directions can coexist in the same part, so the drawing should show the bend line and the material direction if the part is critical. The supplier can then choose the blank orientation rather than discovering the problem at the press.

Surface finish also matters at the outside of the bend. A scratch, a deep die mark or a corrosion pit acts as a stress raiser. Keep the bend area free of deep marks and specify a finish that does not require a heavy grit blast before bending.

Springback and the radius you get

The radius on the drawing is the radius after springback, not the radius of the tool. A hard temper bar springs back more, so the tool has to be made tighter or the bend has to be overbent and re-struck. If the supplier sets the tool to the drawing radius and ignores springback, the part will be open and the measured radius will be larger.

Springback also changes the angle. A 90 degree bend may come out at 92 or 93 degrees, and the error grows with the radius and the yield strength. The first article should record both the angle and the radius, because correcting one can change the other.

For a critical part, ask for a trial bend on the production material before the tool is finished. The trial should use the same temper, the same edge condition and the same grain direction. A trial on a convenient offcut can give a false answer.

1. 01

   State the temper

   Give the alloy and temper, not just the conductivity or the thickness.

2. 02

   State the bend direction

   Flatwise or edgewise, with a direction arrow on the drawing.

3. 03

   Give the inside radius

   Use the finished radius after springback, with a tolerance.

4. 04

   Define the edge

   Deburr or radius the outside edge in the bend zone if the radius is tight.

5. 05

   Trial the bend

   Bend a sample of the production material and record the angle and radius.

6. 06

   Inspect the first article

   Check radius, angle, cracking and the leg lengths after forming.

How to put the radius on the drawing

Give the inside radius and the temper, then add the bend direction and the edge condition. A note such as 'inside radius 10 mm minimum, half hard, deburr outside edge in bend zone' tells the supplier what the design needs. A radius with no temper is only half a call, because the same radius can be safe in soft copper and marginal in hard copper.

If the radius is a minimum rather than a nominal, say so. If it is a maximum because of space, say that too. The supplier can then choose the tool and the bend order. A drawing that gives a nominal radius with a tight tolerance and no temper invites a part that measures correctly on the bench and cracks in service.

• Alloy and temper, not just 'copper'.
• Inside radius, nominal or minimum, with a tolerance.
• Flatwise or edgewise, with a direction arrow.
• Edge condition in the bend zone.
• Any stress relief or post-bend anneal.
• The inspection method for radius and angle.