Sheet metal bending: techniques, press brakes and V-groove folding compared

At YDF, we have been making bespoke metalwork since 1972, folding sheet metal for furnishings, retail and architecture to architects' and designers' drawings, using a press brake and VIVO METALFOLDING®, our sharp-edge system. This guide brings together techniques, machines and calculation concepts, with a section on a topic rarely covered in technical guides: how to fold a piece that remains visible.

Sheet metal bending: comparing techniques

All techniques use the same principle. The metal is stressed beyond its elastic limit: the fibres on the outside of the bend stretch, those on the inside compress, and a zone in the middle retains its length — the neutral axis. What changes is how the punch and die guide the sheet, and with it the precision, force, radius and marking.

Air bending

The punch pushes the sheet into a V-die without reaching the bottom: the sheet touches only the punch tip and the two die shoulders. The final angle depends on how far the punch descends, not on the die shape. This is the most widely used technique: the same tools produce different angles, and the required force is the lowest. In return, springback is more pronounced and the angle is affected by variations in thickness and hardness between batches of sheet metal.

Bottoming

The punch brings the sheet into contact with the sides of the die, which has the same angle as the required bend. This needs more force than air bending but reduces springback and improves angle repeatability. In return, dedicated tooling is needed for each angle, and the greater contact area increases the risk of marks.

Coining

The punch does not stop at contact: it penetrates the material and thins it slightly along the bend line. Plastic deformation affects the full thickness, making springback negligible and the internal radius equal to the punch radius. It requires much higher forces, wears the tooling and leaves visible marks. Today it is mainly used on thin sheets when a very precise angle is required.

Roll bending and sheet metal rolling

When the sheet needs to form a broad curve rather than a sharp edge, it passes between rollers that curve it progressively: this is sheet metal rolling. In our department, rolling is also used for rolled metal walls.

V-groove folding, in the last row, is discussed below in the section 'When the bend is visible'.

The press brake and its tooling

The press brake is the machine used for most sheet metal folds. A moving ram carries the punch, while a fixed bed carries the die; on modern machines, numerical control manages the punch stroke and backgauge position.

• Punch: the upper tool. It can be straight, gooseneck-shaped to bend return flanges without colliding with them, or radiused for softer bends. The tip radius influences the internal bend radius.
• Die: the lower tool, with one or more V-shaped openings. The opening is selected according to thickness. A wider V requires less force and produces a larger radius; a narrower V requires more force, tightens the radius and leaves more marks.
• Backgauge: the stops against which the sheet rests; it determines the flange length.
• Crowning: compensates for deflection of the ram and bed, which would otherwise open the bend at the centre of long workpieces.

Also important is the bending sequence. On a U-section or box, the wrong sequence can cause the sheet to hit the punch, making the next bend impossible. The sequence is therefore checked during engineering, before cutting. In our bespoke press brake bending service this is handled by our in-house technical office.

Radius, springback, K-factor: key concepts

Five concepts are enough to design a folded part that can be manufactured without surprises. Exact values depend on the material, thickness, tooling and machine, and are calibrated in the workshop.

Internal bend radius

This is the radius of the curve on the inside of the bend. With a press brake it is never zero, because the metal deforms across a band rather than a line. In air bending, the radius depends mainly on the die opening; in bottoming and coining, it approaches the punch radius. There is a minimum radius below which the outside stretches too far and may crack. It increases with thickness, is tighter for ductile materials and wider for hard or work-hardened alloys.

Springback

When the punch rises, the sheet recovers part of its deformation and the angle opens slightly. This is springback. It increases with the strength of the material and the ratio of radius to thickness: stainless steel springs back more than mild steel, and a broad bend more than a tight one. It is compensated for by overbending, bottoming or, in the most demanding cases, coining; some presses measure the angle and adjust the stroke.

K-factor and sheet metal flat patterns

Sheet metal is cut flat, often by laser, and then bent. Its flat pattern must be calculated to determine the size of the unfolded piece. The calculation uses the neutral axis, the layer that neither stretches nor compresses during bending. The neutral axis does not stay at the centre of the thickness: it moves towards the inside of the bend. The K-factor expresses its position as the ratio of its distance from the inside face to the sheet thickness; it is therefore usually less than 0.5.

The flat pattern is calculated by adding the straight sections and, for each bend, the arc length measured along the neutral axis: bend angle in radians × (internal radius + K × thickness). The K-factor is not a material constant: it varies with the technique, tooling and radius-to-thickness ratio. Workshops determine it through test bends and enter it into the design software.

Minimum flange

The flange is the section of sheet beyond the bend line. In air bending, it must remain supported by both die shoulders until the end of the stroke; if it is too short, it slips into the V and the bend becomes distorted. As the die opening increases with thickness, so does the minimum flange. For very short flanges, a narrower die or a longer flange trimmed after bending may be considered.

Distance between holes and bends

A hole, slot or cut too close to the bend line falls within the deformation zone: it can become oval, shift or cause the bend to deviate. The minimum distance depends on thickness and radius. Alternatively, the hole can be made after bending. Where bends end at an edge or meet at a corner, a small relief notch is needed to prevent tearing or bulging.

How steel, stainless steel, aluminium and brass are bent

Each metal responds differently to bending. The alloy, supplied condition (annealed, work-hardened or hardened), thickness and rolling direction all matter: bending across the sheet's grain reduces the risk of cracks, while bending along the grain increases it.

• Carbon steel. Consistent and predictable; often painted or treated after bending, so marks are less critical.
• Stainless steel. It requires more force and work-hardens as it deforms. It often arrives already satin-finished or polished, so the surface must be protected throughout processing.
• Aluminium. Soft and easily marked. Harder alloys and tempers tend to crack on tight radii: the alloy must be selected with bending in mind too.
• Brass. Alloys intended for cold forming bend well; leaded alloys, designed for machining, are less suitable. The surface is delicate, so the finish is chosen together with the manufacturing sequence. The finishes we offer are on the page brass finishes.

When the fold is visible

Bending guides discuss components: brackets, casings, frames. In furnishings, retail displays or architectural cladding, the bend is part of the design itself. The edge is visible, as are the light passing over it and the continuity of the finish from one face to the next.

Sharp or radiused edge

A press-brake bend always has a radius. On thin sheet, it appears as a soft line of light; on thicker material, it becomes a noticeable curve and the piece reads as folded sheet metal. A sharp edge, by contrast, makes the form appear solid, as if cut from a block. A radiused edge is stronger and softer to the touch, while a sharp edge is more defined: this is a design choice to specify on the drawing.

Bending sheet that is already satin-finished or polished

When sheet arrives already finished, especially stainless steel:

• protect the visible face with an adhesive film that stays in place during cutting and bending and is removed as late as possible;
• the direction of the satin grain is decided before cutting: if the satin grain runs in different directions on the faces of a box, the light reveals it immediately;
• the tools must be clean: a particle between the die and sheet leaves a mark;
• mirror polishing is the most demanding: every mark shows.

For chemical brass finishes, such as burnished or bronzed, we assess whether to bend first and finish afterwards on a case-by-case basis.

Tool marks

In press brake bending, the outer face slides over the die shoulders and may be marked by two lines parallel to the bend; the inner face can show the punch tip's mark. The higher the force, the more pronounced these marks become. To reduce them, protective films are placed between sheet and die, dies with rounded shoulders or synthetic inserts are used, and wider V-openings are selected where the radius allows.

V-groove milling and VIVO METALFOLDING®

V-groove folding achieves a sharp edge in a different way. A V-shaped groove is milled along the bend line on the back of the sheet. The cut reaches almost to the visible face, leaving a thin layer of metal. The sheet is then folded along the groove, usually to 90°: the two faces of the V close together and the outer edge remains sharp.

Building on this technique, we developed VIVO METALFOLDING®, the YDF system for making boxes and structures without screws or pins: the folds create the volume, without mechanical joints. It is suitable for boxes, displays, counters and cladding. Details on the page sheet metal V-groove folding.

V-groove folding or press brake bending: the differences

V-groove folding brass

Sharp edges are particularly effective in brass: combined with a satin or burnished finish, they define crisp volumes in a warm colour. This is a combination we often use for luxury retail and high-end residential projects.

The two techniques can be combined: in the same piece of furniture, the visible front can use V-groove folding, with the internal structure and reinforcements folded on a press brake.

Common design errors in sheet metal bending

1. Radius not specified. The piece will have the radius of the available tooling. The internal radius must be specified or, if a sharp edge is required, this must be stated explicitly.
2. Holes and slots too close to the bend. They become distorted: move them further away or make them afterwards.
3. Flanges that are too short for the die required by that thickness: the sheet is unsupported and the angle is incorrect.
4. No corner relief. Where two bends meet, the material tears or bulges. On a visible piece, the corner detail must be designed, not left to the workshop.
5. Inconsistent dimensions. Dimensions taken partly from the outside and partly from the inside of the bend, or tolerances intended for machining. A single reference is needed.
6. Impossible bending sequence. Narrow, deep U-profiles or boxes where the final bend leaves no room for the punch.
7. Ignoring the rolling and satin-grain directions. This leads to cracks at tight radii and faces that reflect light differently.
8. Visible faces and finish not specified. Without this information, the workshop does not know which side to protect or where a mark is acceptable.

Our technical office engineers every design for bending: checking flat patterns, radii and sequences, and proposing the necessary changes to the designer while respecting the design.

Projects made by YDF to architects' and designers' drawings can be found under Projects.

Do you have a sheet metal component with visible bends? Send us the drawing: we will advise which technique to use for each edge and, if needed, prepare samples in the chosen material and finish. Request a quote.