How Sheet Metal Bending Works
Sheet metal bending is the manufacturing process by which most enclosures, electrical boxes, brackets and components are formed through the use of a machine known as a CNC pressbrake. (Panel Bending machines can also be used, although their operation is outside the scope of this feature.)
Sheet metal is bent when it is forced between two tools by the pressbrake: an upper tool (known as a punch) and a bottom tool (known as a die). The pressbrake controls the movement of either the punch or the die and provides the press force using hydraulic rams or electrical servo motors. The bend angle is predominantly determined by the depth of penetration of the punch within the die.
The maximum force provided by the pressbrake determines the maximum bend length for a combination of thickness of sheet metal, bend radius and bend angle. The force required to bend sheet metal increases with bend length, external bend angle and sheet metal thickness, and it decreases with increasing bend radius. Hydram’s pressbrakes have varying capabilities and maximum bend length of 4 metres and a maximum force of 250 tonnes are available. The table below illustrates some typical examples for 90 degree bends:
|Mild Steel Thickness||Bend Length||Inner Bend Radius||Required Force|
Sheet metal part design and complexity
Components vary in complexity, from parts with a single bend, through to parts with multiple bends with multiple flange lengths. Modern pressbrakes are equipped with adjustable backstops, driven by servo motors, against which the components are offered by hand or robotic manipulator. The closer the backstop to the tooling, the shorter the resulting flange is and vice versa.
On complex parts, the backstops adjust after each bend to the corresponding distance required for the next bend. The movement of the backstops and the pressbrake tooling is synchronised by a CNC controller. CNC programs can be generated online on the machine user interface or by an offline programming (or CADCAM) software package.
You may be interested in our Sheet Metal Component Design Guide.
A variety of pressbrake tools is available to suit different sheet metal bending tasks. The characteristics of the upper and lower tools are varied according to the requirements of the sheet metal component. A number of bending examples are illustrated below:
Thicker metal is generally processed with a larger bending radius, and this can be achieved by increasing the top tool radius and the distance across the die opening – or V-width.
Tools For Sheet Metal Bending Thick Metals
Components that require a sharp bend angle require “over bending” tooling. Both the top and bottom tools in this case have a more acute angle
Tools For Sheet Metal Over Bending Acute Angles
Components that have more than one bend often require special top tooling to provide clearance for existing flanges. Without this clearance the component would collide with the tool before the subsequent bend operation was complete. This type of tooling is often referred to as gooseneck.
Tools For Sheet Metal Bending Tight Clearance
To provide clearance in extreme cases, the top tool can be suspended from the pressbrake beam using modified clamps. These extended clamps provide far greater clearance for large flanges, so long as the pressbrake has sufficient stroke length to accommodate the overall tool height.
Folded sheet metal blank development
When designing sheet metal components with folds or bends it is necessary to create a flat pattern or blank development of the part. This blank is then laser cut or CNC punched before arriving at the pressbrake for folding. In creating the blank, it is important that the design takes into account the bend radius formed by the pressbrake tooling. The bend radius has the effect of decreasing the developed blank size. The larger the radius, the smaller the blank, as shown in the example below:
The bend radius varies with the material thickness and the tooling used for bending the material. It is therefore essential that the designer is aware what tooling will be used to bend the material and have a good appreciation what affect this has on the bend radius. Likewise, to ensure accuracy of the bent component, the pressbrake operator needs to know what radius the part has been designed with so that the correct tooling choice is made.
The design calculations required for sheet metal blank development involve a “bending allowance” calculation.
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