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4 types of bending processes of tube bending

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pipe bending machine

When it comes to bending pipe or tube, there are several techniques that can be used to achieve a quality, custom-formed part. The primary types of bending used for tubes and pipes are compression bending, roll bending, freeform bending, and mandrel bending (also called rotary draw bending), which is the most common method.

Compression Bending

Compression bending is a rather simple method, the tube is clamped against a stationary bend former and the pressure dies, follower, and sweeps the tube around the bend former to form the bend. that is often used on construction sites for bending electrical conduits or used a lot in the furniture industry, to bend lawn mower handles and generator frames.

For speed and economy, fabricators often use compression bending. This bending mode is used when the roundness of the bend is not critical, and when the objective is more about high output to get the lowest price per tube. However, this type of bending is limited as it can’t make tight (a small radius) bends because the pipe will buckle or break.

In addition to speed, some of the advantages are that no oil is used inside the part and that you don’t use a mandrel inside the tube, which is why the bend is a little out of round. Because you don’t use oil, no cleaning of the part is needed afterward. A lot of times cleaning the part consumes as much time as bending it.

Although furniture, lawn mower handles, and generator frames often are symmetric, a compression bender isn’t limited to symmetrical parts. You can set it up to make a 90-degree bend and a 45-degree bend, for example.

The centerline radius of the tube diameter needs to be approximately two times the diameter of the bend. For example, a 1-in. tube bend requires at least a 2-in. centerline to get acceptable bend quality.

Rotary Draw Bending

Rotary bending process
Rotary bending process

Rotary bending is similar to wiping but the top die is made of a freely rotating cylinder with the final formed shape cut into it and a matching bottom die. On contact with the sheet, the roll contacts two points and it rotates as the forming process bends the sheet. It is typically used to bend a tube three to four times the centerline radius of the tube diameter.

For precision work, rotary draw bending dominates the tube bending landscape, especially for those applications involving tight radii—sometimes down to a CLR that’s just 0.7 times the tube OD (or as tube processors call it, less than 1×D).

pipe bending machine
pipe bending machine

This bending method is typically considered a “non-marking” forming process suitable for pre-painted or easily marred surfaces. This bending process can produce angles greater than 90° in a single hit on the standard press brakes process.

Clamp-die, bend-die, and pressure-die

The rotary draw bending of bending requires 3 pieces of the die are needed.

Bend-die: It forms the tube to the specified bend.
Clamp-die: It holds the tube while bending it.
Pressure-die: As it’s being bent, the part naturally wants to kick out. This tool applies pressure to the back of the tube and follows along as the tube is being bent to prevent this.

Mandrel

If you’re bending a thin-wall tube or a tight radius, may need three other pieces of tooling. will need a mandrel to go inside of it, which keeps the tube round as it is being bent. The bend radius and tube thickness determine the number of balls in the mandrel.
Rotary draw bending hand allows for more control during the tube bending process as use mandrels and dies inside the tube. This control allows to make a very tight bend and can be used to make small dimensional tolerances.

Wiper-die

In addition, a wiper die is needed to bend the thin-wall tube. It prevents the tube from rippling. Normally, when you bend a piece of tube, material gathers on the inside radius, which results in a large lump or ripples at the end. The wiper die wipes, or smooths, those ripples or lumps away, pushing the metal farther from the bend.

Collet

A collet for when making more than one bend on a part. Will need the collet to be sized for the tube diameter. It holds the tube as it is bent and then pushes it forward to get it ready for the next bend.

Most rotary bending machines are oriented for right-handed or left-handed bends, but some are outfitted to bend in both directions.

Roll Bending

Roll-bending is a simpler process. All that is necessary are three specially contoured rollers for each section of steel tube or section bar, through which the material is passed to give it the desired cold process deformation.

Roll bending of open section bars

In the case of open section bars, there are two possible solutions:

Special rollers are prepared that follow the contour of the bar (generally for complex section bars in aluminum).
Nylon blocks are produced to match with the bar for roll-bending, so as to simulate a closed structure like a tubular element.

For tube sections that are more difficult to contour, however, where the esthetics may be jeopardized, microspheres are used. They are not really equipment, but a powder consisting of microscopic spheres that are inserted in the tube so as to fill it completely; once the tube has been filled and sealed at the ends, it will be possible to roll-bend the part without causing significant deformation. This procedure, in view of the time necessary to implement it, will be much more expensive.

Minimum bend radius

The roll-bending is the only technique that can be used to bend the same metal tube with different bend ratios.

To have a more precise idea of the minimum bend radius, we have to multiply the tube diameter by five. If, for example, the tube diameter is Ø50 mm., the minimum roll-bending radius is 250 mm. To obtain a smaller bend radius it will be necessary to bend the tube with a CNC tube bender.

The minimum bend radius is calculated with a special formula that gives us a coefficient; if the coefficient exceeds a certain value it determines the feasibility of bending for a specific radius.

Freeform Bending

Freeform bending uses one set of tooling to bend variable/multiple radii. Normally pipe and tube bending requires a bend die, a clamp die and a pressure die; with freeform bending the material is continuously guided through the machine using one die that is the size of the pipe or tube being bent. Most any radius can be achieved using a single die set.  NISSIN freeform bender

In most types of bending, a straight length is necessary between bends for clamp dies to hold the part in place while in process. Freeform bending does not require clamping, eliminating any possibility of marking the tube or pipe. Minimal tooling is required, helping to reduce lead times and, in some cases, streamline costs. Freeform bending is ideal for creating prototypes or producing complex parts with multiple radii.