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Inside the world of tube bending

Across industries, the demand for complex tube and pipe bending continues unabated. Be it for structural components, mobile medical equipment, frames for all-terrain or utility vehicles, or even metal safety bars in bathrooms, each project is different.

To achieve the desired results requires good equipment and, especially, the right expertise. And like any other manufacturing discipline, efficient tube bending starts with the core vitals, the fundamental concepts that form the foundation of any project.

Tube Terminology
Some core vitals help determine the scope of a tube or pipe bending project. Factors such as the material type, the end use, and the estimated annual usage directly impact the manufacturing processes, the costs involved, and the delivery lead time.

The first core vital is the degree of bend (DOB), or the angle to which the bend is formed. Next is the centerline radius (CLR), which runs along the centerline of the pipe or tube to be bent. As a rule, the tightest achievable CLR is one times the pipe or tube diameter. Double the CLR to calculate the centerline diameter (CLD), the distance from the tube or pipe centerline axis across to the other centerline of a 180-degree return bend.

The inside diameter (ID) is measured across the widest part of the inside opening of the pipe or tube. The outside diameter (OD) is measured across the widest area of a pipe or tube, including the wall. Finally, the nominal wall thickness is measured between the outside and inside surface of a pipe or tube.

Industry standard tolerance is ±1 degree on bend angles. Each company has an internal standard, which may be based on the equipment being used as well as the experience and knowledge of the machine operator.

How to Name Tube and Pipe
Tube is measured and referred to according to its OD and gauge, meaning its wall thickness. Common gauges include 10, 11, 12, 13, 14, 16, 18, and 20. The lower the gauge, the thicker the wall: 10-ga. tube has a 0.134-in. wall, whereas 20-ga. tube has a 0.035-in. wall. A tube with an OD of 1½ in. and a 0.035-in. wall is called out on part prints as “1½-in. 20-ga. tube.”

Pipe is specified by the nominal pipe size (NPS), a nondimensional number in inches that describes the diameter, and a schedule (or Sch.) for wall thickness. Pipe has a variety of wall thicknesses, depending on its use. Popular schedules include Sch. 5, 10, 40, and 80.

A pipe with a 1.66-in. OD and a 0.140-in. wall is called out on a part’s drawing by the NPS followed by the schedule—in this case, “1¼-in. Sch. 40 pipe.” Pipe schedule charts specify the OD and wall thickness for a correlating NPS and schedule.

A Word on Wall Factor
Wall factor, the ratio between the OD and wall thickness, is another vital element in tube bending. Working with thin-wall material—at or below 18 ga.—may require more support at the arc of the bend to prevent wrinkling or collapse. In this case, a good-quality bend will require a mandrel along with other tooling.

The other important element is the D of bend, the diameter of the tube in relation to the bend radii, often called out by however many times larger the bend radius is than the value of D. For example, a 2D bend radius of a 3-in.-OD tube would be 6 in. The higher the D of bend, the easier the bend will be to form. And the lower the wall factor, the easier the bend. This correlation between wall factor and D of bend helps determine what will be needed to begin a tube bending project.

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