Above: A machine operator programs the CNC tube bending machine for the specifi ed operations
December, 2023- Designing products with components that require custom tube bending can be more involved than it may initially appear. Factors such as tooling, material characteristics and labor play a role in the manufacturing approach to tube bending and associated services. Here are some of the technical factors involved in custom tube bending, along with some pointers that can help companies collaborate with their preferred vendors.
Freeform bending is commonly used for parts with angles greater than 180 degrees
Examples of tube bending for the automotive and aerospace aftermarket. Below: Some parts require multiple operations, such as tube bending, tube laser cutting and end forming
TOOLING, RADIUS AND MULTIPLE BENDS
Tube bending requires tooling. The primary tooling used on a bending machine is the bend die, along with a clamp die, pressure die, mandrel and wiper. Most machinery suppliers will have a stable of tooling to choose from; however, in some cases, tooling must be created to accommodate the dimensions of the part. If there is flexibility to adjust the part’s specifications, one alternative is to use an existing bend die with a similar radius to the original part design, thus saving on the additional tooling costs and lead time.
Generally, the tightest achievable center line radius is one times (1X) the pipe or tube diameter. Whenever possible, try to choose a center line radius of one-and-a-half times or greater than the tube diameter to save labor costs. Find out what tooling is available from your vendor so you know what options are available.
It is common to have more than one bend on a part, and these parts require a straight length between the bends to secure the part with a clamp. Standard tooling can accommodate a distance between bends of at least three times the tube diameter. Shorter clamp areas of two times the tube diameter can be achieved but may result in cosmetic marking from clamp modifications. Parts with a short length between bends may be good candidates for freeform bending, which is commonly used for parts with angles greater than 180 degrees, or those that require multi-radius bends with little to no distance in between.
TALK ABOUT TOLERANCES
In general, the tolerance is the stated limit of acceptable deviation from a nominal dimension called out on a drawing. The use of tolerances helps to ensure that the final product is suitable for its end use, especially if the part is going into a larger assembly or must mate up with another piece. When designing a part, there can be a tendency to be safe with dimensional tolerances but for cost efficiency, it can be helpful to keep them only as tight as necessary. That’s because tighter tolerances may add labor but can be achieved with the right machine technology.
The two most common types of tubing used by fabricators to create custom parts are seamless and welded. The choice between the two is determined by application type, material availability and cost. In tube bending, the weld seam must be oriented to the neutral, or zero, axis (top or bottom relative to the bend plane) so that it is not exposed to stretching or contracting. For tight bends, this is especially critical because bending on a tight radius requires more torque, which can cause cracks along the seam. In some cases, where a seam would interfere with the aesthetics of the product, the weld seam location will be called out in the drawing file or communicated to the fabricator.
DIMENSIONS UP, DOWN, LEFT, RIGHT
The ratio between the tube’s outer diameter and wall thickness, called wall factor, is one of the key elements used to assess the difficulty of a tube bend, and that measurement must be known from the outset of the project. A wall that is thin compared to the OD requires more support at the arc of the bend to prevent wrinkling or collapse. During bending, the wall on the outside radius can thin up to 33 percent, depending on the radius and other factors. In this case, use of a mandrel along with other tooling is necessary to achieve good quality.
Known as the “D of Bend,” this is the diameter of the tube in relation to the bend radii, often called out by however many times larger it 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 number for the D of the bend, the easier it is to form the bend. And, the lower wall factor, the easier the bend.This correlation, between wall factor and D of bend, helps determine what resources will be needed for a tube bending project.
PART DRAWING
The drawing for the part, which details its specifications, is necessary for projects that require custom tube bending and laser cutting. This file provides the key pieces of information needed to manufacture the part. The drawing shows all dimensions and measurements, material type, tolerances required, revision information, part number and relevant notes. These details are used to determine costs, to set up how a part will be manufactured and, in some cases, how the full order will be handled or shipped.
Although the practice of tube bending has progressed significantly over the years, many of the variables that impact the simplicity or complexity of the job remain the same. These include the OD and wall thickness, the geometry of the designed part (simple or complex bends), the available tooling and the lead time to secure the type of raw material needed for the project.
Sharpe Products, 800/879-4418, http://sharpeproducts.com/
