The mandrel bending operation begins its cycle.The mandrel is inserted into the inner diameter of the tube.The bending die (left) determines the radius.The clamping die (right) guides the tube around the bending die to determine the angle.
Across industries, the need for complex tube bending continues unabated.Whether it’s structural components, mobile medical equipment, frames for ATVs or utility vehicles, or even metal safety bars in bathrooms, every project is different.
Achieving the desired results requires good equipment and especially the right expertise.Like any other manufacturing discipline, efficient tube bending begins with the core vitality, the fundamental concepts that underlie any project.
Some core vitality helps determine the scope of a pipe or pipe bending project.Factors such as material type, end use, and estimated annual usage directly affect the manufacturing process, the costs involved, and delivery lead times.
The first critical core is the degree of curvature (DOB), or the angle formed by the bend.Next is the Centerline Radius (CLR), which runs along the centerline of the pipe or tube to be bent.Typically, the tightest achievable CLR is double the diameter of the pipe or tube.Double the CLR to calculate the centerline diameter (CLD), which is the distance from the centerline axis of the pipe or pipe through another centerline of a 180-degree return bend.
The inside diameter (ID) is measured at the widest point of the opening inside the pipe or tube.The outside diameter (OD) is measured over the widest area of a pipe or tube, including the wall.Finally, the nominal wall thickness is measured between the outer and inner surfaces of the pipe or tube.
The industry standard tolerance for bend angle is ±1 degree.Every company has an internal standard that may be based on the equipment used and the experience and knowledge of the machine operator.
Tubes are measured and quoted according to their outside diameter and gauge (i.e. wall thickness).Common gauges include 10, 11, 12, 13, 14, 16, 18, and 20.The lower the gauge, the thicker the wall: 10-ga.The tube has a 0.134 inch.wall, and 20-ga.The tube has a 0.035 inch.wall.1½” and 0.035″ OD tubing.The wall is called “1½-in” on the part print.20-ga.tube.”
Pipe is specified by a nominal pipe size (NPS), a dimensionless number describing the diameter (in inches), and a wall thickness table (or Sch.).Pipes come in a variety of wall thicknesses, depending on their use.Popular schedules include Sch.5, 10, 40 and 80.
A 1.66″ pipe.OD and 0.140 inches.NPS marked the wall on the part drawing, followed by the schedule – in this case, “1¼”.Shi.40 tubes.”Pipe plan chart specifies the outer diameter and wall thickness of the associated NPS and plan.
The wall factor, which is the ratio between the outside diameter and the wall thickness, is another important factor for elbows.Using thin-walled materials (equal to or below 18 ga.) may require more support at the bend arc to prevent wrinkling or slumping.In this case, quality bending will require mandrels and other tools.
Another important element is the bend D, the diameter of the tube in relation to the bend radius, often referred to as the bend radius many times larger than the value of D.For example, a 2D bend radius is 3-in.-OD pipe is 6 inches.The higher the D of the bend, the easier the bend is to form.And the lower the wall coefficient, the easier it is to bend.This correlation between Wall Factor and Bend D helps determine what is required to start a pipe bend project.
Figure 1. To calculate percent ovality, divide the difference between the maximum and minimum OD by the nominal OD.
Some project specifications call for thinner tubing or piping to manage material costs.However, thinner walls may require more production time to maintain the shape and consistency of the tube at bends and eliminate the chance of wrinkling.In some cases, these increased labor costs outweigh the material savings.
When the tube bends, it can lose 100% of its round shape near and around the bend.This deviation is called ovality and is defined as the difference between the largest and smallest dimensions of the outer diameter of the tube.
For example, a 2″ OD tube can measure up to 1.975″ after bending.This 0.025 inch difference is the ovality factor, which must be within acceptable tolerances (see Figure 1).Depending on the end use of the part, the tolerance for ovality can be between 1.5% and 8%.
The main factors affecting ovality are elbow D and wall thickness.Bending small radii in thin-walled materials can be difficult to keep ovality within tolerance, but it can be done.
Ovality is controlled by placing the mandrel within the tube or pipe during bending, or in some part specs, using (DOM) tubing drawn on the mandrel from the start.(DOM tubing has very tight ID and OD tolerances.) The lower the ovality tolerance, the more tooling and potential production time is required.
Tube bending operations use specialized inspection equipment to verify that formed parts meet specifications and tolerances (see Figure 2).Any necessary adjustments can be transferred to the CNC machine as required.
roll.Ideal for producing large radius bends, roll bending involves feeding the pipe or tubing through three rollers in a triangular configuration (see Figure 3).The two outer rollers, usually fixed, support the bottom of the material, while the inner adjustable roller presses on the top of the material.
Compression bending.In this fairly simple method, the bending die remains stationary while the counter-die bends or compresses the material around the fixture.This method does not use a mandrel and requires a precise match between the bending die and the desired bending radius (see Figure 4).
Twist and bend.One of the most common forms of tube bending is rotational stretch bending (also known as mandrel bending), which uses bending and pressure dies and mandrels.Mandrels are metal rod inserts or cores that support the pipe or tube when bent.The use of a mandrel prevents the tube from collapsing, flattening, or wrinkling during bending, thereby maintaining and protecting the shape of the tube (see Figure 5).
This discipline includes multi-radius bending for complex parts requiring two or more centerline radii.Multi-radius bending is also great for parts with large centerline radii (hard tooling may not be an option) or complex parts that need to be formed in one full cycle.
Figure 2. Specialized equipment provides real-time diagnostics to help operators confirm part specifications or address any corrections necessary during production.
To perform this type of bending, a rotary draw bender is provided with two or more tool sets, one for each desired radius.Custom setups on a dual head press brake – one for bending to the right and the other for bending to the left – can provide both small and large radii on the same part.The transition between left and right elbows can be repeated as many times as needed, allowing complex shapes to be fully formed without removing the tube or involving any other machinery (see Figure 6).
To get started, the technician sets up the machine according to the tube geometry listed in the bend data sheet or production print, entering or uploading the coordinates from the print along with length, rotation and angle data.Next comes the bending simulation to ensure the tube will be able to clear the machine and tools during the bending cycle.If the simulation shows a collision or interference, the operator adjusts the machine as needed.
While this method is typically required for parts made of steel or stainless steel, most industrial metals, wall thicknesses and lengths can be accommodated.
Free bending.A more interesting method, free bending uses a die that is the same size as the pipe or tube being bent (see Figure 7).This technique is great for angular or multi-radius bends greater than 180 degrees with few straight segments between each bend (traditional rotational stretch bends require some straight segments for the tool to grasp).Free bending does not require clamping, so it eliminates any possibility of marking tubes or pipes.
Thin-walled tubing—often used in food and beverage machinery, furniture components, and medical or healthcare equipment—is ideal for free bending.Conversely, parts with thicker walls may not be viable candidates.
Tools are required for most pipe bending projects.In rotary stretch bending, the three most important tools are bending dies, pressure dies and clamping dies.Depending on the bend radius and wall thickness, a mandrel and wiper die may also be required to achieve acceptable bends.Parts with multiple bends require a collet that grips and gently closes to the outside of the tube, rotates as needed, and moves the tube to the next bend.
The heart of the process is bending the die to form the centerline radius of the part.The die’s concave channel die fits with the outer diameter of the tube and helps hold the material as it bends.At the same time, the pressure die holds and stabilizes the tube as it is wound around the bending die.The clamping die works in conjunction with the pressing die to hold the tube against the straight segment of the bending die as it moves.Near the end of the bend die, use a doctor die when it is necessary to smooth the surface of the material, support the tube walls, and prevent wrinkling and banding.
Mandrels, bronze alloy or chromed steel inserts to support pipes or tubes, prevent tube collapse or kink, and minimize ovality.The most common type is the ball mandrel.Ideal for multi-radius bends and for workpieces with standard wall thicknesses, the ball mandrel is used in tandem with the wiper, fixture and pressure die; together they increase the pressure needed to hold, stabilize and smooth the bend.The plug mandrel is a solid rod for large radius elbows in thick walled pipes that do not require wipers.Forming mandrels are solid rods with bent (or formed) ends used to support the interior of thicker walled tubes or tubes bent to an average radius.Additionally, projects requiring square or rectangular tubes require specialized mandrels.
Accurate bending requires proper tooling and setup.Most pipe bending companies have tools in stock.If not available, tooling must be sourced to accommodate the specific bend radius.
The initial charge to create a bending die can vary widely.This one-time fee covers the materials and production time required to create the required tools, which are typically used for subsequent projects.If the part design is flexible in terms of bend radius, product developers can adjust their specifications to take advantage of the supplier’s existing bending tooling (rather than using new tooling).This helps manage costs and shorten lead times.
Figure 3. Ideal for the production of large radius bends, roll bending to form a tube or tube with three rollers in a triangular configuration.
Specified holes, slots, or other features at or near the bend add an auxiliary operation to the job, as the laser must be cut after the tube is bent.Tolerances also affect cost.Very demanding jobs may require additional mandrels or dies, which can increase setup time.
There are many variables manufacturers need to consider when sourcing custom elbows or bends.Factors such as tools, materials, quantity, and labor all play a role.
Although pipe bending techniques and methods have advanced over the years, many pipe bending fundamentals remain the same.Understanding the fundamentals and consulting with a knowledgeable supplier will help you get the best results.
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