Written by Heather Schultz, Creative Marketing Director and Stacy Cramer, Vice President of Sharpe Products
When it comes to bending pipe or tube, it is not necessarily a “one shoe fits all” process that is used to achieve a quality bent piece. Given the variety of methods and variables that go into producing bends, having a basic understanding of the types of tube bending processes and their capabilities, you will better understand options for designing your part.
There are several methods for bending pipe and tube: compression bending, roll bending, freeform bending, and rotary draw bending/mandrel bending, which is the most common type. Here is more detail about these techniques:
This is a simple method of bending pipe or tube where the bend die stays stationary while a counter die bends or compresses the material around the stationary die (Fig. 1 & 2). This method requires a bend die that is the size of the desired bend radius for the part and the material is then formed around the bend die. There is no tooling (mandrel) inserted inside the tube (Fig. 3), therefore the roundness of the tube may be compromised and tight bend radii cannot be achieved with this method. Compression bending equipment has advanced to include CNC controlled machines and as well as machines that have two bending heads to allow two bends to be produced simultaneously, cutting production time in half. The dual bending head technology is ideal for symmetrical part production. Handles, furniture, and frames of many varieties are typical applications for this type of bending.
Compression bending may be the right bending method for your part if:
- Low cost, high speed production is important
- Radius does not need to be very tight
- Bend appearance or roundness is not a critical element of your bend
- Your part is symmetrical
Fig. 1 Fig. 2 Fig. 3
|Fig. 1 & 2 – The bend die stays stationary while the counter die compresses tubing around the stationary die. (CLICK IMAGES TO ENLARGE)
||Fig. 3 – Mandrels are not used in compression bending, therefore tube roundness may be compromised.
Roll bending is typically used for large radius bending. In this process a piece of pipe or tube is passed through a series of three rollers that are in a triangle configuration. The rollers apply pressure to the material, gradually forming the desired bend radius. Since the desired radius is achieved through varying pressure, not through the use of a bend die, a set of rollers is needed for the pipe or tube OD, and not for the specific bend radius. This method is ideal for large radius bends and is best suited for producing coils or large radius sweeps. (Fig. 4)
Roll bending may be the best bending method for your project if your part is made up of only large bend radii. On the contrary, if your part requires a bend radius smaller than 8 times that of the pipe or tube outside diameter, another method may be better to achieve the desired results.
|Fig. 4 – Roll bending utilizes 3 rollers configured in a pyramid shape to make large radius bends and sweeps. Mandrels and die sets are not needed in this method.
Freeform bending uses single die technology where the pipe or tube is continuously guided through the die according to programmed specifications (Fig. 5). A single die that is the size of the pipe or tube being bent is the only die required, therefore, any radius can be created with this single die, unlike other methods that require a die to be the size of the desired bend radius. This also allows multi-radii bends to be created without the use of multiple bend radius dies. A mandrel is not used with freeform bending.
Other bending methods require straight lengths between bends for clamp dies to hold the part in place during the bending process (Fig. 6). Freeform bending does not use any clamping methods, making parts with no straight lengths between bends possible. The benefit to this is that it eliminates some marking on the exterior surface of the tube or pipe that is common with clamping. Because there is minimal tooling used in this process, it also allows for the production of bends over 180 degrees, which is not possible in other methods that require a bend radius die. Freeform bending is best suited for complex parts requiring multiple radii or no straight lengths between bends, where cosmetics are important, or for prototype parts.
Freeform bending may be the right method for your part if:
- Angles greater than 180 degrees are required
- Multi-radii bends with little to no distance in between are desired
- Minimal tooling costs are desired
- Part design is a prototype
- Part cosmetics are important
- Your part material has a thinner wall
Fig. 5 Fig. 6
|Fig. 5 – Tubing is guided continuously through a single die according to programmed specifications.
||Fig. 6 – No addtional tooling is needed to produce multi-radii bends with little to no straights needed between bends.
Rotary Draw/Mandrel Bending
Rotary draw bending is the most common of the bending methods as it is the most versatile and precise method in creating high quality bends and tight radius 1D and 2D (Fig. 7) bends. To achieve high quality bends and tight radii, this method requires more tooling than the other methods. Rotary bending uses bend die sets which include a bend die, clamp die, and pressure die, along with other various components that work in a rotary action. The rotary action draws the pipe or tube around the bend radius die making the desired bend in the material. Parts can be consistently and accurately produced because the tooling or bend die sets control the tube bending process.
Rotary draw bending is the only method that allows a mandrel to be added to the process, also known as mandrel bending. (Fig. 8) A mandrel is a solid metal tool that is inserted into the pipe or tube prior to bending providing internal support to prevent defects when being bent. Defects that can occur if a mandrel is not used are rippling, flattening, and collapse. Using a mandrel also gives maximum control in keeping the ovality (tube roundness) of the tube in tact especially in thin-walled tubing, and also makes it possible to achieve tighter radii.
Rotary/mandrel tube bending technology and machinery has grown by leaps and bounds over the years. Bending machinery was once run manually and controlled by hydraulics, therefore parts with complex bends were required to be produced manually adding time and labor costs to production.
With CNC technology and all-electric machinery, those same complex parts can be produced much more efficiently. Additionally, rotary draw bending machines have been developed to include other capabilities so that parts that were previously better suited for other bending methods can be competitively produced on a rotary draw or mandrel bending machines.
For example, some modern machines are equipped with the ability to stack multiple bend radius dies allowing for the production of parts with multiple bend radii (Fig. 9). Some mandrel benders have tube generation capabilities as well which allows parts with both tight radius bends and large radius rolls to be produced on one machine through one process, minimizing part handling and costs, and maximizing efficiency. Other machines allow for the bend arm, the part of the machine that actually performs the bend process, to flip to be able to bend in both a left hand and a right hand direction (Fig. 10) allowing for efficient production of symmetrical parts and other geometries that were not previously possible using only one bending machine.
Rotary draw/mandrel bending may be right for your part if:
- Tight radius bends are required. In fact, if your radius is 1-2 times the outside diameter, this is your only option.
- If your part requires multiple radii, including tight and large radii bends
- If your part requires high accuracy and has ongoing production.
- If your part requires high quality bends (roundness and minimal deformation)
Fig. 7 Fig. 8
|Fig. 7 – Mandrel bending is the only method where complex tight radius bends can be produced.
||Fig. 8 – A mandrel is used to minimize defects that can happen during the bending process.
Fig. 9 Fig. 10
|Fig. 9 – Bend die stacking capabilities allow multi-radii bends to be produced in one process, minimizing part handling and increasing efficiency.
||Fig. 10 – Left & right bending processes allow for complex structures to be bent with minimal machine interference.
In addition to knowing your options for which bending process is best for your project, it is also important to understand how product design can impact lead times and labor. Discussing the scope of your project at the beginning stages is one helpful way to understand all of the variables involved in the manufacturing process. Contact your product specialist today to request information that can help you with your design specifications.