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 in accomplishing a quality bent piece. With a variety of methods and variables that go into producing bends, having a basic understanding of the types of tube bending processes and their capabilities and limitations, you will be better equipped to determine the best method of bending to achieve a quality tube bent product in the most economical way.
There are several methods for bending pipe and tube. In this article we will be giving an overview of the following methods, compression bending, roll bending, freeform bending, and rotary draw bending/mandrel bending (the most common type). We will discuss when they are the best option and when they may not be the best option based on the specifics of a bending project.
This is a simple method of bending pipe or tube where the bend die in place 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 great for (but not limited to) 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
Compression bending is not right for your part if:
- Bend roundness and cosmetics are important
- The part is complex with multiple bend radii
- The part requires a tight bend radius
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 pyramid configuration that 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 that would be impossible to manufacture a large bend radius die required by other applications such as compression bending and rotary draw bending. This bending method 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, this application is not right for your part.
|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. However, a mandrel (a tool that is inserted inside the tube) is not used, making it impossible to achieve tight radius bends.
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 lack of clamping on the part during production also eliminates some marking that is common in other types of bending that use 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 a great application for complex parts requiring multiple radii or no straight lengths between bends, or where cosmetics are important.
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 required.
- Minimal tooling costs are desired – One die or tooling set can create any radius, therefore radius modifications can be made as needed easily on the go.
- Internal part marking is unacceptable – A mandrel is not inserted during the bending process.
- Part design is not final – Part alterations are more cost-effective because of minimal tooling investments.
- Part cosmetics are important - Less marking on the part is a result from less tooling being used to produce the part.
Freeform bending is not right for your part if:
- Your part requires tight radii - Typically freeform bending requires a centerline radius of three times the tube diameter.
- Low cost is important – With the increased flexibility and complexity of part geometry, initial machine set-up can be more labor intensive.
- Your part requires a heavy wall thickness - Freeform bending is limited to thinner wall tubing
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 to be bent.
Rotary/mandrel tube bending technology and machinery has grown by leaps and bounds over the years. Bending machinery was run manually and controlled by hydraulics, therefore parts with complex bends were required to be produced manually adding time and labor costs to production. Now with the use of 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 repeatability
- If your part requires high quality bends (roundness and minimal deformation)
Rotary Draw/mandrel bending may not be right for you if:
- You part design is not finalized. Mandrel bending is tooling intensive and significant tooling costs may be incurred if changes needed to be made to bend dies.
- You have multiple radii on the part and none are tighter than three times the outside diameter. This part may be able to be produced on a freeform bender without requiring the expense of multiple bend dies and additional labor costs of setting up multiple bend dies
- Your part is all large radii bends. Rotary draw bending requires a bend die for the bend radius and large radius bend dies can be difficult or impossible to produce.
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 designing a cost-effective part will also aid in the decision of how your project will be produced. Let’s take a look at a few of these points.
Part Design - Another Cost Efficiency to Keep in Mind
Designing cost effective bends for your tube bending projects can be more involved then it may appear. The cost in manufacturing a pipe or tube bend is directly affected by the amount of labor and tooling needed to produce your parts. In order to have the most cost efficient tube bends, the following are some key factors to consider.
- In traditional mandrel pipe and tube bending, a bend die that is built for a specific radius is required to create a bend. These bend dies can range in cost from $2,000 up to $10,000 depending on your pipe and tube size and the radius size. For high volume jobs the cost of the bend die may not be an issue, but for small runs it may be critical. It is possible to avoid these large tooling costs in cases where there is flexibility in the radius. Consulting with your tube bending house to see which radii bend dies they own and then choosing one of these radii to design your bend will save on tooling costs and lead time.
- In addition to choosing a die that already exists selecting a radius that is greater than 1-1/2 times the tube diameter will result in a cost effective bent part because center line radii tighter than 1-1/2 times the tube diameter will be more labor intensive and will have increased part cost. For instance, a 4" O.D. tube bent on a 6" centerline radius (C.L.R.), which is 1-1/2 times the diameter, will be less labor intensive than bending a tighter bend such as ones that are between a 4" and 6" C.L.R.
- Typical pipe and tube bending processes with parts that have more than one bend require a straight length between the bends. Standard tooling can accommodate parts that allow a distance between bends of at least 3 times the tube diameter. Parts that have a distance less than 3 times the diameter are possible to produce, but may require special tooling which in turn increases tooling charges. An exception to this rule exists with a different bending technology such as freeform bending.
- When designing a part there can be a tendency to be safe with dimensional tolerances when specifying your needs, but for cost efficiency it best to keep them only as tight as necessary. Specifying tighter tolerances will make the project more labor intensive and possibly increase your costs. Again, consulting with your bending house to see what bending tolerances can be held while meeting your requirements and without unnecessarily increasing your costs will be beneficial to the design and manufacturing phases of your bent tube project(s).
- Many customers tend to use a thinner wall for their projects to reduce material costs. However, thinner wall tubing may require more labor to bend, (as in some instance it may not be able to hold the roundness of the tube as it is bent and result in ripples or wrinkling in the bend). In some cases the additional labor costs outweigh the material savings therefore it may or may not be beneficial to use a thinner wall material.
Combining Good Part Design and the Proper Bending Method
Even though tube bending machinery and technology has progressed significantly over the years the variables to keep in mind still remain the same. These include, but are definitely not limited to, the O.D. and wall thickness of the pipe or tube, geometry of the designed part (simple or complex bends), and potential machine interference with complex parts.
Other variables can always arise during a tube bending project, but by having a basic understanding of the most popular processes and what they entail, you will better able to design a cost-effective part. And to ensure this, it is always recommended that you consult with a professional bending house to discuss the scope of the project and other details that might be needed to obtain a quality bent part.