In today's competitive environment, there is a relentless push toward greater efficiency. As a result, the tube industry demands that parts be bent faster, cleaner, easier, and at a lower cost, while at the same time improving the quality of the finished product.

Nonmandrel bending is an option for meeting these demands. It dates back about 40 years to the vertical ram-type machines that used what was then referred to as "heart-shaped tooling."

In the 1970s, the heart-shaped tooling evolved into the semiautomated rotary draw bender. However, it was employed with a static pressure die, often with unsatisfactory results.

Nonmandrel bending saw its greatest advancement in the late 1980s with the completely automated computer numerical-controlled (CNC) bending machines. Refinements to the engineering of the shaped tube groove were also made. The tube groove often allows better control of the manufacturing process, and therefore, the finished product.

This article will answer questions about how nonmandrel bending works, when it is applicable, what kind of results can be expected, and what primary machine requirements are necessary to successfully employ nonmandrel bending.

How it works

Nonmandrel bending's primary objective is to bend tubing that would normally require some form of internal support without using that support, a combination of two factors are applied to the tube simultaneously during the bending process.

First, the diameter of the tube is reduced at the point of bend, while at the same time, the circumference of the tube is reshaped into a structural form for support.

When the tubing is hard, such as stainless steel, hard copper, or titanium, good bends can be achieved with a fairly generous structural shape. Soft tubing is much more difficult to control and may require a more radically shaped tube groove.

Because the tube, at the point of bending, is in a state of plastic deformation, that deformation will remain permanent after removing the load that caused. So, after bending, the result is a piece of tubing that used to round, but which has taken on a very new and distinctive shape during the bending process.

Effects on the Tube Structure

What are the effects of deformation on the tube structure?

The first area to consider is wall thinning. In a mandrel bend, the neutral axis is typically about one third of the way outward of the inside radius.

Therefore, material inside of the neutral axis is in compression, and the material outside of the neutral axis is in tension. In other words, the outer wall is being stretched and thinned.

Because of the tube diameter reduction and the absence of the internal support, the tube can be boosted by the machine to a greater degree. The neutral axis will shift outward. The shift is generally toward the centerline radius (CLR), or even outward to the outer half of the tube. In some instances, the shift is nearly all the way out to the outerwall.

Any shift in the neutral axis is going to directly and proportionally affect wall thinning.

The next area to consider is ovality of the bent tube structure. As a result of reshaping the tube during bending, the outer wall is caused to be popped out to minimize outer wall flattening.

In the past, the greatest area of flattening could be measured 90 degrees from the tooling parting line. With non mandrel bending, however, the area with the greatest ovality loss is typically measured about 45 degrees from the tooling parting line this is a result of the tubing being reshaped to a structural form.

Ovality loss due to the restructuring of the tube varies with each application. However, it is somewhat predicable. Calculating that loss will be examined later in this article.

The final area to be considered is how favorably nonmandrel bending will stand up to the issue of finished product quality. Generally, the nonmandrel shaped bend is just as strong and stable as mandrel bends.

Two other areas should be examined regarding quality. First is the issue of flow. Non mandrel bends, as a result of the diameter reduction, are also going to experience a proportional reduction in flow.

Second, as a result of the tube's structural deformation, bent tubes will have a certain amount of out - of - roundness detectable to the eye. This out - of - roundness, in some of the more generous applications, can be minimized to some degree, but not eliminated.

Nonmandrel Tooling

A typical set of nonmandrel tooling will consist of a bend die, clamp die, and a pressure die.

The clamp die will remain unchanged from a round set of tools; that means that it will have a properly sized round tube groove transversing the entire length. Also note that the tube groove will be less thani the tube radius, to allow for gripping the tube.

Corresponding to the clamp die is the grip, or straight portion, of the bend die. The grip also has a properly sized round tube groove. Generally, depth of the round groove is half the tube diameter, which allows for maximum support and control of the tube.

The changes in tooling begin to become more obvious at or near the tangent point of the CLR. At this point, there is a transition from the round groove to the shaped groove.

In most cases, the shaped tube groove will exist throughout the remaining tube groove portion of the bend die itself. There are applications in which the shaped groove exists only in the bend portion, and then makes a transition back to the round to satisfy the demands of a particular application.

The bend die is also the piece of tooling for which a series of controlled wrinkles would be employed in extreme allowable instances.

The pressure die is primarily of traveling nature. Its length will correspond to the desired degree of bend, and will have a shaped tube groove along its entire length.

Note that the pressure die's shaped groove is generally the same shape and depth as the bend die's shaped groove. This is a departure from normal operation of round groove tooling, which has a shallow groove in the pressure die to maintain pressure throughout the bend.

Also note that the shaped tube groove in both the bend die and the pressure die are more shallow than the radius of the tube being bent.

Successfully Operating Nonmandrel Tooling

Because the tube is being resized and reshaped during the nonmandrel bending process, tool alignment is a consideration that is just as important as it is in, for example, thin walled tube mandrel tooling. Tool alignment in nonmandrel bending is achieved by using interlocking tooling.

Note that bending by nonmandrel method requires a closed die operation. This means that the pressure die must close up and operate surface-to-surface with the bend die throughout the bending process. The idea behind this is to achieve complete cavity control, and therefore, maximum material control. Material control is necessary to realize the full potential of the nonmandrel bending process.

The final tooling consideration is the overall quality of the tooling itself. Consider the quality and comprehension of the engineering involved in developing an acceptable, workable shape for the current application. The shape of nonmandrel tooling will vary with the difficulty of the bend, the material being bent, and the application of the finished product.

Also consider the choice of materials for manufacturing the dies. Because of the constant abrasion involved in resizing and reshaping the tube, only high quality tool steel should be considered for nonmandrel tooling.

In case of extreme production, it is also advisable to invest in a titanium nitride (TiN) coating or some other longwearing surface tooling.

Note that medium carbon alloy steels do not belong in the nonmandrel tooling business or on today's high speed bending machines, The additional cost in purchasing a tool manufactured from quality materials is often outweighed by the benefit of added tool life. A properly engineered, quality set of tools is capable of constant, consistent performance.

Machine Requirements for Nonmandrel Bending

To use nonmandrel bending successfully, machine function and rigidity must be considered. The most widely used form of machine is the CNC rotary draw bender, although nonmandrel tooling will also perform satisfactorily on a press bender.

As a minimum requirement, the bending machine should have a traveling pressure die. A more desirable feature would be a machine controlled pressure die assist that works in conjunction with a machine controlled tube boost. The boost can be through the carriage itself or a separate unit.

The timing and control of the boost and the powered pressure die necessitates that they be programmed from a single control.

Power and rigidity are also machine requirements for nonmandrel bending. Usually, machine capacity is rated by a material designation along with a maximum outside diameter (OD) and wall thickness. In some instances, machine capacity may even be listed as a section modules capacity. In general terms, capacity can be thought of as the ability to pull a bend.

While bending with a shaped tube groove will fall into the same basic limits, the power and rigidity needed to resize and reshape the tube during bending affect the situation.

This is because the machine's pressure die bolster must have the power to close the pressure die with the bend die. The pressure die bolster must also have the rigidity to maintain cavity control throughout the bend.

Appropriate Applications

It is now time to examine how to calculate the feasibility of nonmandrel bending and apply that calculation to specific applications.

All feasibility begins with a calculation of the bend difficulty. The formula is :

The estimated ovality of a bent tube will equal about 75 percent of the DF, depending on the material, material hardness, and how severely the tube is being worked.

After calculating a DF for a particular application, the next step is to consider the material to be bent and its relative hardness.

At this point, nonmandrel bending is just about opposite of mandrel bending ideals; harder materials are going to work better. This does not mean that nonmandrel is not applicable to soft materials, but rather, that it is easier to exceed the limits of available structural support in soft materials.

The guidelines pertaining to various materials and their maximum DF's might be stated as follows:

These limits are intended as guide lines only. The other variables, such as machine features and finished products, must be determined on a case by case basis.

Limitations of Nonmandrel Bending

It is now possible to look a little deeper into the limits of nonmandrel bending and expose some fo the possible pitfalls.

One pitfall that may be difficult to diagnose is a concentricity problem in the material being bent. One bend may be good, and the next bend will experience a wrinkle. The problem occurs when the heavier wall is on the outside of the bend; this causes excessive flattering and possibly wrinkling on the inside of the bend.

Another common problem occurs when trying to bend to big of tube on too small of a machine. This could be referred to as a rigidity problem.

Pushing the Limits

What can be done to exceed the limits of nonmandrel bending?

One possibility may be to reengineer for a harder material. However, this may not be an option.

Another option is to consider controlled wrinkles on the inside radius of the bend. These are effective and could raise the feasible difficulty factor to more than 40, but the wrinkles also need to be acceptable to the finished product.

Another possibility might be to employ a shaped tube groove with a properly sized plug mandrel. This is clearly a compromise, but may allow the elimination of a ball mandrel in favor of a simpler operation.

The most important factor affecting the limits of nonmandrel bending is the user's persistence to find better and faster ways to bend tubes. Today's limits are in place because of a driving force to improve.

Production Costs

One factor affecting production cost in nonmandrel bending is the tooling. Generally, nonmandrel tooling is easier to set up, thus providing quicker changeover.

Setup is performed using the interlocking feature, which means simply adjusting the pressure die to the bend die to control the cavity, and resetting the tube boost if needed.

Another factor affecting cost is the elimination of the expense of expendable tools. The mandrel and wiper die are the pieces of a setup that will experience the most attrition and wear. These pieces also most often experience the misadjustment that causes tool failure.

The absence of a mandrel and wiper die saves not only the cost of the tools, but also the downtime associated with replacing a failed tool.

A final factor affecting production cost would be that in nonmandrel bending, parts are bent dry and clean. The realized gain will vary with difficult operations, but in the case of dedicated part washers, the savings can be significant. Dry, clean bending can also eliminate the need for disposing contaminated cleaning solutions.

Incorporating nonmandrel bending into the manufacturing process can create new ways of performing operations. For example, with nonmandrel bending, it is possible to bend complex, multiple operation, multiple bend parts in situations when it usually would not be possible to provide support to one or more bends.

It is also possible to bend parts with prefinished ends or tubes that have had end fittings attached prior to the bending process. Nonmandrel bending also lends itself well to a more fully automated system of operation.

Summary

The success of nonmandrel bending depends on the combination of five factors:

1. OD of the tube

2. Wall thickness of the tube

3. Material being bent

4. Machine doing the bending

5. Function of the finished product

Nonmandrel bending is not the answer to all bending situations, nor has it reached its full potential. Its primary applications are found in the automotive and heating and air conditioning (HVAC) industries. Thoughtful engineering and creative manufacturing can create other applications appropriate for nonmandrel bending.