Tube-bend tooling: square-back wiper dies or wiper inserts?

Tight bend radii, thin walls create need for wiper dies

TPJ - THE TUBE & PIPE JOURNAL® JUNE 2005

November 8, 2005

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Wiper dies are a fundamental requirement in modern tube bending applications in which tubes are bent at increasingly tight bend radii with increasingly thinner wall thicknesses. The design of the wiper die plays a key role in its performance and durability, as does its manufacturing method and the material from which it has been constructed.

Wiper dies are a fundamental requirement in modern tube bending applications in which tubes are bent at increasingly tight bend radii with increasingly thinner wall thicknesses. The design of the wiper die plays a key role in its performance and durability, as does its manufacturing method and the material from which it has been constructed.

Tool Purpose and Function Within the Tooling Set

As the D ratio (the centerline bend radius divided by the tube diameter) decreases and the wall factor (tube diameter divided by the wall thickness) increases, the tube is less resistant to bending. As a result, it has a greater tendency to collapse rather than follow the form of the bend. Consequently, the material must be contained physically—surrounded at all points, internally and externally—where the bend takes place, to prevent tube collapse and any resulting deformities, such as wrinkles. This is needed in the tangent area, predominantly where the tube changes from a straight to a bent section and also where the material flows ahead and behind of the tangent point.

The bend die, clamp die, and pressure die provide this containment to some extent and can be used in applications in which the difficulty factor (wall factor divided by D ratio) is low and the tube is less prone to collapse. However, more complex applications require more support and containment. A mandrel inside the tube and a wiper die behind the tangent point and opposite the pressure die provide adequate containment, prevent collapse, and accommodate material flow (see Figure 1).

Figure 1
Complex applications require a mandrel inside the tube and a wiper die behind the tangent point and opposite the pressure die to provide adequate containment, prevent collapse, and accommodate material flow.

It is a common misconception that these tools remove deformities after they occur. Actually, the wiper die is designed to provide critical containment in the tangent area and to help control the material flow behind the tangent while the tube is being bent. The forces of compression in the tangent area inside the tube extend into the unbent portion of the tube behind the tangent; without the wiper die supporting this area, it would collapse.

Two primary wiper dies styles are square-back wiper dies and wiper inserts. In deciding which style of wiper dies to use, you need to consider several important factors, which depend on the tube parameters and material.

Considerations for Tool Selection

The wiper tip and heel are the most important parts of the wiper die, because they must seat into the bend die groove behind the tangent to contain the tube as much as possible without interfering with the bending process. The heel should be in full contact with the bend die groove to ensure rigidity during the bending process and to prevent unseating or deforming the tip. The wiper die groove is also an important feature, because it is of sufficient length to control the material flow behind the tangent area but not allow it to pass through. Therefore, it follows that the body of the wiper die itself is important in ensuring the wiper is long enough and rigid enough to support the bending process.

Square-back Wiper Die. This die is manufactured from a single piece of material. Typical square-back wipers are slightly wider and thicker than the tube and provide sufficient rigidity for bending (see Figure 2). Based on the standard mounting parameters for the machine, not only are they long enough to contain material flow, they generally are also long enough to accommodate recutting when the tip becomes worn.

Figure 2
The length and rigidity of square-back wiper dies make them particularly suitable for difficult-to-bend grades of metal and bending applications with low to very low D ratios and high to very high wall factors, such as aerospace, performance motorsport, and HVAC industries.

These design parameters make square-back wiper dies acceptable for almost all bending applications. However, their length and rigidity make them particularly suitable for bending applications with low to very low D ratios and high to very high wall factors, such as aerospace, performance motorsport, and HVAC industries. Tubes made of difficult-to-bend grades of metal, such as stainless steel, titanium, INCONEL® alloys, and very soft aluminum, often are more controllable with this type of wiper die design.

Wiper Die Insert. The wiper die insert restricts only the working area of the wiper die, so it usually is shorter than its square-back counterpart. The insert design consists of a steel holder unit that is fixed into position on the machine bracket and a replaceable insert that fits into the holder and contains the tube groove, wiper tip, and heel (see Figure 3).

The insert is usually semicircular in profile, and usually its diameter is a little larger than the tube diameter. Generally along its length there are flats, or raised steps, to provide a rigid seat in the holder unit and a securing thread to fix it into position. As the tip wears, an operator can remove the insert and replace it with another insert quickly and without disturbing the machine setup.

Figure 3
Wiper die inserts can be removed quickly and are suitable for high-volume production and low- to medium-difficulty bends in which the wall factor and D ratios are a little more generous, such as in the automotive industry.

For this reason, wiper die inserts are suitable for high-volume production. Wiper inserts are generally, but not exclusively, used for low- to medium-difficulty, high-volume bending applications in which the wall factor and D ratios are a little more generous, such as in the automotive industry.

Applications. While a square-back wiper die is good for all-around applications and is especially suitable for more difficult bending applications, it is considerably more expensive than an insert system. Because it is machined from a solid block, it is more massive and requires more machining time to manufacture. Initially it must be machined square, with an accurate tube groove along the entire length and an accurate heel through the full thickness of the material block. As the tip wears, the wiper die can be recut by repeating the heel machining process, which shortens the wiper die. Still, it must be noted that if the tube groove itself is worn, it must also be remachined to ensure optimum performance and accurate recutting of the tip and heel.

Wiper inserts usually are less costly to produce than square-back wiper dies, because multiple wiper inserts can be manufactured from a thick-wall tube, and because the shorter and smaller inserts require less material. In addition, changeover time is shorter with an insert than with a square-back wiper die. However, initial setup requires the purchase of the steel holder, and the inserts must be replaced and disposed of more often than the square-back wiper die.

In both cases, the wiper die tip can be machined using one of two different CNC machining methods, depending on the bending application. Considering the way that the wiper die heel and tip seat into the bend die groove, the tip thickness would be zero at tangent, theoretically, and would increase gradually with the distance and angle back from the tangent point (seeFigure 4). Of course, a zero or near zero tip thickness is not physically possible, but there are two alternatives.

Figure 4

The first alternative is to select a minimal material thickness at the tangent point where the wiper die material will remain intact during machining and give optimum performance during bending. This method allows for a more rapid machining process and resultant cost savings. However, as the wiper tip is seated with a material thickness on the theoretically zero thickness tangent point, it can interfere with material containment and may indent tubes with high wall factors. It also may cause excessive drag, possibly causing clamp slippage and deficient bend quality. Nonetheless, this option performs relatively well in most automotive bending applications.

The second alternative is to calculate the optimal material thickness as before and also to calculate where tip thickness falls behind the tangent point on the wiper die tip, working back from the theoretical zero position. The tip then can be machined to the theoretical zero measurement and subsequently trimmed back to the optimal thickness position where the material remains intact. Although this method carries a marginal additional cost over the first method, it does allow for the wiper insert to be seated into the bend die with an optimal containment position, which is more suitable for high-wall-factor materials. As the tip does not interfere with the material flow, it may last longer too.

Additional Considerations. Material selection and lubrication are the remaining considerations for tool selection. Aluminum-bronze alloys perform well, with minimal drag resistance for stainless steel, titanium, INCONEL® alloys, and similar alloys. Mild steels also can be bent with this tool material or other high-quality tool steels. Chrome-plated tool steels perform better on aluminum and copper, with the softer grades benefiting from an additional polytetrafluoroethylene (PTFE) treatment, which is added after chroming to decrease the drag further caused by the tube passing through the wiper die groove.

Most materials benefit from lubrication on the wiper die; however, light oils usually are ineffective. The most effective lubricants are those that do not disperse under the pressures of the bending process. Many gels and drawing oils are suitable.

Barry Rooney is European sales manager for tube forming and tooling solutions, AddisonMckee Ltd., 188 Bradkirk Place, Walton Summit Centre, Bamber Bridge, Preston, Lancashire PR5 8AJ, U.K., +44-0-1772-334511, fax +44-0-1772-323227,bmr@addisonmckee.co.uk; P.O. Box 749, 2695 State Route 73 South, Wilmington, OH 45177, 937-382-4490, fax 937-382-4963, www.addisonmckee.com.



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