Wiper Die Alignment and Setback in Tube Bending

Wrinkles on the inside radius usually appear when the material on the intrados loses lateral stability as it enters the bend zone. During rotary draw bending, the inside wall experiences severe axial compression. If the tube wall ratio is thin relative to the center line radius, compressive force causes the unsupported wall to buckle immediately behind the tangent point. While internal mandrels support the tube profile against cross-sectional flattening, the wiper die fills the gap between the fixed rear tooling and the rotating bend die to prevent compressive wave formation.

Mechanism of Inside-Radius Hump and Wrinkle Formation

As the bend die rotates, material on the outside radius stretches under tension while material on the inside radius compresses. This compression creates localized hoop stress. Without a properly mounted wiper die, the unsupported tube wall immediately behind the point of bend tangent buckles toward the center of the tube or folds outward against the bend die groove. In thin-wall steel, stainless, or aluminum tubing, even minor gaps between the tool geometry and the tube outer diameter allow small ripples that propagate into severe folds as the bending rotation continues.

Precision multi-axis machines like the CNC Boru Bükme Makinesi (Griffin) maintain consistent axis positioning throughout the draw cycle, but mechanical tool fitment remains the defining factor for controlling inside-radius stability. The wiper die must physically contact the tube profile right up to the tangent line to suppress compressional instabilities before they begin.

Setting Wiper Die Rake Angle and Tip Distance

Proper wiper die setup requires precise adjustments to two main geometry factors: setback distance from the tangent point and rake angle relative to the tube centerline. The tip of the wiper die features a thin, feathered edge matched to the exact radius of the bend die groove.

• Setback Distance: The tip of the wiper die should sit slightly behind the tangent point. Placing the tip directly on or past tangent increases wear and risks shearing the tip off during bending. Moving the tip too far back leaves an unsupported gap where material buckles under compression.
• Rake Angle: Rake angle refers to the slight angular offset between the body of the wiper die and the straight tube centerline. Setting a slight rake angle (typically 0.5 to 1.5 degrees depending on material and wall thickness) ensures that only the feathered tip makes contact with the tube near tangent, reducing drag friction along the remaining body of the tool.

On manual or semi-automatic systems such as the Yarı Otomatik Boru Bükme Makinesi (Atlas BR), rake angle and setback are adjusted mechanically using alignment screws on the wiper holder block. Proper mounting requires tightening the wiper firmly into the holder while checking clearance with feeler gauges or optical alignment aids before running test bends.

Identifying Wiper Wear and Friction Conditions

Tooling wear at the feathered tip is a common root cause of sudden bend defects after successful production runs. This is one of the first things we check when a customer reports wrinkles at the start of the bend. Because the tip of the wiper die is thin, concentrated frictional heat and high surface pressure cause rapid wear or localized galling, especially when bending stainless steel or titanium alloys without adequate lubrication.

A worn wiper tip develops a rounded edge or small micro-chips. This alteration increases the effective gap at the tangent point, allowing thin wall material to slip under the tool edge and form characteristic micro-wrinkles. Severe friction can also cause material pickup, where tube material cold-welds to the wiper tip, scoring subsequent tubes along the intrados. Inspecting the feather edge under magnification during scheduled maintenance helps detect tip erosion before part quality drops below tolerance.

Correcting Alignment Parameters in ETU Control Systems

On automated Dural machines, ETU software provides interface monitoring for tooling parameters, positioning offsets, and axis sequence timing. While physical alignment of the wiper die is performed mechanically at the mounting bracket, verifying machine repeatability and force control through ETU helps maintain stable process conditions over high production volumes.

When troubleshooting inside-radius wrinkling on the shop floor, follow this systematic check sequence:

1. Inspect the wiper die feather edge for wear, chipping, or aluminum/steel material pickup.
2. Verify that wiper die radius matches the tube outer diameter without excessive clearance.
3. Check the physical setback distance relative to the bend die tangent point.
4. Confirm adequate supply of high-pressure bending lubricant directly to the wiper contact zone.

To evaluate tooling selection, wiper die material choice, or bending parameters for your specific production requirements, provide your STEP model or scaled drawing, material specification, wall thickness, center line radius, and annual production volume.