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A Different Angle on Wiper Dies

by Bill Tingley, Vice President & General Manager, Bend Tooling Inc.

 

Rake is the angle at which the wiper die is set relative to the tube.  It is a small adjustment to the fixturing of the wiper die in a rotary-draw bending set-up, but it is critical to optimizing bend quality and tool life.  (See illustration below.)

Although some applications will not tolerate any rake of the wiper die (usually high-pressure bending jobs), a wiper should be set up with as much rake as possible to reduce drag on the tube and increase wiper life.  Because the purpose of the wiper is to stop a terminal hump from forming on the inside radius upon the completion of the bend, the wiper only needs to suppress that amount of the height of the terminal hump which exceeds the tubing materialís point of elasticity.  Therefore, raking the wiper preserves its life by reducing the force of the terminal hump against the feathered edge.  Otherwise at zero-rake the feathered edge must suppress the entire height of the terminal hump, including that portion of its height that the tubingís elasticity will reduce naturally on its own at the completion of the bend.

Of course, material variations and other considerations mean, as a practical matter, a wiper will be set at less than the ideal rake.  For materials with little elasticity, especially stainless steels, this means zero-rake.  Such inelastic materials resist compression.  Therefore, the plastic point-of-bend region of the intrados requires complete containment by tooling both ahead of the line of tangency and behind it.  The bend die cavity together with the wiper die cavity at zero-rake provide this containment.  When such a zero-rake application requires high direct pressure, a solid-body wiper die may be preferred to a pocket-style inserted wiper die in order to act as a backstop for the force the pressure die is under.  Whether solid or inserted, the feathered edge should be cut with offset geometry (sometimes called "aero-cut") instead of the standard simple-sweep geometry that accommodates raking.

Instead of an angle, rake is often expressed as a measure of the gap between the back edge of the wiper and the outside diameter of the tube.  However, expressing rake as this particular distance can be problematical.

First of all, it focuses the machine operatorís attention on the non-working end of the wiper.  Setting this gap at the correct distance is no guarantee that the wiperís feathered edge is properly nested in the bend die cavity.  Even if the feathered edge is in contact with the bend die cavity, it may not be in the correct position relative to the line of tangency.

Second, variations in wiper length will result in variations in rake if this gap distance is held.  This may not be critical if reduced rake is the result; the wiper will just not last as long as it could.  However, if excessive rake results, then terminal humping will become a problem.

An alternative to using this gap to set rake is to use the distance between the line of tangency and the feathered edge.  Once the rake is determined with a feathered edge properly nested in the bend die cavity, a linear measurement must be taken between the feathered edge and the line of tangency along the circumference of the bend die cavity.  When the time comes to replace the wiper, the feathered edge of the new wiper is nested in the bend die cavity flush with the line of tangency.  Then the new wiper is slowly rotated away from the line of tangency, maintaining secure contact between the feathered edge and the bend die cavity, until the feathered edge reaches the distance previously measured.

By this means, the established rake for an application can be repeatedly set without regard to variations in the design of the wiper die.

© Bend Tooling Inc., 2005

Past Technical Articles

SPECIFYING MATERIALS FOR TUBE-BENDING TOOLS

THE WIPER FEATHERED EDGE

POPPET LINKS VS. SPLIT LINKS

THE "FORWARD MANDREL" SECRET OF TUBE-BENDING

ASSEMBLY OF MANDRELS WITH POPPET-STYLE LINKS

A DIFFERENT ANGLE ON WIPER DIES


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