Rotary-draw bending, or draw bending for short, is the best means of consistently producing high-quality tube bends, whether the application is heavy-walled stainless steel pipe for a nuclear reactor or delicate precious-metal tubing for jewelry.  This is because, unlike compression or ram bending, the draw method fixes the line of tangency in space   Thus it becomes practical to mount tooling both inside and outside the tube to control the flow of material as the tube becomes plasticized at the point of bend (the active region of the tube before and after the line of tangency).  This is why mandrel and wiper tooling are unique to the draw method.  (See Note #1.)  In fact, the draw method is so closely associated with the use of a mandrel, it is sometimes referred to as “mandrel bending”.  The objective of this article is to reintroduce the tube-bending industry to the central importance of the mandrel in draw bending.

Records from the U.S. Patent Office show that the history of the articulated mandrel for tube-bending can be traced back to the late nineteenth century when a series of segments were chained together to form the curves of a teapot spout.  The modern mandrel was patented by Henry Spates in the late 1950’s and was initially used in the aircraft, shipbuilding, and hot rod industries.  The Spates invention featured sturdy replaceable linkage which permitted the balls attached to the nose of the mandrel to rotate in all directions in a wrist-like motion, whereas earlier mandrels typically restricted the balls to an elbow-like motion in the plane of bend.  The most significant improvement to the modern mandrel has been the inserted nose, a disposable component which takes the brunt of the wear, introduced by Bend Tooling Inc. in 1986.  (See the photo for an example of this innovation.)

Because the mandrel had been recognized as the controlling element in draw bending, this continuum of improvement over the past century focused upon practical and cost-effective features of the mandrel to meet demands of better quality and performance.  (See Note #2.)  However, over the past twenty-five years the widespread use of pressure die assist and pressure die boost, especially with the convenient controls featured by most computer numerically controlled (CNC) machines, has allowed tube benders to apply force to control the flow of material at the point of bend through the pressure die instead of the mandrel.  The effectiveness of this “high pressure” approach is limited; a fact many tube benders are now facing as bend applications become more extreme simultaneously with tightening tolerances.  A return to basics with the aggressive use of the mandrel overcomes this problem, because the mandrel nose becomes once again the workhorse in a rotary-draw bend allowing the operator to reduce the machine pressure.

The key to maximizing the performance of the mandrel in a draw-bending set-up is its nose.  The three factors to consider are the diameter of the nose (n), the radius of the nose (m), and position of the nose relative to the line of tangency (S).  We recommend using the following formulas to calculate these values (see the illustration above for reference):

Mandrel nose diameter:  n = t – ( w x 2.21 ).  (See Note #3.)

Mandrel nose radius:  If F < 50 then m = n x .1 else m = n x .02, where F = t / w.

Set-up position of mandrel nose:  S = sqrt ( ( r + ( t / 2) – w )² – ( r + ( n / 2 ) )² ) + m.

For full information on how to optimize the use of the mandrel in your next set-up, we have detailed instructions and drawings in the “Four-Step Set-Up Procedure” section.  Also visit “The Bend Tooling Encyclopedia” for complete definitions of the terms used in this newsletter.

NOTE #1:  There are isolated cases in which compression bending machines have been fixtured to mount a mandrel with a long chain of balls.  Also, ram bending machines have been retro-fitted with complicated moving double-mandrel systems.  The benefits of such improvements have been limited.

NOTE #2:  Of course, not all draw bending applications necessitate the use of a mandrel.  Most applications suitable for compression or ram bending, will not require a mandrel when produced on a rotary-draw bending machine.  Also, “heart-shape” or “elliptical” die cavities, which slightly flatten the tube into the plane of bend, increase the range of non-mandrel draw bending applications.  However, until there is wider use of the electric (i.e., servo-controlled) rotary-draw bending machine, these non-mandrel applications will remain at the margins of draw bending.

NOTE #3:  This is the formula for single-walled tubing.  For laminated (i.e., double-walled) tubing, use this formula for the mandrel nose diameter:  n = ( t – ( w_o x 2 ) ) – ( w_i x 2.21 ), where w_o = thickness of outside lamination and w_i = thickness of inside lamination.