BEND TOOLING INC.: Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools
The Tools Make the Bend
by Bill Tingley, Vice President & General Manager, Bend Tooling Inc.
Even though we are in the age of computer-numerically controlled (CNC) machinery, the old bromide —the tools make the bend— is truer today than ever before. As applications have become more extreme in rotary-draw bending, the design, set-up, and maintenance of tools has returned to central importance. This article is primarily concerned with the set-up of tube-bending tools on a rotary-draw tube-bending machine. In it will be described four simple steps for setting up a complete set of tools on a machine, which if executed precisely and in the correct order, will maximize bend quality, tool life, and process control.
Modern features on tube-bending machines, especially pressure die assist, have permitted many tube-bending machine operators to rely more upon manipulating machine controls to obtain adequate results than upon a precise, systematic tool set-up. Many optional controls on CNC tube-bending machines, such as variable assist pressure, circumferential boost, and in-cycle mandrel retraction, were created to make the most difficult applications practical. However, they are instead frequently used to push material about at the point of bend to overcome the defects of a poor set-up on routine applications.
This over-reliance on the machine's controls "squeezes" the tube into the desired shape through excessive use of radial force at the point of bend. Because this approach works against the axial tension on the tube that is natural to the rotary-draw process, high machine-actuated pressures must be used to force the part into shape. Use of high pressure at the point of bend often forces the machine operator to trade off tool life or process control in order to achieve acceptable bend quality. A four-step set-up procedure that combines a forward mandrel position with low machine pressure solves these problems.
The Four-Step Set-up
It is not necessary to sacrifice longer tool life or improved process control for better bend quality. All three of these objectives can be maximized — right now. Most tube-bending applications can be immediately "de-pressurized" with existing tool sets through a precise, back-to-basics, four-step set-up procedure. This set-up is known as "Forward Mandrel, Low Pressure".
The specific purpose of this procedure is to precisely position on the machine the critical working surfaces of your tools so that under minimum pressure they will exploit the natural axial flow of the tubing material and guide it into the desired shape.
The four steps in a "Forward Mandrel, Low Pressure" set-up are:
1. Mandrel nose placement
2. Direct pressure die setting
3. Wiper tip rake
4. Pressure die assist setting
The trick to successfully implementing the four-step set-up is understanding that only one aspect of bend quality can be addressed at each step.
Accurate placement of the mandrel nose ensures a stable round cross-section throughout the arc of the bend. A correct direct (or radial) pressure setting of the pressure die stops the buckling of the inside radius. A wiper tip properly raked away from the line of tangency prevents the wrinkle or series of small wrinkles that can form at the terminal end of the inside radius. And finally a balanced pressure die assist setting will push out the outside radius sufficiently to mitigate flattening and to eliminate any terminal hump.
Not all draw-bending applications require a mandrel or a wiper or the use of pressure die assist. But knowing the limits of what can be accomplished at each point of the "Forward Mandrel, Low Pressure" set-up will help you to troubleshoot problems more accurately and quickly — and perhaps determine the need for one of these things if it is absent.
Furthermore applying these steps in the above sequence will help overcome the common troubleshooting hurdle of one tool masking the failure of another — for example, excessive direct pressure die pressure covering up an improperly placed mandrel nose. The following descriptions of each step will clarify these troubleshooting issues. next page
4-Step Set-Up Procedure
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