When There is Trouble Around the Bend

Just as Alan Jackson made mention that it is five o’clock somewhere, you can bet that there is someone in the world every minute of the day wondering just how they are going to get better bend quality.

More About Bend Quality 

As you probably already know, getting the bend quality that you need is not always a 10 minute job. The variables can be endless.  Wall thickness, material, material hardness, bend radius, degree of bend, and on and on.  Most often the job is not on the edge of reality.  That is a good thing.  Maybe you just need to squeeze a little more perfection out of the current tooling.

Real World Example

Take for example a one inch tube on a two inch centerline bend radius (1 on a 2).  Let’s say the wall is .049” and the material is welded steel.  Believe it or not, this is a common bend in the United States.  You will find this type of bend in everything from furniture and heat transfer to lawn and garden applications.  The bending process can be rotary wipe or rotary draw; it doesn’t matter.  Bend quality is the goal.

The bend factors are not too bad.  The wall factor is 1/.049 or 20.4 and the D of bend is 2D (2” CLR /Ø1).  They could be worse like trying to bend a 1” tube on a 2” centerline radius when the wall is .025”.  So let’s be thankful that the wall we are looking at is .049.  That will ensure this article ends on a positive note.

Depending on the level of quality required, the 1 on a 2 using .049 wall most often does not require a mandrel or wipe die.  However, there are several reasons why this bend can be troublesome.  With a worn set of tools, the tube may be slipping in the clamp.  That can cause a kink or wrinkle on the inside of the bend; Figure 1.  If you prevent the tube from slipping in the clamp, the wrinkle may go away.  With sufficient clamping pressure, maybe more pressure from the pressure will make that wrinkle go away.

After making a bend, you may notice that the cross section of the tube in the bend area is not too round; see Figure 1 cross section above.  In the world of tube bending, this would relate directly to the ovality.  Ovality is often specified on a drawing and is a quality control feature.  If the ovality is too large, then the tube is not round enough in the bend area.

This is where a new set of tools may come into play.  A worn set of tools may be past the point of where they can apply the needed pressures (squeeze) on the tube during the bending process.  Take for example the die set in Figure 2.  This die set is mounted on a serpentine tube bender and can bend both CW and CCW.

The tooling consists of a set of two split die assemblies.  The dies have the ability to close or squeeze on tube during the bend operation.  This can make it convenient to change the ovality for the better.  Quite often a little more squeeze can result in better ovality; Figure 3.  In the case of the 1 on a 2, the ovality can improve just by stressing the tube in a direction opposite that it will want to flatten prior to the start of the bend.

If you don’t have a split die actuator on your tube bender, don’t sweat it.  The tooling expert that you have been working with for years will know how to get the same results with a set of standard tooling.  The goal here is to understand what forces you may need to apply to the tube so that you can get past the quality concerns and move on to more prosperous 5 o’clock endeavors.


About the Author

George Winton, P.E. designs and builds CNC tube fabrication equipment for Winton Machine in Suwanee, GA. He can be reached at gwinton@wintonmachine.com or 888.321.1499

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All of our semi-rigid coax and tube fabrication machines at Winton are designed, manufactured, and tested in-house.  We have a large line of standard products as well as the ability to engineer the best solution for our customer’s needs.  Our experienced sales staff makes sure that our customers can justify their capital equipment investment by offering a solution that is exactly what they need in order to manufacture their parts.  Please contact us today to discuss your project.