Bending Tubes Without Wrinkles

When setting up a new tube bending job you may often have concerns about how the setup process is going to go. The biggest question is "how soon can I start making parts?" When the centerline radius is generous and the wall is thick, wrinkles are about the last thing you expect to see on the inside of a bend. Nevertheless, even when the industry standard charts suggest that a wiper die is not needed, wrinkles have been known to appear.

What is Causing My Wrinkles?

To begin with, trust the charts provided by suppliers of your tube bending tooling. These charts have been right on for decades. If the charts say a wiper die is not needed, this should be an indication that you need to be looking elsewhere for the cause of any wrinkles.

How to Reduce Wrinkles in Tube Bending Operations | Winton Machine

The picture above shows a partial wrinkle in two different bends. The bends started out good until about 30± degrees, then the wrinkles started to appear. - But why?

After seeing this, your first reaction should be to increase the pressure on the die; because this will often eliminate the wrinkles. If you have increased this die pressure and found that it has had little to no effect, it is time to consider other possibilities. There are several valid reasons as to why wrinkles may still be present.

Other Ways to Prevent Wrinkles 

When a job is first set up, the pressure holding a die in position is often set when the bend arm is at 0 degrees. Let’s face it, this is a convenient place to set the die pressure. However, as a swing arm rotates thru a bend, any significant runout (T.I.R) of the bend die relative to the pressure die can cause the wrinkles as shown in the picture above.  In short, excessive concentricity of the bend die can cause relative separation between the bend die and pressure die. This separation of the two dies causes the pressure on the tube to taper off as the swing arm rotates. This in turn increases the possibilities for wrinkles to form.

It should be mentioned that the wrinkle condition described above is more common to pressure dies that are held in place with an over-center type mechanism. A direct acting pressure die on the other hand, with ample stroke and the proper control method is often able to compensate for more runout.

The Birth of the Runoff Condition

So, where does a runout condition start? Most often in the machining of one or more components that is used to mount the bend die (see the picture below).  Providing the concentricity of the bend die is a good start, but the clearance between the bolster, centering ring, and bend die should be examined next.

Preventing Wrinkles in Your CNC Tube Bending Operations | Winton Machine

The fit between the centering ring and the bolster should be a snug slip fit. Sometimes the centering ring is machined as one with the bend die. This is a good idea providing the runout between the centering ring and bend groove is very small. Once the fit between the bend die and bolster is considered up to par, put a dial indicator on the centering ring when installed in the bolster. Worn spindle bearings have been known to cause eccentric motion.

The integrity of the fit between the bolster and the bend die is not the only explanation for many partial wrinkles. Undersized machines, worn linkages, and worn dies, and are just to name a few.


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.

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