Tips for Bending Small Diameter Copper Tubing with a CNC Machine
The Market for Copper Tube Bending
Copper tubing is used in several markets including refrigeration, laboratory equipment, and process equipment. Copper is one of the softest metals to bend. Being a soft metal is helpful yet at the same time can be a disadvantage when making a tight radius bend. This is often the case for copper tubing below ¾” in diameter.
What is Tight Radius Bending
Tight radius bending for copper starts when the bend radius is at equal to or less than about 2.5 times the tube’s outside diameter (2.5D). Therefore, a 1.25” centerline bend radius (CLR) on a ½” diameter tube would equate to a 2.5D of bend; (CLR/OD). A significant amount of tubing bent for the refrigeration market is bent between 2D and 3D. For a ½” tube diameter this equates to a CLR between 1” to a 1.5”. Given the price of copper today, a smaller CLR means less copper consumed in the bend. This is attractive from a cost of goods standpoint.
The Wall Factor
The wall factor is a way to evaluate how a tube may react during a bending operation. The wall factor is simply the ratio of the tube’s outside diameter to the wall thickness (D/t). Therefore, a ½” outside diameter tube with a .035” wall would have a wall factor of 14.3.
As the wall factor starts to head north of 15, the need to increase the CLR might be needed for successful bending. The consequences of not increasing the CLR may yield wrinkles on the inside of the bend. Therefore, as a general rule for bending copper without a mandrel on a 2D radius, the wall factor should not climb much above 15. The 1/2” diameter tube in the photo below was bent on a 1” CLR.
It is possible to bend the ½” diameter copper tube above to a smaller radius. However, for consistent production results, keeping the D of bend greater than 2 can disperse of several long-term headaches.
There are other factors that can influence the quality of a bend. The yield strength of the material being bent can directly influence whether there are wrinkles or no wrinkles. When bending copper, the compressive forces on the inside of the bend can tend to give way sooner as the centerline radius becomes smaller.
As an example, a Ø½” aluminum tube with a .035” wall can often be bent to a tighter radius then soft copper. This is because the aluminum can withstand greater compressive forces on the inside of a bend before the inside wall itself starts to wrinkle.
Also, the position of the carriage on a CNC bender can influence wrinkles when bending copper. If the alignment of the carriage is too far away from the pressure die during bending (non-mandrel bending), then wrinkles may occur. This is due, in part, to the softness of some grades of copper. Therefore, adjusting the carriage to a greater CLR can sometimes eliminate wrinkles.
This situation is highlighted when a CNC tube bender’s carriage releases the tube before the last bend is made. Once released, the carriage can no longer support the tail end if the tube during a bend.
The graphical relationship between the CLR and the tube diameter is shown above. With the right tooling, 2D copper bending works fine up to and including a ½” diameter tube. As the tube diameter starts to head north of 9/16” diameter, a minimum CLR of 2.5D is recommended when wall factors are greater than 15.
When bending with a 2D CLR, it may be necessary to use some type of crown tooling to prevent flattening on the outside of the tube in the bend zone. Crown tooling can help prevent the outside of a copper tube from fattening. Flattening is usually realized on the outside of a bend as the CLR becomes smaller given a constant tube diameter and wall thickness.
About the Author
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