It is now time to examine how to calculate the feasibility of nonmandrel bending and apply that calculation to specific applications.
All feasibility begins with a calculation of the end difficulty. The formula is:
Tube OD ÷ Wall Thickness= Wall Factor
CLR ÷ Tube OD = D of Bend
Wall Factor ÷ D of Bend = Difficulty Factor (DF)
An example of this calculation for a tube with .500-inch OD, .028 wall thickness, and .787 CLR would be:
.500 ÷ .028 = 17.86
.787 ÷ .500 = 1.574
17.86 ÷ 1.574 = 11.35 DF
The estimated ovality of a bent tube will equal about 75 percent of the DF, depending on the material, material hardness, and how severely the tube is being worked.
After calculation a DF for a particular application, the next step is the consider the material to be bent and its relative hardness.
At this point, nonmandrel bending is just about opposite of mandrel bending ideals; harder materials are going to work better. This does not mean that nonmandrel is not applicable to soft materials, but rather, that it is easier to exceed the limits of available structural support in soft materials.
The guidelines pertaining to various materials and their maximum DFs might be stated as follows:
Soft Copper or Aluminum: 9-11 DF
Steel: 12-14 DF
Stainless Steel: 13-15 DF
These limits are intended as guidelines only. The other variables, such as machine features and finished products, must be determined on a case-by-case basis.