B = degree of bend

E = feathered edge thickness

Fb = bend difficulty factor

Fd = “D” of bend

Fw = wall factor

Kr = constant for rigidity

Ks = constant for minimum clamp length

Kz = constant for feathered edge

Lc = clamp length

Lp = pressure die length

Mb = mandrel ball diameter

Md = mandrel nose diameter

Mm = mandrel body diameter

Mr = mandrel nose radius

Pe = percentage of elongation at arc

Pt = percentage of wall-thinning

Pw = wall thickness after thinning

R = centerline radius

Ri = inside radius

Ro = outside radius

S = maximum set-up depth

T = tube outside diameter

Ti = tube inside diameter

W = wall thickness

Wi = thickness of inside lamination

Wo = thickness of outside lamination

Ti = T – ( W x 2 )

Ri = R – ( T / 2 )

Outside radius:

Ro = R + ( T / 2 )

Fw = T / W

“D” of bend:

Fd = R / T

Where “Kr” = a constant for material rigidity (assign the same value to “Kr” as you would to calculate pressure die length; a value of 2 is suitable for most applications; click here for more information) and “n1” through “n4” are values to adjust the weight of each factor in the equation (see below for our recommended weighting):

General formula: Fb = [ ( n1 x Kr ) + ( n2 x Fw ) + ( ( n3 x B ) / 180 ) ) ] / [ n4 x Fd ]

Formula with recommended weighting: Fb = [ 2Kr + .2Fw + ( B / 180 ) ] / [ Fd ]

Note: A bend difficulty rating (calculated with our recommended weighting) of 7 or less indicates a bend that is relatively simple to produce with the rotary-draw method. Factors in excess of 7 typically require either additional precision in set-up or close attention during production in order to hold the set-up parameters.

Wall-thinning of extrados at outside radius after bending (rule of thumb only):

Where “Pt” = percentage of wall-thinning and “Pw” = targeted thickness of wall after thinning out from bending:

Pt = ( Ro – R ) / Ro

Pw = W x ( 1 – Pt )

Percentage of elongation at arc of the bend (rule of thumb only):

Pe = ( Ro / R ) – 1

Mandrel nose diameter for single-wall tubing:

Md = T – ( W x 2.21 )

Mandrel nose diameter for double-wall tubing:

Where “Wo” = wall thickness of outside lamination and “Wi” = wall thickness of inside lamination:

Md = ( T – ( Wo x 2 ) ) – ( Wi x 2.21 )

Mandrel nose radius:

if Fw .006* then E = T x Kz else E = .006*

* Inches. For metric applications, substitute .15 millimeters.

Clamp length:

Where “Kr” = a constant for material rigidity (assign a value of 2 to “Kr” for most applications; click here for more information) and “Ks” = a constant limiting the minimum clamp length depending upon the surface of the cavity (assign to “Ks” the value of 2 for smooth cavities and 1 for serrated cavities; click here for more information):

if ( T x ( Kr x 2.5) ) – R