RUMUS BENTANGAN

Bend allowance

The bend allowance (BA) is the length of the arc of the neutral line between the tangent points of a bend in any material. Adding the length of each flange taken between the center of the radius to the BA gives the Flat Pattern length. This bend allowance formula is used to determine the flat pattern length when a bend is dimensioned from 1) the center of the radius, 2) a tangent point of the radius or 3) the outside tangent point of the radius on an acute angle bend.
The BA can be calculated using the following formula:^[7]

Diagram showing standard dimensioning scheme when using Bend Allowance formulas. Note that when dimensions "C" are specified, dimension B = C - R - T

Example
Angle 90 Pl 3.142 Radius 1.5 K-Factor 0.33 Thickness 6
Bend allowance 5.46708

Bend deduction

Diagram showing standard dimensioning scheme when using Bend Deduction formulas.

The outside set back (OSSB) is the length from the tangent point of the radius to the apex of the outside of the bend. The bend deduction (BD) is twice the outside setback minus the bend allowance. BD is calculated using the following formula:^[8]

The above formula works only for right angles. For bend angles 90 degrees or greater the following formula works, where A is the angle in radians (=degrees*π/180)

K-factor

K-factor is a ratio of location of the neutral line to the material thickness as defined by t/T where t = location of the neutral line and T = material thickness. The K-Factor formulation does not take the forming stresses into account but is simply a geometric calculation of the location of the neutral line after the forces are applied and is thus the roll-up of all the unknown (error) factors for a given setup. The K-factor depends on many factors including the material, the type of bending operation (coining, bottoming, air-bending, etc.) the tools, etc. and is typically between 0.3 to 0.5.
The following equation relates the K-factor to the bend allowance:^[9]

The following table is a "Rule of Thumb". Actual results may vary remarkably.

Generic K-Factors	Aluminum		Steel
Radius	Soft Materials	Medium Materials	Hard Materials
Air Bending
0 to Thickness	0.33	0.38	0.40
Thickness to 3 x Thickness	0.40	0.43	0.45
Greater than 3 x Thickness	0.50	0.50	0.50
Bottoming
0 to Thickness	0.42	0.44	0.46
Thickness to 3 x Thickness	0.46	0.47	0.48
Greater than 3 x Thickness	0.50	0.50	0.50
Coining
0 to Thickness	0.38	0.41	0.44
Thickness to 3 x Thickness	0.44	0.46	0.47
Greater than 3 x Thickness	0.50	0.50	0.50

The following formula can be used in place of the table as a good approximation of the K-Factor for Air Bending:
LOG(MIN(100,MAX(20 × R,T) / T)) / LOG(100) / 2