Sheet Metal Repair

Bend Allowance

This calculator can give you the basic layout to bend some parts. The Bend Allowance (BA) needs to be taking into account due to the plastic deformation carry out on the part. Before using the calculator you should know the Radius of the bending machine, the degree of bending (Normally is 90 degrees) and the final lengths. To calculate the Bend Allowance the following equation is used, where BR is Bend Radius, T is thickness and A is Bend Angle:

\(BA = {(2\pi*(BR+(0.5*T)))/(360/A)}\)

The Set Back is calculated using the following equation, where Mt is Material Thickness and K is a contant given by the Bend Angle. When the bending angle is 90, K is equal to 1. For others angles, you need to find the value in a chart. However, there is a basic equation into the code to calculate the K factor when the angle is not 90.

\(SB=K(BR+MT)\)

Thickness [inch]:

Bend Radius [inch]:

Bend Angle [°]:

k value:

Flat A:

Flat B:

Results

Rivets

Let's talk a little bittle about the most common rivets used for small repairs on aircraft as well as some important equation to understant the identification.

Universal Rivets

MS20470 and AN470

Countersunk Rivets

MS20426 and AN426

Identification

MS uses to refer to Military Stantard. But they are others specification like National Aerospace Standard (NAS) and Airforce Navy Standard (AN).

The rivet´s diameter is given in 32 and te lenght in 16.

MS 20470AD-4-4. Diamteter is 4/32 (1/8) and the lenght is 4/16 (1/4).

\(RivetLenght=MT + 1.5D\)

Special Fasteners

Blind Rivets

CherryMax (Mechanical lock)

The identification is CR3. The next two numbers represent the rivet type and Material combination. The Diameter is in 32th of an inch. Available in nomial size and 1/16" oversize. Gold color means nominal size while silver colored is oversize. They are more difficult to remove due to its mechanical locked. First it is necesary to rid the lock ring.

Cherry Friction lock Rivets

Used in non structual applications. CR9163 (Universal Head) and CR9162 (Countersink Head)

Rivnuts

Has internal threads for the machine screws. Usually are made from aluminum allow.