Layout and Forming (Part Two)

in Aircraft Metal Structural Repair

Bending a U-Channel

To understand the process of making a sheet metal layout, the steps for determining the layout of a sample U-channel will be discussed. [Figure 4-124] When using bend allowance calculations, the following steps for finding the total developed length can be computed with formulas, charts, or computer-aided design (CAD) and computer-aided manufacturing (CAM) software packages. This channel is made of 0.040-inch 2024-T3 aluminum alloy.


Figure 4-124. U-channel example.

Figure 4-124. U-channel example.

Step 1: Determine the Correct Bend Radius

Minimum bend radius charts are found in manufacturers’ maintenance manuals. A radius that is too sharp cracks the material during the bending process. Typically, the drawing indicates the radius to use, but it is a good practice to double check. For this layout example, use the minimum radius chart in Figure 4-125 to choose the correct bend radius for the alloy, temper, and the metal thickness. For 0.040, 2024- T3 the minimum allowable radius is 0.16-inch or 5⁄32-inch.

Figure 4-125. Minimum bend radius (from the Raytheon Aircraft Structural Inspection and Repair Manual).

Figure 4-125. Minimum bend radius (from the Raytheon Aircraft Structural Inspection and Repair Manual). [click image to enlarge]

Step 2: Find the Setback

The setback can be calculated with a formula or can be found in a setback chart available in aircraft maintenance manuals or Source, Maintenance, and Recoverability books (SMRs). [Figure 4-126]

Figure 4-126. Setback chart.

Figure 4-126. Setback chart. [click image to enlarge]

Using a Formula to Calculate the Setback

SB = setback
K = K-factor (K is 1 for 90° bends)
R = inside radius of the bend
T = material thickness

Since all of the angles in this example are 90° angles, the setback is calculated as follows:

SB = K(R+T) = 0.2 inches

NOTE: K = 1 for a 90° bend. For other than a 90° bend, use a K-factor chart.

Using a Setback Chart to Find the Setback

The setback chart is a quick way to find the setback and is useful for open and closed bends, because there is no need to calculate or find the K-factor. Several software packages and online calculators are available to calculate the setback. These programs are often used with CAD/CAM programs. [Figure 4-126]

  • Enter chart at the bottom on the appropriate scale with the sum of the radius and material thickness.
  • Read up to the bend angle.
  • Find the setback from corresponding scale on the left.

Example:

  • Material thickness is 0.063-inch.
  • Bend angle is 135°.
  • R + T = 0.183-inch.

Find 0.183 at the bottom of the graph. It is found in the middle scale.

  • Read up to a bend angle of 135°.
  • Locate the setback at the left hand side of the graph in the middle scale (0.435-inch). [Figure 4-126]

Step 3: Find the Length of the Flat Line Dimension The flat line dimension can be found using the formula:

Flat = MLD – SB
MLD = mold line dimension
SB = setback

The flats, or flat portions of the U-channel, are equal to the mold line dimension minus the setback for each of the sides, and the mold line length minus two setbacks for the center flat. Two setbacks need to be subtracted from the center flat because this flat has a bend on either side.

The flat dimension for the sample U-channel is calculated in the following manner:

Flat dimension = MLD – SB
Flat 1 = 1.00-inch – 0.2-inch = 0.8-inch
Flat 2 = 2.00-inch – (2 × 0.2-inch) = 1.6-inch
Flat 3 = 1.00-inch – 0.2-inch = 0.8-inch

Step 4: Find the Bend Allowance

When making a bend or fold in a piece of metal, the bend allowance or length of material required for the bend must be calculated. Bend allowance depends on four factors: degree of bend, radius of the bend, thickness of the metal, and type of metal used.

The radius of the bend is generally proportional to the thickness of the material. Furthermore, the sharper the radius of bend, the less the material that is needed for the bend. The type of material is also important. If the material is soft, it can be bent very sharply; but if it is hard, the radius of bend is greater, and the bend allowance is greater. The degree of bend affects the overall length of the metal, whereas the thickness influences the radius of bend.

Bending a piece of metal compresses the material on the inside of the curve and stretches the material on the outside of the curve. However, at some distance between these two extremes lies a space which is not affected by either force. This is known as the neutral line or neutral axis and occurs at a distance approximately 0.445 times the metal thickness (0.445 × T) from the inside of the radius of the bend. [Figure 4-127]

Figure 4-127. Neutral axis and stresses resulting from bending.

Figure 4-127. Neutral axis and stresses resulting from bending.

The length of this neutral axis must be determined so that sufficient material can be provided for the bend. This is called the bend allowance. This amount must be added to the overall length of the layout pattern to ensure adequate material for the bend. To save time in calculation of the bend allowance, formulas and charts for various angles, radii of bends, material thicknesses, and other factors have been developed.

Formula 1: Bend Allowance for a 90° Bend

To the radius of bend (R) add 1⁄2 the thickness of the metal (1⁄2T). This gives R + 1⁄2T, or the radius of the circle of the neutral axis. [Figure 4-128] Compute the circumference of this circle by multiplying the radius of the neutral line (R + 1⁄2T) by 2π (NOTE: π = 3.1416): 2π (R + 1⁄2T). Since a 90° bend is a quarter of the circle, divide the circumference by 4. This gives:

Figure 4-128. Bend allowance for a 90° bend.

Figure 4-128. Bend allowance for a 90° bend.

This is the bend allowance for a 90° bend. To use the formula for a 90° bend having a radius of 1⁄4 inch for material 0.051- inch thick, substitute in the formula as follows.

The bend allowance, or the length of material required for the bend, is 0.4327 or 7⁄16-inch.

Formula 2: Bend Allowance for a 90° Bend

This formula uses two constant values that have evolved over a period of years as being the relationship of the degrees in the bend to the thickness of the metal when determining the bend allowance for a particular application. By experimentation with actual bends in metals, aircraft engineers have found that accurate bending results could be obtained by using the following formula for any degree of bend from 1° to 180°.

Bend allowance = (0.01743R + 0.0078T)N where:

R = the desired bend radius
T = the thickness of the metal
N = number of degrees of bend

To use this formula for a 90° bend having a radius of .16- inch for material 0.040-inch thick, substitute in the formula as follows:

Use of Bend Allowance Chart for a 90° Bend

In Figure 4-129, the radius of bend is shown on the top line, and the metal thickness is shown on the left hand column. The upper number in each cell is the bend allowance for a 90° bend. The lower number in the cell is the bend allowance per 1° of bend. To determine the bend allowance for a 90° bend, simply use the top number in the chart.

Figure 4-129. Bend allowance.

Figure 4-129. Bend allowance. [click image to enlarge]

Example: The material thickness of the U-channel is 0.040- inch and the bend radius is 0.16-inch.

Reading across the top of the bend allowance chart, find the column for a radius of bend of .156-inch. Now, find the block in this column that is opposite the material thickness (gauge) of 0.040 in the column at the left. The upper number in the cell is (0.273), the correct bend allowance in inches for a 90° bends.

Several bend allowance calculation programs are available online. Just enter the material thickness, radius, and degree of bend and the computer program calculates the bend allowance.