Layout and Forming (Part Three)

in Aircraft Metal Structural Repair

Use of Chart for Other Than a 90° Bend

If the bend is to be other than 90°, use the lower number in the block (the bend allowance for 1°) and compute the bend allowance.


Example:

The L-bracket shown in Figure 4-130 is made from 2024-T3 aluminum alloy and the bend is 60° from flat. Note that the bend angle in the figure indicates 120°, but that is the number of degrees between the two flanges and not the bend angle from flat. To find the correct bend angle, use the following formula:

Bend Angle = 180° – Angle between flanges

Figure 4-130. Bend allowance for bends less than 90°.

Figure 4-130. Bend allowance for bends less than 90°.

The actual bend is 60°. To find the correct bend radius for a 60° bend of material 0.040-inches thick, use the following procedure.

  1. Go to the left side of the table and find 0.040-inch.
  2. Go to the right and locate the bend radius of 0.16-inch (0.156-inch).
  3. Note the bottom number in the block (0.003034).
  4. Multiply this number by the bend angle:

0.003034 × 60 = 0.18204

Step 5: Find the Total Developed Width of the Material

The total developed width (TDW) can be calculated when the dimensions of the flats and the bend allowance are found. The following formula is used to calculate TDW:

TDW = Flats + (bend allowance × number of bends)

For the U-channel example, this gives:

TDW = Flat 1 + Flat 2 + Flat 3 + (2 × BA)
TDW = 0.8 + 1.6 + 0.8 + (2 × 0.27)
TDW = 3.74-inches

Note that the amount of metal needed to make the channel is less than the dimensions of the outside of the channel (total of mold line dimensions is 4 inches). This is because the metal follows the radius of the bend rather than going from mold line to mold line. It is good practice to check that the calculated TDW is smaller than the total mold line dimensions. If the calculated TDW is larger than the mold line dimensions, the math was incorrect.

Step 6: Flat Pattern Lay Out

After a flat pattern layout of all relevant information is made, the material can be cut to the correct size, and the bend tangent lines can be drawn on the material. [Figure 4-131]

Figure 4-131. Flat pattern layout.

Figure 4-131. Flat pattern layout.

Step 7: Draw the Sight Lines on the Flat Pattern

The pattern laid out in Figure 4-131 is complete, except for a sight line that needs to be drawn to help position the bend tangent line directly at the point where the bend should start. Draw a line inside the bend allowance area that is one bend radius away from the bend tangent line that is placed under the brake nose bar. Put the metal in the brake under the clamp and adjust the position of the metal until the sight line is directly below the edge of the radius bar. [Figure 4-132] Now, clamp the brake on the metal and raise the leaf to make the bend. The bend begins exactly on the bend tangent line.

Figure 4-132. Sight line.

Figure 4-132. Sight line. [click image to enlarge]

NOTE: A common mistake is to draw the sight line in the middle of the bend allowance area, instead of one radius away from the bend tangent line that is placed under the brake nose bar.

Using a J-Chart To Calculate Total Developed Width

The J-chart, often found in the SRM, can be used to determine bend deduction or setback and the TDW of a flat pattern layout when the inside bend radius, bend angle, and material thickness are known. [Figure 4-133] While not as accurate as the traditional layout method, the J-chart provides sufficient information for most applications. The J-chart does not require difficult calculations or memorized formulas because the required information can be found in the repair drawing or can be measured with simple measuring tools.

Figure 4-133. J chart.

Figure 4-133. J chart. [click image to enlarge]

When using the J-chart, it is helpful to know whether the angle is open (greater than 90°) or closed (less than 90°) because the lower half of the J-chart is for open angles and the upper half is for closed angles.

How To Find the Total Developed Width Using a J-Chart

  • Place a straightedge across the chart and connect the bend radius on the top scale with the material thickness on the bottom scale. [Figure 4-133]
  • Locate the angle on the right hand scale and follow this line horizontally until it meets the straight edge.
  • The factor X (bend deduction) is then read on the diagonally curving line.
  • Interpolate when the X factor falls between lines.
  • Add up the mold line dimensions and subtract the X factor to find the TDW.

Example 1

Bend radius = 0.22-inch
Material thickness = 0.063-inch
Bend angle = 90º
ML 1 = 2.00/ML 2 = 2.00

Use a straightedge to connect the bend radius (0.22-inch) at the top of the graph with the material thickness at the bottom (0.063-inch). Locate the 90° angle on the right hand scale and follow this line horizontally until it meets the straightedge. Follow the curved line to the left and find 0.17 at the left side. The X factor in the drawing is 0.17-inch. [Figure 4-134]

Figure 4-134. Example 1 of J chart.

Figure 4-134. Example 1 of J chart.

Total developed width = (2 + 2) – .17 = 3.83-inches

Example 2

Bend radius = 0.25-inch
Material thickness = 0.050-inch
Bend angle = 45º
ML 1 = 2.00/ML 2 = 2.00

Figure 4-135 illustrates a 135° angle, but this is the angle between the two legs. The actual bend from flat position is 45° (180 – 135 = 45). Use a straightedge to connect the bend radius (0.25-inch) at the top of the graph with the material thickness at the bottom (.050-inch). Locate the 45° angle on the right hand scale and follow this line horizontally until it meets the straight edge. Follow the curved line to the left and find 0.035 at the left side. The X factor in the drawing is 0.035 inch.

Figure 4-135. Example 2 of J chart.

Figure 4-135. Example 2 of J chart.

Using a Sheet Metal Brake to Fold Metal

The brake set up for box and pan brakes and cornice brakes is identical. [Figure 4-136] A proper set up of the sheet metal brake is necessary because accurate bending of sheet metal depends on the thickness and temper of the material to be formed and the required radius of the part. Any time a different thickness of sheet metal needs to be formed or when a different radius is required to form the part, the operator needs to adjust the sheet metal brake before the brake is used to form the part. For this example, an L-channel made from 2024 –T3 aluminum alloy that is 0.032-inch thick will be bent.

Figure 4-136. Brake radius nosepiece adjustment.

Figure 4-136. Brake radius nosepiece adjustment.

Step 1: Adjustment of Bend Radius

The bend radius necessary to bend a part can be found in the part drawings, but if it is not mentioned in the drawing, consult the SRM for a minimum bend radius chart. This chart lists the smallest radius allowable for each thickness and temper of metal that is normally used. To bend tighter than this radius would jeopardize the integrity of the part. Stresses left in the area of the bend may cause it to fail while in service, even if it does not crack while bending it.

Figure 4-137. Interchangeable brake radius bars.

Figure 4-137. Interchangeable brake radius bars.

The brake radius bars of a sheet metal brake can be replaced with another brake radius bar with a different diameter. [Figure 4-137] For example, a 0.032-inch 2024-T3 L channel needs to be bent with a radius of 1⁄8-inch and a radius bar with a 1⁄8-inch radius must be installed. If different brake radius bars are not available, and the installed brake radius bar is smaller than required for the part, it is necessary to bend some nose radius shims. [Figure 4-138]

Figure 4-138. Nose radius shims may be used when the brake radius bar is smaller than required.

Figure 4-138. Nose radius shims may be used when the brake radius bar is smaller than required. [click image to enlarge]

If the radius is so small that it tends to crack annealed aluminum, mild steel is a good choice of material. Experimentation with a small piece of scrap material is necessary to manufacture a thickness that increases the radius to precisely 1⁄16-inch or 1⁄8-inch. Use radius and fillet gauges to check this dimension. From this point on, each additional shim is added to the radius before it. [Figure 4-139]

Figure 4-139. General brake overview including radius shims.

Figure 4-139. General brake overview including radius shims. [click image to enlarge]

Example: If the original nose was 1⁄16-inch and a piece of .063- inch material (1⁄16-inch) was bent around it, the new outside radius is 1⁄8-inch. If another .063-inch layer (1⁄16-inch) is added, it is now a 3⁄16-inch radius. If a piece of .032-inch (1⁄32-inch) instead of .063-inch material (1⁄16-inch) is bent around the 1⁄8-inch radius, a 5⁄32-inch radius results.

Step 2: Adjusting Clamping Pressure

The next step is setting clamping pressure. Slide a piece of the material with the same thickness as the part to be bent under the brake radius piece. Pull the clamping lever toward the operator to test the pressure. This is an over center type clamp and, when properly set, will not feel springy or spongy when pulled to its fully clamped position. The operator must be able to pull this lever over center with a firm pull and have it bump its limiting stops. On some brakes, this adjustment has to be made on both sides of the brake.

Place test strips on the table 3-inch from each end and one in the center between the bed and the clamp, adjust clamp pressure until it is tight enough to prevent the work pieces from slipping while bending. The clamping pressure can be adjusted with the clamping pressure nut. [Figure 4-140]

Figure 4-140. Adjust clamping pressure with the clamping pressure nut.

Figure 4-140. Adjust clamping pressure with the clamping pressure nut.

Step 3: Adjusting the Nose Gap

Adjust the nose gap by turning the large brake nose gap adjustment knobs at the rear of the upper jaw to achieve its proper alignment. [Figure 4-140] The perfect setting is obtained when the bending leaf is held up to the angle of the finished bend and there is one material thickness between the bending leaf and the nose radius piece. Using a piece of material the thickness of the part to be bent as a feeler gauge can help achieve a high degree of accuracy. [Figures 4-141 and 4-142] It is essential this nose gap be perfect, even across the length of the part to be bent. Check by clamping two test strips between the bed and the clamp 3-inch from each end of the brake. [Figure 4-143] Bend 90° [Figure 4-144], remove test strips, and place one on top of the other; they should match. [Figure 4-145] If they do not match, adjust the end with the sharper bend back slightly.

Figure 4-141. Brake nose gap adjustment with piece of material same thickness as part to be formed.

Figure 4-141. Brake nose gap adjustment with piece of material same thickness as part to be formed.

Figure 4-142. Profile illustration of brake nose gap adjustment.

Figure 4-142. Profile illustration of brake nose gap adjustment. [click image to enlarge]

Figure 4-143. Brake alignment with two test strips 3-inches from each end.

Figure 4-143. Brake alignment with two test strips 3-inches from each end.

Figure 4-144. Brake alignment with two test strips bent at 90°.

Figure 4-144. Brake alignment with two test strips bent at 90°.

Figure 4-145. Brake alignment by comparing test strips.

Figure 4-145. Brake alignment by comparing test strips.