Sheet Metal Brakes and Folding

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 4-132] 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. 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.

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 4-132] Instruction Place a straightedge across the chart connecting the radius on the upper scale and thickness on lower scale. Then, locate the angle on the right hand scale and follow this line horizontally until it meets the straight edge. The factor X is then read on the diagonally curving line. Interpolate when the factor X falls between lines. Bend Radius Thickness Angle Factor X X = Amount to be reduced from sum of flange dimension A + B − X = Developed length Example 0.063 Material 0.12 Bend raduis 45° Angle X = 0.035 0.50 0.47 0.44 0.40 0.38 0.34 0.31 0.28 0.25 0.22 0.19 0.16 0.12 0.09 0.06 0.03 0.00 0.130 0.120 0.110 0.100 0.090 0.080 0.070 0.060 0.050 0.040 0.030 0.020 0.010 0.000 150° 140° 130° 120° 115° 110° 105° 100° 95° 90° 85° 80° 75° 70° 65° 60° 55° 50° 45° 40° 35° 30° A BR Bendangle 1.00 1.20 1.40 1.60 0.90 0.80 0.70 0.05 0.15 0.20 0.25 0.30 0.40 0.50 0.60 0.06 0.07 0.08 0.09 0.10 1.70 0.04 0.03 0.02 0.01 4-67 • 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 4-133] Total developed width = (Mold line 1 + Mold line 2) – X factor 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 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 2.0" 0.5" R = 0.25"135° 2.0" 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.

Total developed width = (Mold line 1 + Mold line 2) – X factor Total developed width = (2 + 2) – .035 = 3.965-inch Using a Sheet Metal Brake to Fold Metal The brake set up for box and pan brakes and cornice brakes is identical. [Figure 4-135] 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.

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 2.00" 0.063" R = 0.22"2.00" 4-68 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. The brake radius bars of a sheet metal brake can be replaced with another brake radius bar with a different diameter.

[Figure 4-136] 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 4-137] 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 4-138] 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 inches from each end and one in the center between the bed and the clamp, adjust clamp UPPER JAW BED LOWER JAW BENDING LEAF NOSE RADIUS BAR Each of these nose radius shims is 0.063 inch thick, which gives radius choices of 1/8", 3/16", and 1/4" This radius shim builds radius to precisely 1/16"R 4-69 Pull forward to clamp (no sponginess felt when evenly set on BOTH sides) Note: Bending leaf counterbalance omitted for clarity Limiting stop Nut to adjust clamping pressure Lifting nut Radius shims Material to be bent 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 4-139] 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 4-140 and 4-141] It is essential this nose gap be Clamping pressure adjustment nut Brake nose gap adjustment knob nut. same thickness as part to be formed. 4-70 Should slip snugly in and out BENDING LEAF NOSE GAP Hold bending leaf at the finished angle of bend 90°(in this case) Scrap of material to be bent perfect, even across the length of the part to be bent. Check by clamping two test strips between the bed and the clamp 3 inches from each end of the brake. [Figure 4-142] Bend 90° [Figure 4-143], remove test strips, and place one on top of the other; they should match. [Figure 4-144] If they do not match, adjust the end with the sharper bend back slightly.

Folding a Box A box can be formed the same way as the U-channel described on in the previous paragraphs, but when a sheet metal part has intersecting bend radii, it is necessary to remove material to make room for the material contained in the flanges. This is done by drilling or punching holes at the intersection of the inside bend tangent lines. These holes, called relief holes and whose diameter is approximately twice the bend radius, relieve stresses in the metal as it is bent and prevent the metal from tearing. Relief holes also provide a neatly trimmed corner from which excess material may be trimmed.

The larger and smoother the relief hole is, the less likely it will be that a crack will form in the corner. Generally, the radius of the relief hole is specified on the drawing. A box and pan brake, also called a finger brake, is used to bend the each end. 4-71