Text-only reference. Published from the recorded official FAA Airframe Chapter 4 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
box. Two opposite sides of the box are bent first. Then, the fingers of the brake are adjusted so the folded-up sides ride up in the cracks between the fingers when the leaf is raised to bend the other two sides. The size of relief holes varies with thickness of the material. They should be no less than 1⁄8-inch in diameter for aluminum alloy sheet stock up to and including 0.064-inch thick, or 3⁄160-inch in diameter for stock ranging from 0.072-inch to 0.128-inch thickness. The most common method of determining the diameter of a relief hole is to use the radius of bend for this dimension, provided it is not less than the minimum allowance (1⁄8-inch).
Relief Hole Location Relief holes must touch the intersection of the inside bend tangent lines. To allow for possible error in bending, make the relief holes extend 1⁄32-inch to 1⁄16-inch behind the inside bend tangent lines. It is good practice to use the intersection of these lines as the center for the holes. The line on the inside of the curve is cut at an angle toward the relief holes to allow for the stretching of the inside flange. The positioning of the relief hole is important. [Figure 4-145] It should be located so its outer perimeter touches the intersection of the inside bend tangent lines. This keeps any material from interfering with the bend allowance area of the other bend. If these bend allowance areas intersected with each other, there would be substantial compressive stresses that would accumulate in that corner while bending. This could cause the part to crack while bending.
Layout Method Lay out the basic part using traditional layout procedures. This determines the width of the flats and the bend allowance. It is the intersection of the inside bend tangent lines that index the bend relief hole position. Bisect these intersected lines and move outward the distance of the radius of the hole on this line. This is the center of the hole. Drill at this point and finish by trimming off the remainder of the corner material. The trim out is often tangent to the radius and perpendicular to the edge. [Figure 4-146] This leaves an open corner. If the corner must be closed, or a slightly longer flange is necessary, then trim out accordingly. If the corner is to be welded, it is necessary to have touching flanges at the corners. The length of the flange should be one material thickness shorter than the finished length of the part so only the insides of the flanges touch.
Open & Closed Bends Open and closed bends present unique problems that require more calculations than 90° bends. In the following 45° and a 135° bend examples, the material is 0.050-inch thick and the bend radius is 3⁄16-inch. 2" Flat Flat Flat Area of bend (BA) 1" 2" Bend relief radius Intersection of inside bend tangent lines 11∕16 1 11∕16 11 ∕16 7∕16 7 ∕16 2 13∕16 2 1 2 13 ∕16 Notice overlapping mold lines (by 1 MG) R = 0.250 ( 1∕4) T = 0.063 ( 1∕16) SB = 0.313 ( 5∕16) BA = 0.437 ( 7∕16) MG = 0.191 ( 3∕16) Normal trim tangent to radius If necessary for flanges to touch If 5∕16 R is required, punch 5∕8 hole 4-72 Open End Bend (Less Than 90°) 1. Look up K-factor in K chart. K-factor for 45° is 0.41421-inch.
2. Calculate setback. SB = K(R + T) SB = 0.41421-inch(0.1875-inch + 0.050-inch) = 0.098-inch 3. Calculate bend allowance for 45°. Look up bend allowance for 1° of bend in the bend allowance chart and multiply this by 45. 0.003675-inch × 45 = 0.165-inch 4. Calculate flats. Flat = Mold line dimension – SB Flat 1 = .77-inch – 0.098-inch = 0.672-inch Flat 2 = 1.52-inch – 0.098-inch = 1.422-inch 5. Calculate TDW TDW = Flats + Bend allowance TDW = 0.672-inch + 1.422-inch + 0.165-inch = 2.259-inch. Observe that the brake reference line is still located one radius from the bend tangent line. Closed End Bend (More Than 90°) 1. Look up K-factor in K chart. K-factor for 135° is 2.4142-inch.
2. Calculate SB. SB = K(R + T) SB = 2.4142-inch(0.1875-inch + 0.050-inch) = 0.57- inch 3. Calculate bend allowance for 135°. Look up bend allowance for 1° of bend in the bend allowance chart and multiply this by 135. 0.003675-inch × 135 = 0.496-inch 4. Calculate flats. 135° 1.52 R .19 0.77 1.52 R .19 45° 135° 0.77 0.05 Flat = Mold line dimension – SB Flat 1 = 0.77-inch – 0.57-inch = 0.20-inch Flat 2 = 1.52-inch – 0.57-inch = 0.95-inch 5. Calculate TDW. TDW = Flats + Bend allowance TDW = 0.20-inch + 0.95-inch + 0.496-inch = 1.65- inch It is obvious from both examples that a closed bend has a smaller TDW than an open-end bend and the material length needs to be adjusted accordingly.
Hand Forming All hand forming revolves around the processes of stretching and shrinking metal. As discussed earlier, stretching means to lengthen or increase a particular area of metal while shrinking means to reduce an area. Several methods of stretching and shrinking may be used, depending on the size, shape, and contour of the part being formed. For example, if a formed or extruded angle is to be curved, either stretch one leg or shrink the other, whichever makes the part fit. In bumping, the material is stretched in the bulge to make it balloon, and in joggling, the material is stretched between the joggles. Material in the edge of lightening holes is often stretched to form a beveled reinforcing ridge around them. The following paragraphs discuss some of these techniques.
Straight Line Bends The cornice brake and bar folder are ordinarily used to make straight bends. Whenever such machines are not available, comparatively short sections can be bent by hand with the aid of wooden or metal bending blocks. After a blank has been laid out and cut to size, clamp it along the bend line between two wooden forming blocks held in a vise. The wooden forming blocks should have one edge rounded as needed for the desired radius of bend. It should also be curved slightly beyond 90° to allow for spring-back. Bend the metal that protrudes beyond the bending block to 4-73 the desired angle by tapping lightly with a rubber, plastic, or rawhide mallet. Start tapping at one end and work back and forth along the edge to make a gradual and even bend.
Continue this process until the protruding metal is bent to the desired angle against the forming block. Allow for spring- back by driving the material slightly farther than the actual bend. If a large amount of metal extends beyond the forming blocks, maintain hand pressure against the protruding sheet to prevent it from bouncing. Remove any irregularities by holding a straight block of hardwood edgewise against the bend and striking it with heavy blows of a mallet or hammer. If the amount of metal protruding beyond the bending blocks is small, make the entire bend by using the hardwood block and hammer.
Formed or Extruded Angles Both formed and extruded types of angles can be curved (not bent sharply) by stretching or shrinking either of the flanges. Curving by stretching one flange is usually preferred since the process requires only a V-block and a mallet and is easily accomplished. Stretching with V-Block Method In the stretching method, place the flange to be stretched in the groove of the V-block. [Figure 4-149] (If the flange is to be shrunk, place the flange across the V-block.) Using a round, soft-faced mallet, strike the flange directly over the V portion with light, even blows while gradually forcing it downward into the V .
Begin at one end of the flange and form the curve gradually and evenly by moving the strip slowly back and forth, distributing the hammer blows at equal spaces on the flange. Hold the strip firmly to keep it from bouncing when hammered. An overly heavy blow buckles the metal, so keep moving the flange across the V-block, but always lightly strike the spot directly above the V . Lay out a full-sized, accurate pattern on a sheet of paper or plywood and periodically check the accuracy of the curve. Comparing the angle with the pattern determines exactly how the curve is progressing and just where it needs to be increased or decreased. It is better to get the curve to conform roughly to the desired shape before attempting to finish any one portion, because the finishing or smoothing of the angle may cause some other portion of the angle to change shape.
If any part of the angle strip is curved too much, reduce the curve by reversing the angle strip on the V-block, placing the bottom flange up, and striking it with light blows of the mallet. Try to form the curve with a minimum amount of hammering, for excessive hammering work hardens the metal. Work- hardening can be recognized by a lack of bending response or by springiness in the metal. It can be recognized very readily by an experienced worker. In some cases, the part may have to be annealed during the curving operation. If so, be sure to heat treat the part again before installing it on the aircraft.
Shrinking With V-Block & Shrinking Block Methods Curving an extruded or formed angle strip by shrinking may be accomplished by either the previously discussed V-block method or the shrinking block method. While the V-block is more satisfactory because it is faster, easier, and affects the metal less, good results can be obtained by the shrinking block method. In the V-block method, place one flange of the angle strip flat on the V-block with the other flange extending upward. Using the process outlined in the stretching paragraphs, begin at one end of the angle strip and work back and forth making light blows. Strike the edge of the flange at a slight angle to keep the vertical flange from bending outward.
Occasionally, check the curve for accuracy with the pattern. If a sharp curve is made, the angle (cross-section of the formed angle) closes slightly. To avoid such closing of the angle, clamp the angle strip to a hardwood board with the hammered flange facing upward using small C-clamps. The jaws of the C-clamps should be covered with masking tape. If the angle has already closed, bring the flange back to the correct angle with a few blows of a mallet or with the aid of a small hardwood block. If any portion of the angle strip is curved too much, reduce it by reversing the angle on the V-block and hammering with a suitable mallet, as explained in the previous paragraph on stretching. After obtaining the 4-74 proper curve, smooth the entire angle by planishing with a soft-faced mallet.
If the curve in a formed angle is to be quite sharp or if the flanges of the angle are rather broad, the shrinking block method is generally used. In this process, crimp the flange that is to form the inside of the curve. When making a crimp, hold the crimping pliers so that the jaws are about 1⁄8-inch apart. By rotating the wrist back and forth, bring the upper jaw of the pliers into contact with the flange, first on one side and then on the other side of the lower jaw. Complete the crimp by working a raised portion into the flange, gradually increasing the twisting motion of the pliers. Do not make the crimp too large because it will be difficult to work out. The size of the crimp depends upon the thickness and softness of the material, but usually about 1⁄4-inch is sufficient. Place several crimps spaced evenly along the desired curve with enough space left between each crimp so that jaws of the shrinking block can easily be attached.
After completing the crimping, place the crimped flange in the shrinking block so that one crimp at a time is located between the jaws. [Figure 4-150] Flatten each crimp with light blows of a soft-faced mallet, starting at the apex (the closed end) of the crimp and gradually working toward the edge of the flange. Check the curve of the angle with the pattern periodically during the forming process and again after all the crimps have been worked out. If it is necessary to increase the curve, add more crimps and repeat the process. Space the additional crimps between the original ones so that the metal does not become unduly work hardened at any one point. If the curve needs to be increased or decreased slightly at any point, use the V-block.
After obtaining the desired curve, planish the angle strip over a stake or a wooden form. Flanged Angles The forming process for the following two flanged angles is slightly more complicated than the previously discussed angles because the bend is shorter (not gradually curved) and necessitates shrinking or stretching in a small or concentrated area. If the flange is to point toward the inside of the bend, the material must be shrunk. If it is to point toward the outside, it must be stretched. Shrinking In forming a flanged angle by shrinking, use wooden forming blocks similar to those shown in Figure 4-151 and proceed as follows: 1. Cut the metal to size, allowing for trimming after forming. Determine the bend allowance for a 90° bend and round the edge of the forming block accordingly.
2. Clamp the material in the form blocks as shown in the block. After bending, tap the blocks slightly. This induces a setting process in the bend. 3. Using a soft-faced shrinking mallet, start hammering near the center and work the flange down gradually toward both ends. The flange tends to buckle at the bend because the material is made to occupy less space. Work the material into several small buckles instead of one large one and work each buckle Form blocks Hardwood wedge block 4-75 out gradually by hammering lightly and gradually compressing the material in each buckle. The use of a small hardwood wedge block aids in working out the buckles. [Figure 4-152] 4. Planish the flange after it is flattened against the form block and remove small irregularities. If the form blocks are made of hardwood, use a metal planishing hammer.
If the forms are made of metal, use a soft-faced mallet. Trim the excess material away and file and polish. Stretching To form a flanged angle by stretching, use the same forming blocks, wooden wedge block, and mallet as used in the shrinking process and proceed as follows: 1. Cut the material to size (allowing for trim), determine bend allowance for a 90° bend, and round off the edge of the block to conform to the desired radius of bend. 2. Clamp the material in the form blocks. [Figure 4-153] 3. Using a soft-faced stretching mallet, start hammering near the ends and work the flange down smoothly and gradually to prevent cracking and splitting. Planish the flange and angle as described in the previous procedure, and trim and smooth the edges, if necessary.
Curved Flanged Parts Curved flanged parts are usually hand formed with a concave flange, the inside edge, and a convex flange, the outside edge. The concave flange is formed by stretching, while the convex flange is formed by shrinking. Such parts are shaped with the aid of hardwood or metal forming blocks. [Figure 4-154] These blocks are made in pairs and are designed specifically for the shape of the area being formed. These blocks are made in pairs similar to those used for straight angle bends and are identified in the same manner. They differ in that they are made specifically for the particular part to be formed, they fit each other exactly, and they conform to the actual dimensions and contour of the finished article.
4-76 The forming blocks may be equipped with small aligning pins to help line up the blocks and to hold the metal in place or they may be held together by C-clamps or a vise. They also may be held together with bolts by drilling through form blocks and the metal, provided the holes do not affect the strength of the finished part. The edges of the forming block are rounded to give the correct radius of bend to the part, and are undercut approximately 5° to allow for spring-back of the metal. This undercut is especially important if the material is hard or if the bend must be accurate. The nose rib offers a good example of forming a curved flange because it incorporates both stretching and shrinking (by crimping). They usually have a concave flange, the inside edge, and a convex flange, the outside edge. Note the various types of forming represented in the following figures. In the plain nose rib, only one large convex flange is used. [Figure 4-155] Because of the great distance around the part and the likelihood of buckles in forming, it is rather difficult to form. The flange and the beaded (raised ridge on sheet metal used to stiffen the piece) portion of this rib provide sufficient strength to make this a good type to use.
In Figure 4-156, the concave flange is difficult to form, but the outside flange is broken up into smaller sections by relief holes. In Figure 4-157, note that crimps are placed at equally spaced intervals to absorb material and cause curving, while also giving strength to the part. In Figure 4-158, the nose rib is formed by crimping, beading, putting in relief holes, and using a formed angle riveted on each end. The beads and the formed angles supply strength to the part. The basic steps in forming a curved flange follow: [Figures 4-159 and 160] 1. Cut the material to size, allowing about 1⁄4-inch excess material for trim and drill holes for alignment pins.
2. Remove all burrs (jagged edges). This reduces the possibility of the material cracking at the edges during the forming process. Flange Holes Crimps 3. Locate and drill holes for alignment pins. 4. Place the material between the form blocks and clamp blocks tightly in a vise to prevent the material from moving or shifting. Clamp the work as closely as possible to the particular area being hammered to 4-77