Chapter 4 Metal Structure Repair PDF

identification. Use part numbers to identify these fasteners. Hi-Lite® Fastening System The Hi-Lite® fastener is similar in design and principle to the Hi-Lok® fastener, but the Hi-Lite® fastener has a shorter transition area between the shank and the first load-bearing thread. Hi-Lite® has approxim...

identification. Use part numbers to identify these fasteners. Hi-Lite® Fastening System The Hi-Lite® fastener is similar in design and principle to the Hi-Lok® fastener, but the Hi-Lite® fastener has a shorter transition area between the shank and the first load-bearing thread. Hi-Lite® has approximately one less thread. All HiLite® fasteners are made of titanium. These differences reduce the weight of the Hi-Lite® fastener without lessening the shear strength, but the Hi-Lite® clamping forces are less than that of a Hi-Lok® fastener. The Hi-Lite® collars are also different and thus are not interchangeable with Hi-Lok® collars. Hi-Lite® fasteners can be replaced with Hi-Lok® fasteners for most applications, but Hi-Loks® cannot be replaced with Hi-Lites®. CherryBUCK® 95 KSI One-Piece Shear Pin The CherryBUCK® is a bimetallic, one-piece fastener that combines a 95 KSI shear strength shank with a ductile, titanium-columbium tail. Theses fasteners are functionally interchangeable with comparable 6AI-4V titanium alloy two-piece shear fasteners, but with a number of advantages. Their one piece design means no foreign object damage (FOD), it has a 600 °F allowable temperature, and a very low backside profile. Lockbolt Fastening Systems Also pioneered in the 1940s, the lockbolt is a two-piece fastener that combines the features of a high-strength bolt and a rivet with advantages over each. [Figure 4-105] In general, a lockbolt is a nonexpanding fastener that has either a collar swaged into annular locking grooves on the pin shank or a type of threaded collar to lock it in place. Available with either countersunk or protruding heads, lockbolts are permanent type fasteners assemblies and consist of a pin and a collar. A lockbolt is similar to an ordinary rivet in that the locking collar, or nut, is weak in tension and it is difficult to remove once installed. Some of the lockbolts are similar to blind rivets and can be completely installed from one side. Others are fed into the workpiece with the manufactured head on the far side. The installation is completed on the near side with a gun similar to blind rivet gun. The lockbolt is easier and more quickly installed than the conventional rivet or bolt and eliminates the use of lockwashers, cotter pins, and special nuts. The lockbolt is generally used in wing splice fittings, landing gear fittings, fuel cell fittings, longerons, beams, skin splice plates, and other major structural attachment. Often called huckbolts, lockbolts are manufactured by companies such as Cherry® Aerospace (Cherry® Lockbolt), Alcoa Fastening Systems (Hucktite® Lockbolt System), and SPS Technologies. Used primarily for heavily stressed structures that require higher shear and clamp-up values than can be obtained with rivets, the lockbolt and Hi-lok® are often used for similar applications. Lockbolts are made in various head styles, alloys, and finishes. The lockbolt requires a pneumatic hammer or pull gun for installation. Lockbolts have their own grip gauge and an installation tool is required for their installation. [Figure 4-106] When installed, the lockbolt is rigidly and permanently locked in place. Three types of lockbolts are commonly used: pull-type, stump-type, and blind-type. The pull-type lockbolt is mainly used in aircraft and primary and secondary structure. It is installed very rapidly and has approximately one-half the weight of equivalent AN steel bolts and nuts. A special pneumatic pull gun is required for installation of this type lockbolt, which can be performed by one operator since buckling is not required. The stump-type lockbolt, although not having the extended stem with pull grooves, is a companion fastener to the pulltype lockbolt. It is used primarily where clearance does not permit effective installation of the pull-type lockbolt. It is driven with a standard pneumatic riveting hammer, with a hammer set attached for swaging the collar into the pin locking grooves, and a bucking bar. 4 2 36 44 6 46 8 48 4 38 6 42 8 2 10 12 14 16 18 20 22 INCH SCALE 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 Figure 4-105. Lockbolts. Shear and tension stump-type pins 24 GRIP SCALE Shear and tension pull-type pins 26 28 30 32 34 36 38 40 42 44 46 48 40 30 32 34 22 24 26 28 14 16 20 18 6 12 4 10 2 8 The blind-type lockbolt comes as a complete unit or assembly and has exceptional strength and sheet pull-together characteristics. Blind-type lockbolts are used where only one side of the work is accessible and generally where it is difficult to drive a conventional rivet. This type lockbolt is installed in a manner similar to the pull-type lockbolt. Figure 4-106. Lockbolt grip gauge. 4-51 The pins of pull- and stump-type lockbolts are made of heattreated alloy steel or high-strength aluminum alloy. Companion collars are made of aluminum alloy or mild steel. The blindtype lockbolt consists of a heat-treated alloy steel pin, blind sleeve, filler sleeve, mild steel collar, and carbon steel washer. These fasteners are used in shear and tension applications. The pull-type is more common and can be installed by one person. The stump type requires a two-person installation. An assembly tool is used to swage the collar onto the serrated grooves in the pin and break the stem flush to the top of the collar. Inspect the lockbolt as follows: 1. The head must be firmly seated. 2. The collar must be tight against the material and have the proper shape and size. 3. Pin protrusion must be within limits. Lockbolt Removal The best way to remove a lockbolt is to remove the collar and R Z The easiest way to differentiate between tension and shear pins is the number of locking grooves. Tension pins normally have four locking grooves and shear pins have two locking grooves. The installation tooling preloads the pin while swaging the collar. The surplus end of the pin, called the pintail, is then fractured. Installation Procedure Installation of lockbolts involves proper drilling. The hole preparation for a lockbolt is similar to hole preparation for a Hi-Lok®. An interference fit is typically used for aluminum and a clearance fit is used for steel, titanium, and composite materials. [Figure 4-107] Lockbolt Inspection After installation, a lockbolt needs to be inspected to determine if installation is satisfactory. [Figure 4-108] 1 Placed the pin in the hole from the back side of the work and slip the collar on. The hold-off head must be toward the gun. This allows the gun to preload the pin before swaging. Then apply the gun; the chuck jaws engage the pull grooves of the projecting pintail. Hold the gun loosely and pull the trigger. 2 The initial pull draws the work up tight and pulls that portion of the shank under the head into the hole. Y T Lockbolt/Collar Acceptance Criteria Nominal Fastener Diameter Y Z (Ref.) R Max. T Min. 5/32.324/.161.136.253.037 3/16.280/.208.164.303.039 1/4.374/.295.224.400.037 5/16.492/.404.268.473.110 3/8.604/.507.039.576.120 Figure 4-108. Lockbolt inspection. 3 Further pull swages the collar into the locking grooves to form a permanent lock. 4 Continued force breaks the pin and ejects the tail. Anvil returns and disengages from the swaged collar. Figure 4-107. Lockbolt installation procedure. 4-52 drive out the pin. The collar can be removed with a special collar cutter attached to a drill motor that mills off the collar without damaging the skin. If this is not possible, a collar splitter or small chisel can be used. Use a backup block on the opposite side to prevent elongation of the hole. The Eddie-Bolt® 2 Pin Fastening System The Eddie-Bolt® 2 looks similar to the Hi-Lok®, but has five flutes, equally spaced along a portion of the pin thread area. A companion threaded collar deforms into the flutes at a predetermined torque and locks the collar in place. The collar can be unscrewed using special tooling. This fastening system can be used in either clearance or interference-fit holes. Blind Bolts Bolts are threaded fasteners that support loads through predrilled holes. Hex, close-tolerance, and internal wrenching bolts are used in aircraft structural applications. Blind bolts have a higher strength than blind rivets and are used for joints that require high strength. Sometimes, these bolts can be direct replacements for the Hi-Lok® and lockbolt. Many of the new generation blind bolts are made from titanium and rated at 90 KSI shear strength, which is twice as much as most blind rivets. Determining the correct length of the fastener is critical to correct installation. The grip length of a bolt is the distance from the underhead bearing surface to the first thread. The grip is the total thickness of material joined by the bolt. Ideally, the grip length should be a few thousands of an inch less than the actual grip to avoid bottoming the nut. Special grip gauges are inserted in the hole to determine the length of the blind bolt to be used. Every blind bolt system has its own grip gauge and is not interchangeable with other blind bolt or rivet systems. Blind bolts are difficult to remove due to the hardness of the core bolt. A special removal kit is available from the manufacturer for removing each type of blind bolt. These kits make it easier to remove the blind bolt without damaging the hole and parent structure. Blind bolts are available in a pull-type and a drive-type. Pull-Type Blind Bolt Several companies manufacture the pull-type of blind bolt fastening systems. They may differ in some design aspects, but in general they have a similar function. The pull-type uses the drive nut concept and is composed of a nut, sleeve, and a draw bolt. Frequently used blind bolt systems include but are not limited to the Cherry Maxibolt® Blind Bolt system and the HuckBolt® fasteners which includes the Ti-Matic® Blind Bolt and the Unimatic® Advanced Bolt (UAB) blind bolt systems. Cherry Maxibolt® Blind Bolt System The Cherry Maxibolt® blind bolt, available in alloy steel and A-286 CRES materials, comes in four different nominal and oversized head styles. [Figure 4-109] One tool and pulling head installs all three diameters. The blind bolts create a larger blind side footprint and they provide excellent performance in thin sheet and nonmetallic applications. The flush breaking stem eliminates shaving while the extended grip range accommodates different application thicknesses. Cherry Maxibolts® are primarily used in structures where higher loads are required. The steel version is 112 KSI shear. The A286 version is 95 KSI shear. The Cherry® G83, G84, or G704 installation tools are required for installation. Huck Blind Bolt System The Huck Blind Bolt is a high strength vibration-resistant fastener. [Figure 4-110] These bolts have been used successfully in many critical areas, such as engine inlets and leading edge applications. All fasteners are installed with a combination of available hand, pneumatic, pneudraulic, or hydraulic pull-type tools (no threads) for ease of installation. Huck Blind Bolts can be installed on blind side angle During the Maxibolt® installation sequence, the Cherry® shift washer collapses into itself, leaving a solid washer that is easily retrieved. Figure 4-109. Maxibolt® Blind Bolt System installation. 4-53 Break neck Drive anvil washer Expander Gold color = Nominal diameter Silver color = Offset diameter Pull grooves Retention splines Lockring (visible after installation) 1 Rivet inserted into clearance hole—tool is engaged. 2 Expander enters sleeve—upset starts to form. 3 Upset continues to form—lock starts to form. 4 Upset complete—lock completely formed. lock collar fills the sleeve lock pocket to prevent leakage or corrosion pockets (crevice corrosion). Flush head blind bolts are designed to install with a flush stem break that often requires no trimming for aerodynamic surfaces. The Huck Blind Bolt is available in high-strength A286 CRES at 95KSI shear strength in 5⁄32-inch through 3⁄8inch diameters in 100° flush tension and protruding head. Also available are shear flush heads in 3⁄16-inch diameter. A286 CRES Huck Blind Bolts are also available in 1⁄64-inch oversize diameters for repair applications. Drive Nut-Type of Blind Bolt Jo-bolts, Visu-lok®, Composi-Lok®, OSI Bolt®, and RadialLok® fasteners use the drive nut concept and are composed of a nut, sleeve, and a draw bolt. [Figure 4-111] These types of blind bolts are used for high strength applications in metals and composites when there is no access to the blind side. Available in steel and titanium alloys, they are installed with special tooling. Both powered and hand tooling are available. During installation, the nut is held stationary while the core bolt is rotated by the installation tooling. The rotation of the core bolt draws the sleeve into the installed position and continues to retain the sleeve for the life of the fastener. The bolt has left hand threads and driving flats on the threaded end. A break-off relief allows the driving portion of the bolt to break off when the sleeve is properly seated. These types of bolts are available in many different head styles, including protruding head, 100° flush head, 130° flush head, and hex head. Use the grip gauge available for the type of fastener and select the bolt grip after careful determination of the material thickness. The grip of the bolt is critical for correct installation. [Figure 4-112] Installation procedure: 1. 5 Install the fastener into the hole, and place the installation tooling over the screw (stem) and nut. Pin breaks flush, lock visible—installation complete. Figure 4-110. Huck Blind Bolt system. surfaces up to 5° without loss of performance. The stem is mechanically locked to provide vibration-resistant FOD-free installations. The locking collar is forced into a conical pocket between stem and sleeve, creating high tensile capability. The Figure 4-111. Drive nut blind bolt. 4-54 Sleeve Bolts Sleeve bolts are used for similar purposes as tapered shank bolts, but are easier to install. Sleeve bolts, such as the two piece SLEEVbolt®, consist of a tapered shank bolt in an expandable sleeve. The sleeve is internally tapered and externally straight. The sleeve bolt is installed in a standard tolerance straight hole. During installation, the bolt is forced into the sleeve. This action expands the sleeve which fills the hole. It is easier to drill a straight tolerance hole than it is to drill the tapered hole required for a tapered shank bolt. Rivet Nut Figure 4-112. Drive nut blind bolt installation tool. 2. Apply torque to the screw with the installation tool while keeping the drive nut stationary. The screw continues to advance through the nut body causing the sleeve to be drawn up over the tapered nose of the nut. When the sleeve forms tightly against the blind side of the structure, the screw fractures in the break groove. The stem of Jo-bolts, Visu-lok®, and Composi-Lok® II fasteners does not break off flush with the head. A screw break-off shaver tool must be used if a flush installation is required. The stem of the newer Composi-Lok3® and OSI Bolt® break off flush. Tapered Shank Bolt Tapered shank bolts, such as the Taper-Lok®, are lightweight, high strength shear or tension bolts. This bolt has a tapered shank designed to provide an interference fit upon installation. Tapered shank bolts can be identified by a round head (rather than a screwdriver slot or wrench flats) and a threaded shank. The Taper-Lok® is comprised of a tapered, conical-shank fastener, installed into a precision tapered hole. The use of tapered shank bolts is limited to special applications such as high stress areas of fuel tanks. It is important that a tapered bolt not be substituted for any other type of fastener in repairs. It is equally as important not to substitute any other type of fastener for a tapered bolt. The rivet nut is a blind installed, internally-threaded rivet invented in 1936 by the Goodrich Rubber Company for the purpose of attaching a rubber aircraft wing deicer extrusion to the leading edge of the wing. The original rivet nut is the Rivnut® currently manufactured by Bollhoff Rivnut Inc. The Rivnut® became widely used in the military and aerospace markets because of its many design and assembly advantages. Rivet nuts are used for the installation of fairings, trim, and lightly loaded fittings that must be installed after an assembly is completed. [Figure 4-113] Often used for parts that are removed frequently, the rivet nut is available in two types: countersunk or flat head. Installed by crimping from one side, the rivet nut provides a threaded hole into which machine screws can be installed. Where a flush fit is required, the countersink style can be used. Rivet nuts made of alloy steel are used when increased tensile and shear strength is required. Hole Preparation Flat head rivet nuts require only the proper size of hole while flush installation can be made into either countersunk or dimpled skin. Metal thinner than the rivet nut head requires a dimple. The rivet nut size is selected according to the thickness of the parent material and the size of screw to be used. The part number identifies the type of rivet nut and the maximum grip length. Recommended hole sizes are shown in Figure 4-114. Tapered shank bolts look similar to Hi-Lok® bolts after installation, but the tapered shank bolts do not have the hex recess at the threaded end of the bolt. Tapered shank bolts are installed in precision-reamed holes, with a controlled interference fit. The interference fit compresses the material around the hole that results in excellent load transfer, fatigue resistance, and sealing. The collar used with the tapered shank bolts has a captive washer, and no extra washers are required. New tapered shank bolt installation or rework of tapered shank bolt holes needs to be accomplished by trained personnel. Properly installed, these bolts become tightly wedged and do not turn while torque is applied to the nut. Figure 4-113. Rivet nut installation. 4-55 Rivnut® Size Drill Size Hole Tolerance No. 4 5/32.155–.157 No. 6 #12.189–.193 No. 8 #2.221–.226 Figure 4-114. Recommended hole sizes for rivet nut. Correct installation requires good hole preparation, removal of burrs, and holding the sheets in contact while heading. Like any sheet metal fastener, a rivet nut should fit snugly into its hole. Blind Fasteners (Nonstructural) This shaping process is called forming and may be a simple process, such as making one or two holes for attaching; it may be a complex process, such as making shapes with complex curvatures. Forming, which tends to change the shape or contour of a flat sheet or extruded shape, is accomplished by either stretching or shrinking the material in a certain area to produce curves, flanges, and various irregular shapes. Since the operation involves altering the shape of the stock material, the amount of shrinking and stretching almost entirely depends on the type of material used. Fully annealed (heated and cooled) material can withstand considerably more stretching and shrinking and can be formed at a much smaller bend radius than when it is in any of the tempered conditions. Pop Rivets Common pull-type pop rivets, produced for non-aircraftrelated applications, are not approved for use on certificated aircraft structures or components. However, some homebuilt noncertificated aircraft use pull-type rivets for their structure. These types of rivets are typically made of aluminum and can be installed with hand tools. When aircraft parts are formed at the factory, they are made on large presses or by drop hammers equipped with dies of the correct shape. Factory engineers, who designate specifications for the materials to be used to ensure the finished part has the correct temper when it leaves the machines, plan every part. Factory draftsmen prepare a layout for each part. [Figure 4-116] Pull-Through Nutplate Blind Rivet Nutplate blind rivets are used where the high shear strength of solid rivets is not required or if there is no access to install a solid rivet. The 3⁄32-inch diameter blind rivet is most often used. The nut plate blind rivet is available with the pullthrough and self-plugging locked spindle. [Figure 4-115] Forming processes used on the flight line and those practiced in the maintenance or repair shop cannot duplicate a manufacturer’s resources, but similar techniques of factory metal working can be applied in the handcrafting of repair parts. Cherry ® The new Rivetless Nut Plate, which replaces standard riveted nutplates, features a retainer that does not require flaring. This proprietary design eliminates the need for two additional rivet holes, as well as reaming, counterboring, and countersinking steps. Forming Process Before a part is attached to the aircraft during either manufacture or repair, it has to be shaped to fit into place. Figure 4-115. Rivetless pull-through nutplate. Forming usually involves the use of extremely light-gauge alloys of a delicate nature that can be readily made useless by coarse and careless workmanship. A formed part may seem outwardly perfect, yet a wrong step in the forming procedure may leave the part in a strained condition. Such a defect may hasten fatigue or may cause sudden structural failure. Of all the aircraft metals, pure aluminum is the most easily formed. In aluminum alloys, ease of forming varies with Figure 4-116. Aircraft formed at a factory. 4-56 the temper condition. Since modern aircraft are constructed chiefly of aluminum and aluminum alloys, this section deals with the procedures for forming aluminum or aluminum alloy parts with a brief discussion of working with stainless steel, magnesium, and titanium. Most parts can be formed without annealing the metal, but if extensive forming operations, such as deep draws (large folds) or complex curves, are planned, the metal should be in the dead soft or annealed condition. During the forming of some complex parts, operations may need to be stopped and the metal annealed before the process can be continued or completed. For example, alloy 2024 in the “0” condition can be formed into almost any shape by the common forming operations, but it must be heat treated afterward. Forming Operations & Terms Forming requires either stretching or shrinking the metal, or sometimes doing both. Other processes used to form metal include bumping, crimping, and folding. Stretching Stretching metal is achieved by hammering or rolling metal under pressure. For example, hammering a flat piece of metal causes the material in the hammered area to become thinner in that area. Since the amount of metal has not been decreased, the metal has been stretched. The stretching process thins, elongates, and curves sheet metal. It is critical to ensure the metal is not stretched too much, making it too thin, because sheet metal does not rebound easily. [Figure 4-117] horizontal flange of the angle strip over a metal block causes its length to increase (stretched), making that section longer than the section near the bend. To allow for this difference in length, the vertical flange, which tends to keep the material near the bend from stretching, would be forced to curve away from the greater length. Shrinking Shrinking metal is much more difficult than stretching it. During the shrinking process, metal is forced or compressed into a smaller area. This process is used when the length of a piece of metal, especially on the inside of a bend, is to be reduced. Sheet metal can be shrunk in by hammering on a V-block or by crimping and then using a shrinking block. To curve the formed angle by the V-block method, place the angle on the V-block and gently hammer downward against the upper edge directly over the ”V.” While hammering, move the angle back and forth across the V-block to compress the material along the upper edge. Compression of the material along the upper edge of the vertical flange will cause the formed angle to take on a curved shape. The material in the horizontal flange will merely bend down at the center, and the length of that flange will remain the same. [Figure 4-118] To make a sharp curve or a sharply bent flanged angle, crimping and a shrinking block can be used. In this process, crimps are placed in the one flange, and then by hammering the metal on a shrinking block, the crimps are driven, or shrunk, one at a time. Stretching one portion of a piece of metal affects the surrounding material, especially in the case of formed and extruded angles. For example, hammering the metal in the Cold shrinking requires the combination of a hard surface, such as wood or steel, and a soft mallet or hammer because a steel hammer over a hard surface stretches the metal, as opposed to shrinking it. The larger the mallet face is, the better. Figure 4-117. Stretch forming metal. Figure 4-118. Shrink forming metal. 4-57 Bumping Bumping involves shaping or forming malleable metal by hammering or tapping—usually with a rubber, plastic, or rawhide mallet. During this process, the metal is supported by a dolly, a sandbag, or a die. Each contains a depression into which hammered portions of the metal can sink. Bumping can be done by hand or by machine. Crimping Crimping is folding, pleating, or corrugating a piece of sheet metal in a way that shortens it or turning down a flange on a seam. It is often used to make one end of a piece of stove pipe slightly smaller so that one section may be slipped into another. Crimping one side of a straight piece of angle iron with crimping pliers causes it to curve. [Figure 4-119] Folding Sheet Metal Folding sheet metal is to make a bend or crease in sheets, plates, or leaves. Folds are usually thought of as sharp, angular bends and are generally made on folding machines such as the box and pan brake discussed earlier in this chapter. Layout & Forming Terminology The following terms are commonly used in sheet metal forming and flat pattern layout. Familiarity with these terms aids in understanding how bend calculations are used in a bending operation. Figure 4-120 illustrates most of these terms. Base measurement—the outside dimensions of a formed part. Base measurement is given on the drawing or blueprint or may be obtained from the original part. Leg—the longer part of a formed angle. Flange—the shorter part of a formed angle—the opposite of leg. If each side of the angle is the same length, then each is known as a leg. Grain of the metal—natural grain of the material is formed as the sheet is rolled from molten ingot. Bend lines should be made to lie at a 90º angle to the grain of the metal if possible. Bend allowance (BA)—refers to the curved section of metal within the bend (the portion of metal that is curved in bending). The bend allowance may be considered as being the length of the curved portion of the neutral line. Bend radius—the arc is formed when sheet metal is bent. This arc is called the bend radius. The bend radius is measured Figure 4-119. Crimping metal. T Thickness (T) FL AN GE F L A T Bend tangent line dimension (BTLD) A MLD Leg Radius (R) Bend tangent line (BL) Mold line (ML) Bend allowance (BA) Mold point Setback (90° bend) R FLAT R+1 Base measurement SB Mold point B SB C BTLD MLD Figure 4-120. Bend allowance terminology. 4-58 from a radius center to the inside surface of the metal. The minimum bend radius depends on the temper, thickness, and type of material. Always use a Minimum Bend Radius Table to determine the minimum bend radius for the alloy that is going to be used. Minimum bend radius charts can be found in manufacturer’s maintenance manuals. Bend tangent line (BL)—the location at which the metal starts to bend and the line at which the metal stops curving. All the space between the bend tangent lines is the bend allowance. Neutral axis—an imaginary line that has the same length after bending as it had before bending. [Figure 4-121] After bending, the bend area is 10 to 15 percent thinner than before bending. This thinning of the bend area moves the neutral line of the metal in towards the radius center. For calculation purposes, it is often assumed that the neutral axis is located at the center of the material, although the neutral axis is not exactly in the center of the material. However, the amount of error incurred is so slight that, for most work, assuming it is at the center is satisfactory. Mold line (ML)—an extension of the flat side of a part beyond the radius. Mold line dimension (MLD)—the dimension of a part made by the intersection of mold lines. It is the dimension the part would have if its corners had no radius. Mold point—the point of intersection of the mold lines. The mold point would be the outside corner of the part if there were no radius. K-Factor—the percentage of the material thickness where there is no stretching or compressing of the material, such as the neutral axis. This percentage has been calculated and is one of 179 numbers on the K chart corresponding to one of the angles between 0° and 180° to which metal can be bent. [Figure 4-122] Whenever metal is to be bent to any angle other than 90° (K-factor of 90° equal to 1), the corresponding K-factor number is selected from the chart and is multiplied Neutral line Figure 4-121. Neutral line. by the sum of the radius (R) and the thickness (T) of the metal. The product is the amount of setback (see next paragraph) for the bend. If no K chart is available, the K-factor can be calculated with a calculator by using the following formula: the K value is the tangent of one-half the bend angle. Setback (SB)—the distance the jaws of a brake must be setback from the mold line to form a bend. In a 90° bend, SB = R + T (radius of the bend plus thickness of the metal). The setback dimension must be determined prior to making the bend because setback is used in determining the location of the beginning bend tangent line. When a part has more than one bend, setback must be subtracted for each bend. The majority of bends in sheet metal are 90° bends. The K-factor must be used for all bends that are smaller or larger than 90°. SB = K(R+T) Sight line—also called the bend or brake line, it is the layout line on the metal being formed that is set even with the nose of the brake and serves as a guide in bending the work. Flat—that portion of a part that is not included in the bend. It is equal to the base measurement (MLD) minus the setback. Flat = MLD – SB Closed angle—an angle that is less than 90° when measured between legs, or more than 90° when the amount of bend is measured. Open angle—an angle that is more than 90° when measured between legs, or less than 90° when the amount of bend is measured. Total developed width (TDW)—the width of material measured around the bends from edge to edge. Finding the TDW is necessary to determine the size of material to be cut. The TDW is less than the sum of mold line dimensions since the metal is bent on a radius and not to a square corner as mold line dimensions indicate. Layout or Flat Pattern Development To prevent any waste of material and to get a greater degree of accuracy in the finished part, it is wise to make a layout or flat pattern of a part before forming it. Construction of interchangeable structural and nonstructural parts is achieved by forming flat sheet stock to make channel, angle, zee, or hat section members. Before a sheet metal part is formed, make a flat pattern to show how much material is required in the bend areas, at what point the sheet must be inserted into the forming tool, or where bend lines are located. Bend lines must be determined to develop a flat pattern for sheet metal forming. 4-59 Degree K Degree K Degree K Degree K Degree K 1 0.0087 37 0.3346 73 0.7399 109 1.401 145 3.171 2 0.0174 38 0.3443 74 0.7535 110 1.428 146 3.270 3 0.0261 39 0.3541 75 0.7673 111 1.455 147 3.375 4 0.0349 40 0.3639 76 0.7812 112 1.482 148 3.487 5 0.0436 41 0.3738 77 0.7954 113 1.510 149 3.605 6 0.0524 42 0.3838 78 0.8097 114 1.539 150 3.732 7 0.0611 43 0.3939 79 0.8243 115 1.569 151 3.866 8 0.0699 44 0.4040 80 0.8391 116 1.600 152 4.010 9 0.0787 45 0.4142 81 0.8540 117 1.631 153 4.165 10 0.0874 46 0.4244 82 0.8692 118 1.664 154 4.331 11 0.0963 47 0.4348 83 0.8847 119 1.697 155 4.510 12 0.1051 48 0.4452 84 0.9004 120 1.732 156 4.704 13 0.1139 49 0.4557 85 0.9163 121 1.767 157 4.915 14 0.1228 50 0.4663 86 0.9324 122 1.804 158 5.144 15 0.1316 51 0.4769 87 0.9489 123 1.841 159 5.399 16 0.1405 52 0.4877 88 0.9656 124 1.880 160 5.671 17 0.1494 53 0.4985 89 0.9827 125 1.921 161 5.975 18 0.1583 54 0.5095 90 1.000 126 1.962 162 6.313 19 0.1673 55 0.5205 91 1.017 127 2.005 163 6.691 20 0.1763 56 0.5317 92 1.035 128 2.050 164 7.115 21 0.1853 57 0.5429 93 1.053 129 2.096 165 7.595 22 0.1943 58 0.5543 94 1.072 130 2.144 166 8.144 23 0.2034 59 0.5657 95 1.091 131 2.194 167 8.776 24 0.2125 60 0.5773 96 1.110 132 2.246 168 25 0.2216 61 0.5890 97 1.130 133 2.299 169 10.38 26 0.2308 62 0.6008 98 1.150 134 2.355 170 11.43 27 0.2400 63 0.6128 99 1.170 135 2.414 171 12.70 28 0.2493 64 0.6248 100 1.191 136 2.475 172 14.30 29 0.2586 65 0.6370 101 1.213 137 2.538 173 16.35 30 0.2679 66 0.6494 102 1.234 138 2.605 174 19.08 31 0.2773 67 0.6618 103 1.257 139 2.674 175 22.90 32 0.2867 68 0.6745 104 1.279 140 2.747 176 26.63 33 0.2962 69 0.6872 105 1.303 141 2.823 177 38.18 34 0.3057 70 0.7002 106 1.327 142 2.904 178 57.29 35 0.3153 71 0.7132 107 1.351 143 2.988 179 114.59 36 0.3249 72 0.7265 108 1.376 144 3.077 180 Inf. 9.514 Figure 4-122. K-factor. When forming straight angle bends, correct allowances must be made for setback and bend allowance. If shrinking or stretching processes are to be used, allowances must be made so that the part can be turned out with a minimum amount of forming. Making Straight Line Bends When forming straight bends, the thickness of the material, its alloy composition, and its temper condition must be considered. Generally speaking, the thinner the material is, the more sharply it can be bent (the smaller the radius of bend), and the softer the material is, the sharper the bend is. Other factors that must be considered when making straight line bends are bend allowance, setback, and brake or sight line. The radius of bend of a sheet of material is the radius of the bend as measured on the inside of the curved material. The minimum radius of bend of a sheet of material is the sharpest curve, or bend, to which the sheet can be bent without critically weakening the metal at the bend. If the radius of bend is too small, stresses and strains weaken the metal and may result in cracking. 4-60 A minimum radius of bend is specified for each type of aircraft sheet metal. The minimum bend radius is affected by the kind of material, thickness of the material, and temper condition of the material. Annealed sheet can be bent to a radius approximately equal to its thickness. Stainless steel and 2024-T3 aluminum alloy require a fairly large bend radius. will be discussed. [Figure 4-123] 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. Bending a U-Channel 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 4-124 to choose the correct bend radius for the alloy, temper, and the metal thickness. For 0.040, 2024T3 the minimum allowable radius is 0.16-inch or 5⁄32-inch. To understand the process of making a sheet metal layout, the steps for determining the layout of a sample U-channel 1.0.04.16 R= 2.0 Left view Scale: 3:2 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 4-125] Isometric view Scale: 3:2 Figure 4-123. U-channel example. CHART 204 MINIMUM BEND RADIUS FOR ALUMINUM ALLOYS Thickness 5052-0 6061-0 5052-H32 7178-0 2024-0 5052-H34 6061-T4 7075-0 6061-T6 7075-T6 2024-T3 2024-T4 2024-T6.012.03.03.03.03.06.06.016.03.03.03.03.09.09.020.03.03.03.12.09.09.025.03.03.06.16.12.09.032.03.03.06.19.12.12.040.06.06.09.22.16.16.050.06.06.12.25.19.19.063.06.09.16.31.22.25.071.09.12.16.38.25.31.080.09.16.19.44.31.38.090.09.19.22.50.38.44.100.12.22.25.62.44.50.125.12.25.31.88.50.62.160.16.31.44 1.25.75.75.190.19.38.56 1.38 1.00 1.00.250.31.62.75 2.00 1.25 1.25.312.44 1.25 1.38 2.50 1.50 1.50.375.44 1.38 1.50 2.50 1.88 1.88 Bend radius is designated to the inside of the bend. All dimensions are in inches. Figure 4-124. Minimum bend radius (from the Raytheon Aircraft Structural Inspection and Repair Manual). 4-61 T SB = DIstance from mold line to bend line BA = Line to bend line BA = Bend angle R = Bend radius T = Thickness Bend line BA Outside mold line R Setback (SB) 1. Enter chart at bottom on appropriate scale using sum T + R 2. Read up to bend angle 3. Determine setback from corresponding scale on left Example: T (0.063) + R (0.12) = 0.183 BA = 135° Setback = 0.453 Bend line Bend Angle (BA) 160° 150° 140° 135° 130° 120° 2.0 1.0 5.0 170° 0.18 0.90 4.5 110° 0.16 0.80 4.0 100° 70° Bend Angle (BA) 0.08 0.10 0.40 0.453 0.50 2.0 2.5 0.12 0.60 3.0 80° 60° 50° 0.06 0.30 1.5 Setback Distance (SB) 0.14 0.70 3.5 90° 0.04 0.20 1.0 45° 40° 30° 0.02 0.10 0.50 20° 10° 0.50 1.0 0.10 0.183 0.20 0.02 0.04 1.5 0.30 0.06 2.0 0.40 0.08 2.5 0.50 0.10 3.0 0.60 0.12 3.5 0.70 0.14 Thickness (T) + Radius (R) Flat Pattern Setback Graph Figure 4-125. Setback chart. 4-62 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 4-125] 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 4-125] 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 4-126] 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 4-127] 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: 4-63 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°. 0445T Distance from inner radius of bend Shrinking Bend allowance = (0.01743R + 0.0078T)N where: Neutral axis Stretching Figure 4-126. Neutral axis and stresses resulting from bending. 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.16inch for material 0.040-inch thick, substitute in the formula as follows: T Bend allowance = (0.01743 × 0.16) + (0.0078 × 0.040) × 90 = 0.27 inches B Radius Use of Bend Allowance Chart for a 90° Bend R + 1/2T C 90° Figure 4-127. Bend allowance for a 90° bend. 2π (R + 1⁄2T) 4 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 0.051inch thick, substitute in the formula as follows. Bend allowance = (2 × 3.1416)(0.250 + 1⁄2(0.051)) 4 = 6.2832(0.250 + 0.0255) 4 = 6.2832(0.2755) 4 = 0.4327 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 In Figure 4-128, 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. Example: The material thickness of the U-channel is 0.040inch 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. 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 4-129 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 4-64 RADIUS OF BEND, IN INCHES Metal Thickness 1/32.031 1/16.063 3/32.094 1/8.125 5/32.156 3/16.188 7/32.219 1/4.250 9/32.281 5/16.313 11/32.344 3/8.375 7/16.438 1/2.500.020.062.113.000693.001251.161.001792.210.002333.259.002874.309.003433.358.406.455.003974.004515.005056.505.005614.554.006155.702.799.603.006695.007795.008877.025.066.116.000736.001294.165.001835.214.002376.263.002917.313.003476.362.410.459.004017.004558.005098.509.005657.558.006198.705.803.607.006739.007838.008920.028.068.119.000759.001318.167.001859.216.002400.265.002941.315.003499.364.412.461.004040.004581.005122.511.005680.560.006221.708.805.609.006762.007862.007862.032.071.121.000787.001345.170.001886.218.002427.267.002968.317.003526.366.415.463.004067.004608.005149.514.005708.562.006249.710.807.611.006789.007889.008971.038.075.126.00837.001396.174.001937.223.002478.272.003019.322.003577.371.419.468.004118.004659.005200.518.005758.567.006299.715.812.616.006840.007940.009021.040.077.127.000853.001411.176.001952.224.002493.273.003034.323.003593.372.421.469.004134.004675.005215.520.005774.568.006315.716.813.617.006856.007955.009037.051.134.001413.183.002034.232.002575.280.003116.331.003675.379.428.477.004215.004756.005297.527.005855.576.006397.723.821.624.006934.008037.009119.064.144.001595.192.002136.241.002676.290.003218.340.003776.389.437.486.004317.004858.005399.536.005957.585.006498.732.830.634.007039.008138.009220.072.198.002202.247.002743.296.003284.436.003842.394.443.492.004283.004924.005465.542.006023.591.006564.738 836.639.007105.008205.009287.078.202.002249.251.002790.300.003331.350.003889.399.447.496.004430.004963.005512.546.006070.595.006611.745.840.644.007152.008252.009333.081.204.002272.253.002813.302.003354.352.003912.401.449.498.004453.004969.005535.548.006094.598.006635.745.842.646.007176.008275.009357.091.212.002350.260.002891.309.003432.359.003990.408.456.505.004531.005072.005613.555.006172.604.006713.752.849.653.007254.008353.009435.094.214.002374.262.002914.311.003455.361.004014.410.459.507.004555.005096.005637.558.006195.606.006736.754.851.655.007277.008376.009458.102.268.002977.317.003518.367.004076.416.464.513.004617.005158.005699.563.006257.612.006798.760.857.661.007339.008439.009521.109.273.003031.321.003572.372.004131.420.469.518.004672.005213.005754.568.006312.617.006853.764.862.665.008394.008493.009575.125.284.003156.333.003697.383.004256.432.480.529.004797.005338.005678.579.006437.628.006978.776.873.677.007519.008618.009700.355.003939.405.004497.453.502.551.005038.005579.006120.601.006679.650.007220.797.895.698.007761.008860.009942.417.004747.476.525.573.005288.005829.006370.624.006928.672.007469.820.917.721.008010.009109.010191.568.617.006313.006853.667.007412.716.007953.863.961.764.008494.009593.010675.156.188.250 Figure 4-128. Bend allowance. 6" R = 3 1.1 120° 0.1 1.98 0.04 Figure 4-129. Bend allowance for bends less than 90°. 4-65 from flat. To find the correct bend angle, use the following formula: 0.27 0.27 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. Br 0.80 1.60 Flat 3 Go to the left side of the table and find 0.040-inch. Flat 2 Bend allowance 1. Flat 1 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. Bend allowance Bend Angle = 180° – Angle between flanges 0.80 Figure 4-130. Flat pattern layout. 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 4-130] Step 7: Draw the Sight Lines on the Flat Pattern The pattern laid out in Figure 4-130 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 4-131] Now, clamp the brake on the metal and raise the leaf to make the bend. The bend begins exactly on the bend tangent line. 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. ak es e Be nd ta ng en tl in Sight line looking straight down the nose radius bar Sig ht lin e The sight line is located one radius inside the bend tangent line that is placed in the brake. Brake nose Bend tangent lines Figure 4-131. Sight line. 4-66 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] Bend Radius 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 X = Amount to be reduced from sum of flange dimension A + B − X = Developed length 1.60 1.40 0.063 0.12 45° X= B R Material Bend raduis Angle 0.035 ngle da Example Be n A 1.70 150° 1.20 1.00 140° 130° 0.90 120° 115° 110° 105° 100° 95° 0.80 Factor X 0.70 0.60 0.50 0.40 90° 85° 0.30 80° Angle 0.50 75° 0.25 70° 0.20 65° 60° 0.15 55° 50° 0.10 45° 0.04 0.09 0.08 40° 0.03 0.07 0.02 0.01 35° 0.06 0.05 30° 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. 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 Thickness Figure 4-132. J chart. 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. 0.5" Interpolate when the X factor falls between lines. 0" 2. Add up the mold line dimensions and subtract the X factor to find the TDW. 135° R = " 25 0. 2.0" Figure 4-134. Example 2 of J chart. 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 Figure 4-134 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. 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 0.063" 2.00" ".22 R =0 2.00" Figure 4-133. Example 1 of J chart. Figure 4-135. Brake radius nosepiece adjustment. 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.063inch 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. Figure 4-136. Interchangeable brake radius bars. Place test strips on the table 3 inches from each end and one in the center between the bed and the clamp, adjust clamp This radius shim builds radius to precisely 1/16"R UPPER JAW NOSE RADIUS BAR BENDING LEAF Each of these nose radius shims is 0.063 inch thick, which gives radius choices of 1/8", 3/16", and 1/4" LOWER JAW BED Figure 4-137. Nose radius shims may be used when the brake radius bar is smaller than required. 4-69 Pull forward to clamp (no sponginess felt when evenly set on BOTH sides) Radius shims Limiting stop Material to be bent Lifting nut Nut to adjust clamping pressure Note: Bending leaf counterbalance omitted for clarity Figure 4-138. General brake overview including radius shims. 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 Figure 4-139. Adjust clamping pressure with the clamping pressure nut. Figure 4-140. Brake nose gap adjustment with piece of material same thickness as part to be formed. 4-70 Scrap of material to be bent Should slip snugly in and out BENDING LEAF NOSE GAP Hold bending leaf at the finished angle of bend 90°(in this case) Figure 4-141. Profile illustration of brake nose gap adjustment. 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. Figure 4-142. Brake alignment with two test strips 3 inches from each end. 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 Figure 4-143. Brake alignment with two test strips bent at 90°. Figure 4-144. Brake alignment by comparing test strips. 4-71 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. R T SB BA MG 1" 2" = = = = = 0.250 (1∕4) 0.063 (1∕16) 0.313 (5∕16) 0.437 (7∕16) 0.191 (3∕16) If 5∕16 R is required, punch 5∕8 hole 2" ∕ 11 16 Intersection of inside bend tangent lines Flat Area of bend (BA) Normal trim tangent to radius Flat 2 13∕16 ∕ ∕ 1 11∕16 11 16 7 16 Bend relief radius Area of bend (BA) Flat ∕ 7 16 If necessary for flanges to touch 1 2 2 13∕16 Notice overlapping mold lines (by 1 MG) Figure 4-145. Relief hole location. Figure 4-146. Relief hole layout. 4-72 Open End Bend (Less Than 90°) Figure 4-147 shows an example for a 45° bend. 1. 2. Look up K-factor in K chart. K-factor for 45° is 0.41421-inch. 135° 77 0. 9 R.1 45° 1.52 Calculate setback. 0.05 SB = K(R + T) SB = 0.41421-inch(0.1875-inch + 0.050-inch) = 0.098-inch 3. 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°) Figure 4-148 shows an example of a 135° bend. 1. 2. Look up K-factor in K chart. K-factor for 135° is 2.4142-inch. Calculate SB. SB = K(R + T) SB = 2.4142-inch(0.1875-inch + 0.050-inch) = 0.57inch 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. Flat = Mold line dimension – SB 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. Figure 4-148. Closed bend. Calculate flats. R.19 135° 77 0. 1.52 Figure 4-147. Open bend. 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 = 1.65inch 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 springback 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. Workhardening 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. Figure 4-149. V-block forming. 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. After obtaining the desired curve, planish the angle strip over a stake or a wooden form. 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. Flanged Angles 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. 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 Figure 4-151, and bend the exposed flange against 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 Hardwood wedge block Form blocks Figure 4-150. Crimping a metal flange in order to form a curve. Figure 4-151. Forming a flanged angle using forming blocks. 4-75