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4-54. HEAT TREATMENT OF ALUMINUM ALLOY PARTS. All structural aluminum alloy parts are to be heat-treated in accordance with the heat-treatment instruction issued by the manufacturers of the part. In the case of a specified temper, the sequence of heat-treating operations set forth in MIL-HDBK-5 or MMPDS-10 and corresponding specifications. If the heat-treatment produces warping, straighten the parts immediately after quenching. Heat-treat riveted parts before riveting, to preclude warping and corrosion.
a. Quenching. Quench material from the solution heat-treating temperature as rapidly as possible after removal from the furnace. Quenching in cold water is preferred, although less drastic chilling (hot or boiling water, or airblast) is sometimes employed for bulk sections, such as forgings, to minimize quenching stresses.
b. Reheating at Temperatures Above Boiling Water. Reheating of 2017 and 2024 alloys above 212 °F tend to impair the original heat treatment. Therefore, reheating above 212 °F, including the baking of primers, is not acceptable without subsequent complete and correct heat treatment.
4-55. BENDING METAL. When describing a bend in aviation, the term “bend radii” (BR) is used to refer to the inside radius. Requirements for bending the metal to various shapes are frequently encountered. When a metal is bent, it is subjected to changes in its grain structure, causing an increase in its hardness.
a. The minimum radius is determined by the composition of the metal, its temper, and thickness. Table 4-6 shows the recommended radius for different types of aluminum. Note that the smaller the thickness of the material, the smaller the recommended minimum BR, and that as the material increases in hardness, the recommended BR increases.
b. When using layout techniques, the mechanic must be able to calculate exactly how much material will be required for the bend. It is easier to lay out the part on a flat sheet before the bending or shaping is performed. Before bending, smooth all rough edges, remove burrs, and drill relief holes at the ends of bend lines and at corners to prevent cracks from starting. Bend lines should preferably be made to lie at an angle to the grain of the metal (preferably 90 degrees).
c. BR in inches for a specific metal composition (alloy) and temper is determined from Table 4-6. For example, the minimum BR for 0.016 thick 2024-T6 (alloy and temper) is found is found to be 2 to 4 times the material thickness, or 0.032 to 0.064.
4-56. SETBACK.
a. Setback is a measurement used in sheet metal layout. It is the distance the jaws of a brake must be setback from the mold line to form a bend. For a 90-degree bend, the point is back from the mold line to a distance equal to the BR plus the metal thickness. The mold line is an extension of the flat side of a part beyond the radius. The mold line dimension of a part is the dimension made to the intersection of mold lines, and is the dimension the part would have if its corners had no radius. (See Figure 4-2.)
Par 4-56 Page 4-17 TABLE 4-6. RECOMMENDED RADII FOR 90-DEGREE BENDS IN
ALUMINUM ALLOYS Alloy and Temper Approximate Sheet Thickness (t) (inch)
0.016 0.032 0.064 0.128 0.182 0.258 2024-01 0 0–1t 0–1t 0–1t 0–1t 0–1t 2024-T31, 2 1½t–3t 2t–4t 3t–5t 4t–6t 4t–6t 5t–7t 2024-T61 2t–4t 3t–5t 3t–5t 4t–6t 5t–7t 6t–10t 5052-0 0 0 0–1t 0–1t 0–1t 0–1t 5052-H32 0 0 ½t–1t ½t–1½t ½t–1½t ½t–1½t 5052-H34 0 0 ½t–1½t 1½t–2½t 1½t–2½t 2t–3t 5052-H36 0–1t ½t–1½t 1t–2t 1½t–3t 2t–4t 2t–4t 5052-H38 ½t–1½t 1t–2t 1½t–3t 2t–4t 3t–5t 4t–6t 6061-0 0 0–1t 0–1t 0–1t 0–1t 0–1t 6061-T4 0–1t 0–1t ½t–1½t 1t–2t 1½t–3t 2½t–4t 6061-T6 0–1t ½t–1½t 1t–2t 1½t–3t 2t–4t 3t–4t 7075-0 0 0–1t 0–1t ½t–1½t 1t–2t 1½t–3t 7075-T61 2t–4t 3t–5t 4t–6t 5t–7t 5t–7t 6t–10t 1 Alclad sheet may be bent over slightly smaller radii than the corresponding tempers of uncoated alloy.
2 Immediately after quenching, this alloy may be formed over appreciably smaller radii.
FIGURE 4-2. SETBACK FOR A 90-DEGREE BEND
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b. To determine setback for a bend of more or less than 90 degrees, a correction known as a K-factor must be applied to find the setback.
(1) Table 4-7 shows a chart of K-factors. To find the setback for any degree of bend, multiply the sum of the BR and metal thickness by the K-value for the angle through which the metal is bent.
TABLE 4-7. K-CHART FOR DETERMINING SETBACK FOR BENDS OTHER THAN 90 DEGREES Deg. K Deg. K Deg. K Deg. K Deg. 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 9.514 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.
(2) Figure 4-3 shows an example of a piece of 0.064-inch sheet metal bent through 45 degrees to form an open angle of 135 degrees. For 45 degrees, the K-factor is 0.41421. The setback, or the distance from the mold point to the bend tangent line, is:
Setback = K(BR + MT) = 0.41421 (0.25 + 0.064) = 0.130 inches FIGURE 4-3. METHODS OF DETERMINING SETBACK FOR BENDS OTHER THAN 90 DEGREES
• T = 0.064"
- R = 0.250
SETBACK
SETBACK This is a 90-degree angle.
Setback = BR + MT = 0.250 + 0.064 = 0.314" T = 0.064" R = 0.250
SETBACK This is a 135-degree open angle. The metal has only been bent 45°.
SB = (BR + MT)K = (0.250 + 0.064) 0.4121 = 0.130" T = 0.064 -
SETBACK
- R = 0.250
SETBACK This is a 45-degree closed angle.
Setback = (BR + MT)K = (0.250 + 0.064)2.4142 = 0.758"
(3) If a closed angle of 45 degrees is formed, the metal must be bent through 135 degrees. The K-factor for 135 degrees is 2.4142, so the setback, or distance from the mold point to the bend tangent line, is 0.758 inch.
4-57. RIVETING.
a. The two major types of rivets used in aircraft are the common solid shank rivet, which must be driven using an air-driven rivet gun and bucking bar; and special (blind) rivets, which Par 4-56 Page 4-19
Par 4-57 Page 4-20 are installed with special installation tools. Design allowables for riveted assemblies are specified in MIL-HDBK-5 or MMPDS-10.
(1) Solid shank rivets are used widely during assembly and repair work. They are identified by the material of which they are made, the head type, size of shank, and temper condition.
(2) The material used for the majority of solid shank rivets is aluminum alloy. The strength and temper conditions of aluminum alloy rivets are identified by digits and letters similar to those used to identify sheet stock. The 1100, 2017-T, 2024-T, 2117-T, and 5056 rivets are the six grades usually available. Army/Navy (AN)-type aircraft solid rivets can be identified by code markings on the rivet heads. A rivet made of 1100 material is designated as an “A” rivet, and has no head marking. The 2017-T alloy rivet is designated as a “D” rivet and has a raised teat on the head. Two dashes on a rivet head indicate a 2024-T alloy designated as a “DD” rivet.
The 2117-T rivet is designated as an “AD” rivet, and has a dimple on the head. A “B” designation is given to a rivet of 5056 material and is marked with a raised cross on the rivet head. Each type of rivet is identified by a part number to allow the user to select the correct rivet.
The numbers are in series and each series represents a particular type of head. (See Figure 4-4 and Table 4-8.)
FIGURE 4-4. RIVET IDENTIFICATION AND PART NUMBER BREAKDOWN
RIVET IDENTIFICATION The material can be identified by the head marking Material Rivet Code Head Marking Material O¢ A PLAIN (Dyed) 1100 AD DIMPLED 2117 D RAISED DOT 2017T DO TWO RAISED DASHES 2024 RAISED CROSS (Dyed) 5056 RAISED CIRCLE 7050 M TWO DOTS Monel
PART NUMBER BREAKDOWN
• MS20470 AD 4-7
•• MS20470 • AD Type Head Material Diameter Length Code Measured Measured in 32nds in 16ths of an inch of an inch Length is measured from the top of the flush head and the underside of the universal head.
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TABLE 4-8. AIRCRAFT RIVET IDENTIFICATION Material 1100 2117T 20171 2017T-HD 2024T 5056T 7075-T73 Head Plain Dimpled Raised Raised Raised Raised Three Marking Dot Dot Double Cross Raised Dash Dashes AN
AD D D DD Material Code AN425 78• Counter- Sunk Head AN426 100• Counter- Sunk Head MS20426 AN427 100• Counter- Sunk Head MS20427 AN430 Round Head MS20470 AN435 Round Head MS20613 MS20615 AN 441 Flat Head AN 442 Flat Head MS20470 AN 455 Brazier × Head MS20470 AN 456 Brazier Head MS20470 AN 470 Universal Head MS20470 Heat Treat Before No No Yes No Yes No No Using Shear Strength 10000 30000 34000 38000 41000 27000 psi Bearing Strength 25000 100000 113000 126000 136000 90000 psi Par 4-57 Page 4-22
TABLE 4-8. AIRCRAFT RIVET IDENTIFICATION (CONTINUED) Material Carbon Corrosion- Copper Monel Monel Brass Titanium Steel Resistant Nickel- Steel Copper Alloy Head Recessed Recessed Plain Plain Recessed Plain Recessed Marking Triangle Dash Double Large and Dots Small Dot AN Material F M c Code AN425 78• Counter- Sunk Head AN426 100• Counter- MS 20426 Sunk Head MS20426 AN427 100• Counter- Sunk Head MS20427 AN430 Round Head MS20470 AN435 Round Head MS20613 MS20615 MS20613 MS20613 MS20615 MS20615 AN 441 Flat Head AN 442 Flat Head MS20470 AN 455 Brazier Head MS20470 AN 456 Brazier Head MS20470 AN 470 Universal Head MS20470 Heat Treat Before Us- No No No No No No No ing Shear Strength 35000 65000 23000 49000 49000 95000 psi Bearing Strength 90000 90000 psi Par 4-57 Page 4-23
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(3) An example of identification marking of a rivet follows:
MS 20470AD3-5 Complete part number MS Military standard number 20470 Universal head rivet AD 2117-T aluminum alloy 3 3/32 inch in diameter 5 5/16 inch in length
(4) Countersunk head rivets (MS20426 supersedes AN426 100-degree) are used where a smooth finish is desired. The 100-degree countersunk head has been adopted as the standard in the United States. The universal head rivet (AN470 superseded by MS20470) has been adopted as the standard for protruding-head rivets, and may be used as a replacement for the roundhead, flathead, and brazier head rivet. These rivets can also be purchased in half sizes by designating a “0.5” after the main length (e.g., MS20470 AD4-3.5).
b. Replace rivets with those of the same size and strength whenever possible. If the rivet hole becomes enlarged, deformed, or otherwise damaged; drill or ream the hole for the next larger size rivet. However, make sure that the edge distance and spacing is not less than minimums listed in subparagraph 4-57c. Rivets may not be replaced by a type having lower strength properties, unless the lower strength is adequately compensated by an increase in size or a greater number of rivets. It is acceptable to replace 2017 rivets of 3/16 inch diameter or less, and 2024 rivets of 5/32 inch diameter or less with 2117 rivets for general repairs, provided the replacement rivets are 1/32 inch greater in diameter than the rivets they replace.
c. Rivet edge distance is defined as the distance from the center of the rivet hole to the nearest edge of the sheet. Rivet spacing is the distance from the center of the rivet hole to the center of the adjacent rivet hole. Unless structural deficiencies are suspected, the rivet spacing and edge distance should duplicate those of the original aircraft structure. If structural deficiencies are suspected, the following may be used in determining minimum edge distance and rivet spacing.
(1) For single row rivets, the edge distance should not be less than 2 times the diameter of the rivet and spacing should not be less than 3 times the diameter of the rivet.
(2) For double row rivets, the edge distance and spacing should not be less than the minimums shown in Figure 4-5.
(3) For triple or multiple row rivets, the edge distance and spacing should not be less than the minimums shown in Figure 4-5.
FIGURE 4-5. RIVET HOLE SPACING AND EDGE DISTANCE FOR SINGLE-LAP
SHEET SPLICES
STRENGTH THROUGH THIS SECTION IS 75% OF SHEET WITHOUT HOLES.
D = DIAMETER OF
• RIVET (PREFER- ABLY ABOUT 3x 7 THICKNESS OF
THICKER SHEET) 20 → L40= -20 (A) DOUBLE ROW
STRENGTH THROUGH THIS SECTION IS 83% OF SHEETL WITHOUT HOLES.- 6D 3D 20- +3D+30=<30+ * 2D (B) TRIPLE OR MULTIPLE ROWS
d. The 2117 rivets may be driven in the condition received, but 2017 rivets above 3/16 inch in diameter and all 2024 rivets are to be kept packed in dry ice or refrigerated in the "quenched" condition until driven, or be reheat-treated just prior to driving, as they would otherwise be too hard for satisfactory riveting. Dimensions for formed rivet heads are shown in Figure 4-6, item (a), together with commonly found rivet imperfections.
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FIGURE 4-6. RIVETING PRACTICE AND RIVET IMPERFECTIONS
(a) DIMENSIONS FOR FORMED RIVET HEADS.
65D (ID MIN.) (MIN.) (TO MIN.)
(b) RIVETING TOOLS
INCORRECT CORRECT SHAPE FOR RIVET SET.
BUCKING BAR
OR DOLLY (C) RIVET IMPERFECTIONS
RIVET DRIVEN AT SLANT, RIVET DRIVEN CORRECTLY, DOLLY RIVET FLAT ON ONE SIDE OR DOLLY HELD HEAD AT SLANT. FLAT.
BODY OF RIVET TOO RIVET NOT PULLED RIVET TIGHT, PLATES SHORT. CLOSING HEAD TIGHT, CLINCHES BULGED ON ACCOUNT SHAPED TOO MUCH BETWEEN PLATES, OF POOR FIT.
WITH SNAP DIE. CLOSING HEAD TOO FLAT.
RIVETED TOO MUCH. RIVET BODY CLINCHED TOO MUCH, HEAD CRACKED.
PLATES CLINCHED AT RIVET MATERIAL TOO AND DRIVEN APART. HARD WHEN
RIVETING TOOL FORMED
DAMAGED PLATE
e. When solid shank rivets are impractical to use, then special fasteners are used. Special fastening systems used for aircraft construction and repair are divided into two types, special and blind fasteners. Special fasteners are sometimes designed for a specific purpose in an aircraft structure. The name "special fasteners" refers to its job requirement and the tooling needed for installation. Use of special fasteners may require an FAA field approval.
f. Blind rivets are used under certain conditions when there is access to only one side of the structure. Typically, the locking characteristics of a blind rivet are not as good as a driven rivet. Therefore, blind rivets are usually not used when driven rivets can be installed.
NOTE: Blind rivets shall not be used:
• In fluid-tight areas; or
• On aircraft in air intake areas where rivet parts may be ingested by the engine, on aircraft control surfaces, hinges, hinge brackets, flight control actuating systems, wing attachment fittings, landing gear fittings, on floats or amphibian hulls below the water level, or other heavily stressed locations on the aircraft.
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Par 4-57 Page 4-27 CAUTION: Fo r metal repairs to the airframe, the use of blind rivets must be specifically authorized by the airframe manufacturer or approved by a representative of the FAA.
(1) Self-plugging friction-lock cherry rivets. This patented rivet may be installed when there is access to only one side of the structure. The blind head is formed by pulling the tapered stem into the hollow shank. This swells the shank and clamps the skins tightly together. When the shank is fully upset, the stem pulls in two. The stem does not fracture flush with the rivet head and must be trimmed and filed flush for the installation to be complete. Because of the friction-locking stem, these rivets are very sensitive to vibrations. Inspection is visual, with a loose rivet standing out in the standard “smoking rivet” pattern. Removal consists of punching out the friction-locked stem and then treating it like any other rivet. (See Figure 4-7.)
FIGURE 4-7. SELF-PLUGGING FRICTION-LOCK CHERRY RIVETS (
2) Mechanical-lock rivets have a device on the puller or rivet head which locks the center stem into place when installed. Many friction-lock rivet center stems fall out due to vibrations; this, in turn, greatly reduces its shear strength. The mechanical-lock rivet was developed to prevent that problem. Various manufacturers make mechanical-lock fasteners, such as Bulbed CherryLOCK, CherryMAX, Olympic-Loks, and Huck-Loks.
(3) Bulbed CherryLOCK Rivets. One of the earlier types of mechanical-lock rivets developed were Bulbed CherryLOCK blind rivets. These blind rivets have as their main advantage the ability to replace a solid shank rivet size for size. (See Figure 4-8.)
(a) A Bulbed CherryLOCK consists of three parts: a rivet shell, a puller, and a lockring. The puller or stem has five features which are activated during installation: a header, a shank expanding section, a lockring indent, a weak or stem fracture point, and a serrated pulling stem. Carried on the pulling stem, near the manufactured head, is the stem lockring. When the rivet is pulled the action of the moving stem clamps together the sheets of metal and swells the shank to fill the drilled hole. When the stem reaches its preset limit of travel, the upper stem breaks away (just above the lockring) as the lockring snaps into the recess on the locking stem.