formed, the rolls must be clean and free of imperfections.
The rear roll must be adjusted to give the proper curvature to the part being formed. There are no gages that indicate settings for a specific diameter;
therefore, trial-and-error settings must be used to obtain the desired curvature.
The metal should be inserted between the rolls from the front of the machine. Start the metal between the rolls by rotating the operating handle in a clockwise direction.
A starting edge is formed by holding the operating handle firmly with the right hand and raising the metal with the left hand. The bend of the starting edge is determined by the diameter of the part being formed. If the edge of the part is to be flat or nearly flat, a starting edge should not be formed.
Be sure that fingers or loose clothing are clear of the rolls before the actual forming operation is started. Rotate the operating handle until the metal is partly through the rolls and change the left hand from the front edge of the sheet to the upper edge of the sheet. Then roll the remainder of the sheet through the machine.
If the desired curvature is not obtained, return the metal to its starting position by rotating the handle counterclockwise. Raise or lower the rear roll and roll the metal through the rolls again. Repeat this procedure until the desired curvature is obtained, then release the upper roll and remove the metal.
If the part to be formed has a tapered shape, the rear roll should be set so that the rolls are closer together on one end than on the opposite end. The amount of this adjustment will have to be deter- If the job being formed has a wired edge, the distance between the upper and lower rolls and the distance between the lower front roll and the rear roll should be slightly greater at the wired end than Forming Processes Before a part is attached to the aircraft during either manufacture or repair, it has to be shaped to fit into place. This shaping process is called forming. Forming may be a very simple process, such as making one or two holes for attaching, or it may be exceedingly complex, requiring shapes with complex curvatures.
Parts are formed at the factory on large presses or by drop hammers equipped with dies of the correct shape. Every part is planned by factory engineers, who set up specifications for the materials to be used so that the finished part will have the correct temper when it leaves the machines. A layout for each part is prepared by factory draftsmen.
Forming processes used on the flight line and those practiced in the maintenance or repair shop are almost directly opposite in the method of procedure. They have much in common, however, and many of the facts and techniques learned in the one process can be applied to the other.
Forming is of major concern to the airframe mechanic and requires the best of his knowledge and skill. This is especially true since forming usually involves the use of extremely light-gage alloys of a delicate nature which 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 the temper condition. Since modern aircraft are constructed chiefly of aluminum and aluminum alloys, this section will deal with the procedures for forming aluminum or aluminum alloy parts.
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 have to be stopped and the metal annealed before the process can be continued or completed. Alloy 2024 in the "O" condition can be formed into almost any shape by the common forming operations, but it must be heattreated afterward.
When forming, use hammers and mallets as sparingly as practicable, and make straight bends on bar folders or cornice brakes. Use rotary machines whenever possible, If a part fits poorly or not at all, do not straighten a bend or a curve and try to re-form it, discard the piece of metal and start with a new one.
When making layouts, be careful not to scratch aluminum or aluminum alloys. A pencil, if kept sharp, will be satisfactory for marking purposes.
Scribers make scratches which induce fatigue failure; but they may be used if the marking lines fall 144
outside the finished part, that is, if the scribed line will be part of the waste material. Keep bench tops covered with material hard enough to prevent chips and other foreign material from becoming imbed.
ded in them. Be sure also to keep bench tops clean and free from chips, filings, and the like. For the protection of the metals being worked, keep vise jaws covered with soft metal jaw caps.
Stainless steel can be formed by any of the usual methods but requires considerably more skill than is required for forming aluminum or aluminum alloys. Since stainless steel work-hardens very readily, it requires frequent annealing during the forming operations. Always try to press out stainless steel parts in one operation. Use dies, if possible.
FORMING OPERATIONS AND TERMS The methods used in forming operations include such sheetmetal work processes as shrinking, stretching, bumping, crimping, and folding.
Bumping Shaping or forming malleable metal by hammering or pounding is called bumping. 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 Folding, pleating, or corrugating a piece of sheet metal in a way that shortens it is called crimping.
Crimping is often used to make one end of a piece of stovepipe slightly smaller so that one section may be slipped into another. Turning down a flange on
• a team is also called crimping. Crimping one side of a straight piece of angle iron with crimping pliers will cause it to curve, as shown in figure 5-19.
Stretching Hammering a flat piece of metal in an area such as that indicated in figure 5-19 will cause the material in that area to become thinner. However, since the amount of metal will not have been decreased, it will cover a greater area because the metal will have been stretched.
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 horizontal flange of the angle strip over a metal block, as shown in figure 5-19, would cause its length to be increased (stretched);
therefore, that section would become 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 During the shrinking process, material is forced or compressed into a smaller area. The shrinking 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 two ways: (1) By hammering on a V-block (figure 5-20), or (2) by crimping and then shrinking on a shrinking block To curve the formed angle by the V-block method, place the angle on the V-block and gently B.
FIGURE 5-19. Crimping and stretching.
FIGURE 5-20. Shrinking on a V-block.
hammer downward against the upper edge directly over the "V" (figure 5-20). 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.
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 will be driven out (shrunk out) one at a time.
Folding Making bends in sheets, plates, or leaves is called folding. Folds are usually thought of as sharp, angular bends; they are generally made on folding machines.
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, the sharper it can be bent (the smaller the radius of bend), and the softer the material, the sharper the bend. 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 will weaken the metal and may result in cracking.
A minimum radius of bend is specified for each type of aircraft sheet metal. The kind of material, thickness, and temper condition of the sheet are factors affecting it. Annealed sheet can be bent to a radius approximately equal to its thickness. Stainless steel and 2024-T aluminum alloy require a fairly large bend radius (see fig. 5-28).
Bend Allowance When making a bend or fold in a sheet of metal, the bend allowance must be calculated. Bend allowance is the length of material required for the bend. This amount of metal must be added to the overall length of the layout pattern to assure adequate metal for the bend Bend allowance depends on four factors: (1) The degree of bend, (2) The radius of the bend, (3) The thickness of the metal, and (4) The 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 will be 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 will be greater, and the bend allowance will be greater. The degree of bend will affect the overall length of the metal, whereas the thickness influences the radius of bend.
Bending a strip 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 XT) from the inside of the radius of the bend (figure 5-21).
When bending metal to exact dimensions, the length of the neutral line must be determined so that sufficient material can be allowed for the bend.
To save time in calculation of the bend allowance, formulas and charts for various angles, radii of bends, material thicknesses, aud other factors have been established. The bend allowance formula for a 90° bend is discussed in the following paragraphs.
- 445T DISTANCE FROM INNER
RADIUS OF BEND
• Bend allowance Neutral line
• Radius of bend
• SHRINKING o NEUTRAL AXIS
STRETCHING
• 0.445 × T FIGURE 5-21. Neutral axis.
Method #1 Formula #1 To the radius of bend (R) add one half the thickness of the metal, (½ T). This gives R + ½ T, or the radius of the circle of approximately the neutral axis.
Since a 90° bend is a quarter of the circle, divide the circumference by 4. This gives:
2T (R+½ I) 4 Therefore, bend allowance for a 90° bend is 2т (R+ ½ T) Radius appe 1/2 T FIGURE 5-22. Bend allowance, 90° bend.
Compute the circumference of this circle by multiplying the radius of curvature of the neutral line (R+½ T in figure 5-22) by 27:
2т (R+½ T).
Note: #=3.1416.
To use the formula in finding the bend allowance for a 90° bend having a radius of ¼ in. for material 0.051-in. thick, substitute in the formula as follows:
Bend allowance = 2X3.1416 (0.250 + 1/2X0.051) 4
6.2832 (0.250 + 0.02555) 4 = 6.2832 (0.2755) 4 =0.4323.
Thus, if necessary, bend allowance or the length of material required for the bend is 0.4323, or 7/16 The formula is slightly in error because actually the neutral line is not exactly in the center of the sheet being bent. (See figure 5-22.) However, the amount of error incurred in any given problem 147
is so slight that, for most work, since the material used is thin, the formula is satisfactory.
Method #2 Formula #2 This formula uses two constant values which have evolved over 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°.
Bend allowance = (0.01743XR+0.0078XT) XN where:
R=The desired bend radius, T= Thickness of the material, and N = Number of degrees of bend.
BA = Bend allowance BA =0.01743 X 20° directly below the radius figures. The top number in each case is the bend allowance for a 90° angle, whereas the lower placed number is for a 1° angle.
Material thickness is given in the left column of To find the bend allowance when the sheet thickness is 0.051 in., the radius of bend is ¼ in.
(0.250-in.), and the bend is to be 90°. Reading across the top of the bend allowance chart, find the column for a radius of bend of 0.250 in. Now find the block in this column that is opposite the gage of 0.051 in the column at left. The upper number in the block is 0.428, the correct bend allowance in inches for a 90° bend.
Method #4 use of chart for other than a 90° Bend If the bend is to be other than 90°, use the lower and compute the bend allowance Therefore. if the number in the block (the bend allowance for 1º) The lower number in this case is 0.004756.
bend is to be 120°, the total bend allowance in inches will be 120X0.004756, or 0.5707 in.
B Bend allowance (Z) = (0.01743R + 0.0078T) × (No, of degrees of bend) T = thickness of metal R = radius of bend Z = bend allowance X = A - (R + T) Y = B - (R + T) Total developed length = X + Y + Z FIGURE 5-23. Computing bend allowance.
Method #3 Use of 90° Bend Chart Either formula may be used in the absence of a bend allowance chart. To determine bend allowance for any degree of bend by use of the chart (figure 5-24), find the allowance per degree for the number of degrees in the bend.
adius of bend is given as a decimal fraction o ne top line of the chart. bend allowance is give
SETBACK When bending a piece of sheet stock, it is necessary to know the starting and ending points of the bend so that the length of the "flat" of the stock can be determined. Two factors are important in determining this, the radius of bend and the thickness of the material.
In figure 5-27, note that setback is the distance from the bend tangent line to the mold point. The mold point is the point of intersection of the lines extending from the outside surfaces, whereas the bend tangent lines are the starting and end points of the bend. Also note that setback is the same for the vertical flat and the horizontal flat.
Another way to look at setback is this: If the mandrel in a cornice brake is adjusted to the edge of the bed, a piece of metal is inserted, and a 90° bend is to be made, when the bending leaf is raised to 90°, the metal will be cut due to the compressing action of the leaf. The mandrel must be "set back" from the edge of the bed one thickness of the metal for a 90° bend. This permits the metal to flow thereby forming a correct bend.
Calculating Setback, Formula #1 To calculate the setback for a 90° bend, merely add the inside radius of the bend to the thickness of the sheet stock, i.e.
Setback = R+T.
Example:
Calculate the setback for a 90° bend, if the 148
RADIUS OF BEND IN INCHES GAGE 32.031 Lre.063 32.094 k8 .125 32.156 18.188 32.219 ¼.250 .250|32.281 18-313 132-344/8 .375 116.438 ½.500 .020 .062 .113 161 .210 .259 ,309 358 .406 455 ] .505 .554 .603 .702 799 .000693 001251 001792 .002333 .002874 .003433 003974 .004515 005056 005614 .006155 .006695 .007795 .008877 025 .066 .116 .165 .214 -263 .313 .362 .410 .459 _509 .558 ,607 .705 .803 .000736|.001294 001835 .002376 .002917 003476 .004017 004558 .00509฿|.005657 .006198 .006739 007838 _008920 .028 000759.001318 .068 .119 001859 _167 002400 .216 .002941 .265 .003499 .315 1.004040 .364 004581 .412 .005122 .461 .005680 .511 .006221 .560 .006762 609 .007862 .708 .007862 805 .032 .071 .121 .170 218 .267 .317 .366 .415 „403 .514 .562 .611 .710 .807 000787 .001345 .001886 .002427 002968 .003526.004067 .004608 .005149 1.005708 .006249 006789. 007889 1.008971 .038 000837 075 .001396 .126 .001937 .174 .002478 .223 003019 .272 .003577 .322 .004118 .371 004659 .419 1.005200 160 005758 006299 .518 .006840 - 616 .007940 715 009021 .812 .040 .077 .127 .176 .224 .273 .323 .372 .421 .459 .520 .568 .617 716 .813 000853 .001411.001952 .002493 .003034 003593 .004134 004675 .005215 .005774 006315 .006856 .007955 .009037 .051 - 134 .183 .232 .280 331 .379 428 471 .527 -57% .624 723 .821 .001413 .002034 .002575 .003116 .003675 004215 .004756 005297 .005855 .006397 .006934 ..008037 .009119 .064 .144 -192 .241 .290 .340 .389 .437 .486 .536 $85 .634 7 32 .830 001595 002136 002676 .003218 003776. 004317 004858 .005399 .005957 006498 007039 008138 .009220 .072 .198 .247 .296 .436 .394 .443 .492 .542 .591 .639 .738 .836 .002202 .002743 003284 .003842 .004283 .004924 005465 .006023 .006564 .007105 .008205 .009287 .078 .202 002790 .300 350 .447 003512 006590 006615 007152 0087252 009333 .002249 .003331 003889 004430 004963 .081 -204 002853 .302 003912 .401 .449 .498 .548 -598 .046 006275 .841 002272 .003354 004453 004969 005535 .006094 006635 .007176 009357 .091 212 002890 1003492 003950 .408 .456 505 -555 -023 752 .849 .002350 004531 .005072 005613 .006172 .007254 .008353 .009435 .094 .214 .262 .311 .361 .410 .459 .507 558 -655 .754 .831 .002374 -002914 003455 .004014 004555 005096 005637 006195 006736 .007277 008376 .009458 _102 002977 .003318 _367 001615 005158 $13 .363 006798 007339 .001 008459 .857 .004076 005699 1.006257 009521 .109 .273 .321 -372 .420 ,469 005554 006312 006833.008365 008453 .862 003031 .003572 004131 .004672 .005213 .009575 .125 -284 .333 .383 004737 .480 005&20 .579 006928 007317 .776 .87 3 003156. 003697 .004256 005338 .006437 008618 .009700 .156 355 .405 453 502 .551 .60 .698 797 .895 003939 004497 005038 005579 .006120 .006679 007220 .007761 .008880 .009942 .188 006737 476 .525 573 .624 .672 .721 .820 .005288 .005829 006370 .006928 007469 008010 .009109 .010191 .250 006313 .006853.007412 007953 1.008494 009563 ,90T 010675 FIGURE 5-24. Bend allowance chart.
material is 0.051-in. thick and the radius of bend is specified to be ⅛ in. (0.125).
Setback =R+T.
=0.125 + 0.051 =0.176 in.
Calculating Setback, Formula #2 To calculate setback for angles larger or smaller than 90°, consult standard setback charts (figure 5-25), or "K" chart, for a value called "K", and then substitute this value in the formula.
Setback= K (R+T).
The value of K varies with the number of degrees in the bend.
Example:
Calculate the setback for a 120° bend with a radius of bend of 0.125 in. in a sheet 0.032-in.
thick.
Setback= K (R+T).
=1.7320 (0.125 + 0.032) =0.272 in.
Brake or Sight Line The brake or sight line is the mark on a flat sheet which is set even with the nose of the radius bar of the cornice brake and serves as a guide in bending. The brake line can be located by measuring out one radius from the bend tangent line closest to the end which is to be inserted under the nose of the brake or against the radius form block.
The nose of the brake or radius bar should fall directly over the brake or sight line as shown in figure 5-26.
Bend Allowance Terms Familiarity with the following terms is necessary for an understanding of bend allowance and its application to an actual bending job. Figure 5-27 illustrates most of these terms.
Leg. The longer part of a formed angle.
Flange. The shorter part of a formed anglethe opposite of leg. If cach side of the angle is the same length, then each is known as a leg.
* Bend tangent lines 00873 61ª _58904 121° .01745 62° 60086 122° .02618 .61280 123° _03492 62487 045 052 -07870 10° .08749 130°.
11° 09629 131* Setback 12° .10510 132° (R + T) 13° •11393 14° .12278 15ª -13165 75° Setback .14054 (R + T) .14945 18° .15838 Mold point 19° .16734 20° .17633 80° 140° FreURE 5-25 B. Setback, 90° bend 21* .16534 81* 141° 22° .19438 82° 142° 23° 24° .21256 20345 83 84° 144° 143ª Mold Line (ML). The line formed by extend- 25° 22169 85° 145° ing the outside surfaces of the leg and flange. (An 26° .23087 86° 146° imaginary point from which real base measure.
27° 28° .24933 •24008 87° B8° 147° 148° ments are provided on drawings.)
29° .25862 89° 149° Bend Tangent Line (BL). The line at which 30° 26795 9Ọ° 150° the metal starts to bend and the line at which the 31° 27732 91ª 1.0176 151° metal stops curving. All the space between the 32° .28674 92° 1.0355 152° band tangent lines is the bend allowance.
33° 34ª .29621 .30573 94° 93° 1,0538 1.0724 153° 154* 4,3315 Bend Allowance (BA). The amount of material 35° .31530 95° 1.0913 155° 4.5107 consumed in making a bend (figure 5-12).
36ª .32492 96° 1.1106 156° 4.7046 Radius (R). The radius of the bend-always 38° 37° .33459 .34433 97° 98° 1.1303 1.1504 157° 158° 4.9151 5.1455 to the inside of the metal being formed unless 39* .35412 99° 1.1708 159° 5.3995 otherwise stated. (The minimum allowable radius 40* .36397 100° 1.1917 160° 5.6713 for bending a given type and thickness of material 41* .37388 101° 1.2131 161* 5.9758 should always be ascertained before proceeding 42° 38386 102° 1.2349 162° 6.3137 with any bend allowance calculations.)
43° 44° 39391 40403 103° 104° 1,2799 1,257 2 164° 163° 7,1154 6,6911 Setback (SB). The setback is the distance from 45° 41421 105° 1.3032 165° 7.5957 the bend tangent line to the mold point. In a 46° 42447 106° 166° 8.1443 90-degree bend SB=R+T (radius of the bend 168° 169° 10.385 167° 8.7769 9.5144 plus thickness of the metal). The setback dimen- sion must be determined prior to making the bend 50° 110* 170° 11.430 as it (setback) is used in determining the location 111° 171* 12.706 of the beginning bend tangent line (figure 5-27).
112° 172° 14.301 Bend Line (also called Brake or Sight Line).
113° 114° 115° 173°. 16.350 174* .19.081 22.904 The layout line on the metal being formed which is set even with the nose of the brake and serves 116° 26.636 as a guide in bending the work. (Before forming 117° 38.188 a bend, it must be decided which end of the ma- 59* -55431 36577 118° 119° 1.6977 114.390 57.390 terial can be most conveniently inserted in the 60° 57735 120° 1.7320 180° Infinite brake. The bend line is then measured and marked off with a soft-lead pencil from the bend tangent line closest to the end which is to be placed under the brake. This measurement should be equal to FIGURE 5-25A Setback (K) chart. the radius of the bend. The metal is then inserted 150
BRAKE
LINE SETBACK
MANDREL JAW BEND - THCENESS / 11
TANGENT LINE BEND -
ALLOWANCE FLANGE BEND
- TANGENT LINES PIECE
TO BE BENT BED
BENDING LEAF FICURE 5-26. Setback - locating bend line in brake.
in the brake so that the nose of the brake will fall directly over the bend line, as shown in figure
5-26.)
Flat (short for flat portion). The flat portion or flat of a part is that portion not included in the bend. It is equal to the base measurement minus the setback.
Base Measurement. The outside dimensions of a formed part. Base measurement will be given on the drawing or blueprint, or may be obtained from the original part.
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.
RADIL'S (R)
MOLD LINE OMI - BEND ALLOWANCE (RA) FLAT MOLD POINT -SET BACK 190° BEND -BASE MEASUREMENT FIGURE 5-27. Bend allowance terms.
"K" No. One of 179 numbers on the "K" chart corresponding to one of the angles between 0 and 180° to which metal can be bent. Whenever metal is to be bent to any angle other than 90° ("K" No.
of 1.0), the corresponding "K" No. is selected from the chart and is multiplied by the sum of the radius and the thickness of the metal. The product is the amount of setback for the bend.
MAKING LAYOUTS It is wise to make a layout or pattern of the part before forming it to prevent any waste of material and to get a greater degree of accuracy in the finished part. Where straight angle bends are concerned, correct allowances must be made for setback and bend allowance. If the shrinking or stretching processes are to be used, allowances must Gage Designation
0.020 0.025 0.032 0.040 0.050 0.063 0.071 0.080 2024-0 1/32 1/16 1/18 1/18 1/16 3/32 1/8 1/8 2024-T4 1/16 1/16 3/32 3/32 1/8 5/32 7/32 1/4 5052-0 1/32 1/32 1/16 1/18 1/16 1/16 1/8 1/8 5052-H34 1/32 1/16 1/18 1/16 3/32 3/32 1/8 1/8 6061-O 1/32 1/32 1/32 1/10 1/16 1/16 3/32 3/32 6061-T4 1/32 1/32 1/32 1/16 1416 3/32 5/32 5/32 6061-18 1/16 1/16 1/18 3/32 3/32 1/8 3/18 3/16 7075-0 1/16 1/16 1/18 1/18 3/32 3/32 5/32 3/16 7075-W 3/32 1/32 1/8 5/32 3/16 9/32 5/16 7075-T6 1/8 1/8 1/8 3/18 1/⅖ 5/18 3/8 7/16 FIcURE 5-28. Minimum bend radii for aluminum alloys.
be made so that the part can be turned out with a Sight line minimum amount of forming.
The layout procedures can be put into three general groups: (1) Flat layout, (2) Duplication of pattern, and (3) Projection through a set of points.
All three processes require a good working knowl- 90° Radius bar edge of arithmetic and geometry. This presentation will discuss only two processes, fat layout and duplication of pattern.
Referring to the "K" chart, figure 5-27, it is Bend tangent line noted that the "K" value for 90° is equal to IT One radius of bend (thickness of metal). Further observation will show that for an angle of less than 90° the setback is less than IT, for an angle of more than 90° the Bend allowance setback is more than IT.
The use of IT setback in a bend of less than 90° (open angle) would result in the flange of the bend Sight line being too long. Conversely in an angle of over 90° with less than IT setback the flange would be too Bend tangent line short.
Break Line One radius Flat Pattern → K Setback Developed Length of Pattern = A+B. To determine the developed Length of the flat pattern, deduct the "K" dimension from the sum of the dimensions A+B.
FIGURE 5-29. Setback development.
Flat Layout Assume that it is necessary to lay out a flat pattern of a channel (figure 5-31) in which the left-hand flat, A, is to be 1 in. high, the right-hand flat, C, is to be 1-1/4 in. high, and the distance between the outside surface of the two flats, B, is to be 2 in. The material is 0.051-in. thick, and the radius of bend is to be 3/16 in. (0.188). The angles are to be 90°. Proceed as follows:
(1) Determine the setback to establish the distance of the flats.
152 Bend allowance FIGURE 5-30. Brake or sight line.
(a) The setback for the first bend:
Setback = R + T = 0.188 + 0.051 = 0.239.
(b) The first flat A is equal to the overall dimension less setback:
Flat A = 1.000 - 0.239 = 0.761 in.
(2) Calculate the bend allowance for the first bend by using the bend allowance chart (figure 5-24). (BA =0.3307 or 0.331.)
(3) Now lay off the second flat, B. This is equal to the overall dimension less the setback at each end, or B minus two setbacks: (See figure 5-31.)
Flat B = 2.00 - (0.239 + 0.239) = 2.000 - 478 = 1.522 in.
(4) The bend allowance for the second bend is Bend tangent lines the same as that for the first bend Flat layout Sight lines
(0.331). Mark off this distance. (See of a channel figure 5-31.)
(5) The third flat, C, is equal to the overall dimension less the setback. Lay off this distance. (See figure 5-31.)
Flat C = 1.250 - 0.239 .761' 7/051" = 1.011 in.
(6) Adding the measurements of flats A, B, and C, and both bend allowances, (0.761 + 0.331 + 1.522 + 0.331 + 1.011), the sum is 3.956, or approximately 4.00 inches. Totaling the three flats, A, B, and C, 1 in., 2 in., and 1-1/4 in., respectively, the sum is 4.250 in. of material length.
This illustrates how setback and bend allowance affect material lengths in forming straight line bends. In this case, the reduction is approximately 1/4 in.
After all measurements are calculated, cut the material and mark off the brake or sight lines as shown in figure 5-31.
Duplication of Pattern When it is necessary to duplicate an aircraft part and blueprints are not available, take measurements directly from the original or from a duplicate part.
In studying the following steps for laying out a part to be duplicated, refer to the illustrations in figure
5-32.
Draw a reference (datum) line, AB, on the sample part and a corresponding line on the template material (example 1, figure 5-32).
Next, with point A on the sample part as a center, draw an arc having a radius of approximately 1/2 in. and extending to the flanges (example 2, figure 5-32).
Draw similar arcs each with a radius 1/2 in.
greater than the previous one until the entire part is marked. In case there is an extremely sharp curve in the object, decrease the distance between the arcs to increase the number of arcs. This procedure will increase the accuracy of the layout. An arc must Folded channel / 3/16R 90° FIGURE 5-31. Flat layout of a channel.
pass through every corner of the part; one arc may pass through more than one corner (example 3, figure 5-32).
Locate the coordinate point on the layout by measuring on the part with dividers. Always measure the distance from the reference point to the beginning of the bend line on the flange of the part.
After locating all points, draw a line through them, using a French curve to ensure a smooth pattern (example 4, figure 5-32).
Allow for additional material for forming the flange and locate the inside bend tangent line by measuring, inside the sight line, a distance equal to the radius of bend of the part.
Using the intersection of the lines as a center, locate the required relief holes. Then cut out and form as necessary.
Relief Holes Wherever two bends intersect, material must be removed to make room for the material contained in the flanges. Holes are therefore drilled at the intersection. These holes, called relief holes, prevent strains from being set up at the intersection of the 153
FIGURE 5-32. Duplicating a pattern.
inside bend tangent lines which would cause the metal to crack. Relief holes also provide a neatly trimmed corner from which excess material may be trimmed.
The size of relief holes varies with thickness of the material. They should be not less than 1/8 in. in diameter for aluminum alloy sheet stock up to and including 0.064-in. thick, or 3/16 in. for stock ranging from 0.072 in. to 0.128 in. in 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 in.).
Relief holes must touch the intersection of the inside bend tangent lines. To allow for possible error in bending, make the relief holes so they will extend 1/32 to 1/16 in. behind the inside bend tangent lines. It is good practice to use the intersection of these lines as the center for the holes (figure 5-33). 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.
Lightening Holes Lightening holes are cut in rib sections, fuselage frames, and other structural parts to decrease weight. To keep from weakening the member by Inside bend tangent lines Relief holes FIGURE 5-33. Locating relief holes.
removal of the material, flanges are often pressed around the holes to strengthen the area from which the material was removed.
Lightening holes should never be cut in any structural part unless authorized. The size of the lightening hole and the width of the fange formed around the hole are determined by design specifications. Margins of safety are considered in the specifications so that the weight of the part can be decreased and still retain the necessary strength.
Lightening holes may be cut by any one of the following methods:
(1) Punching out, if the correct size punch die is available.
(2) Cutting out with a fly cutter mounted on a drill.
(3) Scribing the circumference of a hole with dividers and drilling around the entire circumference with a small drill, allowing enough clearance to file smooth.
(4) Scribing the circumference of the hole with dividers, drilling the hole inside the circumference large enough to insert aviation snips, cutting out excess metal, and filing smooth.
Form the flange by using a flanging die, or hardwood or metal form blocks. Flanging dies consist of two matching parts, a female and a male die. For flanging soft metal, dies can be of hardwood, such as maple. For hard metal or for more permanent use, they should be made of steel. The pilot guide should be the same size as the hole to be flanged, and the shoulder should be the same width and angle as the desired flange.
When flanging lightening holes, place the material between the mating parts of the die and form it by hammering or squeezing the dies together in a vise or in an arbor press. The dies will work more smoothly if they are coated with light machine oil.
Note that in the two form blocks shown on the left side of figure 5-34, the hole in the upper block is the same size as the hole to be flanged and is chamfered to the width of the flange and the angle desired, whereas in the lower block, the hole is the same diameter as that of the flange. Either type may be used. When using the upper block, center the material to be flanged and hammer it with a stretching mallet, around and around, until the flange conforms to the chamfer. When using the lower block, center the lightening hole over the hole in the block, then stretch the edges, hammering the material into the hole, around and around, until the desired flange is obtained. Occasionally, the chamfer is formed with a cone-shaped male die used in conjunction with the form block with which the part was formed.
HAND FORMING All forming revolves around the process of shrinking and stretching, and hand forming processes are no exception. If a formed or extruded angle is to be curved, either stretch one leg or shrink the other, whichever will make 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.
Straight Line Bends Chamfered flanging block 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 Cone-shaped or metal bending blocks by proceeding as explained male die in the following paragraphs.
After laying out and cutting a blank to size, clamp it rigidly along the bending line between two Forming block wooden blocks held in a vise. The wooden forming block should have one edge rounded for the desired radius of bend. It should also be curved slightly beyond the 90° point to allow for springback.
By tapping lightly with a rubber, plastic, or rawhide mallet, bend the metal protruding beyond the Plain flanging block bending blocks to the desired angle. Start tapping at one end and work back and forth along the edge FIGURE 5-34. Flanging form blocks. to make a gradual and even bend.