Module 7 (Maintenance Practices) Sub Module 7.14 (Material Handling).pdf

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PIA TRAINING CENTRE (PTC) Module 7 - MAINTENANCE PRACTICES Category A/B1 Sub Module 7.14 Material Handling

PTC/CM/B1.1 Basic/M7/03 Rev. 00 7.14 Mar 2014

MODULE 7

Sub Module 7.14

MATERIAL HANDLING



PTC/CM/B1.1 Basic/M7/03 Rev. 00 7.14 - i Mar 2014

Contents SHEET METAL WORK ............................................................................. 1

MARKING OUT ...................................................................................... 2

BENDING AND CALCULATION OF BEND ALLOWANCE .......................... 3

FORMING OF SHEET METAL PARTS....................................................... 8

INSPECTION OF SHEET METAL WORK ................................................... 9

BONDING PRACTICES .......................................................................... 11

INSPECTION METHODS ....................................................................... 14



PTC/CM/B1.1 Basic/M7/03 Rev. 00 7.14 - ii Mar 2014

MODULE 7

Sub Module 7.14.1

SHEET METAL



PTC/CM/B1.1 Basic/M7/03 Rev. 00 7.14 - 1 Mar 2014

SHEET METAL WORK While the majority of metals can be rolled into sheet form, consideration is confined here solely to the working with sheets of the light alloys, which are encountered on aircraft and, in particular, those formed from aluminium alloy ingots. Safe working procedures were covered adequately, in the Workshop and Hangar Safety Section of the SAFETY PRECAUTIONS topic, but there are several additional points, which need highlighting, with regard to working with sheets of aluminium alloy. By definition, sheets of aluminium alloy are comparatively thin in cross-section and, as such, they not only pose a health hazard, through cuts, when being handled but they are, also, prone to buckling and creasing if handled carelessly. Large sheets of aluminium alloys are, usually stored upright, on their longest edge and supported, clear of the floor, in a wooden framework so they are protected from damage and corrosion. Care must be taken when removing a large sheet from its storage rack a task which normally involves at least two persons and good communication between the carriers is important so that the task is completed in a safe manner and no damage is done to the sheet metal.

Some sheets are covered, on one or both surfaces, with a thin protective plastic membrane and, if possible, it may be beneficial to leave at least the underneath protection in place while the marking out is done, to minimise the possibility of the surface sustaining undesirable scratch marks. If no protective membrane is applied to the sheet, then care must be taken over the condition of the surface of the table, or workbench, upon which the sheet is to be laid for the marking out procedures. Other factors, which should be considered (as with all work) concern the requirements to ensure that: Material wastage is kept to a minimum The task is done correctly, first time, so that valuable time,

also, is not wasted. The first point is usually obvious, due to the cost of the materials involved, but the second point quite often gets forgotten, when work is being done, but the actual labour costs far outweigh the material costs on a high percentage of tasks. Repair or modification drawings must be studied very carefully, to ensure there is no doubt about the data and dimensions provided, so that the marking out is correctly done and the approved metal is shaped in exactly the manner that the designer of the drawing intended.




MARKING OUT Having carefully studied all the data and dimensions on the relevant drawing, the technician, after confirming that the correct metal (to the appropriate heat-treatment standard) is being used, can proceed with marking out the pattern for the part which is being formed. Firstly the overall dimensions of the part must be computed and, where necessary, a bare outline drawn on the large sheet, so that the metal can be cut and, thus, allow an easier, smaller piece upon which to work. It must be remembered that, the metal should be cut so that any identification markings remain on the larger piece, for future users of the sheet and that scribers must only be used to mark lines which are going to be removed from the surface. Scribed lines penetrate the aluminium cladding of Alclad alloys, which can lead not only to subsequent corrosion, but also can create stress raisers and the initiation of cracks in the material. The drawing of the outline is achieved by establishing a datum line or point on the surface of the metal and taking all dimensions from the datum so that errors, due to chaining of dimensions, are eliminated. The drawing surface of the metal should be cleaned of any protective oil (or plastic membrane) before marking out commences and the sheet should be laid flat on a clean, firm workbench or table in good lighting conditions.

In some instances it may be advantageous to rub chalk on the surface or to apply a thin coat of zinc chromate, to make it easier to distinguish the marking out lines, which (if they are not going to be removed) should be made with a soft pencil. Once the outline is completed, the sheet may be (carefully) moved to the squaring shears, or guillotine and the outline cut from the main sheet. The square edge, created by the squaring shears, will make the use of such tools as engineers squares, combination sets and Vernier protractors etc. easier, to achieve parallel and appropriately angled lines during completion of the marking out. Note: Before any centre punch marks are made (for the location of the centres of radii or holes) it must be confirmed that they are in the required location. The punch should be only lightly tapped with a hammer (or a suitably adjusted automatic centre punch used), so that the punch marks do not distort the thin metal sheet.




BENDING AND CALCULATION OF BEND ALLOWANCE As previously stated, the sheet metal used for aircraft construction and repair, is generally formed from an ingot of aluminium alloy that has been processed through a series of rollers. This process reduces the thickness of the material to a dimension that meets the requirements of the design drawing. As a result of this process, the metal assumes a grain structure, which can easily be detected in a sample of sheet aluminium alloy. When planning any sheet metal work process, the orientation of the metal is to be taken into account so that any bends formed will, where practical and achievable, be made across the grain. Where, however, strength is required along the length of a long, channel section, then, regardless of any bends, the grain should flow along the length of the channel. Great care must be taken, before bending aluminium alloy, to ensure that it is of the correct designation and heat-treatment standard. The subject of the heat-treatments of aluminium alloys was discussed fully in Module 6 Materials and Hardware in the topic on AIRCRAFT MATERIALS NON-FERROUS.

Some alloys must be subjected to either an annealing, or to a solution treatment procedure before (and, again, after) bending but, as this is, usually, beyond the scope of maintenance technicians, mention of it is merely made here to draw attention to its requirement and for the need for vigilance when bending sheets of aluminium alloy. Bending of aluminium alloys is achieved either by the use of: Specially-shaped bending bars: used for small pieces and

larger angles and between which, the sheet is clamped, in a vice, while the metal is bent, by hitting with a hide-faced or similarly soft-headed hammer

A large, free standing, bending machine (or bending brake): in which the metal sheet is clamped and the bend made, in one movement, by means of a hinged bending leaf.

Caution must also be exercised when forming a bend, using the bending bars and soft-headed hammer method, because too many blows with the hammer will cause work-hardening of the metal, or the metal, in the bend, will become too thin and stretched. Subsequent cracking of the metal will result from these faults For this reason the bending brake is preferred but, in a similar manner to the squaring shears, only the approved thicknesses of metals should be bent in these machines, as any distortion will destroy the accuracy of the bends.




Square (or sharp) angles, in aluminium alloys, are only formed by adhesive, casting, extrusion or welding methods. Whether it be the bending bars or the bending brake method, which is used to bend aluminium alloy sheet, the bend will always be formed around a radius, as it is not possible to create square angles by bending without cracking the metal. It is recommended that the radii of bends, in aircraft-grade, aluminium alloy sheets, be not less than three times the thickness (3t) of the metal, in sheets thicker than 22 SWG (0.7 mm) and should, preferably, be greater if possible. It is, therefore, usual to create bends, in sheets of 20 SWG (0.9 mm), of not less than 3 mm (3 x 0.9 mm = 2.7 mm) radius and bends in sheets of 18 SWG (1.2 mm), of not less than 4 mm (3 x 1.2 mm = 3.6 mm) radius. As an example, if it were required to form a right angled curve (10 mm radius) in an 18 SWG aluminium alloy sheet, to provide two legs, effectively 76.2 mm (3 in) in length (refer to Fig. 1), it can be seen that the actual length of metal involved is obviously less than 2 x 76.2 mm (6 in). The total length of the metal, required for the curve, is deduced by using the formula: LT = L1 + BA + L2 Where LT = Total Length of Metal Required L1 = X (r + t) L2 = Y (r + t) BA = Bend Allowance X and Y = Effective Lengths of Unbent Sections r = Radius of Bend t = Thickness (SWG) of Metal

X = 76.2 mm

Y =

76.2

mm

r = 10.0 mm

t = 1.2 mm

L 1

L 2

A

B

Total Length of Metal in a Curve Fig. 1




The lengths of both L1 and L2 can be found by subtracting the sum of the radius and the metal thickness from the effective lengths of the unbent sections. In this instance, therefore, the lengths are both 76.2 11.2 = 65 mm. To calculate the length of metal in the bend (Bend Allowance) it is necessary to consider the fact that, when metal is bent, the metal on the inside of the bend will be compressed while that on the outside of the bend will be subjected to tension or stretching so that the length of metal on the inside and outside of the bend will be different. It may, however, be considered that there is a neutral line (at approximately half thickness) where the compression and tensile forces cancel out. It is this line which is taken, to calculate the length of the arc of the circumference of the circle, which would be described by the radius of the curve. The Bend Allowance is, thus calculated, using the formula: BA = 2 (r + t) 360 Where r = Radius of Bend t = Thickness of Metal = Angle of Bend Note: Some books use the formula: BA = (D + t) 360 Where D = the diameter of the circle

However, as curves are normally shown as radii, in engineering drawings, the previously given formula is preferred here. Substituting figures, in the preferred formula, it will be seen that the bend allowance equates to 16.65 mm (0.66 in). When this figure is added to lengths L1 and L2, it can be seen that the total length of metal, required to form the curve, is only 146.65 mm (5.77 in) and not 152.4 mm (6 in). In a simple, single curve, this represents a saving in metal of only 5.75 mm (0.23 in) but, in a multiple bend component, and with larger radii involved, considerable savings of metal can be made by using these formulas to calculate the correct amount of metal required to forms bends in sheet metal. Table 1 shows data relating to bend allowances for forming 90 curves of various radii in both 20 SWG and 18 SWG metals. Using the preferred formula, the student should be able to calculate the bend allowances and enter them in the empty spaces in the table.




Table 1 BEND ALLOWANCES (BA) FOR A 90 BEND

Inside Bend Radius BA - 20 SWG (0.9 mm) 3 mm 4 mm 5 mm Inside Bend Radius BA - 18 SWG (1.2 mm) 4 mm 5 mm 6 mm

When the total length of sheet metal, required to form a curve, has been calculated, it will be necessary to draw the development (refer to Fig. 2) of the intended shape, so that the bend lines can be seen.

16.65 65 65

Unbent Section

Unbent Section

Bend Allowance

Bend Lines

Development of Shape Fig. 2




An additional line must be drawn on the development drawing before the metal is placed in the bending brake. This line (refer to Fig. 3), is referred to as the sighting line or the brake reference line. It is drawn at a distance, equal to the radius of the curve (in this instance, using the figure of 10 mm from the previous example), parallel to, but away from, the bend line, which is under the clamping nib of the bending brake and towards the bend line which is free of the clamping nib. The sighting line, as the name implies, is then used, to align with the front of the clamping nib and, in this way, allowance is made for the thickness of the metal in the formation of the curve.

Use of Sighting or Brake Reference Line Fig. 3

Bend Leaf Bends Counter-clockwise

Clamping Nib

Brake Bed

Hinge Point

Metal to be Bent

Sighting Line

Bend Lines




FORMING OF SHEET METAL PARTS Once the marking out has been verified as being correct, the forming of the final shape of the sheet metal component can be achieved by the use of appropriate cutting and, if necessary, bending tools. Cutting While metal-cutting tools were discussed in the earlier topic on TOOLS, mention is made here of the manner in which the relevant tools should be used when working with sheet aluminium alloys. The squaring shears has already been used to produce a convenient size upon which to work and, of course, to provide an accurate straight edge from which to make measurements. Note: The squaring shears must only be used to cut metal of the approved thickness (recommended by its manufacturer) and must never be used on sheets (or strips) of metal thicker than those specified. The alignment of the blade will be distorted and the accuracy of its cut will be degraded if this caution is ignored. When using shears (whether squaring or the hand type), then the cut must be made slightly above the line. This allows for filing down to the line, which will eliminate the possibility of stress raisers being formed at the edges of the metal, due to the shearing action of the various types of shears.

Care must be taken when drilling aluminium sheet, due to the danger of cutting enlarged holes in the soft, thin metal and to the tendency to distortion, caused by the application of too great a weight on unsupported aluminium sections. Twist drills must be of the correct type and size, with accurately-ground points, and their passage, through the metal, must be carefully controlled at all times. Off cuts of scrap wood should be placed behind (or underneath) sheet metal parts while drilling is in progress and both the backing piece and the part must be firmly held, to prevent movement during the drilling procedures. Similarly, scrap wood should be used, as backing, when hack-sawing or filing sheet metal and protection must be given, against possible damage, when such components are held in the jaws of vices, by the use of soft vice clamps. Obviously fine-toothed hacksaws (32 tpi) and second cut and/or smooth files (used with long, smooth strokes), are the cutting tools, used in the shaping of sheet metal parts. Files, as discussed in the TOOLS topic, must be regularly cleaned, to prevent the build up of pinnings, and the use of file cards and chalk, for this task, has also been, earlier, mentioned.




INSPECTION OF SHEET METAL WORK As far as aircraft maintenance technicians are concerned, the inspection of sheet metal work is confine to visual or assisted visual methods. Personnel who have approval may also perform dye penetrant procedures in the search for cracks in suspect areas. Specially trained and approved NDT personnel may use Eddy Current, Ultrasonic or Radiographic procedures to detect faults in aluminium alloy sheet metal work.




MODULE 7

Sub Module 7.14.2

COMPOSITE AND NON - METALLIC




BONDING PRACTICES Bonding, by the use of adhesives, is the third method of achieving permanent joints between surfaces, to be considered in this part of the course. Comprehensive coverage of adhesives and sealants is provided in Module 6 - (Materials and Hardware), along with details of composite materials, the detection of typical defects and the methods used in their repair, therefore consideration here will be limited merely to a summary of: Bonding terminologies Methods of bonding The inspection and testing of bonded joints. Bonding, in the aerospace industry, is employed to form permanent joints between materials ranging from composites, fabrics, metals and metal alloys, to plastics, - all of which are referred to as adherends. The surface texture of a particular adherend, the type of joint required, and the manner in which loads are applied to the joint will dictate the type of adhesive to be used, and the method to be employed, in effecting the joint. Synthetic resins (and some elastomers) are mainly used as adhesives in the bonding of aircraft structures and associated components and, while most of them are used at the manufacturing stages, some may well be used, by aircraft servicing technicians, during routine maintenance tasks.

WARNING: CONTROLLED VENTILATION, PROTECTIVE CLOTHING, AND ANTI-FIRE/EXPLOSION PRACTICES, ARE ABSOLUTELY ESSENTIAL, WHEN WORKING WITH ADHESIVES AND SEALANTS. ALTHOUGH MANY OF THE ADHESIVES IN CURRENT USE ARE SUPPLIED IN FILM FORM, SOME ARE LIQUIDS OR PASTES, FROM WHICH, TOXIC AND FLAMMABLE VAPOURS ARE EMITTED, PRIOR TO CURING. MANY OF THE NECESSARY, SURFACE PREPARATION SOLVENTS, ALSO GIVE OFF TOXIC/FLAMMABLE VAPOURS. The warning is reproduced from Module 6 (Materials and Hardware), where it also states that the two major groups of adhesives, are: Flexible adhesives: used where some flexing or slight

relative movement, of the joint is required, and where high load-carrying properties are not paramount. These adhesives are, generally, based on flexible plastics or elastomers

Structural adhesives: used in applications where high loads must be carried without excessive creep and which are relatively rigid without being excessively hard or brittle. These adhesives are based on resins (commonly of the epoxy or of the polyester types).

Note: Another group of adhesives is the two-polymer type, which has a reasonably even balance of resin and elastomer. This results in a flexible, yet fairly strong, adhesive.




BONDING METHODS While the two major groups of adhesives are designated as flexible or structural, they are further classified as being of the thermoplastic or of the thermosetting types. Each types characteristics will influence the method employed in its use as a bonding agent. Thermoplastic adhesives Thermoplastic materials are those which soften on heating and harden when cooled but will, again soften and harden as often as the heat/cool cycle is repeated. Thermoplastic adhesives consist of thermoplastic materials (which may be either acrylic-, cellulose-, epoxy-, rubber- or vinyl-based), in solution with a volatile solvent and which may surface of adherends in the form of: Direct application adhesives Contact (or impact) adhesives. Direct application adhesives, are spread over the area of both surfaces of the joint before the joint is closed and the solvent continues to evaporate. This method can create problems if the joint area is large, as all of the solvent may not evaporate and a weak joint will result.

Contact adhesives are also applied to both surfaces to be joined but, with these adhesives, the solvent is allowed to evaporate until the adhesive feels tacky, when the surfaces are, then, brought into contact and a complete joint is achieved. Thermosetting adhesives Thermosetting materials (thermosets), once set, cannot be reformed by the application of heat and they create permanent heat-resisting bonds. Thermosetting adhesives consist of epoxy- and phenolic-based materials in addition to polyesters, polyurethanes, and silicones. Thermosets require a curing process (which is achieved by the application of heat), to cause them to harden. The heat can be obtained by placing the components being joined into an oven or into an autoclave (a pressurised oven). Alternatively, the adhesive in the joint can be heated by the chemical (exothermic) reaction of a hardening agent, which is added to the adhesive, prior to the joint being made. Thermosetting adhesives are the types most widely used in the aerospace industry.




INSPECTION OF BONDED JOINTS The inspection of bonded joints may be done (as discussed in Module 6) visually, usually in good lighting conditions and, possibly, with the aid of magnifying glasses or small microscopes. Delamination and de-bonding of aircraft honeycomb panels and control surfaces may be detected by percussion (ring) testing or coin tapping, while more sophisticated methods, such as ultrasonic and radiographic procedures, may be used by suitably trained and approved personnel. Where repairs are done to composite structures, then samples of the adhesives used are kept for testing, while peel tests are done on adhesives which are used to attach de-icing or anti-icing elements to the leading edges of propellers or flying control surfaces.




INSPECTION METHODS Today's composite inspection techniques and non-destructive testing (NDT) methods typically involve the use of multiple methods to accurately determine the airworthiness of an aircraft structure. Fortunately, many metal inspection and NDT methods transfer to composite applications. Composite structures require ongoing inspection intervals along with non-scheduled damage inspection and testing. When a composite structure is damaged, it must first be thoroughly inspected to determine the extent of the damage, which often extends beyond the immediate apparent defect. Proper inspection and testing methods help determine the classification of damage, which is, whether the damage is repairable or whether the part must be replaced. In addition, classifying the damage helps to determine the proper method of repair. The manufacturer's structural repair manual outlines inspection procedures, damage classification factors, and recommended repair methods. Some of the more common composite inspection and testing methods are visual inspection, tap testing, and ultrasonic testing along with several other more advanced NDT methods.

Visual inspection Visual inspection is the most frequently used inspection method in aviation. Ideally, pilots, ground crew and maintenance technicians visually inspect the aircraft on a daily basis. Where composite materials are concerned visual inspection is generally used to detect resin-rich areas, resin starvation, edge delamination, fiber breakage, cracks, blistering, and other types of surface irregularities. A strong light and magnifying glass are useful tools for visual inspection. In extremely critical cases a small microscope is helpful in determining whether the fibers in a cracked surface are broken, or if the crack affects the resin only. Shining a strong light through the structure, called backlighting, helps in the identification of cracked or broken fibers, and, in some cases, delamination. The delaminated area may appear as a bubble, an indentation in the surface, or a change in color if viewed from the side opposite the light. However, backlighting does not detect entrapped water. In addition, to properly inspect a composite using the backlight method, you must strip the surface of all paint. Many times, visual inspection alone is not adequate to accurately determine the soundness of a composite structure. In the case of visually inspecting a sandwich structure, many times core crush is not evident from the surface. The surface may not show any residual damage and may have sprung back to its original shape and location, which is one of the main problems with inspecting composite materials. Internal damage is not always evident from the surface, which further necessitates the use of additional, more advanced methods of inspection when damage is suspected.




The maintenance technician is generally the first person to assess damage using visual inspection techniques. After this initial inspection, more often than not advanced forms of inspection and testing may be required to determine the extent of the damage. Taptest / ring test This is one of the simplest methods used to detect damage in bonded parts. The laminated part is tapped with a coin or small metallic object, such as a ring or a tap hammer to detect delamination. The tap test is an acoustic test, one in which you listen for sound differences in the part, and is not the most accurate test method. The tap test detects delaminations close to the surface in addition to transitions to different internal structures. A properly prepared, undamaged laminated area produces a sharp, even pitch as compared to a delaminated area, which produces a dull sound. However, changes in the thickness of the part, reinforcements, fasteners and previous repairs may give false readings when using the tap test. Tap testing will not indicate subsurface delamination if the defect is well below the surface, especially in thick laminated parts. Thus tap test should be limited to near surface inspection of bond-line defects. Inspection of the bond-line by tap testing becomes less and less effective as the depth of the bond-line from the surface increases. (Fig. A)

Ultrasonic inspection Ultrasonic inspection is the most common instrumental NDT method used on composites today. An ultrasonic tester is useful for detecting internal damage such as delaminations, core crush, and other subsurface defects. Two common methods of ultrasonic testing are the Pulse echo and through transmission methods (Refer Fig. B) In the pulse echo method, the tester generates ultrasonic pulses, sends them through part, and receives the return echo. The echo patterns are displayed on an oscilloscope. An advantage to the pulse echo method is that it only requires access to one side of the structure. However, near-surface defects do not readily allow sound to pass through them, making it difficult to detect defects located under the first defect. The pulse echo method works well on laminates because they do not reduce the magnitude of sound waves as much as a bonded core structure. The through transmission method uses two transducers. One transducer emits ultrasonic waves through the part and the other receives them. Defects located at multiple levels throughout the structure are more easily detected because the receiver, located on the backside of the part, receives the reduced amount of sound waves that pass through the defects. The ratio of the magnitudes of sound vibrations transmitted and received determines the structure's reliability.




Testing bonded-core structures usually requires the through transmission method due to the fact that sound waves reduce in magnitude as they travel through the sandwich structure. To effectively test this type of structure, the use of a receiver on the backside of the part dramatically increases the likelihood of detecting a defect. (Fig. C)




Radiography Radiography or x-ray inspection is used to detect differences in the thickness or physical density compared to the surrounding material of a composite. It can be used to detect surface as well as internal cracks. Radiography also detects entrapped water inside honeycomb core cells. In addition to detecting the actual defect, it can also detect the extent and size of the damage, unlike ultrasonic or tap testing. X-ray inspection will also detect foreign objects in the composite structure if the object's density is different from the composite structure. Thermography Thermography locates flaws by temperature variations at the surface of a damaged part. Heat is applied to the part and the temperature gradients are measured using an infrared camera. Thermography requires knowledge of the thermal conductivity of the test specimen and a reference standard for comparison purposes.

Dye penetrant Dye penetrant successfully detects cracks and other defects in metallic surfaces, but should not be used on composite structure unless called for by the manufacturer of that particular part. If a dye penetrant is used on the composite structure and allowed to sit on the surface, the wicking action of the fibers may absorb the penetrant. Absorbed penetrant does not allow fibers to bond to new material. The entire area affected by the dye penetrant would have to be removed before a patch could be applied, which could extend the damaged area of the part to a size that would make the part unserviceable. Acoustic emission testing Another nondestructive testing technique used to detect composite defects is acoustic emission testing. Presently, this type of test is more commonly found in production facilities rather than in maintenance. Acoustic emission testing is a comparison test. Thus to detect flaws a good test sample must be available to compare the test results of the composite structure. It measures the sounds of a structure and any subsequent defects. Basically, acoustic emission testing picks up the "noise" of the defect and displays it on an oscilloscope. This type of testing detects entrapped water, cracks, delamination, and other subsurface flaws.

Module 7 (Maintenance Practices) Sub Module 7.14 (Material Handling).pdf

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