7.14 Marking Out Sheet Metal Flashcards
What is important to understand when manufacturing sheet metal components?
It is important to understand the physical changes that occur to the metal that is formed or bent.
How is the correct component size achieved after forming/bending?
By calculating the bend allowance and adding this to the overall dimension.
What is the Base Measurement?
The outside dimension of a formed part, given on the engineering drawing or obtained from the original part.
What is a Leg in sheet metal terminology?
The longer part of a formed angle.
What is a Flange?
The shorter part of a formed angle, opposite to the leg.
What does the Grain of the Metal refer to?
The natural grain of the material formed as the sheet is rolled from a molten ingot.
What are Bend Lines?
Lines that must lie at a 90° angle to the grain of the metal if possible.
What is Bend Allowance (BA)?
The curved section of the bend, considered the length of the curved portion of the neutral line.
What is Bend Radius (BR)?
The measurement of the arc formed when the metal is bent, measured from a radius centre to the inside surface of the metal.
What is a Bend Tangent Line (BL)?
The line where the metal starts to bend and stops curving; the space between the BL is the bend allowance.
What is the Neutral Axis?
An imaginary line that has the same length after bending as it had before bending.
What is a Mould Line (ML)?
The extension of the flat side of a part beyond the radius.
What is a Mould Line Dimension (MLD)?
The dimension of a part made by the intersection of the mould lines.
What is a Mould Point?
The point of intersection of the MLs, representing the outside corner of the part if there were no radius.
What is the K-Factor?
The percentage of the material thickness where there is no stretching or compressing of the material.
What is Setback (SB)?
The distance the jaws of a folder must be set back from the mould line to form a bend.
What is a Sight Line?
The layout line on the metal being formed that serves as a guide in bending the work.
What is Flat in sheet metal terminology?
The portion of a part that is not included in the bend, equal to the base measurement (MLD) minus the setback.
What is a Closed Angle?
An angle that is less than 90° when measured between legs, or more than 90° when the amount of bend is measured.
What is an Open Angle?
An angle that is more than 90° when measured between legs, or less than 90° when the amount of bend is measured.
What is Total Developed Width (TDW)?
The width of material measured around the bends from edge to edge.
What must be used for all bends that are smaller or larger than 90°?
The K-Factor.
What should be used when marking out flat pattern developments on aircraft sheet metal?
Authorised soft markers and pencils.
Why should engineer’s scribers not be used for marking out flat pattern developments?
They can cause stress raisers and are not authorised for this purpose.
How is the K-factor calculated for bends other than 90°?
It is multiplied by the sum of the Radius (R) and the Thickness (MT) of the metal.
What is the formula for calculating the K-factor?
K-factor = tangent of half the bend angle.
What is the purpose of making a flat pattern?
To prevent material waste and achieve greater accuracy in the finished component.
What factors must be considered when forming a straight line bend?
Thickness of the material, alloy composition, and material’s temper (hardness).
What happens if the radius of a bend is too small?
It can weaken the metal and result in cracking.
Where can minimum bend radius charts be found?
In the manufacturer’s maintenance manuals or Structural Repair Manual (SRM).
What is the bend radius designated to?
The bend radius is designated to the inside of the bend. All dimensions are in inches.
What factors affect the bend allowance?
The bend allowance relies on four factors: Degree of bend, Radius of the bend, Thickness of the metal, Type of material used.
How does the radius of the bend relate to the thickness of the material?
The radius of the bend is normally proportional to the thickness of the material being bent.
What happens to the material when bending a piece of metal?
Bending compresses the material on the inside of the radius and stretches the material on the outside.
What is the neutral line or axis?
The neutral line or axis occurs at a distance of approximately 0.445 times the metal thickness from the inside of the radius of the bend.
What is the purpose of determining the length of the neutral axis?
The length of the neutral axis must be determined to calculate sufficient material for the bend, known as the bend allowance.
What is the formula for calculating the bend allowance for a 90° bend?
Determine the radius of the bend (R) and add the thickness of the metal. Calculate the circumference of the neutral axis circle and divide by 4.
What is the bend allowance for a 90° bend with a radius of 0.250 in and thickness of 0.051 in?
The bend allowance is 0.43 in (10.92 mm).
How is the bend allowance for angles other than 90° calculated?
The formula uses the relationship of the degrees in the bend to the thickness of the metal.
What is the formula for calculating the setback?
SB = setback, K = K-factor (K is 1 for 90° bends), R = inside radius of the bend, MT = material thickness.
How is the flat line dimension calculated?
Flat = Mould Line Dimension (MLD) – Setback (SB).
What is the significance of the bend sight line?
The bend sight line must be located one bend radius from the bend tangent line to ensure accurate bending.
What is the consequence of incorrect positioning of the bend sight line?
Incorrect positioning results in the bend being formed at the wrong location within the bend allowance.
What is the process of shaping sheet metal parts called?
The process of shaping sheet metal parts is called forming.
What can forming processes produce?
Forming processes can produce curves, flanges, and irregular shapes by stretching or shrinking the material.
How does the type of material affect forming?
The amount of shrinking and stretching depends on the type of material used; annealed materials can be formed to smaller bend radii than tempered materials.
What equipment is used for mass-manufacturing aircraft parts?
Large hydraulic/pneumatic presses equipped with dies of the correct shape are used for mass-manufacturing aircraft parts.
What is a potential defect in handcrafting formed parts?
A missed step in the forming process can leave the part in a strained condition, increasing component fatigue or causing sudden structural failure.
Which metal is the most easily formed for aircraft parts?
Pure aluminium is the most easily formed of all metals used on an aircraft.
What is necessary for extensive forming or making complex curves?
For extensive forming or making complex curves, the metal must be in the annealed condition.
What is the minimum bend radius for sheet metal?
Consult the SRM (Structural Repair Manual) for the minimum bend radius.
What is the purpose of radius bars in a sheet metal folder?
Radius bars assist with producing the correct bend radius.
How is clamping pressure set in a sheet metal folder?
Clamping pressure is set by sliding a piece of material under the folder radius bar and adjusting the clamping pressure nut.
What is the process for hand forming a bend?
Clamp the sheet metal along the bend line between two wooden forming blocks and tap it with a mallet to achieve the desired angle.
What is the preferred method for curving by stretching?
Curving by stretching one flange is usually preferred and can be accomplished using a V-block and a mallet.
How is shrinking accomplished using the V-block method?
In the V-block method, one flange of the angle section is placed flat on the V-block, and light blows are made to curve the flange.
What is a joggle in sheet metal forming?
A joggle is formed on a part to allow clearance for a sheet or another mating part, maintaining the smooth surface of a joint.
What is the general rule for calculating the allowance for a joggle?
The general rule of thumb for calculating the allowance is four times the material thickness.
What is the procedure for forming a joggle on a flat piece of metal?
Lay out the boundary lines, bend the sheet in the folder, and check the joggle for correct dimensions and clearance.
What materials are preferred for making joggle dies?
Steel or aluminium alloy dies are preferred for producing a large number of joggles due to their durability and reduced surface damage.
What is the purpose of a joggle block?
A joggle block is used to create a joggle in thin aluminium alloy.
What should be done after forming a joggle?
The joggle blocks are turned over in the vice, and the bulge on the opposite flange is flattened with a wooden mallet.
What materials are suitable for making dies for joggles?
Hardwood dies are satisfactory for limited use, while steel or aluminium alloy dies are better for larger production.
Aluminium alloy dies are preferred as they are easier to fabricate and cause less surface damage.
What should be done when using joggling dies for the first time?
Test them for accuracy on a test piece and keep the surfaces of the blocks free from dirt and filings.
How is a joggle plate used?
A joggle plate is used to produce a joggle by placing the metal between the slots and hitting it with a mallet on a hard surface.
What is the thickness requirement for a joggle plate?
The thickness of the joggle plate must represent the depth of the joggle required, using the four times the material thickness rule for allowance.
What types of damage can be caused by the impact from foreign objects on aircraft?
Types of damage include:
* Load damage from heavy landings
* Distortion or failure of the substructure
* Corrosion
* Cracks
* Dents
* Erosion
* Chattering
* Gouges
* Pitting
* Scratches
* Stains
Each type of damage may require specific inspection techniques.
What is a smoking rivet?
A rivet that shows a dark, greasy residue or deterioration of paint around the head, indicating potential looseness or movement.
This condition can result from fretting corrosion.
What should be inspected during a visual inspection of sheet metal work?
Inspect for:
* Evidence of corrosion
* Damage from impacts
* Condition of rivets and bolts
* Elongated holes
* Structural integrity
Pay special attention to corners and areas where moisture can accumulate.
What are the signs of warped or buckled wings?
Indicated by parallel skin wrinkles running diagonally across the wings.
This condition can result from violent maneuvers, extreme turbulence, or hard landings.
What is the purpose of recoating aluminium alloy surfaces?
To prevent the formation of corrosion on exposed metal surfaces.
This is especially important for aluminium clad (Alclad™) surfaces.
Fill in the blank: A _______ is a physical separation of two adjacent portions of metal caused by excessive stress.
Crack
Cracks may extend inward from the surface.
What are the conditions under which rivet head cracks are acceptable?
Acceptable if:
* Depth < 0.32 cm (1/8 in) of shank diameter
* Width < 0.16 cm (1/16 in) of shank diameter
* Length confined within a circle of max diameter 1 to 1/4 times shank diameter
* Cracks must not intersect
These criteria help maintain structural integrity.
True or False: A loose rivet will always overload adjacent rivets to the point of cracking.
False
Airframe cracking is not necessarily caused by defective rivets.
What inspection techniques can be used to identify hidden damage in aircraft structures?
Eddy current and ultrasonic inspection techniques.
These Non-Destructive Testing (NDT) methods are essential for thorough inspections.
What is burnishing?
Polishing of one surface by sliding contact with a smooth and harder surface, usually without displacement or removal of metal.
This can occur during normal operation or contact with other surfaces.
What should be done if rivets show signs of critical loading?
Drill off the head and carefully punch out the shank to examine for joggles and misalignment.
This helps determine if the rivet has sheared and what corrective action is needed.
What is the significance of keeping drain holes clean during aircraft inspections?
Prevents moisture and salt spray accumulation, which can lead to corrosion.
This is crucial for maintaining the integrity of internal surfaces.
What does a gouge indicate on a metal surface?
It indicates metal loss or material displacement due to contact with foreign material under heavy pressure.
Gouges can compromise structural integrity.
What inspection is needed for riveted joints?
Examine both shop and manufactured heads, surrounding skin, and structural parts for deformities.
This can reveal issues like shifted or loose rivets.
What is bonding in the aerospace industry?
Bonding is carried out to form permanent joints between materials such as composites, fabrics, metals, and plastics, known as adherents.
What factors determine the type of adhesive used in bonding?
The surface texture of the adherent, the type of joint required, and the manner in which loads are applied to the joint.
What are the main types of adhesives used?
Adhesives are mainly synthetic resins.
When is bonding typically performed?
Most bonding is done during the manufacturing stage, but it can also be done by mechanics during routine maintenance.
What forms do adhesives come in?
Adhesives can be supplied in film form, liquids, or pastes.
What caution should be taken when using adhesive pastes or liquids?
They emit toxic and flammable vapours before curing.
What symptoms can result from exposure to adhesives and sealants?
Symptoms include headaches, breathing problems, and irritated eyes.
What safety measures should be taken when using adhesives?
Ensure proper ventilation, wear Personal Protective Equipment (PPE), read the Material Safety Data Sheet (MSDS), and keep adhesives away from ignition sources.
What is an advantage of bonded repairs?
They do not add stresses to the structure by drilling fastener holes.
What is a disadvantage of bonded repairs?
Most repair materials require special storage, handling, and curing procedures.
When are bolted repairs typically used?
Bolted repairs are quicker and easier and are usually used on composite skins thicker than 6 mm (0.125 in).
Why are bolted repairs not allowed on honeycomb sandwich assemblies?
Due to the potential for moisture ingression through fastener holes causing core degradation.
What type of repair is most effective for honeycomb sandwich parts?
A bonded scarf (tapered) type repair.
What are the five basic types of composite repairs?
- Bolted metal or pre-cured composite patches
- Bonded metal or pre-cured composite patches
- Resin injection
- Laminating new repair plies through one surface
- Laminating new repair plies through both surfaces.
Why are bolted and bonded surface patches not recommended?
They do not restore the full strength to the original composite structure and can cause aerodynamic changes.
What is the purpose of resin injection repairs?
To fill holes or voids by injecting resin into the damaged area.
What is the best type of permanent repair for a composite structure?
To laminate new repair plies in the damaged area.
What are the basic steps to carry out a composite repair?
- Find and clean the damaged area
- Assess the extent of the damage
- Define the repair procedure
- Fabricate the repair plies
- Prepare the structure
- Lay up/install, vacuum bag, and cure the repair plies
- Clean up
- Inspect the repair
- Document the repair.
What tools are used to prepare fabrics for composite repairs?
Heavy-duty scissors, squeegees, brushes, markers, rulers, and circle templates.
What type of power tools are best suited for composite materials?
Pneumatic power tools such as drill motors, routers, and grinders.
What is the purpose of a caul plate in composite repairs?
To support the composite structure during the cure cycle.
What is vacuum bagging in composite repairs?
A technique where a plastic bag is sealed around the repair area and air is removed to create a secure bond.
What are release agents used for?
To ensure that the part comes off the tool or caul plate easily after curing.
What is the function of breather material in vacuum bagging?
To provide a path for air to escape from the vacuum bag.
What does a bleeder ply do?
Creates a path for air to escape and collects excess resin.
What is the purpose of perforated release film?
To allow air out of the repair while preventing the bleeder ply from sticking.
What is the role of layup tapes in vacuum bagging?
To seal the vacuum bag to the component and create an air-tight seal.
What is the disadvantage of vacuum bag materials?
They are one-time use materials and can crush the part if not handled properly.
What is the function of a vacuum pump in composite repairs?
To remove air from the vacuum bag, allowing atmospheric pressure to compact the plies.
What are typical oven cure temperatures for composite repairs?
120 °C (250 °F) and 175 °C (350 °F).
What is the purpose of an autoclave in composite repairs?
To contain pressure, heat the gas stream, apply vacuum, and control operating parameters.
What are the major elements of an autoclave system?
- A vessel to contain pressure
- Sources to heat and circulate gas
- A vacuum subsystem
- A control subsystem
- A loading subsystem.
What is the typical heating method for aircraft repairs?
Electrical resistance heat blankets and infrared heat lamps.
What is a heat bonder?
A portable device used to control the temperature of the cure cycle and monitor vacuum pressure.
What is a heat blanket?
A flexible heater used to apply heat to repairs, made of silicon rubber with a metal resistance heater.
What is the purpose of thermocouples in composite repairs?
To accurately measure temperatures during the repair process.
What are the two types of thermocouples?
Type J and Type K, named for their type of wire.
What is important about thermocouple placement?
They must be placed close to the repair material without becoming embedded.
What is the wet lay up process?
Involves impregnating dry fabric with resin and applying it to the repair.
What is the typical overlap for wet lay up repair plies?
1 inch (25 mm).
What is prepreg?
A fabric impregnated with the correct amount of resin during manufacturing.
How should prepreg materials be stored?
In a freezer below -18 °C (-0.4 °F) to prevent curing.
What should be done when thawing prepreg materials?
Allow them to warm to room temperature without opening the sealed bag.
What should be done before opening prepreg material bags?
Do not open these bags until the material is completely thawed, to prevent contamination of the material by moisture.
What is the overlap for prepreg repair plies?
Prepreg repair plies generally have an overlap of 1/2” (12.5mm).
What is the maximum storage life for prepreg materials at low temperature?
The maximum time allowed for storing a prepreg at low temperature is typically between six months to a year.
What is the mechanical life of prepreg materials?
The maximum time allowed for a material to be at room temperature before the material cures.
What is the handling life of prepreg materials?
The recommended time at room temperature to complete the lay up process, which is shorter than the mechanical life.
What is the importance of ply orientation in composite repairs?
The strength and stiffness of a composite panel or structure depend on the ply orientation.
What are the two lay up methods for repairs?
One method is to use the largest ply first, while the more common method is to use the smallest ply first and build up sizes.
What is the typical curing temperature for composite materials?
The typical curing temperature for composite materials is 120 °C ‑ 175 °C (248 °F ‑ 347 °F).
What are the three damage classifications for composite repairs?
- Temporary repair 2. Interim repair 3. Permanent repair
What is a potted repair?
A potted repair is for minor core damage smaller than 12 mm (0.5 in) in diameter, filled with a recommended potting compound.
What must be done to remove water from the damaged area before repair?
Any traces of water need to be removed from the core before the part can be successfully repaired.
What is the purpose of a vacuum bag in the repair process?
A vacuum bag is used to consolidate the plies and to bleed off excess resin.
What is the typical process for curing a repair?
The repair is cured at the dedicated cure cycle, with parts that can be removed from the aircraft cured in an oven or autoclave.
What inspections are performed post-repair?
Visual, tap, and/or ultrasonic inspection are used to inspect the cured repair.
What is the relationship between controlling the repair environment and the integrity of the repair?
Controlling the repair environment is directly related to the integrity and endurance of the repair.
Why must bonded areas be kept clean?
Bonded areas must be kept clean for the full potential of adhesion to be successful.
What is a peel ply and its purpose in bonding?
A peel ply is a thin nylon or fibreglass layer cured onto bonding surfaces to ensure surface cleanliness prior to bonding.
What happens when the peel ply is removed?
When the peel ply is stripped away, a pristine surface becomes available.
How can high resin peaks from peel ply be addressed?
Light sanding removes any high resin peaks produced by the peel ply weave.
What temperature conditions are preferred for bonded repairs?
Elevated temperatures are preferred for most repair methods that produce the highest quality bond.
What methods can be used to maintain temperature during repairs on aircraft?
Heat bonders, heat lamps, hot air, or heat blankets can be used to maintain the correct temperature.
What is vacuum bagging?
Vacuum bagging is a common method to remove air from the plies of a repair layup.
How do typical aircraft fuels and oils affect cured epoxy resin?
Typical aircraft fuels and oils have little effect on cured epoxy resin but can saturate underlying core materials.
What is the effect of military hydraulic fluids on cured epoxy resin?
Military hydraulic fluids do not have a direct chemical effect on cured epoxy resin.
What is the effect of Skydrol™ hydraulic fluids on cured epoxy?
Some types of Skydrol™ brand hydraulic fluids can attack cured epoxy and weaken it.
What is the effect of de-icing/anti-icing fluids on composite structures?
De-icing/anti-icing fluids have no known effects on composite structures.
How do weaker acids affect cured epoxy panels?
Weaker acids do not affect cured epoxy panels but can attack glass fibres.
What is the effect of strong acids on cured epoxy?
Strong acids can attack cured epoxy and destroy it.
Do solvents harm cured epoxies?
Most ordinary solvents do not harm cured epoxies, except for Methylene Chloride.
What is the effect of chemical paint strippers on composites?
Chemical paint strippers, largely based on methylene chloride, cannot be used on composites.
Why is electrical contact important in aircraft?
Electrical contact between all metallic and composite parts is required to prevent arcing or fibre damage.
How does lightning protection work in aircraft?
Aluminium provides a conductive path for the dissipation of electrical energy during a lightning strike.
What happens if there is no lightning protection in composite components?
Without lightning protection, the resins in the composite evaporate, leaving a bare cloth.
What methods are used for electrical bonding in composite structures?
Methods include weaving aluminium wires, laminating a fine aluminium screen, bonding aluminium foil, flame spraying aluminium, or bonding a piece of metal.
What must be performed after repairs to composite lightning protection methods?
An electrical continuity check must be performed to verify conductivity between the repair and the surrounding mesh/wires.
What must be done after a composite repair has been cured?
A thorough inspection of the repair must be carried out and the results must be documented.
Who performs the initial inspection after vacuum bagging materials are removed?
The engineer performs the initial inspection.
Who conducts the second level inspection before releasing the component back to service?
The supervisor/certifier conducts the second level inspection.
What are the general methods of inspection for composite repairs?
Methods of inspection are generally visual, but instrumented methods such as ultrasound can also be required.
What conditions must be recognized during post-repair inspections?
Conditions include resin dry portions, resin rich areas, delamination, debonding, foreign object inclusion, evidence of overheating, and evidence of undercure.
What must also be inspected in addition to the repair area?
The area immediately surrounding the repair, exposed to elevated temperatures, must also be inspected.
What should be done if defects are discovered during the inspection?
A standard damage assessment must be carried out and component-allowable damage limits must be applied.
How are the results of non-destructive inspections documented?
Results are recorded in a written report attached to the repair documentation.
What is the purpose of inspection documentation?
It serves as evidence to support certification and release to service, and to prove that the correct Structural Repair Manual (SRM) repair was carried out.
What are some non-destructive inspection (NDI) techniques available?
Techniques include tap testing, visual inspection, bond testing, thermography, shearography, holography, acoustic emission, ultrasonics, and radiography.
Which NDI techniques are most commonly used?
Ultrasonics and radiography are the most commonly used methods.
What is tap testing used for?
Tap testing is used for the quick evaluation of accessible aircraft composite or bonded surfaces to detect delamination or debonding.
What are the three commonly used tap testing methods?
The methods are coin tap test, instrumented tap test (woodpecker), and tap test with a light hammer.
What does a flat or dull response indicate during tap testing?
It indicates voids or delamination/debonding at the bond line, which is unacceptable.
What is a limitation of tap testing?
It is limited to finding relatively shallow defects and requires two-sided access for complete inspection of honeycomb structures.
What should be done if a disbond is detected using tap testing?
An ultrasonic inspection must follow to determine the extent of the damage.
What does a visual inspection primarily look for?
It looks for resin starvation, edge delamination, fibre break-out, and other types of defects on the exterior.
What tools can aid visual inspection?
Tools include 10 times magnification, borescopes, and mirrors.
What is the purpose of a visual inspection before other NDI methods?
It helps identify variables that can affect the NDI and visible defects.
What does a visual inspection of a laminate detect?
It can detect resin starvation appearing as porosity on the surface.
What is shearography used for?
Shearography is used for the rapid assessment of flaws in surfaces of composite structures.
How does shearography detect flaws?
It detects flaws by applying stress to the part and analyzing the fringe pattern created.
What is ultrasonic inspection used for?
Ultrasonic inspection is used for evaluating subsurface flaws in composite parts.
What affects the speed of ultrasonic waves?
The density and elasticity of the medium affect the wave speed.
What is radiography used for?
Radiography is used for quick and low-cost inspection of composite structures.
What are the hazards associated with radiography?
Exposure to high levels of X-rays and gamma rays is hazardous and can be fatal.
What does a radiograph represent?
A radiograph is a photographic record produced by the passage of X-rays or gamma rays through a test object.
