Sheet Metal Forming and Bending

Sheet Metal Forming and Bending

• Most aircraft skins made of sheet metal require bends or curves.
• Forming/bending tools can be manually, electrically, pneumatically, or hydraulically powered.
• These tools create straight and compound curves.
• Folding machines provide the easiest, most accurate bending method.

Folding Machines

• Machine types:
  • Press brake
  • Cornice brake
  • Bar folding machine
  • Box brake

Press Brakes

• Utilize a fixed female die and a male die powered by an electric motor-driven flywheel.
• Commonly used for repetitive production in aircraft and commercial sectors.
• During use, the material is moved over the female die, and the male die lowers to form a bend.
• Various die types accommodate almost any sheet metal bend.
• Well-suited for economical mass production, where a skilled worker sets up the machine for others or robots.

Cornice Brakes

• Capable of bending large sheets of metal with varying thicknesses.
• Standard capacities typically range from 12 to 22 gauge mild steel sheets (0.110 to 0.032 in.) that are 3 to 12 ft wide.
• Feature a sharp nose bar that accommodates bars of any radius.
• Include a bed to hold the job and an adjustable clamping nose bar operated by a handle to accommodate different material thicknesses.
• An adjustable, counterweighted bending leaf aids in bending, with higher raising resulting in larger angles.
• Metal must be bent past the required angle to account for spring-back.
• Tapered radius bars are employed for greater bending angles and require a sight line one radius from the bend line.

Bar Folder

• Designed for bending edges of light material up to 22 gauge thick and 42 inches in length.
• Equipped with adjustment screws to adjust material thickness at both ends.

Box Brake

• Known as a pan or finger brake.
• Similar to a cornice brake, but incorporates removable fingers on its top leaf.
• It can fabricate boxes and fold adjacent sides of partially formed boxes due to its adjustable fingers.
• Requires at least 75% of the leaf to be fitted during operation.

Safety Precautions for Bending Machines

• Body parts should be kept away from clamping jaws.
• Stay clear of counterweights.
• Clear communication between operators is essential.
• Prior to use, verify the correct machine setup.
• Avoid folding materials other than sheet metal or plastic.
• Lightly oil metal parts to prevent rust.
• Regular maintenance ensures safe and accurate machine operation.

Shrinking and Stretching Machines

• Form contours by expanding (stretching) or compressing (shrinking) metal.
• Stretchers expand metal's edges to create outside curves, while shrinkers contract metal for inside curves.
• Both types have two sets of gripping jaws operated manually with hand levers or foot pedals.
• The shrinker's jaws grip and compress the metal edge by moving inward.
• The stretcher's jaws grip and spread the metal apart.
• Progressive working of metal over distance creates curves.
• The jaws compress or stretch the metal without buckling or tearing.
• They are available in hand-operated or power-operated configurations.

Bumping and Hand Forming

• Used to form compound curves in angles, channels, ribs, and non-structural streamlined fairings.
• Extruded angles are shaped using a wooden or plastic mallet around a hardwood block to form convex or concave shapes.
• Concave curves in angles utilize flange shrinking over a hardwood V-block.
• Compound curved channels employ two hardwood blocks clamped in a vice, with the material shaped using a mallet over a radius block.
• Metal should be backed with a hardwood forming bar to control the bend with a radius block to accommodate spring-back.
• Complete bends are formed using a mallet and wood block.

Hand Forming Methods

• Hand shrinking and stretching (bumping) are used to form many components, such as nose ribs.
• Non-structural streamline fairings are hand-formed in wooden female forming blocks with a hold-down plate and mallet.
• If the material work hardens, it should be annealed before trimming to size.
• Compound curves can be created using a wooden or plastic mallet over a leather lead or shot-filled bag.

Joggling

• Used to overlap metal pieces to create a flat surface, such as when fitting a repair doubler over an angle.
• One piece is bent to clear the other, then bent back.
• Parts should be joggled rather than pulled in with rivets.
• Joggling methods include:
  • Pressing metal to shape using metal dies.
  • Forming with a metal brake.

Rollers

• Used to put gentle curves in sheet metal.
• Three hard steel rollers are positioned in a framework.
• A crank handle drives one roller, with an adjustable clamp roller assisting in pulling the metal.
• The radius roller is adjustable to change the curve's radius.
• Rollers can be adjusted 'out of parallel' to create cone shapes.
• To use, place metal between the drive and clamp rollers and crank it through, adjusting the radius as needed.
• Safety measures:
  • Keep fingers and clothing clear of the rollers.
  • Roll wire or rod only in the designated slots.
  • Keep rollers lightly oiled.

Presses

• Used to manufacture components ranging from small stampings to large compound curved skins.

Hydropress

• Used for forming smaller compound curved parts.
• A blank is placed over a metal male die and held with pins.
• The die is placed on the hydropress, a rubber-covered ram lowers, and water pressure is applied.

Cutting Sheet Metal

• Hand or machine shears are commonly used.
• Hand shears include aviation and tin snips.
• Shears are frequently used to notch out corner material when folding in an aircraft workshop.
• The treadle guillotine, a foot-operated machine, cuts flat sheet metal also known as squaring shear, and has various material cutting capacities.

Squaring Shears

• Employ side fences for square cuts and an adjustable stop fence for setting the depth of cut.
• Utilize a clamp to hold the job tight.
• Stepping on the treadle clamps the metal, and the blade cuts the metal with minimal burrs.
• Safety measures:
  • Keep fingers clear of the clamp and blade.
  • Keep shears lubricated.

Power Guillotine

• Capable of cutting wider and thicker flat sheet metal than treadle guillotines.
• Operated using a foot pedal and assisted by flywheel inertia or hydraulic power.

Throatless Shears

• Hand-operated tool for greater mobility and can easily cut metal of any length or shape.
• Operates similarly to scissors.

Scroll Shears

• Used for making irregular cuts inside a sheet without cutting through the edge.
• The upper blade remains stationary, while the bottom blade moves upwards when the handle is operated.
• Functions similarly to a can opener.

Bending Considerations

• Physical changes within the sheet metal during bending must be considered.
• To achieve the correct component size, account for the material that will be used in the bend.
• This is achieved by calculating the bend allowance and adding it to overall dimensions.

Bend Allowance Calculation

• To calculate it, you need to know:
  • Bend radius
  • Material thickness
  • Bend angle
  • Neutral axis
  • Mould line and mould point
  • Bend tangent line
  • Flats
  • Setback
• Bend radius (BR) is the curve amount measured on the bend's inside.
• BR is usually tight in sheet metal bending.
• Hard metals such as tempered aluminium alloys need to avoid sharp bends to prevent cracking.

Minimum Bend Radius

• Minimum BR prevents cracking in tempered metals.
• Minimum BR charts can help select bend allowance based on the material and thickness.
• Bend angle is the angle at which the metal is bent.
• Material thickness (MT) is the metal's thickness to be bent.
• During bending, the material stretches on the outside and compresses on the inside.
• The neutral axis is the point where no shrinking or stretching occurs.
• It is located approximately 44.53% of the MT from the bend's inside.

Setback (SB) and K-Value

• Setback (SB) differs for bends greater or less than 90°, therefore calculate SB using this formula: SB = K(BR +MT)
• A K-value multiplier is applied to the SB formula.
• For a 90° bend, the K-value is 1.
• K charts determine the K value for angles.
• If a K chart isn't available, then: K = Tangent of half of the Bend Angle
• Example: for a 60° bend, K is: K 60º = tan 30º = 0.5773
• The flat is the unbent portion of the metal and the distance between bend tangent lines (BTLs) or the BTL and edge.

Calculating a Flat (Example)

• Fabrication of a channel from 0.040-in. 2024-T3, 3 in. wide, with 90° sides and a 1/8 in. bend radius.
• The material thickness (MT) is 0.040 inches.
• The bend radius (BR) is 1/8 inches (0.125 inches).
• The sides must be 90° (vertical), as you are fabricating a channel with two sides.
• Determines the true length of the flat layout is needed.
• The flat will be the width (3 in.) minus the two setbacks (one for each side).

Calculating the Setback

• SB 90º = K(BR+MT) = 1(0.125 + 0.040) = 0.165 in
• To calculate the flat, subtract 2 x setback from the overall width: Flat = 3 in – (2 x 0.165) = 3 in – 0.330 = 2.67 in
• The flat is one measurement needed for flat pattern layouts.

Bend Allowance

• The other measurement is the bend allowance (BA).
• It equals the neutral axis length between between bend tangent lines (BTLs).

Neutral Axis Centered Bend Allowance (NA BA)

• One BA estimation method is to put the neutral axis in the metal's centre.
• The BA is the neutral axis circumference obtained by: Circumference = pi (2BR + MT)
• Find material required to form a full circle.
• Calculate with provided values where BR = 0.250 in. and MT = 0.064 in.: C = pi (2BR + MT) = 3.1416 (2 X 0.250 + 0.064) = 3.1416 X 0.564 = 1.77186 inches

Material Required to Form a Bend

• This is the length of the metal needed for a full circle.
• How much material is required for just the 90° bend?
• Finding the material per degree: 1º = Circumference / 360 = 1.77186 in / 360 = 0.0079218 in
• Multiplying it by 90 degrees for the 90 degree bend: 90º = 0.0049218 in x 90 = 0.44296 in = BA = 0.44296 in
• A formula for BA with a centred neutral axis: Bend Allowances = (2 x pi (BR + ½MT) / 360) x Bend Angle
• This BA calculation is close and ideal for most cases.

Empirical Formula Bend Allowance Approach

• The method places the neutral axis at 44.5% of MT from the bend's inside.
• Formula: Bend Allowances = (0.0078 MT + 0.07143 BR) x Bend Angle
• Calculate BA for a bend radius, material thickness and angle of: BR = 0.250 inches, MT = 0.064 inches, bend angle is 90°. Formula: Bend Allowances = (0.0078 MT + 0.07143 BR) x Bend Angle = (0.0078 x 0.064 + 0.01743 x 0.250) x 90º = 0.437 inches