Sheet Metal 3:Bending Terminology & Sheet Metal Forming

Bending Terminology

• Sheet metal forming and flat pattern layouts commonly use specific terminology.
• Understanding these terms supports accurate bend calculations in bending operations.

Term Definitions

• Flat is the outside dimension of a formed part and either appears on the blueprint or is derived from the original part.
• A leg is the longer portion of a formed angle, while a flange is the shorter portion, opposite the leg.
• If both sides are equal in length, each side is a leg.

Setback (SB)

• Setback refers to the distance the jaws of a brake are set back from the mold line to create a bend.
• For a 90° bend, SB = R + T which means Setback equals the Radius of the bend plus the Thickness of the metal.
• Most sheet metal bends are 90° bends.
• For bends that are smaller or larger than 90°, the K-factor must be used, noted as SB = K(R+T).
• The setback dimension must be determined to accurately locate the beginning bend tangent line.
• Setback must be subtracted for each bend if a part contains more than one bend.

Calculated Flat

• Calculated Flat refers to the part of the material not included in the bend
• Calculated flat equals flat minus the setback, noted as Calculated Flat = Flat – SB.

Bend Allowance (BA)

• Bend allowance is the curved section of metal within the bend.
• Bend allowance can be considered as the length of the curved portion of the metal.

Bend Radius

• Bend radius is the arc formed when bending sheet metal.
• The bend radius is measured from a radius center to the inside surface of the metal.
• The minimum bend radius depends on temper, thickness, and material.
• Manufacturer's maintenance manuals contain minimum bend radius charts.

Total Developed Width (TDW)

• Total developed width is the width of the material measured around the bends from edge to edge.
• Finding the TDW is necessary to determine the size of the material to be cut.
• The TDW is less than the sum of flat dimensions because the metal is bent on a radius.

Sight Line

• A sight line, also known as the bend or brake line, is the layout line set even with the nose of the brake.
• The sight line serves as a guide in bending work.

K-Factor

• K–Factor represents the percentage of material thickness where no stretching or compressing occurs.
• This percentage, already calculated, appears as one of 179 numbers on the K chart for angles 0° to 180°.
• When bending metal to any angle other than 90° (K-factor of 90° equals 1), select the corresponding K-factor number from the chart.
• The K-factor is multiplied by the sum of the radius (R) and the thickness (T) of the metal.

Grain and Bend Tangent Line

• The natural grain forms as the sheet is rolled from molten ingot.
• Bend lines should be at a 90° angle to the grain of the metal, if possible.
• The bend tangent line shows where the metal starts to bend and where it stops curving
• The space between the band tangent lines is the bend allowance.

Angle Definitions

• A closed angle measures less than 90° between legs or more than 90° when measuring the bend.
• An open angle measures more than 90° between legs, or less than 90° when measuring the bend.

Neutral Axis

• The neutral axis is an imaginary line which maintains the same length throughout the bending process.
• The bend area becomes 10–15% thinner after bending.
• This thinning moves the neutral line towards the radius center.
• For calculation purposes, the neutral axis is assumed to be located at the center of the material for simplicity.

Mold Line

• Mold line refers to the extension of the flat side of a part beyond the radius.
• Mold line dimension is the measurement of a part made by the intersection of mold lines.
• The mold point indicates the intersection of the mold lines and the outside corner of a part without a radius.

Layout or Flat Pattern

• The construction of structural and nonstructural parts is done by forming flat sheet stock into various shapes like channels, angles, zees, or hat sections.
• A flat pattern layout prevents material waste and promotes accuracy in the finished part.
• Bend lines must be determined when developing a flat pattern for sheet metal.
• Setback and bend allowance must be made when forming straight angle bends.

Straight Line Bends

• The material thickness, alloy, and temper condition should be considered when forming bends.
• Thinner, softer materials allow for sharper bends with smaller radii.
• A bend radius that is too small can weaken the metal and even result in cracking.
• Measure the radius of the bend of sheet material on the inside of the curved material.

U-Channel Layout Example

• Steps for determining a sample U-channel's layout can help to understand the process of making a sheet metal layout.

Determining Bend Radius

• The necessary bend radius of a sheet is the sharpest curve that doesn't critically weaken the material during bending.
• Drawings often indicate the needed radius, but it is good to double check.
• Minimum bend radius charts are found in manufacturers’ maintenance manuals.
• When performing layout, choose the correct bend radius for properties like metal thickness, alloy, and temper.
• For 0.040, 2024-T3, the minimum allowable radius is 0.16-inch or 5⁄32” inch.

Finding Setback Procedure

• The setback can be calculated with a formula or located on a setback chart.
• Setback charts may be found in aircraft maintenance manuals or Standard Maintenance Practices (SMPs).
• SB = K(R+T) which means Setback equals the the K-factor multiplied by the inside radius of the bend plus material thickness.
• For angles, a K-factor chart should be used for other than 90° angles.
• SB = 1(0.16 + 0.040) = 0.20 inches is the calculation for the example channel.

Calculated Flat Dimension

• The calculated flat dimension can be found using: Calculated Flat = Flat – SB
• Two setbacks must be subtracted from the center flat because it has a bend on either side.
• Flat 1 (1.00-inch) – 0.2-inch = 0.8-inch, Flat 2 (2.00-inch) – (2 × 0.2-inch) = 1.6-inch, and Flat 3 (1.00-inch) – 0.2-inch = 0.8-inch were the calculations for the example channel.

Bend Allowance

• The bend allowance is the length of material required for the bend/fold, which must be calculated.
• Bend allowance depends on the degree and radius of the bend, and the thickness of the metal.
• Metal compresses on the inside of the curve with bending while stretching the curve's outside.
• The distance between the compressions and stretches is the neutral line or neutral axis.

Calculating Bend Allowance

• The length of the neutral axis is determined to ensure adequate material for the bend: this is the bend allowance.
• Formulas and charts for various angles, radii of bends, thicknesses, etc. help save time in calculating the bend allowance during calculation.
• For the bend allowance for a 90° bend, formula 1: To the radius of bend (R) add 1⁄2 the thickness of the metal (1⁄2T), noted as 2π (R + 1⁄2T) / 4.
• For a 90° bend with 1/4 inch radius and 0.051 inch material, the bend allowance equals 0.4327 or 7⁄16-inch.
• Formula 2 uses two constant values: Bend allowance equals (0.01743R + 0.0078T)N, where R equals desired bend radius, T equals the thickness, and N equals the number of degrees of bend.
• For a 90° bend with a radius of 0.16 inch for 0.040 inch material, Bend allowance = (0.01743 × 0.16) + (0.0078 × 0.040) × 90 = 0.27 inches.

Bend Allowance Chart

• A bend allowance chart displays the radius of bend and metal thickness.
• In the chart's cells, the upper number is the bend allowance for 90°, and the lower number is the allowance per 1°.
• Use the top number in the chart to determine the bend allownace for 90°.
• Reading across the top of the bend from the U-channel example finds a needed radius at . 156-inch for its .040-inch alloy.
• The correct bend allowance is (0.273) which occurs in this cell for 90° bends.
• Many online bend allowance calculation programs are available: material thickness, radium, and degree of bend inputs enables it to calculate the bend allowance.
• To compute the bend allowance for other than 90°, use the bottom number in the calculation chart (bend allowance for 1°).

Total Developed Width of Material

• Calculate Total Developed Width by TDW equals Flats plus bend allowance, multiplied by the number of bends.
• Calculated Flat 1 + Calculated Flat 2 + Calculated Flat 3 + (2 × BA) equals the TDW for the sample U-channel.
• 1. 202-inch example channel.
• TDW = 3.74-inches for the example channel.
• The metal is less than the dimensions of the outside of the channel (4 inches).
• Metals follow the radius of the bend instead of going from Flat to Flat.
• The calculated TDW is generally smaller than the total Flat dimensions.
• Incorrect math can occur if the calculated TDW is larger than the Flat dimensions.

Flat Pattern Layout Process

• After laying out the pattern, add a sight line to help position the bend tangent line where bending starts.
• The line should be drawn inside the bend allowance area at one bend radius from the bend tangent line under the brake nose bar.
• Begin creating a flat pattern layout of bend-relevant information by cutting material to size.
• Sight lines can be drawn on the cut material.
• Metals should be placed in a brake clamp and adjusted until the sight line is right below the radius bar's edge.
• The brake can then be clamped and raised to create the bend.
• The sight line must be one radius away from the bend tangent line under the brake nose bar.

Sheet Metal Brakes

• A sheet metal brake set up ensures successful bending on the dimensions, temper, and radius of parts.
• Changing metal thicknesses and required radii requires an updated sheet metal break before use.
• Replacing any brake radius bar with one sized differently is doable.
• 0.032-inch 2024-T3 L channels might require an adjusted 1/8" radius option.
• Calibrating radius and fillet gauges checks this dimension.

Open End Bends

• Lookup the K-factor in the K-chart, here K-factor is at .41421
• Using SB = K(R + T) the following is determined: SB = 0.41421-inch (0.1875-inch + 0.051-inch) = 0.098-inch
• Using bend allowance chart results with: BA (.00375 inch times 45) equals 0.165 inch
• To calculate flats, use: Calculated Flat equals Flat - SB
• Flat 1: .77 inch minus 0.098 inch = 0.672 inch
• Flat 2: 1. 52 inch minus 0.098 inch = 1.422 inch
• To calculate TDW use: TDW equals Calculated Flats plus Bend allowance
• TDW equals 0.672 inch plus 1.422 inch plus 0.165 inch =2.259

Closed End Bends

• Lookup K - factor chart and find that K factor for this measurement will be 2.4142 in
• Using SB = K(R + T) the following is determined: SB = 2.4142-inch(0.1875-inch + 0.051-inch) = 0.57

Bending Straight Lines by Hand

• Straight bends commonly use a brake.
• Comparatively short sections can be bent by hand, when the machines are unavailable.
• Bent by hand requires aid from wooden/metal bending blocks.
• Metals should be laid, cut, and clamped over to two forming blocks stored in a vise
• The edges of bending blocks should then be rounded as needed for a certain radius.
• Metals should also be curved slightly over 90° for spring back aid.
• Tapping metals over the block to certain angles requires rubber, plastic, or rawhide mallets

Metal Bending Rules for by Hand Technique

• First, tap one side and make sure to work along the edge to ensue consistent results.
• Continue this technique until the angle is at the forming block
• Be aware of Springback. This can be fixed by measuring past the set angle.
• Irregularities can be removed by the forming block. A hammer can be used to aid.

Joggling

• A joggle, often found at the intersection of stringers and formers, is the offset formed on a part; it allows for clearance between sheet/mating portions.
• The joggle maintains the smooth surface of a joint/splice.
• Offsets have a small amount of depth, therefore, depths are specified in thousandths of an inch.
• Thickness needs to be cleared and governs depth of the joggle.
• Jogglers usually have an extra 1/16 inch allowance that gives enough added clearance between parts.
• The allowance equals the distance between the two bends of a joggle.
• The allowance for the bends is four times the thickness.
• Follow these guidelines so that it is important to follow the drawing

Joggling Technique

• Forming methods include cornice breaks for straight-flange or flat materials
• Follow sight lines where the bends need to occur on sheets
• Insert sheets into the brake and bend upwards approximately 20-30 degrees.
• Release any braking system and remove the part.
• Turn components over and clamp them into 2nd bending line of the break
• Bend parts until correct height is reached.
• Remove parts and confirm dimensions.

Joggling by Hand

• Curved parts with the need of a curves flange use blocks made of hard steel Aluminum
• Parts can be inserted and squeezed with a clamping device
• Joggle blocks are able to be reversed to flattened out with rawhide and mallets