Sheet Metal Bending Terminology

• Terms related to bending sheet metal and flat pattern layout are used to help understanding bend calculations.

Terminology

• Flat (base measurement) refers to the outer dimensions of shaped part which is specified on the drawing or blueprint, or determined from the original component.
• Leg denotes the longer segment of a formed angle.
• Flange is the shorter segment of a formed angle, opposite the leg. Both sides are considered legs if the angle's sides are of equal length.
• Setback (SB) is the measurement necessary to move the brake's jaws away from the mold line in order to create a bend.
• For a 90° bend, SB = R + T, where R stands for bend radius and T represents the metal's thickness.
• Sheet metal bends are mostly 90°.
• When bends are smaller or greater than 90°, the K-factor must be used alongside the formula SB = K(R+T).
• Determining the setback dimension before bending helps establish where the bend tangent line will start.
• Setback must be deducted for each bend in a component with several bends.
• Calculated Flat indicates the portion of a component that is not bent, equal to the Flat less the setback (Calculated Flat = Flat – SB).
• Bend allowance (BA) describes the curved section of metal inside the bend, representing the length of metal that is curved while bending.
• Bend radius refers to the arc created as sheet metal is bent.
• The arc is called the bend radius.
• The bend radius is measured from the center of a radius to the metal's inside surface.
• Material type influences the minimal bend radius, as well as thickness and temper, and it can be shown using manufactuer's maintenance manuals/charts.
• Total developed width (TDW) refers to the material's width when measured around its bends from one edge to another.
• To determine the material's size for cutting, the TDW must be ascertained.
• In comparison to dimensions of flats, TDW is less, because bending forms a radius instead of a square angle.
• Sight line, otherwise known as the bend or brake line, is the layout line marked on the metal that needs to be aligned with the nose of the brake and acts as a bending guide.
• K-Factor is the percentage of material thickness unaffected by stretching or compressing.
• Its value is is on the K chart with 179 numbers, according to degrees from 0° and 180° for bending metal.
• Choose the right K-factor number on the chart and multiply it with the sum of the metal's radius and thickness whenever bending metal at an angle other than 90° (where K-factor equals 1).
• Grain of the metal is the material's natural grain, created while rolling it out of molten ingot.
• If feasible, bend lines must have a 90° angle respective to the metal's grain.
• Bend tangent line (BL) indicates where the metal begins to bend and where the bend itself ceases.
• The space between bend tangent lines accounts for the bend allowance.
• Closed angle refers to an angle is less than 90° when measured between its legs, yet more than 90° when evaluating the amount of bend.
• Open angle refers to an angle is more than 90° when measured between its legs, but less than 90° when assessing the extent of the bend.
• Neutral axis denotes an imaginary line sustaining consistent length during bending.
• Bending thins the bend area by 10–15% which drives the metal's neutral line inwards to central radius.
• It is assumed that the neutral axis is at the material's center for calculation convenience only.
• For most tasks, such a slight error from assuming its centering is enough.
• Mold line (ML) serves as an extension of a component's flat side.
• Mold line dimension (MLD) provides measurements for a component based on the intersection of mold lines, showing size as if corners weren't radiused.
• Mold point marks where the mold lines meet, showing where the part's outside corner would be without any radius.

Layout or Flat Pattern

• Structural and nonstructural components are made by shaping flat sheet stock to form a channel, angle, zee, or hat section.
• Layouts of flat patterns needs to be calculated before forming to reduce waste and improve accuracy of the final product.
• Bend lines guides the flat patterns for sheet metal forming.
• Correct allowances must be set for setback and bend allowance when making straight angle bends.

Making Straight Line Bends

• When bending material considerations must be the thickness, alloy composition, and temper.
• Softer, thinner materials allow for sharper bends (smaller radius of bend).
• Cracks can occur if the bend radius is too small, causing metal weakening due to stresses and strains.
• The bend radius of a sheet of material is the radius of the bend measured internal to the curved material.

Steps for sample U-channel layout

• Step 1: For the metal thickness, alloy and temper, choose the correct radius via manufactuer's charts or maintenance manuals.
• 0.040, 2024-T3 requires a minimum allowable radius of 0.16-inch or 5/32" inch.
• Step 2: Calculate setback using a formula or setback chart.
• SB = K(R+T)
• K = K-factor (K is 1 for 90° bends)
• For angles other than 90° a K-factor chart must be used
• For this U-bend calculations, SB = 1(0.16 + 0.040) = 0.20 inches
• Step 3: Solve for calculated flat dimension using the formula Calculated Flat = Flat – SB
• Flat 1 (1.00-inch) – 0.2-inch = 0.8-inch
• Flat 2 (2.00-inch) – (2 × 0.2-inch) = 1.6-inch
• Flat 3 (1.00-inch) – 0.2-inch = 0.8-inch
• Subtracted setbacks will only be from the areas with bends
• Step 4: Identify bend allowance, which is the length of material needed for a bend
• This depends on degree of bend, radius of the bend, and thickness of the metal.
• The neutral line/axis is unaffected by bending's compression and tension and must be long enough for a proper amount of material to be available for the bend.
• Time can be saved by using formulas and charts for angles, bend radii, material thicknesses etc.
• Bend Allowance formula uses:
• Pi = 3.1416
• Bend allowance = (2π (R + ½T))/4
• 0.27 inches bend allowance using formula is
• Bend allowance =(0.01743 × 0.16) + (0.0078 × 0.040) × 90
• Bend allowance chart factors in if radius of bend and metal thickness is known
• Allowance for 90 degree bend is shown, and one for per degree of bend, use top allowance, and the chart factors in metal thickness

Use of Bend Allowance Chart for a 90° Bend

• Use the chart to factors and find
• Material thickness
• Radius
• Degree of bend

Use of Chart for Other Than a 90° Bend

• Calculate allowance for bends that are not 90 degrees
• Step 5: Total Developed Width (TDW = Flats + (bend allowance x number of bends))
• The following solves for total developed width of the U-channel as TDW = 0.8 + 1.6 + 0.8 + (2 × 0.27), which is a total of 3.74 inches
• TDW should be smaller than totals of flat dimensions

Flat Pattern Lay Out

• Sight line to position the bend tangent line
• Placed one bend radius away from the bend tangent line to be placed under the brake nose bar

Step 6: Flat Pattern Lay Out

• After calculating parameters a layout can be made
• Material must be cut to the correct size
• Sight line needs to be present of material
• Put metal in brake for clamp
• Position metal to have sight line directly below the edge of the radius bar
• Clamp brake and raise leaf

Using a Sheet Metal Brake to Bend Metal

• Setup of brake is required to get a proper and accurate bend
• Setup relies on the thickness, required radius, temper, of the material to be formed
• When those parameters above change, operators need to be aware that adjustments must be taken prior to use in forming

Using a Sheet Metal Brake to Fold Metal

• 0.032-inch 2024-T3 L channel, to be able to bend it with radius, the radius bar must be installed
• Checks must be performed using radius and fillet gauges

How To Bend Open End Bend (Less Than 90°)

• Step 1: Look up K-factor in K chart
• Step 2: Calculate setback
• SB = K(R + T)
• SB = 0.41421-inch (0.1875-inch + 0.051-inch) = 0.098-inch
• Step 3: Calculate bend allowance
• BA = (0.003675-inch × 45) = 0.165" inch
• Step 4: Calculate flats
• Flat 1 = .77-inch – 0.098-inch = 0.672" inch
• Flat 2 = 1.52-inch – 0.098-inch = 1.422" inch
• Step 5: Calculate TDW
• Calculated Flats + Bend allowance, 0.672-inch + 1.422-inch + 0.165-inch = 2.259' inch

How To Bend Closed End Bend (More Than 90°)

• Step 1: Look up K-factor in K chart
• Step 2: Calculate setback
• SB = K(R + T), SB = 2.4142-inch(0.1875-inch + 0.051-inch) = 0.57" inch
• Step 3: Calculate bend allowance
• BA = (0.003675-inch × 135) = 0.496-inch
• Step 4: Calculate flats
• Flat 1 = 0.77-inch – 0.57-inch = 0.20-inch
• Flat 2 = 1.52-inch – 0.57-inch = 0.95-inch
• Step 5: Calculate TDW
• TDW = Calculated Flats + Bend allowance
• TDW = 0.20-inch + 0.95-inch + 0.496-inch = 1.65" inch
• Open-end bends smaller TDW (material) than closed-end bends.

Straight Line Bends

• Brakes generally make straight bends in materials.
• When these are not available, bending short sections can be achieved using wooden or metal bending blocks.

Straight Line Bends by Hand

• Clamp metal along bend line between two wooden forming blocks in a vise if brake is unavailable.
• Forming blocks must have one edge rounded so that bending radius needs to be desired.
• Slightly curve beyond 90° to compensate for spring-back.
• Tap protruding metal with rubber/plastic/mallet to achieve desired angle.
• Tap to the edge and work back and forth along to get even bend.
• Continue until the protruding metal is bent to the desired angle against the forming block, allowing spring back.
• Use hardwood and a mallet or hammer for any irregularities.

Joggling

• It's offset formed on parts to clearances for mating parts, its where stringers and formers often meet.
• Joggling maintains the smooth surface of joints or splices.
• Joggle depth is calculated as thousandths of an inch.
• Extra 1/16 inches when calculating clearance between joggled, overlapping parts.
• Allowance is the measurement between joggle bends.
• Dimension is shown on the technical drawing.
• A general rule of thumb is four times the thickness displacement of flat sheets.
• For extrusions allowance can be as much as 12 times material thickness.

Forming joggles on cornice break (Lab)

• Sight line of joggle for bends on a sheet.
• Insert sheet on brake metal, bend approximately 20 - 30 degrees
• Remove part
• Part must be turned over and clamped second
• Adjust until correct heigh of joggle
• Check the joggle size dimension and clearance is correct

Joggling by Hand

• Forming blocks/dies can be made to have joggle, on a curved or curved part/flange
• Squeeze joggle block
• Bulges can be turned facing down vice and flattening can be achieved using wooden mallet