BIZOMAT MAKEOS · Fabrication
Bent Bracket Plan (Flat Blank, Bend Force, Springback)
For this bent sheet-metal bracket, what flat blank do I cut, what press brake force do the bends need, and to what angle must I bend to allow for springback?
Plan for a press-braked bracket with one or two bends: bend allowances, setbacks, bend deductions and the flat blank length from the outside flange dimensions (as the sheet-metal flat pattern tool), the air-bending force for the bend length in your V-die against the press capacity (as the press brake force tool), and the angle to bend to so the bend springs back to the drawing angle (as the bending springback tool), with blank mass.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thickness | mm | Sheet thickness. |
| Inside bend radius | mm | Inside radius of the bends. |
| K-factor | K-factor locating the neutral axis in the bend. | |
| Flanges (outside dimensions) and bends | ||
| Flange 1 | mm | First flange, outside dimension. |
| Bend 1 angle | ° | Angle of the first bend. |
| Flange 2 | mm | Second flange, outside dimension. |
| Bend 2 angle (0 = one bend) | ° | Angle of the second bend (0 for a single bend). |
| Flange 3 | mm | Third flange, outside dimension (used with two bends). |
| Bend length (part width) | mm | Length of the bend line (the width of the part). |
| Material and press | ||
| Tensile strength | MPa | Tensile strength of the sheet. |
| Yield strength | MPa | Yield strength of the sheet. |
| Elastic modulus | GPa | Elastic modulus. |
| Density | kg/m³ | Density of the sheet. |
| V-die opening | mm | Opening of the V-die. |
| Die factor | Die factor for air bending. | |
| Press brake capacity | kN | Rated force of the press brake. |
| Least end flange for the die | mm | Shortest flange that sits across the V-die. |
| Least inside radius (× thickness) | Smallest inside radius the material bends to without cracking, as a multiple of thickness. | |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Thickness
- What it is
- The thickness of the sheet the bracket is folded from.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bend force rises with the square of thickness.
- Get it right
- Use the measured or certified thickness; nominal gauge sizes can differ from the actual sheet.
What this means: Inside bend radius
- What it is
- The inside radius of the bends, the same for every bend, as formed by the punch or the air-bend in the V-die.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.1 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger radius increases springback and the bend allowance.
- Get it right
- In air bending the radius depends on the die opening as well as the punch; use the radius your tooling actually produces.
What this means: K-factor
- What it is
- The K-factor: the position of the neutral axis in the bend as a fraction of the thickness measured from the inside face.
- Units and format
- Enter a plain number with no unit. The form accepts 0.2 to 0.55. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher K-factor increases the bend allowance and lengthens the flat blank.
- Get it right
- Take it from your press-brake tooling data or measure it on a test bend; it is a fraction below 1, not a percentage.
Flanges (outside dimensions) and bends
What this means: Flange 1
- What it is
- The outside dimension of the first flange, measured from its free edge to the outside face of the adjoining flange.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The flat blank length increases one for one with each flange.
- Get it right
- All flange dimensions are outside dimensions; convert inside dimensions by adding the thickness.
What this means: Bend 1 angle
- What it is
- The angle the sheet is bent through at the first bend, which is 90° for a right-angle bend.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 1° to 175°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the angle bent through, not the included angle between flanges: a 120° included angle is a 60° bend.
What this means: Flange 2
- What it is
- The outside dimension of the second flange, between the outside faces of the flanges on either side; with one bend it is the end flange.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be longer than the setbacks of the bends at its ends.
What this means: Bend 2 angle (0 = one bend)
- What it is
- The angle the sheet is bent through at the second bend; set it to zero for a bracket with a single bend.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 175°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 means one bend: flange 3 is then ignored.
What this means: Flange 3
- What it is
- The outside dimension of the third flange, from the outside face of flange 2 to the free edge, used only when there is a second bend.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Ignored when the second bend angle is 0.
What this means: Bend length (part width)
- What it is
- The length of the bend line, which is the width of the part along the brake.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bend force and blank mass rise in direct proportion to this length.
- Get it right
- Use the full length of material being bent at once.
Material and press
What this means: Tensile strength
- What it is
- The ultimate tensile strength of the sheet material, from the mill certificate or material standard, used for the bending force.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 450 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bend force rises in direct proportion to tensile strength.
- Get it right
- Use tensile strength, not yield strength, for the force.
What this means: Yield strength
- What it is
- The yield strength of the sheet material, from the mill certificate or standard, used to work out springback.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 250 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher yield strength increases springback, so the bend must be overbent more.
- Get it right
- Do not swap it with the tensile strength; yield sets how much the bend opens after release.
What this means: Elastic modulus
- What it is
- The elastic (Young’s) modulus of the sheet material, used in the springback calculation.
- Units and format
- Enter a number in GPa: gigapascals (1 GPa = 1000 MPa). The form accepts 0.000001 GPa to 1000 GPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 GPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher modulus reduces springback.
- Get it right
- Enter gigapascals, not megapascals.
What this means: Density
- What it is
- The density of the sheet material, used to work out the mass of the flat blank.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 20000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7850 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Blank mass rises in direct proportion to density.
What this means: V-die opening
- What it is
- The width of the V-die opening across the top of the die, between its shoulders.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 16 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bend force falls in inverse proportion to the die opening.
- Get it right
- Use the opening width, not the depth of the V.
What this means: Die factor
- What it is
- The die factor in the air-bending force formula, which allows for the die opening relative to thickness, from your press-brake or tooling data.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.33 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bend force rises in direct proportion to the die factor.
- Get it right
- Take it from the tooling supplier for your die opening; it is a multiplier, not a force.
What this means: Press brake capacity
- What it is
- The rated force of the press brake, from its nameplate or manual.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter kilonewtons; convert tonnes-force by multiplying by about 9.81. Check the rating per metre if the bend is short.
What this means: Least end flange for the die
- What it is
- The shortest end flange that can still sit across both shoulders of the V-die during bending.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Base it on the die opening of your tooling; both end flanges are checked against it.
What this means: Least inside radius (× thickness)
- What it is
- The smallest inside bend radius the material can take without cracking, expressed as a multiple of the sheet thickness.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from the material supplier for the grade, temper and bend direction relative to the rolling direction.
Method and formulas
- Bend allowance = θ (R + K t); setback = (R + t) tan(θ ÷ 2); deduction = 2 × setback − allowance; flat length = Σ flanges − Σ deductions (as the sheet-metal flat pattern tool).
- Force = k L t² UTS ÷ V (as the press brake force tool).
- Springback ratio from R S_y ÷ (E t); bend-to angle = final angle ÷ K_s (as the bending springback tool).
- Example: cut the blank 101.94 × 300 mm (0.48013 kg); each bend needs 44.888 kN; bend to 90.382° for 90°.
Limitations
- One radius and K-factor for all bends; air bending in a V-die.
- Springback from elastic–plastic theory; confirm with a trial bend.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Prepare a blank and brake set-up for a bracket
- Check a press brake can bend a part
- Set the overbend for springback
Who it is for
- Press brake operators
- Sheet-metal designers
- Fabrication supervisors
What you get
- Flat blank length and mass
- Bend force and press use
- Angle to bend to for springback
Interpreting the result
- A wider V-die lowers the force but raises the inside radius and springback.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1346–1353. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · Library collection
Designing sheet metal parts for economical manufacture: bends, holes, flat patterns, fasteners and tolerances
In Manufacturing Operations and Design for Manufacture
What it covers: Sheet metal parts are cheap when designed for the cutter and press brake. How to set bend radii, place holes and flanges, get flat patterns right and tolerance folded parts.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Bent Bracket Plan (Flat Blank, Bend Force, Springback) calculator