Bump bending, also called step bending, approximates a large-radius curve with successive small bends. Choosing more steps can reduce visible facets, but the proposed spacing must still suit the die support, material, tool clearance and gauging method. Define the required finished profile and its datums before choosing a step count. A geometric chord calculation describes an ideal shape; it does not supply a CNC program, a flat pattern or a guaranteed part tolerance.
Separate the radius, sweep and bend length
On the section drawing, identify the inside or outside radius, sheet thickness, arc sweep and the tangent straight sections. The sweep is the angle travelled around the curve, not automatically the included angle displayed by the controller. Record the bend length along the machine separately from distances around the curved section. An outside radius, an inside radius and a neutral-axis radius describe different surfaces; do not substitute one for another in a flat-pattern calculation.
Choose a forming route against the finished part
WILA describes step bending with conventional tooling and cautions that smaller spacing is constrained by the die's minimum bendable flange. Wilson Tool's radius-bending guidance identifies springback, thickness variation and marking as separate application variables. Compare the alternatives below with the supplier using the actual drawing; neither manufacturer's catalogue establishes Rucheng's supplied configuration.
Technical references: WILA; Wilson Tool.
| Route | Review before selection | Evidence to retain |
|---|---|---|
| Multiple small bends | Acceptable faceting, supported spacing, repeatable gauge contact and clearance at every intermediate shape. | Approved step layout and program revision, tooling, material lot and measured profile. |
| Dedicated radius tooling | Tool radius, die opening, springback, surface finish, removal path and the complete assembly's permissible load. | Supplier-reviewed tool drawings and a representative formed sample; tool radius alone is insufficient. |
| Rolling or another forming route | Review long continuous arcs, available straight ends, section geometry and subsequent flange operations with the process supplier. | Agreed process sequence, equipment scope and drawing-based acceptance criteria. |
A geometric check for facet size
For an ideal circle of radius R divided into N equal chord spans over a sweep Θ, let Δθ = Θ/N. Each chord has length c, and the maximum radial gap between that chord and the circle is e. The formulas below use angles in radians inside sine and cosine. They describe a zero-thickness, inscribed polygon at the selected reference surface. Real bends have finite radii and springback; N is a count of ideal spans, not a universal count of press strokes.
Δθ = Θ/N; c = 2R sin(Δθ/2); e = R[1 − cos(Δθ/2)]
Example: a 100 mm reference radius over 90°
For illustration, use R = 100 mm and Θ = π/2 radians (90°) at one named reference surface. The table compares ideal polygon geometry only. It contains no material, thickness, tooling or machine assumptions and cannot establish manufacturability. The rounded values are not recommended setup dimensions.
| Ideal spans N | Angle per span | Chord c (mm) | Ideal gap e (mm) |
|---|---|---|---|
| 6 | 15° | 26.105 | 0.856 |
| 12 | 7.5° | 13.081 | 0.214 |
| 24 | 3.75° | 6.544 | 0.054 |
Increasing N from 6 to 24 reduces the theoretical gap, while shrinking each chord from 26.105 mm to 6.544 mm. That smaller spacing may fail the selected die's support requirement. A 0.054 mm geometric gap is not a promise of 0.054 mm production accuracy. Do not enter c directly as a back-gauge increment: the machine program must account for its angle convention, actual bend radii, material development, datum and end treatment.
Check the complete sequence before a trial
Review the first and last bend separately: tangent transitions and available straight material can differ from the repeated middle steps. Show how the evolving curve contacts the gauge and supports without colliding with the punch, holders or machine. Protocase publishes its own step-bend size and U-profile clearance limits; those are useful examples of process-specific restrictions, not universal limits or Rucheng specifications.
Technical references: Protocase.
Do not solve an unsupported short spacing simply by choosing a narrower V die. Reassess the process-specific force, local forming radius and permissible loading of the punch, die, holders and machine. Follow the equipment manufacturer's setup and safeguarding procedures. Geometry approval and load approval are separate checks, and a successful graphical simulation does not replace a representative trial.
Inspect the curve, not just its total angle
- Agree the drawing revision, reference surface and datum alignment. Specify profile tolerance, end positions, tangent transitions and cosmetic requirements independently; a radius dimension alone may not define them all.
- Choose a suitable inspection method, such as an agreed profile template or coordinate measurement against the drawing. Record section locations along the bend length and how the released part is supported. State whether inspection is free-state or under a drawing-defined restraint.
- Compare the complete curved region and both ends, not one local radius or the total sweep alone. Record local facets, overall contour, twist and surface marks as separate observations, using the agreed acceptance criteria.
- Retain the accepted material identification, actual thickness, tool combination, program revision and measured results. After a material, tooling or sequence change, review which first-piece checks must be repeated before batch release.
Prepare a useful supplier review package
For a Rucheng Machinery inquiry, provide the formed drawing and CAD model, material grade and condition, thickness, bend length, quantity and finish. Mark the required curve, datums and tolerances; identify nearby holes, mating surfaces and straight ends. Request a proposed machine/tooling route and trial scope separately. A controller feature list does not prove that a particular curved part is feasible.
Discuss press brake requirementsBump-bending questions
Is step bending the same as making a joggle?
Here, step bending means repeated small bends approximating a curve. A joggle or offset is an opposing bend pair creating a change in level. Use a section drawing to remove ambiguity when a supplier uses the word step.
Does adding more steps always improve accuracy?
It reduces the facet gap in the ideal geometric model. Actual accuracy also depends on support, material variation, tool condition, positioning and accumulated bend errors. Validate the complete profile rather than approving a step count alone.
Can a radius gauge alone accept the part?
Only if the drawing and agreed inspection plan make that sufficient. A local fit does not by itself verify the whole curve, end locations or twist. Define the acceptance method and datums before the trial.