Radius bending. Hit by hit.

01 — Bump bendingA radius made of many bends.

To get a large radius — larger than a normal punch would give — you don't need a dedicated radius tool: you make many small bends close together, all equal, at a constant pitch. This is multistep radius bending (or bump bending). Each hit bends a few degrees; adding up the hits forms the curve. The closer the hits, the smoother the curve.

The advantage is flexibility: with the same V-die and a standard punch you get any radius, without buying a tool for every size. The price is cycle time (many hits) and a slightly faceted surface, which the calculator helps you keep under control.

On PG presses this is a CNC function, radius (bump) bending: it's the press's CNC that generates the sequence of sub-bends from the few values you set. This calculator reproduces the Circle mode — radius + angle → circular trajectory — and uses the same fields: radius, angle, step, number of hits, development. PG's radius (bump) bending function also offers 3-points and custom modes.

02 — The formulasAngle, pitch, development.

Three numbers govern everything. The angle per hit is the total arc divided by the number of hits. The pitch is how much to advance the back gauge between hits, measured on the development (neutral axis at Ri + K·S). The arc development is the sum of the pitches:

Example: inner radius 100 mm, arc 90°, thickness 3, 45 hits → angle/hit = 2°, pitch ≈ 3.5 mm, arc development ≈ 159 mm. These are public formulas (DIN for the K-factor, arc geometry), applied here to the full table of hits, hit by hit. The pitch is the arc development divided by the number of hits — the same logic as PG's radius (bump) bending function.

03 — How many hitsSmooth vs fast.

This is the real trade-off of bump bending. More hits = smoother radius but longer cycle; fewer hits = faster but faceted surface. The shop rule: keep the angle per hit around 1.5–2° and the pitch between 2 and 5 mm. The deviation from radius the calculator shows is the faceting depth, in millimetres: how far the polyline of hits strays from the theoretical radius, i.e. the part's tolerance. The look, instead, depends on the angle per hit, because every hit leaves an edge that catches the light.

That is why the traffic light next to the deviation judges the angle per hit: green up to 2°, amber up to 4°, red beyond, where the radius stays visibly faceted.

04 — The tableOne row per hit.

The hit sequence above is what you take to the machine. Each row is one hit, in the order you strike it: the cumulative angle (where you are along the arc), the distance from the start of the arc and the pitch from the previous hit. The distance is measured on the development, from the point where the flange resting on the back-gauge fingers ends: the value to set on the CNC is this distance plus the flange length. If you enter the flange in the optional field, the table gives you the back-gauge position directly. You start from the hit farthest from the fingers and the back gauge advances one pitch per hit: the flange on the fingers stays flat and the already curved part stays on the operator's side. No mental maths and no cumulative errors: the list is ready to follow.

05 — LimitsWhen to validate.

The calculation is geometric and indicative for air bending on mild steel. Springback adds up hit after hit: in practice you set a small over-bend per hit, to be tuned on the material. The choice of die and punch, the behaviour on stainless and high-strength steels and very tight radii shift the result. Use this tool to set up the job, then verify on a sample or with our technical office. For force, die and machine switch to the bending parameters calculator.

On long, thick sheet, large-radius bump bending is done on the hydraulic press brake DCA, with up to 16.2 m of bending length. For roofing and architectural sheet metal up to 3 mm and from 6.5 to 12.5 m long there is the sheet metal press brake.