Principle of work and method of using the CNC bending machine

• 1
  Key takeaways
• 2
  How the machine forms the bend
• 3
  Choosing the method: air bending, bottoming or coining
• 4
  Bend allowance, deduction and the flat pattern
• 5
  What goes wrong and why
• 6
  Bending versus machining: picking the right process
• 7
  Step by step: using the CNC bending machine
• 8
  Bending method comparison

A press brake is a two-part tool: a punch mounted on a moving ram and a die with a V-shaped opening fixed to the bed. The operator places the sheet against the backgauge, the ram descends, and the punch presses the sheet into the V. The material first bends elastically, then yields, and finally flows plastically around the punch tip. The inside radius is set mostly by the punch tip radius, not by the die opening, as long as the die opening is wide enough.

The die opening controls how much force is needed. A wider V needs less tonnage but leaves a longer unsupported span between the shoulders, which raises the minimum bend radius the material can hold without cracking. A rule that holds up well in practice: the V opening should be at least 6 to 8 times the sheet thickness for mild steel, and 8 to 10 times for stainless. Go narrower and tonnage climbs fast.

The controller does not bend metal. It positions the ram and the backgauge repeatably. On a modern machine the Y axis sets ram depth, X moves the backgauge in and out, R sets its height, and Z offsets the gauge fingers left and right for parts with several bends. Crowning, either a hydraulic bed or a set of shims, compensates for the ram and bed deflecting under load so the middle of a long part does not open up.

This is why the same drawing can produce a good part on one machine and a bad one on another. If the ram deflects, the bed is not crowned, or the tooling is worn, the angle drifts along the length of the part even though the program is identical.

Air bending leaves the sheet supported only at the die shoulders. The punch tip never touches the bottom of the V, so the angle depends on how far the ram travels. The advantage is flexibility: one punch and die handle many angles and thicknesses, and tonnage stays low. The trade-off is that angle accuracy depends on material consistency, so springback must be dialed in per batch.

Bottoming drives the punch until the sheet touches the die walls. The angle is then fixed by the tool geometry, so repeatability is better and springback matters less. Tonnage rises, and the tooling must match the target angle. Coining goes further and presses the material to the full tool shape, which gives the tightest angle control and the highest tonnage, but it marks the surface and shortens tool life.

For most job-shop work, air bending is the right default. Choose bottoming when the part has a tight angle tolerance and the volume justifies dedicated tooling. Choose coining only when the angle must be held very tightly and the cosmetic surface is not critical. On thin sheet under 1.0 mm, air bending with a small V is usually the only practical route because tonnage limits rule out coining.

One more decision belongs here: whether the part should be bent at all. If the geometry needs a closed box, a hemmed edge with a very small inside radius, or a bend line that crosses a machined pocket, the sequence and tool clearance get difficult. Sometimes it is cheaper to machine the feature from solid.

When sheet metal bends, the outer surface stretches and the inner surface compresses. Somewhere between them is the neutral axis, which keeps its original length. The flat length of the blank equals the sum of the straight flange lengths plus the arc length of the neutral axis through the bend. That arc length is the bend allowance, and it depends on the inside radius, the angle and the K-factor.

The K-factor describes where the neutral axis sits, expressed as a fraction of the thickness measured from the inside surface. For air bending in mild steel it commonly falls between 0.33 and 0.45. Tight radii push it lower, wide radii push it higher. If your CAD system uses a fixed K-factor of 0.5 for everything, flat patterns will run long on tight bends and short on generous ones.

Bend deduction is the number most press brake operators use at the machine. It is the difference between the sum of the outside dimensions and the flat length. Once the deduction is known for a given material, thickness, radius and die, it can be stored in the controller and reused. This is why a shop that bends the same families of parts every week can program a new job in minutes.

A practical check: cut one test blank, bend it, and measure the outside flange length against the drawing. If it is long, the deduction is too small. Adjust by the difference and re-cut. Two iterations usually land within ±0.1 mm on flange length, which is well inside most sheet metal tolerances.

Angle drift along the length of a part points to deflection. The ram and bed bow under load, so the middle of a 2,000 mm bend opens by a degree or more unless crowning is applied. Hydraulic crowning adjusts the bed curve to match the load. Manual shimming works but takes time and is easy to get wrong. Check crowning before you touch the program.

Cracking on the outside of the bend means the inside radius is too tight for the material. The minimum bend radius for 6061-T6 aluminium is far larger than for 5052, and 7075 is worse still. Annealed tempers bend easily; hardened tempers do not. If the drawing calls for a tight radius in a hard temper, either change the temper or relieve the bend line before forming.

Springback shows up as an angle that opens after the ram retracts. Mild steel recovers roughly 1–3°, stainless 2–5°, and aluminium can recover more. The fix is to overbend: set the ram depth so the loaded angle is smaller than the target by the expected recovery. Most controllers store a springback correction per material and thickness, so the operator adjusts once and the value is reused.

Other common problems: the backgauge finger marks the sheet because it is set too high, the flange is short because the gauge position ignored the bend deduction, and tool marks appear on the inside radius because the punch tip radius is smaller than the sheet thickness. Each has a setup answer, and none of them need a new machine.

Press brakes are fast and cheap per part once the tooling is set. A simple bracket with two bends can be formed in seconds, and the tooling cost is low. That is why sheet metal enclosures, chassis, brackets and covers almost always start as a flat blank and get bent.

Machining wins when the geometry cannot be formed. Deep pockets, tight internal corners, threaded features, bearing bores and faces that must be flat within ±0.005 mm are not press brake work. A bend also cannot cross a hole without distorting it unless the hole is added after forming, which adds an operation.

Volume changes the answer. Below a few hundred parts, bending plus a bit of machining is usually cheapest. Above that, a dedicated die or a progressive stamping tool may pay off, but the tooling cost and lead time are real. For prototypes and low-volume runs, laser cutting or machining the blank and then bending it is the practical route.

At GreatLight we run both. Sheet metal parts are formed and then finished, and parts that need tight tolerances on machined features go onto the 5-axis or mill-turn centers. If a design mixes both, we split the operations and sequence them so the bend does not disturb a critical machined face.