CNC folding machine basic knowledge

• 1
  How a CNC folding machine actually forms an angle
• 2
  Springback is the error you have to plan for
• 3
  Bend radius, die opening and minimum flange
• 4
  Where folding stops and machining takes over
• 5
  Choosing between air bending, bottoming and coining
• 6
  Air bending vs bottoming vs coining

A press brake has three moving relationships: the punch moves down, the sheet sits on two die shoulders, and the material between them is forced to yield. The angle you get is set by how deep the punch enters the die, called the bend depth. On a manual brake the operator watches the part and adjusts. On a CNC folding machine the controller calculates that depth from the material, thickness and target angle, then commands the ram to a position.

The bending itself is plastic deformation. Steel yields at roughly 60% of its tensile strength, aluminium at a lower fraction, so the force needed differs by material even at the same thickness. Bending a 2 mm 304 stainless sheet takes far more tonnage per meter than bending 2 mm 6061 aluminium, and the tooling has to handle that load without deflecting.

Every metal springs back a little after the punch lifts. Elastic recovery happens because the outer fibers were stretched past yield while the neutral axis stayed elastic. Carbon steel at 90° typically springs back 1° to 2°. Stainless 304 can recover 2° to 4°. 7075 aluminium and titanium alloys go higher, sometimes 5° or more, and thicker sheet adds to it.

The two ways to fight it are overbending and bottoming. Overbending means the controller asks for 92° to get 90°. Bottoming presses the material against the die walls so the springback is largely removed at the cost of much higher tonnage. Coining, the extreme version, forces the material to take the full tool shape; that needs four to eight times the air-bend force and usually marks the surface.

Real-time angle measurement changes the picture. A laser or contact sensor reads the bend as it forms and the control adjusts depth on the next stroke. That closed loop is why a modern machine can hold ±0.5° on carbon steel without an operator tweaking every part. The material still varies from heat to heat, so the first-off part usually gets checked, not trusted.

The inside bend radius is not something you pick freely. In air bending it settles at roughly 16% of the die opening for mild steel, so a 12 mm V-die in 3 mm plate gives an inside radius near 2 mm. Ask for a tighter radius than that and the punch starts to coin the material, which raises tonnage and can crack high-strength alloys.

Die opening itself follows a rule of thumb: 6 to 8 times the sheet thickness for mild steel, 8 to 10 times for stainless. Narrower dies cut the minimum flange length, which matters on small brackets. The minimum flange is about 70% of the die opening in air bending. Go below it and the part tilts into the die instead of sitting flat.

Bend relief and hole distance also belong in the design review. A hole placed closer than 2.5 times the material thickness to the bend line will distort. Slots need more clearance than round holes. These are geometry problems the software can catch before a blank is cut, and catching them there is far cheaper than catching them at the brake.

Sheet metal folding gives you a constant wall thickness and a fast cycle. It cannot give you a pocket, a threaded boss, a tight bore or a true 3D contour. Those features come from milling or turning. A sensor housing, for example, often starts as a folded shell and then gets its mounting holes, sealing face and connector thread cut on a 5-axis machine.

There is also a thickness ceiling. Press brakes handle 0.5 mm to around 6 mm comfortably in a shop setting; beyond that tonnage and tooling cost climb fast, and thick plate is usually better cut and welded or machined from solid. Very thin foil below 0.3 mm is a different problem: it wrinkles and needs special tooling.

Corner quality is another boundary. A folded corner is a bend, so it carries a radius. If the design calls for a sharp 90° external corner with a crisp edge, folding will not deliver it. Say so early, because the fix is either a welded corner that gets dressed, or a machined part.

GreatLight runs folding alongside 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and a 4,000 mm maximum processing size. That mix lets us fold the shell and machine the critical features to ±0.005 mm without shipping the part between suppliers.

Air bending is the default. It needs the least tonnage, works with one punch and die set across a range of angles, and modern controls hold it well. The trade-off is that the angle depends on material consistency, so springback compensation has to be right. For most brackets, panels and enclosures, air bending is the correct call.

Bottoming suits parts where the angle must not drift across a long run, and the tonnage is available. The die angle is machined to match the target, the punch bottoms out, and the angle repeats. It costs more in tooling and force. Coining is rare outside of specific high-volume jobs because the tonnage and the surface marking are hard to justify.

Material choice pushes the decision too. 5052 and 6061 aluminium bend cleanly in the soft tempers; 7075 in T6 will crack at a tight radius and needs a larger die opening or a pre-bend anneal. 304 stainless work-hardens as it bends, so a slow ram and a generous radius help. Titanium and Inconel need even more care, and we usually run a trial blank before committing to a setup.