Press Bending Data

Springback and Bend Allowance in Press Bending

Every bend gives part of its angle back when the load comes off, and the blank has to be developed for the length the bend actually takes. This page collects the neutral axis rules behind bend allowance, the factors that decide how much a bend springs back, and the die methods that hold the angle.

Bend allowance and the neutral axis The exact bend allowance is the arc length of the true neutral axis, and the neutral axis itself can only be approximated, so practice sits on a set of working assumptions.

Case	Working assumption
Inside radius below twice the stock thickness	The neutral axis is assumed to sit 1/3 of the stock thickness from the inside radius of the bend.
Inside radius of twice the stock thickness and above	The neutral axis is assumed to sit 1/2 of the stock thickness from the inside radius.
Range reported by many experts	The true neutral axis moves between 0.2 and 0.5 times the stock thickness from the inside radius.

Metal is stretched above the neutral axis and compressed below it, and the position of the axis moves with the bending method. A bend made by a tightly wiped flange needs less metal than an air bend on a press brake, because wiping stretches the metal.
The exact length of a bend is determined by trial and error, which is also why close tolerance stampings are developed on the press rather than on paper.

Bend angle	Coefficient	How it is used
90 degree bends	1.57	The number of radians in 90 degrees, multiplied by the assumed neutral axis.
Bends at other angles	0.0175 per degree	Multiply the number of degrees of bend by 0.0175, the number of radians in one degree, and use the result in place of 1.57.

Springback Springback is elastic recovery, and it is caused by residual stress left in the metal by cold working. In a simple bend the inside carries residual compressive stress and the outside carries residual tensile stress, and when the pressure is released the metal springs back until the residual stress forces are balanced by the stiffness of the material.

What stiffness means here

Stiffness is a function of the modulus of elasticity, which is why mild steel, with a high modulus relative to its tensile strength, springs back less than a material with a lower modulus at a comparable strength level.

The amount of springback is difficult to predict. For close tolerance work the figure for a specific material and forming method is developed under actual production conditions, and if the compensation is left out of the die it is corrected later by the repair facility. The springback behind a large wipe steel clearance can run to several degrees or more.

요인	Why it increases springback
Higher material strength	More stress stays locked in the bend.
Thinner material	Less section to hold the bend.
Lower modulus of elasticity	The material recovers more of the strain when the load comes off.
Larger die radius	A longer arc stores more elastic energy.
Greater wipe steel clearance	The bend is worked less, so less of the springback is taken out in the tool.
Less irregularity in the part outline	A flange that is irregular in outline or surface contour holds a slight springback.
Flatter surface contour	The same effect along the length of the bend.

The three ways a bend is held to angle The tool either bends past the angle, presses the bend to the bottom, or wipes the flange against a controlled radius.

Overbending

The most common method is overbending. The material is bent beyond the desired angle and allowed to spring back to the target. Air bending, the simplest operation of the three, needs the least tonnage for the work it does and depends on exact repeatability of ram travel, so the amount of over travel is set experimentally.

Overbending alone does not hold a bend in a material that varies, which is where the two methods below come in.

Coining

Coining, also called bottom bending, brings the whole thickness of the metal up to the yield point in the bend area. It produces sharp, accurate bends with less sensitivity to material conditions than air bending, and it removes the residual stresses that cause springback in the first place.

The price is force and die wear. Coining may need five to ten times the tonnage of simple air bending, machine deflection grows with the tonnage, and the die wears faster. Where a bend has to be accurate along its whole length, the ram adjustment may not be enough and the bed has to be shimmed. The tool steel for a coining die is a shortlist of its own, and it is set out on the coining die selection page.

Wipe flanging and the improved method

The common method of holding springback in a wipe flange is to coin the top of the bend with the flange steel. Its limit is the compensation it can carry, which leaves a distorted bend angle in some work. The coining also leaves a score mark that can weaken the stamping, and if the pressure is pushed too far the metal at the top of the bend extrudes into a weak and distorted condition.

The better method relieves the radius in the flange steel so it never touches the top of the bend radius. One way is to relieve the flange steel at about 20 degrees tangent to the radius. Another is to machine it to a radius larger than the outside of the bend. The flange steel is then positioned so the tightest point falls 45 to 60 degrees beyond the top of the radius, and the side of the form steel is relieved 5 degrees or more so the material can be overbent. The result is more tolerant of press adjustment and of material variation than coining the top of the bend.

What makes the bend angle move Once the tool is right, the remaining variation comes from the press, the stock and the room.

원인	What it does to the bend
Changes in stock yield strength	A different lot bends to a different angle on the same setting.
Variation in stock thickness	Thickness moves the neutral axis and the tonnage together.
Machine variation from temperature changes	The press is not the same machine at the end of a long run.
Machine deflection	Most visible on long press brake beds, where the middle of the bend gives more than the ends.

Compensation

A change in any condition that affects the bend angle is normally answered with a ram travel adjustment, and press brake bed deflection is answered with shims. Some press brakes carry automatic deflection compensating devices such as hydraulic cylinders built into the bed. Where high force is required because of thicker or harder stock, a simple ram adjustment may not be enough.

Forming die notes from the same chapter The forming side of press tooling is designed around the same elastic behaviour, and the source gives a short set of rules for the tool itself.

The radius being formed should be at least twice the metal thickness, to keep stress concentration out of the part.
The form blocks are made wider than the workpiece, and their height follows an approximate ratio of 1 to 1.5.
A solid forming die puts a great deal of side pressure on the female die block. One option is a female block made from two pieces of tool steel held together by prestressed tie rods, which also protects the tool if a double blank is fed. A relief under the center of the block transfers much of the tensile strain to the more robust bottom.
Where coining pressures are reached, running the die in a hydraulic press limits the pressure applied to it.
A good polish on the form blocks protects the stamping surface, and the surface roughness chart covers the finishes the work is measured against.
The punch is made of a good grade of tool steel, heat treated for toughness. The source names S5 or S7 as good choices for severe work, and the S7 page covers the grade in detail. The forming and bending family as a whole is collected under forming, drawing and bending dies.

Hole flanging is the same problem at 90 degrees

A flange formed around a previously pierced hole at 90 degrees is a stretch flange at that angle, and the pierced hole size is calculated with the same principles as a 90 degree bend. A flange stretched more than 2.5 times the metal thickness in height can split. Burring the edge of the hole before the extruding operation helps, and coining the hole edge is a good way to strengthen it, particularly in progressive die work.

Which steel the tool calls for

Bending and forming dies are chosen on the same basis as the rest of the press tooling, with wear on one side and toughness on the other. The shortlist for this class of tool is on the bending and forming die selection page, and the wider family is collected under cold work tool steels.

Confirm before quoting

These figures are for general reference only. Springback and bend allowance move with the material, the tooling and the press, and the source recommends trial and error for close tolerance work. Confirm against your own trial or contact Aobo Steel.