BENDING

BENDING:-
Bending is a method of producing shapes by stressing metal beyond its yield strength, but not past its ultimate tensile strength. The forces applied during bending are in opposite directions, just as in the cutting of sheet metal. Bending forces, however, are spread farther apart, resulting in plastic distortion of metal without failure.

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PRINCIPLES OF BENDING-
• Any elastic material when deformed within its elastic limit regains its shape as soon as the force is withdrawn.
• But if deformed beyond its elastic limit it remains permanently in the formed shape. (Fig 1)

• When a material is subjected to bending its outer layer experiences tensile stress.
• Their length increases.
• The inner layers experience compressive stresses.
• Their length shortens.
• The plane in the material in between the outer and inner layers experiences  no stress.
• This is called neutral plane. It experiences no change in length.
• The material fibre at this plane is called neutral fibre.
• Neutral fibre represents the original length of the material before they  were subjected to bending. (Fig 2)

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The principles of bending involve:-
• Selection of material of length equal to neutral fibre.
• Stressing it beyond elastic limit.

Plastic deformation due to bending:
• The act of bending causes the portion of the material which is within the area of bend to become distorted.
• Material beyond the bend area is also slightly affected, but it can be neglected.
• The distortion is called plastic deformation.
• For illustrative purposes, the crystalline structure of the metal is represented by cubical units equal in shape and size.
• After bending the units are displaced and deformed. (Fig 3)

• Material towards the inner bend surface is under compression and the material towards the outer bend surface is under tension.
• Units outside the neutral plane are stretched longitudinally.
• Their area gets reduced.
• The units inside the neutral plane are compressed longitudinally.
• Their cross sectional area is increased.
• In the compressed part of the bend the material bulges wider than its original width.
• On the tension side the material is reduced in both width and thickness.
• Thinning of the material on the tension side is more than the bulging on the compression side.

• This is because the resistance of the material to compression is more than the resistance to tension.
• Because of this internal movement takes place on the tension side of the bend.
• This shifts the neutral plane towards the compression side of the bend. (Fig 4)

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Factors influence the bend severity.

• Increasing the stock material thickness, increases the severity.
• Increasing the bend angle increases the severity.
• Decreasing the bend radius increases the severity.

THEORY OF METAL FLOW

In an analysis of bending, it is helpful to think of the metal part to be formed as being made up of a number of longitudinal fibers enclosed in the part's cross section.
Such a part, undergoing a bending action perpendicular to its longitudinal axis, behaves according to the known laws of physics, in a predictable manner: As the part is bent, its fibers experience a distortion such that those nearer its outside, convex surface are forced to stretch. Thus, a portion of the part's cross section is put in tension and another portion in compression.

BEND ELEMENTS

Bending nomenclature.
Bending terms are defined in the following glossary:

Bend allowance
Length of the curved strip comprising a bend, measured along the neutral axis from one bend tangent line to another,
Bend angle
a. Usually, the "included" angle of the work piece.
b. Also, the angle through which a bend is performed that is, the
    supplementary angle to that formed by the two bend tangent lines or planes.
Bend radius
The inside radius of a bent section.
Bend tangent
A tangent line where the flat, straight section of the part stops and the radius of the bend begins.
Neutral Plane
The neutral plane is theoretical plane originated by inherent bending stresses. The Neutral plane occurs at a distance of 0.33 to 0.5 S,  from the inner surface.

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Neutral Axis:-

Somewhere in the cross section, a plane of demarcation separates the tension and  compression zones. The fibers lying in this plane are affected by the bending in a neutral manner, neither forced to stretch nor to compress.
This plane, situated in the cross section parallel to the surface around which the part is bending, is called the neutral axis of the part's cross section. The neutral axis is shown schematically in previous slides. Although not precisely true under all conditions; for purposes of analysis, it can be assumed that the neutral axis of a cross section coincides with the center of gravity. The location of the center of gravity for a given cross section is determined by the geometry of its configuration.
Therefore, for practical applications, the same can be said of the cross-section's neutral axis. The center of gravity of a symmetrical cross section falls exactly on its centerline, while being displaced from the centerline in the case of an unsymmetrical cross section.

Fiber Deformation

Knowing the location of the neutral axis of a part helps in analyzing the results that take place when bending occurs. It is the deformation of the part's fibers during a bending action that is significant. The outer and inner surfaces are of particular interest, and the deformation of their fibers is greatly influenced by the location of the neutral axis in the part's cross section. The extent to which a fiber distorts, whether in compression or tension, can be considered to be proportional to the fibers perpendicular distance from the neutral axis. This distance acts on the fibers as a lever, using the neutral axis as a fulcrum. Thus, the outer surface and inner surface fibers experience the most distortion, while the other fibers of the cross section are subjected to only a proportionate share.
When bent around a die, a thin part, such as a sheet, experiences little distortion of its outer fibers, perhaps not enough to reach the yield state. On the other hand, the outer fibers of an  extrusion of substantial depth, when bent around the same die, might undergo sufficient elongation to cause rupture. The reason is that the lever distances acting on the fibers of the sheet are small, while those acting on fibers of the deeper sectioned extrusion are great.

Since the neutral axis accompanies the center of gravity, in a geometrical sense, the inner fibers of an unsymmetrical extrusion, such as aT-section with flanges of unequal thickness, may have only slight compression if the part is formed with the heavy flange inward against the die. Consequently, wrinkling of the inner fibers does not occur The outer fibers, on the other hand, elongate considerably. This is because the center of gravity of the section, and hence its neutral axis, is close to the inner fibers and relatively far from the outer fibers. The lever distances, causing distortion of the inner and outer fibers, therefore are different. Such parts, having their neutral axis located very near the inside concave surface, can be formed by bending alone.
If the part were bent the other way, with the heavy flange outward, the situation would be reversed. The outer fibers might not be elongated enough to reach yield, while the inner fibers, with their long lever distances, could be caused to compress to such an extent that the inner flange would buckle, as well as wrinkle.
Thus, it is apparent that the neutral axis location is a major factor in determining metal flow and forming characteristics of a part during bending operations, and that the reactions of the inner and outer surfaces depend on the part's geometry as a key determinant for the center of gravity and the neutral axis.

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Bend allowance:-

Bend allowance is the dimensional amount added to a part through elongation during the bending process. It is used as a key factor in determining the initial blank size.
The length of the neutral axis, or bend allowance, is the length of the blank. Since the length of the neutral axis depends upon its position within the bend area, and this position is dictated by the material type and thickness and the radius and degree of bend,it is impossible to use one formula for all conditions. However, for simplicity, a reasonable approximation with sufficient accuracy for practical usage when bending is given by the following equation:

Where Lo = Developed length.
a = Angle of bend
       A + B = Unbend length or Length of straight
             Ri = Inner radius
             S = sheet thickness
            ξ = Correction factor ( to be chosen from graph )

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CALCULATION FOR DEVELOPED LENGTH

For Problems Click Here

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CALCULATION OF MAXIMUM & MINIMUM RADIUS

The work piece straightness if 'b' is less than y (yield stress) in order to obtain a permanent bend the stress on bending area must be higher than the yield point stress of the material.

In order to obtain a permanent set the stress which  occurs on bending must be higher than yield point of the material. The bellow formula therefore gives the condition for R max. Radius which produces a permanent set.

CALCULATION OF MAXIMUM & MINIMUM RADIUS

The R max value of radius to which a particular material could be bent b is just equal to y according to the previous formula,

σ y = SE / (2 ri + S)
i.e,    2 R max . + S = ( SE /σ y) - S
R max  = ( SE  / 2σ y ) - ( S / 2)
In this case S/2 can be neglected comparing to the value of R max.
Therefore,

There is also a limit for  bend radius on minimum side. If the bend radius compared with the thickness of the sheet is below a certain, the stress in the outside fiber exceeds the ultimate tensile stress therefore rupture occurs.
So, R min could be calculated by following formula.

Where,
C =Constant referred to the following table.
If ri is greater than the Rmax, no permanent deformation takes place.

Sl . No	Material	Constant
01	Mild steel	1.5
02	Deep drawing tool	0.5
03	Construction steel	2.0
04	Copper	0.27
05	German Silver	0.45
06	Brass	0.4
07	Aluminum hard	0.4
08	Aluminum pure	0.7
09	Aluminum half hard	1.4
10	Gun Metal	1.2
11	Stainless Steel	0.5
12	Brass	0.3

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Bending force requirements.

When simple flanges are air bent, forming loads are easily determined. Many load charts similar to the one illustrated are available listing loads for mild steel in various thicknesses for a range of V-die openings.
The force required to bend metal depends upon the type of material and its physical properties, thickness of the stock, length of the bend, width of the die, whether or not a lubricant is used, and the amount of wiping, ironing, or coining.
V-dies in which the punch does not bottom (air bending), commonly used in press brakes, require the least force. Bending tonnage (force) required varies directly with tensile and yield strengths of various materials.

Force Calculation

Two general types of bending are used in modern press working.
One is V-die bending, which is used extensively in brake die operations as well
as in stamping die operations. The other is wiping die bending.

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BENDING FORCE:-

FOR ‘V’ Bending dies:-

Where,
C = constant
b = sheet width
s = sheet thickness
W = width of the Die

The value of  constant ‘C’ can be takes from the graph

                                                    (or)

can be calculated using the formula,

[ The formula can be used up to W=20 S]

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FOR ‘U’ Bending dies

Where,

C = Constant
b = width of the bend
s = Sheet thickness
σ = Ultimate tensile stress
cb = Bending clearance
R1 = Die Radius
R2 = Punch radius

For Problems Click Here

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Notes:-

Air Bending
In air bending the punch does not seat fully in the die; the sheet metal, supported by high points of the die, wraps around the tip of the punch to form the bend. Air bending is a versatile operation; a large variety of parts can be made from a single set of dies. Accuracy of the parts must be closely monitored, however, because spring back is a factor.
Angular accuracy is obtained in air bending by over bending and then permitting the material to spring back to the desired angle. Depending on the material,  mayor may not be consistent. Low-carbon steels, for instance, may have widely varying tolerances that affect the Spring back consistency. An advantage of air bending is that it requires considerably less press-brake tonnage (force) to produce a given bend-four to six times less than in bottoming bending. Thus, some shops prefer air bending even if they have to rerun rejected parts to obtain the desired angle. With the air bending method, the formed angle can be specified anywhere from 180 to the included angle of the female die. The sharpness of a bend is a function of the distance between the two edges of the female die and the distance that the punch tip travels into the die.
Once a female die opening is selected, the repeatable accuracy of bending each successive piece part is determined by how consistently the punch tip penetrates the die. Variations in punch travel are particularly pronounced in forming lighter gages of sheet metal. A variation of 0.005“ (0.13 mm), for instance, while forming l6-gage (0.063"; 1.60 mm) mild steel may result in angular deviations of up to 70 when making 90˚ bends. In other words, air bending accuracy of a press brake is directly related to its ability to bring the punch tip to the same lowest point repeatedly during each stroke.

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Bottom Bending:-
Bottom bending and coining form bends by letting the punch penetrate the female die as far as the dies and the formed material will permit, Generally, bottom bending results in more consistently accurate parts than air bending. Furthermore, a radius smaller than metal thickness can be obtained with bottom bending and coining.
To help overcome spring back, the clearance between the punch and the die is set slightly less than the material thickness. The resulting coining action counteracts the spring back, provided a sufficient dwell time at the bottom of the stroke is used to allow lhe material to make a compressive shift. While bottom bending results in consistent part quality, three to five times the press brake tonnage is required to produce a given part in comparison to air bending. Furthermore, to avoid damage from overloading (particularly with press brakes using mechanical drives), clearances between punches and dies must be set very carefully: If the clearance is too loose, reject parts will be formed 'if the clearance is too tight, full length overloading may occur. In practice, a press brake usually is set up with material thickness clearance between the upper and lower dies.
For this reason, bottom bending should' be used only where it is really needed-in applications requiring a high degree of accuracy and sharp corners. For example, metal furniture, cabinets, and partitions usually require bottom bending. Because of the higher tonnage (force) requirement, bottom bending is generally limited to bending steel that is no heavier than 12 gage (0.109"; 2.77 mm).

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SPRING BACK:-

In bending operations the elastic limit of the metal in process is exceeded but is ultimate strength is not.

Therefore some of the original elasticity of the stock material will be present on the material after bending operation is over.

Because of this when the force (punch) is withdrawn the material on the compression side of the bend, tends to expand slightly and the material on the compression side of the bend tends to expand slightly and the material on the tension side is tends to contract.

The combined result is that the work piece tends to resume its original shape.

This causes the bend to spring open a small amount.

This reaction of the material is called spring back.

The spring back varies according to the thickness, type and condition of the  stock material.

It also varies directly in proportion to the size of the bend radius. The larger the bend radius the greater the spring back.

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HOW TO AVOID SPRING BACK?

4 METHODS
to
OVERCOME SPRING BACK
in
‘V’ BENDING DIES

1. Over Bending
2. Corner Setting
3. Offset Punch Method
4. Angular Punch Relief

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1. Over bending IN ‘V’ BENDING:-

Over bending is the simplest way to correct spring back. It is done by making the punch angle (angle m) smaller by the required amount.

For soft steel, Brass, aluminum or copper spring back  =  0 to 18
For ¼ to ½ hard material   = 1 to 58
For hardened material  = 12 to 158 or more

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2. Corner Setting on ‘V’ Dies:-

This is the most effective way of avoiding the spring back. This is a method of eliminating spring back than making compensating allowances.

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3. Offset Punch Method:-
The face of the punch is offset in order to achieve coining penetration in the bend area. Offset dimensions should not be made unnecessarily deep, as this can weaken the piece part. An offset depth of 5% of ‘S’ is normally used.

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4. Angular Punch Relief:-
An angular differential is provided between the included angle of the punch and the included angle of the die opening.

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5 METHODS to
OVERCOME SPRING BACK
in
‘U’ BENDING DIES

1.Convex pad method.
2. Punch sidewall relief - Angular
3. Punch sidewall relief - straight under cut.
4. Over Bending
5. Corner setting

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1. Convex pad method:-
1.For compensating spring back, over bending can be used in u bending.
2.In the convex pad method the spring back in another area is utilized.
3.The web (bottom) of the work piece is formed.
4.The elastic limit of the stock material is not exceeded.
5.When the work piece is removed from the tool the web spring back to a flat plane.
6.This causes the bend legs to pivot inward around the bend axis.
7.This compensates for the spring back of the bend legs.
8. For large work pieces punch side wall relief method is used.

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2. Punch sidewall relief:-
• For compensating spring back, over bending can be used in u bending.
• In the convex pad method the spring back in another area is utilized.
• The web (bottom) of the work piece is formed.
• The elastic limit of the stock material is not exceeded.
• When the work piece is removed from the tool the web spring back to a flat Plane.
• This causes the bend legs to pivot inward around the bend axis.
• This compensates for the spring back of the bend legs.
• For large work pieces punch side wall relief method is used.

There are two methods
1.Punch side wall relief-angular.
2.Punch side wall relief -straight undercut.

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4. Over bending:-
• Over bending is the simplest way to correct spring back.
• It is done by making the punch angle (angle B) smaller by the required amount.
• For soft steel, brass, aluminum or copper spring back 0 to 1º for 1/4 hard 1/2 hard materials 1 to 5°.
• For hardened material 12 to 15° or more.
• These values are only approximate values, because many variables influence the spring back.
• Correct values can be found out only by experiment.

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5. Corner setting:-
• The face of the punch is offset in order to achieve a coining penetration in the  bend area.
• If offset dimensions are unnecessarily deep, it will weaken the piece part.
• An offset depth of 5% of sheet thickness is normally used.

• This is the most effective way of avoiding spring back.
• This is the method of eliminating spring back than of making compensating allowance.
• Setting is accomplished by coining (squeezing) the stock material at the bend area.
• The coining effect causes additional compressive strain within the material.
• The extra compression strains overcome the spring back tendencies.
• Since the set is made in the bend, the practice of setting in bending operation is referred to as comer setting.

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HOW TO STRIP THE BENT COMPONENT?

U bending operation requires two opposed stripping actions.

1.Stripping the work piece out of the die opening.
The pressure pad performs this function.

2. Stripping the work piece from the punch.
1.Positive knockout offs actuated by the knockout system of the press
2.The work piece is knocked off the punch.

SPRING ACTUATED PLUNGER:-

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HOOK STRIPPERS:-

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POSITIVE KNOCK OFF:-

EFFECT OF GRAIN DIRECTION:-
The most favorable condition exists when the axis of the bend is perpendicular to the grain direction.
The most reverse bends practical for the type of material can be made in this direction.
The least favorable condition exists when the axis of the bend is parallel to the grain direction.
The ability of the material to withstand bending strain as the angle approach 908.

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EFFECT OF BURR SIDE:-
It is undesirable for the burr side to be located in the outer surface of the formed piece part, because the burr drags around the bend radius and into the die opening.

This causes excessive wear in the die members. If the piece part is loaded such that burr is located on the inner surface of the formed piece part, the burr will face towards the punch.

Since there is no drag between the work piece and the punch, burr cannot erode the punch.

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BENDING IN PROXIMITY TO PIERCED HOLES:-
Holes pierced before bending will be distorted if they are very close to the bend area. As a rule distortion will be minimized if the distance P is held to minimum of 1.5 s

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BENDING DIES

BENDING DIES
1. ‘V’ Bending Dies
2. ‘U’ Bending Dies
3. Multiple Bending Dies
4. ‘L’ Bending Dies

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‘V’ BENDING DIES:-

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‘U’ BENDING DIES:-

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‘L’ BENDS ON PRESSURE PAD DIES:-
‘L’ Bends are produced in V dies. They are also produced in pad type dies. An L bend is one side of U bend. Since the other leg of U is missing, self-equalizing qualities of U bend are not available.

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Rotary Bending
Traditionally, press brake bending has been performed in one of three ways: with a V-die, with a wiping die, or with a U-die. In recent years, however, another (patented) method, rotary bending, is gaining acceptance. The main advantage of rotary bending is that it significantly reduces the force required to perform bending.
The rotary bending design eliminates the need for any type of hold-down pad or device. It provides its own  inherent holding action at the same time the bending operation is proceeding.
.

The rotary bender is comprised of three components: the saddle (punch), the adjustable rocker, and the die anvil. The rocker is cylindrical in shape with an 88˚ V-notch cut out along the length. The edges of the rocker jaws are flatted and radius to minimize marking; Three stages of a rotary bender operation are illustrated in Fig. In view a, the material is clamped and the rocker rotation has begun; view b shows that humping is controlled and limited to space between edges of the rocker; and view c shows how the rocker clamps the work piece in position and Over bends it sufficiently to allow for spring back. The primary application for rotary benders is in progressive dies. Z-bends and short leg bends can be made in a single operation; and where needed, dart stiffeners can be rolled into the work piece at the same time it is being bent.

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Draw backs of ‘L’ Bending:-
The two problems encountered are:
The one sided lateral thrust imposed upon the punch.
The work piece tends to pullout of the die opening.

‘V’ BENDING IN PRESS BRAKES:-

PRESS BRAKES OPERATIONS:-

MATERIALS FOR BENDING   HARD     Avery stiff; springy, cold rolled strips intended for flat work, where ability to withstand cold forming is not required.

HALF HARD

A moderately stiff cold-rolled strip suitable for limited bending. Right angled bends may be at 90 to the grain direction around a radius equal to thickness

QUARTER HARD:-
A medium soft cold-rolled strip suitable for limited bending, forming, forming and drawing. May be bent to 180 across the grain and to the 90 parallel with the grain and a radius equal to the thickness.

SOFT:-

A soft, ductile cold rolled strip suitable for fairly deep drawing operations where surface disturbances such as stretcher strains are objectionable. Strip of this temper is capable of being bent flat upon it self in any direction.

DEAD SOFT

A soft ductile, cold rolled strip produced without definite control of stretcher and fluting it is suitable for difficult draw applications where such surface disturbances may be tolerated. It is suitable for bending flat upon itself in any direction.

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‘V’  BENDING TOOL:-
In ‘V’ bending tool the shape of the punch and dies is in the form of the letter ‘V’.  It is used to produce the ‘V’ bending in the components.

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U BENDING TOOL:-
U Bending Tool is shown in Figure
The component is located in the nest.  When the punch enters the die, the shedder acts as a pressure pad and supports the component during bending
.  Once the bending operation is over and the punch withdraws from the die, the compression spring pushes the shedder up.  The component is thus ejected.
Figure

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Top plate:-
     This is the plate to which the punch holder or punch is held.  Top plate also holds the shank and guide bushes.

Bottom Plate
     It is the palate on to which the die and pillars are fitted

Bending Punch and Bending die
     These are the bending elements of the tool.  These are made of High Carbon steel and are hardened and tempered to 55-58 HRC

Locating Plates
     The component to be bent is located by means of locating plates or nest pins depending upon the shape of the component

Guide pillars and Guide bushes:
     To achieve a well guided movement of the moving parts with respect to the fixed parts, the guide pillars and bushes are used.

Shank
     It is the part which connects the moving half of the tool to the press  ram.

Screws and Dowels
     The main fastening member which holds the plates together is the screws.  The locations of each plate accurately is done by the dowels.  The number and size of screws and dowels depends upon the size of plates and load on them