BENDINGSTEEL SHEET ANDSTRIP PRODUCTS

TECHNICAL BULLETIN TB-F4Rev 2, November 2003

This issue supersedes all previous issues

FORMING

1 BENDING OPERATIONSSimple straight bending is probably the mostcommon operation used for forming shapesfrom flat steel sheet. However, the type ofbend can range from a single slightdeformation to multiple bending of aworkpiece in one or more directions.

Types of machinery used are:

• Pan brakes

• Electro Magnetic folders

• Press brakes

• Punch presses

• Automated Bending Systems

• Roll-formers

Using special tools, sheet metal workers canmanually produce neat bends in steel sheet ina particularly skillful manner.

The choice of device or method usuallydepends on quantity and complexity of thework to be done as well as on machineryavailability. Given a choice, punch presses areused when close tolerances are to be met,parts are small, quantities are large or whenother operations such as notching can becombined. Press brakes are ideal for bendingrelatively long, narrow parts and for smallerquantity jobs. Pan brakes can be used similarlyto press brakes but can achieve selectivebending not possible with press brakes.

Forming in a pan brake is a slow, labourintensive process but has very little set-up costso it is suitable for smaller and single piecesand for small production runs.

Large quantities and long length product willdictate roll-forming (refer to Technical BulletinTB-F5 Roll-Forming of Steel Strip) as the bestmethod of forming.

Details of capacities and capabilities of thevarious pieces of equipment are readilyobtainable from equipment manufacturers orsuppliers for calculation of costs to enable adecision on the forming method to be used.

2 FOLDINGIn a folding machine, sheet is clamped alongits entire length and a folding apron ismanually pivoted to deform the sheet to therequired angle (Figure 1).

The bending and clamping edges of theequipment can consist of fingers or segmentsin varying widths to enable bends to be madeat right angles to each other for making boxor pan shapes. The latter gives the name panbrake to this type of equipment.

A more recent addition to this type of foldingequipment are electro magnetic folders. Thesefolders provide great folding flexibility for thejobbing shop, with the profiles achievablebeing almost infinite. The versatility can beattributed to the fact that the clamp bar ismagnetically clamped to the material beingfolded, hence folding profile depth isunlimited compared to the conventional panbrake folders which are restricted by theirmechanical clamping structure.

Clamping may be a problem for close bendsand the length of the product is limited to thatof the folder. Accuracy of the finished articledepends largely on the skill and care of theoperator.

Figure 1 – Electro Magnetic folder orPan brake diagrammatic

3 PRESS-BRAKE FORMINGPress-brake forming is a process in which ablank or workpiece is placed over a straightopen die and is deformed down into the dieby a punch actuated by a ram. There isprobably no more widely used machine in thesheet metal industry than the press brakeunless it is the guillotine shears necessary toprepare the feed for it.

The press brake is commonly used forforming relatively long, narrow componentsthat are difficult or impossible to press form,and for applications in which productionquantities are too small to warrant contourroll-forming tooling costs.

Press-brake forming normally produces onebend per ram stroke but multiple bends arepossible with special tools. The length ofproduct is limited by the machine itself tonormally about 4 m but machines have beenbuilt for lengths of 10 m. The latter presswould typically be of 600 tonnes capacity. It isnot uncommon practice to mount two pressbrakes in tandem, exercising great care inensuring that they are both accurately aligned.

The versatility of the press brake isdemonstrated in that it can be used forproducing members with varying cross-sectionsuch as tapered members. Cold roll-formingmethods can generally only producecomponents with a constant cross-section.

The main advantages of press brakes areversatility and with computer control, the easeand speed with which they can be changedover to a new set-up.

4 PRESS BENDINGPower presses (of the inclinable C-type) areused for bending steel sheet in themanufacture of parts where productionquantities are large and where part size isrelatively small. Apart from the size and speedof the bending operation and sometimes theorientation of the dies, this process has manysimilarities to bending in a press brake. Thesame types of materials are worked and thesame allowances made.

There are several basic types of powerpresses. Equipment suppliers should beconsulted for advice on whether hydraulic ormechanical presses are best suited and, if thelatter, whether a direct drive or a geared driveis necessary.

5 BENDING METHODSBends can be accomplished in a number ofways. In the most common method, a flatsheet is forced by a top tool (or punch or knife)into the opening of a bottom tool or die(Figure 2). In another method used forproducing bends near the edge of a sheet andfor forming channels, a blade forces sheetaround a radiused bend with a wiping action(Figure 3).

Figure 2 – Simple V-bend

Figure 3 – Wipe bend

5.1 V-BendsThe angle of the bend can be varied by theamount of penetration of the top tool or thesize of the opening, W, in the die (V-notchopening in Figure 4).

Figure 4 – V-notch opening

The opening, W, must suit the thickness andgrade of steel being formed. For normalcommercial grades (with a yield strength of220-350 MPa) used in press-brake forming, apractical rule of thumb is:

Equation 1W=12 – 15t

Where t = thickness of the sheet being formed.

An excessive V-notch opening will give badlyshaped bends and a gradual rather than a neattransition from bend into flat surface. Variablespring back is also more likely to occur. Toosmall a V-notch opening requires a greaterbending force and the edge of the openingmay mark the workpiece surface or causeexcessive edge wear. However, a smaller thanusual V-notch opening is sometimesdeliberately used to achieve a short lip on theedge of a sheet.

The two most common methods of formingare air bending and hard bottoming. Whichmethod is used depends largely on the natureof the final product and the accuracy required.

punch

die

punch

Hold down

W

5.1.1. AIR BENDINGIn air bending, the sheet surface is touched bythe tools at three points only (the punchsurface and the corners of the open die)throughout the whole operation.

Figure 5 – Air bending in a press brake

The shape and nose radius of the punch arevaried to suit the workpiece. The requiredangle is produced on the workpiece byadjusting the depth to which the punch entersthe die opening. This allows the sheet to bebent beyond the design angle to produce thecorrect profile after spring back.

When the metal is deformed plastically (thatis, the punch force exceeds the yield strength ofthe steel) the radius formed bears a definiterelation to the opening in the die. A small dieopening produces a small radius. The use of alarge die opening increases the radius butcompensation must be made by overbendingfor increased spring back. Changing the sizeof the die opening also changes the amount offorce needed to make the bend. As the dieopening is increased, less force is required.Conversely, as the die opening decreases,bending leverage is less and thus more forceis required.

Figure 6 – Variation of C1 factor for V-Bends with W/t ratio.

The press size to handle single bendingoperations can be calculated from Equation 2:

Equation 2Required capacity

C1 Rm L t2F = ––––––––– x 10-3

W

where F = bending force, kN

C1 = constant for die width/steel thickness combination (Figure 6)

Rm = yield strength of steel, MPa

L = bend length, mm

t = steel thickness(steel base if coated sheet), mm

W = V-notch opening, mm

This value is only for air bending in which therequired shape is reached without the sheetreaching the bottom of the V-notch. Therequired angle is obtained by adjusting thedepth to which the top tool enters the bottomtool. Air bending is widely used as it canallow for the degree of overbending necessaryto compensate for spring back which isinevitable in steel sheet. The degree of springback must be checked by trial and error onprototypes before production runs commence.

A further advantage of air bending is theability to produce a great variety of bendangles with fewer tools as the angle on theworkpiece, the top tool angle and the V-notchneed not be matched.

5.1.2. BOTTOMING AND COININGTwo alternatives to air bending are bottomingand coining in which a top tool, workpieceand V-notch are in solid contact when thebend is completed.

Figure 7 – Bottoming and Coining

Both methods are used for achieving greaterbend accuracy than is achievable with airbending. Bottoming is more commonly usedas it requires less pressure than coining. The Vwidth for bottoming is 6 - 12t. Coining, whichuses high surface and bite by a sharp punchto control springback, uses a V width of 5 - 6t.

punch

die limit of stroke

1.63

1.50

1.38

1.33

1.301.28

1.24

1.20

6 7 8 10 12 14 16

C1

FAC

TO

R

WIDTH/THICKNESS COMBINATION

punch t

W

t t

90º 90º

V = 6–12 x t V = 5–6 x t

Bottoming Coining

These methods produce more accurate bendswith consistent spring back and neat transitionfrom bend to flat.

Inherent disadvantages with these techniquesrestrict their use. These include:a) The top and bottom angle must be

accurately matched so more tools are required for the same work range compared to air bending

b) Great care must be taken in tool alignment and penetration setting. Insufficient penetration may cause inconsistent shape; excess penetration may damage the press brake

c) Steel thickness must be constant. The use of different thicknesses has the sameeffect as varying penetration settings

d) Press capacity is required to be up to five times greater than for air bending.

5.2 Wipe BendingIn wipe bending (Figure 3) a wiping diebends the sheet without any kick-back ofmaterial in front of the press. Otheradvantages are better accuracy and safety.Tooling is more expensive and requiresadjustment for different thicknesses and typesof material.

The bending force required for wipe bendingwork must take into account punch/sheetsurface and punch/die friction. This isestimated to be 20% of the bending force.When a hold down is necessary, the forcerequired is assumed to be 50% of the bendingforce so the cumulative sum of forces requiredis 1.7 x bending force.

Equation 3F1 = 0.17 Rm L t x 10-3

F2 = 0.17 F1

where F1 = bending force (without allowance for friction), kN

Rm = tensile strength of sheet, MPa

L = length of bend, mm

t = thickness of sheet, mm

F2 = total forming force, kN

Examples of tooling for channels areillustrated in Figure 8 where at least twooperations are necessary. A spring actuatedpressure pad and ejector are normal fordimensional accuracy and to assist in removalof the formed piece.

Figure 8 – Wiping dies for channels

5.2.1 SIMPLE CHANNELThe force (F1) required to form the simplechannel in Figure 8a has been given as twicethat for a single wipe bend. In addition, anallowance (F2) must be made for friction,taken as 50% of the bending force, plus anallowance (F3) equal to the total tensilestrength of the steel sheet for the strippingpad force (Equation 4).

Equation 4

F1 = 0.3 Rm L t x 10-3

F1F2 = ––––2

1.3 Rm L t2F3 = –––––––––– x 10-3

W

F4 = F1 + F2 + F3

where F1 = forming force, kN

Rm = tensile strength, MPa

L = length of channel, mm

t = steel thickness, mm

F2 = force allowance for friction, kN

F3 = force allowance for pressure pad, kN

W = die width, mm

F4 = total force, kN

punch

punch

pad

pad

die

die

(a) Simple channel

(b) Winged channel

5.2.2 WINGED CHANNELForming a winged channel requires additional force:

Equation 5

F1 = 2 Rp L t x 10-3

F1F2 = –––

5

2RP L t2

F3 = –––––––– x 10-3

3W

F4 = F1 + F2 + F3

where Rp = yield strength of steel sheet, MPa

There is a rapid increase in forming force on awinged channel if bottoming of dies isallowed to occur in the flanges. No allowancehas been given for this eventuality.

6 DIE DESIGNA considerable range of standard tools isavailable and suppliers of press equipmentshould be consulted for stock designs beforespecial tools are considered.

Off-set or cranked dies enable more complexshapes to be formed. In forming channelswith standard tools of the types illustrated inFigure 9, it is necessary that the channel legbe a length equal to at least a half of the dieopening.

Figure 9 – Shapes produced with off-set dies

6.1 Special ShapesOther tools are produced to make specialshapes both as single tools or combinationtools (Figures 10 and 11).

Figure 10 – Single tool set for hemming

Figure 11 – Combination curling tool

6.2 Combination ToolsCombination tools increase the capacity of apress brake at a cost which must be balancedagainst the improvement in productivity.

Figure 12 shows a wiping die where rightangle and return bends can be made quicklyand accurately.

Figure 12 – Wiping the die for return bends

punch

first stage

second stage

die

third stage

First stage Second stage

punch punch

die die

6.3 Tandem Press BrakesPress brakes can be mounted side by side toenable extra long lengths of bend to be made.Accuracy of alignment is essential to ensureadequate quality of work.

6.4 Flexible DiesElastomer type material can replaceconventional multiple dies by using a simpleunderground 50 mm V-block. This deformsunder load, matching the shape of the punch(Figure 13 & 14). This versatility eliminates thehigh cost of the standard multiple die blockform and the cost of repeated and sometimesfrequent tool changes.

Important advantages with this die material are:a) Extreme accuracy without additional

tooling costs since every bend carried out is, in principle, a coining operation (the material will follow exactly the formof the top tool)

b) Ability to provide formed products free from tool marks

c) Production of a short flange with a 90°angle (Figure 15)

d) If any part of the die becomes damaged,the damaged section can be cut out and replaced with a new section without impairing bending performance.

Figure 13 – Flexible die

Figure 14 – Varying thickness steel formed using a flexible die

Figure 15 – Short flange produced with flexible die

Disadvantages are:

a) Flexible dies are generally expensive butthe economics of the whole situation should be discussed with equipment suppliers

b) A larger capacity machine is necessary for the same job (two to three times that for air bending).

7 PRECAUTIONSProducts from a correctly operated press brakeare accurate and consistent provided machinesand tools are kept in the best possible conditionto maintain close dimensions. General purposetooling is seldom built for precision work andfrequently is given hard usage. Uneven wearaggravates quality of work.

It is general practice for press-brakes to bendthe legs of formed sheet upwards when thepunch or blade forces the sheet downwardsinto the die. If a wide sheet is being formedthe inertia of the sheet mass causes a kink toform near the bend. This is called kick-backor whip-up, which is undesirable in a panelwhere surface quality is important. There areseveral possible remedies:a) Adjust the press-brake folding speed so

that the sheet will not swing upwards too quickly.

b) Lift the wide leg of the sheet at the moment of deformation to overcome theinertia of the sheet.

c) Use a wiping die if a bend is formed near the edge of a sheet, for example,a flange (Figure 12).

8 MATERIAL CHARACTERISTICS AND ALLOWANCES

8.1 MaterialsA wide range of sheet steels can be formed ina press-brake. Limitations are the ductility ofthe metal and the deforming force required.Generally minimum bend radii necessary for asatisfactory bend increase with a larger angleof bend and thicker steels.

Rolling direction and edge condition areamong several factors which have an effect onforming characteristics.

punch

punch

punch

punchpunch

8.1.1 ROLLING DIRECTIONThe formability of steel sheet varies with thedirection for the axis of bend. Sharper bendscan usually be made across rather thanparallel to the rolling direction (generally thelong dimension) of the sheet withoutincreasing the probability of cracking. Thisoccurs because of the inherent metallurgicalcharacteristics of steel sheet.

Figure 16 – Orientation of bends

Orientation of bends with respect to themetallurgical grain structure affects not onlythe severity of the bend that can be made butalso the service life of that bend undervibration and fatigue conditions (Figure 16).

This difference is not so evident in BlueScopeSteel Limited ductile steels compared to someof the high strength steels.

8.1.2 EDGE CONDITIONExcessive edge burr and work hardening arenot conducive to good bending performanceof steel sheet. The best solution is thecomplete removal of distorted steel but if thisis not practical the sheet should be turnedover so that it is formed with the burr on theinside of the bend. This is necessary as theburr will crack when stretched around theoutside of a bend, act as a stress concentrationpoint and initiate failure into the base metal inthe bend itself. This effect is worse in thinnersheet, sheet of reduced ductility and in narrowstrips less than eight times the sheet thicknessin width.

8.2 Bend Allowances(not applicable to stretch bending)

Most modern day fabrication workshopspossess CAD (Computer Aided Drafting)packages which readily compute blank sizesdirectly from either 2 or 3 dimensionaldrawings of sheet metal parts. However, in the

event that a blank size needs to be accuratelydetermined manually, the following approachmay be taken.

To determine the size of the blank needed toproduce a specified formed part, the blankdimension at 90° to the bend axis can bedeveloped on the basis of the dimensionalong the neutral axis or line whichtheoretically does not change its length duringthe bending operation. As a general guide,this line can be taken at a distance of at infrom the surface of the inside of to the bend.

Figure 17 – Neutral axis at a distance of at fromthe inside surface of the bend

Equation 6

R R Ra = [√ –– (1+––)]– ––t t t

where R = inside radius of bend, mm

t = metal thickness, mm

For values of a for some specific R & tcombinations (expressed as R/t) see Table 1.

Table 1 Values of a (ratio of distance of neutral axis from the inside bend surface to the metal thickness) for specific R & t combinations.

R/t 0.5 1 2 3 4 5 10 20

a 0.366 0.414 0.449 0.464 0.472 0.477 0.488 0.494

The width of strip required to make a givenshape can be determined by adding togetherthe lengths of the straight components and thelengths of each bend.

The length of each bend (L) can bedetermined from the formula:

Equation 7

L = 0.01745 u(R +at)

where u = angle of bend, degrees

t = metal thickness, mm

R = inside radius of bend, mm

a = ratio (refer Equation 6)

(a) Better formability - transverse bend

(b) Reduced formability - longitudinal bend

rolli

ng

dire

ctio

n

rolli

ng

dire

ctio

n

surface intension

surface incompression

neutral axis orline in bend

t

t

R

t = Metal thicknessR = Inside radius

The exact amount of metal required in even astandard bend varies with such factors as itshardness, uniformity of thickness and thespeed with which the bend is made. For accurate results the final blank widthshould be checked by a practical test.

8.3 Allowances for springback (not applicable to stretch bending)

Generally, as the bend radius becomes larger,the allowance for spring back must be moregenerous, because less of the bent metal hasbeen stressed beyond its yield strength. Verylarge radii cannot easily be formed by ordinarybending, but must be stretch formed.

Spring back in bending low carbon steel sheethas to be considered only when closedimensional control is needed. It normallyranges from 0.5° to 1.5° and can be controlledby overbending or by bottoming/coining thebend area.

As a general guideline, the spring back in abending operation can be estimated fromEquation 8.

Equation 8

R u1— = — =1 - 3f + 4f 3 for f <0.5R1 u

R— = 0 for f >0.5R1

where

syRf = —— x 10-3

Et

R = pre-spring back inside bend radius, mm

R1 = post-spring back inside bend radius, mm

u = pre-spring back angle of bend (angle of former)

u1 = post-spring back angle of bend

sy = yield strength of sheet, MPa

E = Young’s Modulus of sheet, GPa(200 for steel)

t = thickness of sheet, mm

Alternatively, spring back my be expressed interm of a change in bend radius as follows:

Equation 9R1 – R

Spring back = ——— x 100%R

1= (————— – 1) x 100%

1 - 3f + 4f 3

However, numerous other factors also affectspring back, including angle of bend

(degrees of bend from flat), method of bending (V-bending or wiping) and amount ofcompression in the bend zone, tool wear andadjustment and power input variations. Becauseof the number of factors involved the degree ofthe spring back should be checked by tests afteran estimation is made.

9 BENDING ORGANIC FINISHED STEELThe performance of composite materials isgoverned by the characteristics of the substrateand the surface layer. The steel base cansometimes limit the performance of thecomposite while in other cases the coating willset the fabrication parameters.

Organic-finished steel sheet can, in mostinstances, be formed in the same basic toolingand presses that are used for uncoated steel butmethods and tooling may have to be modifiedto avoid damaging or removing the coating. Theperformance of the coating during press formingoperations is directly equated to the steelsubstrate hardness (or stiffness). If the substrateis thicker or stronger, a greater pressure isrequired to carry out forming. This in turnsubjects the coating to greater abrasion, surfaceshear and die pressure. On the credit side theuse of organic coatings usually increases tool lifeand reduces the need for press lubricants.

When straight-line forming organic-finishedsteels, the coating is required to stretchconsiderably more than the substrate andcreates a situation where coating can be eitherthin at the outside of the bend or slightly pullback from a sheared edge. This condition in noway impairs the quality of bond between thesteel and the coating. To keep the tendency tothin or pull back to a minimum it isrecommended that the maximum permissibleinside radius be used. When components arebeing formed with the main surface organiccoating on the inside the opposite situationarises as the coating is then under compressionand will be required to compress more than thesteel substrate.

For press-brake forming it is preferable, wherepossible, to use dies made from polyurethanerubber or to cover the V of metal dies withpolyethylene film tape as this will protect thecoating and enable forming of the coated sheetinto the most complex shapes without abrasionor damage.

High gloss paint finish films are particularlynotable in reducing tool wear. The forming ofthe high gloss materials often becomes lesscomplicated if tools are used that are designedwith the material in mind. The tools should befree from any sharp edges that are likely todamage the coating. The forming edges, wherepossible, should be highly polished and thetooling and pressings should be handledcarefully.

To calculate the correct clearance betweenpunch and die for forming organic-finishedsteel it is preferable, where possible, to allowfor overall thickness (substrate plus coating)plus 5%, as the coatings perform better whenthey are not over compressed. If there is achoice it is a preferable to form organic-finished steels on a slow-acting press. Rapidbending should be discouraged as should theforming of short flanges close to a cut edge.

A clear polyethylene CORSTRIP® film issometimes applied to the surface ofCOLORBOND® prepainted strip at the laststage of manufacture. The film is used forprotecting the paint surface during formingoperations where tool condition is not ofadequate standard. It acts as a solid lubricantduring forming operations and is designed tobe left on high quality surfaces until ready forthe ultimate end use. CORSTRIP® strippablecoating is easily removed after finalfabrication. COLORBOND® steel sheet coveredwith CORSTRIP® coating should not, however,be left in open sunlight as ultraviolet light maycause adhesion of the film to the paintsurface.

10 AUTOMATED BENDING SYSTEMSThere are a number of sophisticated flexibleautomated bending systems being used by themanufacturing industry. These bendingsystems may operate in a stand alone modesimilar to press brakes, however they aremore commonly integrated into computercontrolled production lines designed toautomatically produce sheet metalcomponents for small to large volumeproduction runs from either sheet or coil feedstocks.

These systems are computer controlled and atthe upper end of the spectrum are capable ofproducing complex sheet metal componentsgenerated from CAD drawings. Material is thenprogressed automatically through punching,shearing, folding and joining operations tocompleted parts without the requirement forman handling.

Literature relevant to these technologies isextensive and outside the scope of thispublication, however, may be readily accessedthrough the selling agents for flexiblemanufacturing systems such as Salvagnini,which manufacture parts from sheet stock, orPivatic flexible manufacturing systems, whichmanufacture sheet metal components usingcoil feed.

ACKNOWLEDGMENTAcknowledgment is made and thanks given toJohn Heine and Son Pty Limited - Sydney, andBliss Welded Products Limited - Sydney forkind assistance in preparation of notes in thisbulletin.

BlueScope Steel (Malaysia) Sdn Bhd Telephone: (603) 3250 8333

BlueScope Steel (Thailand) Limited Telephone: (66 38) 685 710

PT BlueScope Steel Indonesia Telephone: (62 21) 570 7564

BlueScope Steel Southern Africa (Pty) Limited Telephone: (27 21) 555 4265

The information and advice contained in this Bulletin is of a general nature only, and has not been prepared withyour specific needs in mind. You should always obtain specialist advice to ensure that the materials, approach andtechniques referred to in this Bulletin meet your specific requirements. BlueScope Steel Limited makes no warranty as to the accuracy, completeness or reliability of any estimates, opinionsor other information contained in this Bulletin, and to the maximum extent permitted by law, BlueScope SteelLimited disclaims all liability and responsibility for any loss or damage, direct or indirect, which may be suffered byany person acting in reliance on anything contained in or omitted from this document.

COLORBOND® and CORSTRIP® are registered trade marks of BlueScope Steel Limited.BlueScope is a trade mark of BlueScope Steel Limited.

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