Bending (Metalworking)

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Bending (metalworking)

“Metal bending” redirects here. For the form of stagemagic, see Spoon bending .

Bending is a manufacturing process that produces a

Bending

a chimney starter a sample product of bending

V-shape, U-shape, or channel shape along a straight axisin ductile materials, most commonly sheet metal .[1] Com-

monly used equipment include box and pan brakes , brakepresses , and other specialized machine presses . Typi-cal products that are made like this are boxes such as

Press brake

electrical enclosures and rectangular ductwork .

1 Process

Bending process

In press brake forming, a work piece is positioned overthe die block and the die block presses the sheet to forma shape. [1] Usually bending has to overcome both tensilestresses and compressive stresses . When bending is done,the residual stresses cause the material to spring back to-wards its original position, so the sheet must be over-bentto achieve the proper bend angle. The amount of springback is dependent on the material, and the type of form-ing. When sheet metal is bent, it stretches in length. The

bend deduction is the amount the sheet metal will stretchwhen bent as measured from the outside edges of thebend. The bend radius refers to the inside radius. The

1

https://en.wikipedia.org/wiki/Compressive_stresshttps://en.wikipedia.org/wiki/Tensile_stresshttps://en.wikipedia.org/wiki/Tensile_stresshttps://en.wikipedia.org/wiki/Duct_(HVAC)https://en.wikipedia.org/wiki/Electrical_enclosurehttps://en.wikipedia.org/wiki/Machine_presshttps://en.wikipedia.org/wiki/Brake_presshttps://en.wikipedia.org/wiki/Brake_presshttps://en.wikipedia.org/wiki/Box_and_pan_brakehttps://en.wikipedia.org/wiki/Sheet_metalhttps://en.wikipedia.org/wiki/Ductilehttps://en.wikipedia.org/wiki/Chimney_starterhttps://en.wikipedia.org/wiki/Manufacturinghttps://en.wikipedia.org/wiki/Spoon_bending


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2 2 TYPES

formed bend radius is dependent upon the dies used, thematerial properties, and the material thickness.

The U-punch forms a U-shape with a single punch. [1]

2 Types

PUNCH

WORKPIECE

BACK GAUGE

DIE

A schematic of air bending with a backgauge .

There are three basic types of bending on a press brake,each is dened by the relationship of the end tool posi-tion to the thickness of the material. These three are AirBending, Bottoming and Coining. The conguration ofthe tools for these three types of bending are nearly iden-tical. A die with a long rail form tool with a radiusedtip that locates the inside prole of the bend is called apunch. Punches areusually attached to the ram of thema-chine by clamps and move to produce the bending force.A die with a long rail form tool that has concave or Vshaped lengthwise channel that locate the outside proleof the form is called a die. Dies are usually stationaryand located under the material on the bed of the machine.Note that some locations do not differentiate between thetwo different kinds of dies (punches and dies.) The othertypes of bending listed use specially designed tools or ma-chines to perform the work.

2.1 Air bending

This bending method forms material by pressing a punch

(also called the upper or top die) into the material, forcingit into a bottom V-die, which is mounted on the press.The punch forms the bend so that the distance between

the punch and the side wall of the V is greater than thematerial thickness (T).

Either a V-shaped or square opening may be used in thebottom die (dies are frequently referred to as tools ortooling). Because it requires less bend force, air bending

tends to use smaller tools than other methods.Some of the newer bottom tools are adjustable, so, by us-ing a single set of top and bottom tools and varying press-stroke depth, different proles and products can be pro-duced. Different materials and thicknesses can be bentin varying bend angles, adding the advantage of exibil-ity to air bending. There arealso fewer tool changes, thus,higher productivity. [2]

A disadvantage of air bending is that, because the sheetdoes not stay in full contact with the dies, it is not as pre-cise as some other methods, and stroke depth must bekept very accurate. Variations in the thickness of the ma-

terial and wear on the tools can result in defects in partsproduced. [2]

Air bending’s angle accuracy is approximately ±0.5 deg.Angle accuracy is ensuredbyapplying a value to thewidthof the V opening, ranging from 6 T (six times materialthickness) for sheets to 3 mm thick to 12 T for sheetsmore than 10 mm thick. Springback depends on materialproperties, inuencing the resulting bend angle. [2]

Depending on material properties, the sheet may be over-bended to compensate for springback. [3]

Air bending does not require the bottom tool to have the

same radius as the punch. Bend radius is determined bymaterial elasticity rather than tool shape. [2]

The exibility and relatively low tonnage required by airbending are helping to make it a popular choice. Qual-ity problems associated with this method are counteredby angle-measuring systems, clamps and crowning sys-tems adjustable along the x and y axes, and wear-resistanttools. [2]

The K-Factorapproximations given below are more likelyto be accurate for air bending than the other types ofbending due to the lower forces involved in the formingprocess....

2.2 Bottoming

In bottoming, the sheet is forced against the V openingin the bottom tool. U-shaped openings cannot be used.Space is left between the sheet and the bottom of the Vopening. The optimum width of the V opening is 6 T(T stands for material thickness) for sheets about 3 mmthick, up to about 12 T for 12 mm thick sheets. The bend-ing radius must be at least 0.8 T to 2 T for sheet steel.Larger bend radius require about the same force as larger

radii in air bending, however, smaller radii requiregreaterforce—up to ve times as much—than air bending. Ad-vantages of bottoming include greater accuracy and less

https://en.wikipedia.org/wiki/Backgauge


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2.7 Rotary bending 3

springback. A disadvantage is that a different tool set isneeded for each bendangle, sheet thickness, and material.In general, air bending is the preferred technique. [2]

2.3 CoiningIn coining, the top tool forces the material into the bottomdie with 5 to 30 times the force of air bending, causingpermanent deformation through the sheet. There is lit-tle, if any, spring back. Coining can produce an insideradius is as low as 0.4 T, with a 5 T width of the V open-ing. While coining can attain high precision, higher costsmean that it is not often used.

2.4 Three-point bending

Three-point bending is a newer process that uses a diewith an adjustable-height bottom tool, moved by a servomotor. The height can be set within 0.01 mm. Adjust-ments between the ram and the upper tool are made us-ing a hydraulic cushion, which accommodates deviationsin sheet thickness. Three-point bending can achieve bendangles with 0.25 deg. precision. While three-point bend-ing permits high exibility and precision, it also entailshigh costs and there are fewer tools readily available. It isbeing used mostly in high-value niche markets. [2]

2.5 Folding

In folding, clamping beams hold the longer side of thesheet. The beam rises and folds the sheet around a bendprole. The bend beam can move the sheet up or down,permitting the fabricating of parts with positive and neg-ative bend angles. The resulting bend angle is inuencedby the folding angle of the beam, tool geometry, and ma-terial properties. Large sheets can be handled in this pro-cess, making the operation easily automated. There islittle risk of surface damage to the sheet. [2]

2.6 Wiping

In wiping, the longest end of the sheet is clamped, thenthe tool moves up and down, bending the sheet aroundthe bend prole. Though faster than folding, wiping hasa higher risk of producing scratches or otherwise damag-ing the sheet, because the tool is moving over the sheetsurface. The risk increases if sharp angles are being pro-duced. Wiping on press brakes. [2]

This method will typically bottom or coin the material to

set the edge to help overcome springback. In this bendingmethod, the radius of the bottom die determines the nalbending radius.Manual setup

2.7 Rotary bending

Rotary bending is similar to wiping but the top die ismade of a freely rotating cylinder with the nal formedshape cut into it and a matching bottom die. On contactwith the sheet, the roll contacts on two points and it ro-tates as theforming process bends thesheet. This bendingmethod is typically considered a “non-marking” formingprocess suitable to pre-painted or easily marred surfaces.This bending process can produce angles greater than 90°in a single hit on standard press brakes process.

2.8 Roll bending

Roll bending

Main article: Roll bending

The roll bending process induces a curve into bar or plateworkpieces. There should be proper pre-punching al-lowance.

2.9 Elastomer bending

In this method, the bottom V-die is replaced by a at padof urethane or rubber. As the punch forms the part, theurethane deects and allows the material to form aroundthe punch. This bending method has a number of advan-tages. The urethane will wrap the material around thepunch and the end bend radius will be very close to theactual radius on the punch. It provides a non-marringbend and is suitable for pre-painted or sensitive materi-als. Using a special punch called a radius ruler with re-lieved areas on the urethane U-bends greater than 180°can be achieved in one hit, something that is not possiblewith conventional press tooling. Urethane tooling shouldbe considered a consumable item and while they are notcheap, they are a fraction of the cost of dedicated steel

tooling. It also has some drawbacks, this method requirestonnage similar to bottoming and coining and does not dowell on anges that are irregular in shape, that is where

https://en.wikipedia.org/wiki/Roll_bending


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4 3 CALCULATIONS

the edge of the bent ange is not parallel to the bend andis short enough to engage the urethane pad.

2.10 Joggling

A joggle bend in sheet metal and a joggling tool

Joggling ,[4] also known as joggle bending , is an offsetbending process in which the two opposite bends are eachless than 90° (see following section for how bend angleis measured), and are separated by a neutral web so thatthe offset (in the usual case where the opposite bends areequal in angle) is less than 5 workpiece thicknesses. [5] Of-ten the offset will be one workpiece thickness, in order toallow a lap joint which is smooth on the 'show-face'.

3 Calculations

Many variations of these formulas exist and are readilyavailable online. These variations may often seem to beat odds with one another, but they are invariably the sameformulas simplied or combined. What is presented hereare the unsimplied formulas. All formulas use the fol-lowing keys:

• Lf = at length of the sheet

• BA = bend allowance

• BD = bend deduction

• R = inside bend radius

• K = K-Factor, which is t / T

• T = material thickness

• t = distance from inside face to the neutral line [6]

• A = bend angle in degrees (the angle through whichthe material is bent)

The neutral line (also called the neutral axis ) is an imag-inary line that can be drawn through the cross-section ofthe workpiece that represents the locus where no tensilenor compressive stresses are present on the work. Its lo-cation in the material is a function of the forces used toform the part and the material yield and tensile strengths.In the bend region, the material between the neutral lineand the inside radius will be under compression duringthe bend. The material between the neutral line and theoutside radius will be under tension during the bend.

Bend allowance and bend deduction are quantities usedto determine the at length of sheet stock to give the de-sired dimension of the bent part. Both bend deductionand bend allowance represent the difference between theneutral line or unbent at pattern (the required length ofthe material prior to bending) and the formed bend. Sub-tracting them from the combined length of both angesgives the at pattern length. The question of which for-mula to use is determined by the dimensioning methodused to dene the anges as shown in the two diagramsbelow.

3.1 Bend allowance

The bend allowance (BA) is the length of the arc of theneutral line between the tangent points of a bend in anymaterial. Adding the length of each ange taken betweenthe center of the radius to the BA gives the Flat Patternlength. This bend allowance formula is used to determinethe at pattern length when a bend is dimensioned from1) the center of the radius, 2) a tangent point of the radiusor 3) the outside tangent point of the radius on an acuteangle bend..

The BA can be estimated using the following formula,which incorporates the empirical K-factor: [7]

BA = A π180

(R + K × T )

3.2 Bend deductionThe bend deduction BD is dened as the difference be-tween the sum of the ange lengths (from the edge tothe apex) and the initial at length. The outside set back (OSSB) is the length from the tangent point of the radiusto the apex of the outside of the bend. The bend deduc-tion (BD) is twice the outside setback minus the bend al-lowance. BD is calculated using the following formula: [8]

BD = 2 ( R + T ) tan A2

− BA

The above formula works only for right angles. For bendangles 90 degrees or greater the following formula works,where A is the angle in radians (=degrees*π/180)


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5

A

t

R

NEUTRAL

LINE

TBA

BC

BC

Diagram showing standard dimensioning scheme when using

Bend Allowance formulas. Note that when dimensions “C” arespecied, dimension B = C - R - T

A

t

R

NEUTRAL

LINE

TB

B

OSSB

OSSBAPEX

Diagram showing standard dimensioning scheme when usingBend Deduction formulas.

BD = R (2 − A) + T (2 − kA )

3.3 K-factor

K-factor is a ratio of location of the neutral line to the ma-terial thickness as dened by t/T where t = location of theneutral line and T = material thickness. TheK-Factor for-mulation does not take the forming stresses into accountbut is simply a geometric calculation of the location ofthe neutral line after the forces are applied and is thusthe roll-up of all the unknown (error) factors for a given

setup. The K-factor depends on many factors includingthe material, the type of bending operation (coining, bot-toming, air-bending, etc.) the tools, etc. and is typically

between 0.3 to 0.5.

In sheet metal design, the K-factor is used to calculatehow much sheet metal one needs to leave for the bend inorder to achieve particular nal dimensions, especially forbetween the straight sides next the bend. Use the known

k-factor and the known inner bending radius to calcu-late the bending radius of the neutral line. Then use theneutral bending radius to calculate the arc length of theneutral line (“circumference of circle” multiplied by the“bend angle as fraction of 360deg”). The arc length ofthe neutral line is the length of the sheet metal you haveto leave for the bend.

The following equation relates the K-factor to the bendallowance: [9]

K =

− R + BAπA /180T

The following table is a “Rule of Thumb”. Actual resultsmay vary remarkably.

The following formula can be used in place of the table asa good approximation of the K-Factor for Air Bending:

K = log(min (100, max (20 R,T )T ))

2log (100)

4 Material considerations

Material sheet thickness varies from 0.79 to 12.7 mm(0.03 to 0.5 in) in with length from 150 mm (6 in) to 6 m(20 ft). Ductile materials are best suited for the pressinglike aluminum, mild steel and new plastic materials. [1]

5 Advantages

Bending is a cost effective process when used for low tomedium quantities,

6 See also

• Bending (mechanics)

• Tube bending

• Press brake

• Brake (sheet metal bending)

• Bending Machine (at metal bending)

https://en.wikipedia.org/wiki/Bending_Machine_(flat_metal_bending)https://en.wikipedia.org/wiki/Brake_(sheet_metal_bending)https://en.wikipedia.org/wiki/Press_brakehttps://en.wikipedia.org/wiki/Tube_bendinghttps://en.wikipedia.org/wiki/Bendinghttps://en.wikipedia.org/wiki/Mild_steel


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6 8 EXTERNAL LINKS

7 References

[1] Manufacturing Processes Reference Guide, IndustrialPress Inc., 1994.

[2] F., M. (August 2008), “Press Brake Bending: Methods

and Challenges” (PDF), Metalforming : 38–43.[3] Tool and Manufacturing Engineers Handbook , Volume 2,

Forming , 4th Edition, Society of Manufacturing Engi-neers, 1984

[4] 3-81. DRAW FORMING

[5] http://www.toolingu.com/definition-410130-35505-joggle-bend.html

[6] http://www.ciri.org.nz/bendworks/bending.pdf

[7] How to Calculate Bend Allowance for Your Press Brake ,archived from the original on 2010-02-24, retrieved 2010-

02-24.[8] Sheet metal bend deduction , archived from the original on

2010-02-24, retrieved 2010-02-24.

[9] Diegel, Olaf (July 2002), BendWorks , archived from theoriginal (PDF) on 2010-02-24, retrieved 2010-02-24.

7.1 Bibliography

• Benson, Steve D. Press Brake Technology: A Guideto Precision Sheet Metal Bending. Society of Man-ufacturing Engineers, 1997. ISBN 978-0-87263-

483-1

• Todd, RobertH.; Allen, Dell K.; Alting, Leo (1994),Manufacturing Processes Reference Guide , IndustrialPress Inc., ISBN 0-8311-3049-0 .

8 External links

• Latang, Paul. “Bending Made Easy” Fabricating &Metalworking, February 2010.

• Bend allowance and deduction calculator

http://www.fxsolver.com/solve/share/ubQyjDc8Vf8DqR539KwOHA==/http://www.fandmmag.com/print/Fabricating-and-Metalworking/Bending-Made-Easy--Advancing-the-CNC-Press-Brake-Control-/1$1907https://en.wikipedia.org/wiki/Special:BookSources/0-8311-3049-0https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttps://books.google.com/books?id=6x1smAf_PAcChttps://en.wikipedia.org/wiki/Special:BookSources/9780872634831https://en.wikipedia.org/wiki/Special:BookSources/9780872634831http://www.ciri.org.nz/bendworks/bending.pdfhttp://www.ciri.org.nz/bendworks/bending.pdfhttp://www.webcitation.org/5nmu2Ljx7http://www.mechengcalculations.com/jmm/metw004.htmlhttp://www.webcitation.org/5nmuYx6gWhttp://www.sheetmetalguy.com/bend-allowance.htmhttp://www.webcitation.org/5nmuupS8Lhttp://www.ciri.org.nz/bendworks/bending.pdfhttp://www.toolingu.com/definition-410130-35505-joggle-bend.htmlhttp://www.toolingu.com/definition-410130-35505-joggle-bend.htmlhttp://www.tpub.com/content/aviationandaccessories/TM-43-0106/css/TM-43-0106_172.htmhttp://archive.metalformingmagazine.com/2008/08/Press_Brake_Bending.pdfhttp://archive.metalformingmagazine.com/2008/08/Press_Brake_Bending.pdf


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9 Text and image sources, contributors, and licenses

9.1 Text• Bending (metalworking) Source: https://en.wikipedia.org/wiki/Bending_(metalworking)?oldid=719398030 Contributors: Bender235,

The wub, DVdm, Chris Capoccia, Brotulix, SmackBot, Jokl, Gilliam, Borowski~enwiki, Kleuske, Tompot~enwiki, Peter Horn, Wiz-ard191, Iridescent, Cydebot, Thijs!bot, Serpent’s Choice, JAnDbot, Rich257, Violask81976, A Nobody, Biscuittin, Roesser, Yintan, RFN-EFF, Flyer22 Reborn, Svick, Denisarona, Slightlymighty, AlanBottomley, DumZiBoT, WikHead, SilvonenBot, Download, OlEnglish,Luckas-bot, Rubinbot, B137, Materialscientist, Waterman67, Kierkkadon, Mstrogoff, Strano.m, EmausBot, ی ن -Wilhkar, Dissym ,م metry, Liamwillco, Geredog, ClueBot NG, Jnorton7558, ForwardChange, Mmarre, Helpful Pixie Bot, Pmayer05, LCS check, Macyber,Ginsuloft, Pokemaster3421, KH-1, Makertum and Anonymous: 74

9.2 Images• File:Absetzzange-blech.jpg Source: https://upload.wikimedia.org/wikipedia/commons/0/0e/Absetzzange-blech.jpg License: CC

BY-SA 3.0 Contributors: Own work Original artist: Nutzdatenbegleiter (talk)

• File:Bend_allowance.svg Source: https://upload.wikimedia.org/wikipedia/commons/7/78/Bend_allowance.svg License: CC BY-SA 3.0Contributors: Own work Original artist: Wizard191

• File:Bend_deduction.svg Source: https://upload.wikimedia.org/wikipedia/commons/5/59/Bend_deduction.svg License: CC BY-SA 3.0Contributors: Own work Original artist: Wizard191

• File:Bending,_Constructionwork.jpg Source: https://upload.wikimedia.org/wikipedia/commons/0/03/Bending%2C_Constructionwork.jpg License: CC BY-SA 3.0 Contributors: Own work Original artist: Kleuske

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• File:Chimney_starter.JPG Source: https://upload.wikimedia.org/wikipedia/commons/8/8e/Chimney_starter.JPG License: CC BY-SA3.0 Contributors: Own work Original artist: ی ن م

• File:Press_Brake.jpg Source: https://upload.wikimedia.org/wikipedia/en/4/4a/Press_Brake.jpg License: PD Contributors: ? Original artist: ?

• File:Press_brake_schematic.svg Source: https://upload.wikimedia.org/wikipedia/commons/7/70/Press_brake_schematic.svg License: CC-BY-SA-3.0 Contributors:

• Gesenkbiegen_ungebogen.svg Original artist: Gesenkbiegen_ungebogen.svg : URMEL

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Bending (Metalworking)