design of a dual operating mode sheet folding machine

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Int. J. Mech. Eng. & Rob. Res. 2013 Gwangwava et al., 2013

DESIGN OF A DUAL OPERATING MODE SHEETFOLDING MACHINE

Gwangwava N1*, Mugwagwa L1 and Ngoma S1

*Corresponding Author: Gwangwava N,[email protected]

A sheet folding machine that can be operated through hydraulics by two hydraulic cylinders ormanually (with the cylinders disengaged) was designed. The design need emanated from thestrained national electrical grid system that has recently seen industrialists and households inZimbabwe experiencing major power cuts. The machine enables manufacturers to scheduleheavier jobs during periods when power supply is up and lighter jobs during power cut periodshence run their workshops throughout the daily production shifts. The two hydraulic cylinderscan be disengaged from the machine’s folding beam so that manual operation can be donethrough a manual clamping lever system. The folding force at full capacity is 294.6 KN (29.46Ton), total bending length of 1.8 m and working height of 1 m. The folding force decreasessignificantly in manual operating mode to 500 N, considering that on average an operate canmanually exert that force. A student version of Simulation X 3.5 was used to simulate the hydraulicoperation of the machine.

Keywords: Sheet metal folding, Folding machine, Sheet metal bending, Press brake

INTRODUCTIONSheet metal bending and folding accounts forthe production of a wide range of consumerdurable goods. The demand of goods that fullyor at least partly comprise of bent sheet metalparts promises to remain high. Typicalproducts made from sheet metal folding includeenclosures, electric boxes, casings forelectrical and electronic gadgets, trays, lids,troughs, air ducts, and chimneys. The paper

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Int. J. Mech. Eng. & Rob. Res. 2013

1 Department of Industrial and Manufacturing Engineering, National University of Science and Technology, Bulawayo, Zimbabwe.

will articulate the design of the components ofthe folding machine which comprise of thefolding beam, the clamping beam, thehydraulic system, selection of a pump topower the hydraulic system, design of thehydraulic cylinder connections such that theycan be easily disengaged from hydraulic tomanual mode. Currently the bulk of availablesheet metal folding machines are operatedmanually whilst few hydraulic operated ones

Research Paper

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exist. The operation of hydraulic foldingmachines is greatly affected by the power cutshence the need for a machine that operatesboth on hydraulic and manual mode.Designing a dual operation mode machinegives the manufacturers more flexibility onlimited resources as compared to buying twomachines with different operating modes sothat the other can be shelved when there is nopower supply. To deal with this problem, thisarticle focuses on the design of dual-modesheet metal folding machine that has the abilityto be disengaged from hydraulic mode tomanual mode in the shortest period of time.With the growth of Small and Medium scaleEnterprises (SMEs) worldwide, the foldingmachine can be used to produce a wide rangeof products in small factories for local marketas well as foreign markets with low powerconsumption since the operators can chooseto switch into the manual mode even duringperiods when there is power supply.

CLASSIFICATION OF SHEETMETAL BENDINGPROCESSESThere are various sheet metal processingoperations, for instance, laser cutting andbending, punching, deep drawing andredrawing, bending, incremental forming,shearing and blanking, stretch forming, rubberhydroforming, spinning and explosive forming(Groover, 2010). Bending along a straight lineis the most common of all sheet formingprocesses; it can be done in various ways suchas forming along the complete bend in a die,or by wiping, folding or flanging in specialmachines, or sliding the sheet over a radius ina die (Marciniak, 2002). The terms folding andbending are loosely used in the sheet-metal

industry and largely interchangeable incommon parlance, to be precise, the term‘folding’ refers to sharp corners with a minimumbend radius and the term ‘bending’ refers todeflections of relatively large corner radii.Folding and bending involve the deformationof material along a straight line in twodimensions only (Timings, 2008).

Bending by Press BrakesBending is a metal forming process in whicha force is applied to a piece of sheet metalcausing bending of it to an angle and formingthe desired shape (Manar, 2013). The processis typically performed on a machine called apress brake which can be manually orautomatically operated. To bend sheet metal,a bottom tool (die) is mounted on a lower,stationary beam (bed) and a top tool (punch)is mounted on a moving upper beam (ram)(Simons, 2006). The opposite configuration isalso possible. Bending produces a V-shape,U-shape, or channel shape along a straightaxis in ductile materials, most commonly sheetmetal. Commonly used equipment include boxand pan brakes, brake presses, and otherspecialized machine presses. A typical pressbrake is illustrated in Figure 1.

Figure 1: Press Brake

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V-die bending can be used in two differentways: for “air bending” or for “bottoming”. Inair bending the punch stops a certain distanceabove the bottom. The die set can be used forbending at any angle larger than 85°. Thebottoming die bends the sheet metal at anangle of the die, which may be 90° or any otherangle. Both types of V-die bending allow forover bending, which means that the bendsunder 90° can be produced. Figure 2 showsV-bending die sets.

and one that operates manually. Manualfolding machines however have somedisadvantages in that they are not conduciveto higher production rates, quality orrepeatability; however they are suitable forlow scheduling light workloads. Hydraulicpowered sheet metal folding machinescounteract the disadvantages of manuallyoperated sheet metal folding machines, butthey have a limitation in that they are affectedby power cuts.

Hence the design on a folding machinethat operates both on hydraulic and manualmode will minimize the impact of power cutson the production rate and also at the sametime improving on quality work throughscheduling of light workloads during periodsof power cuts and scheduling of otherdemanding workloads during periods inwhich electricity is available.

Hydraulic Power SystemSheet metal working machines can beclassified according to energy supply. Fivecategories can be identified as follows;

Mechanical: Where the work force is suppliedby some mechanical means such as a cam orlever.

Hydraulic: These utilize the pressure of wateror other fluid media.

Steam: They use pressurized steam.

Electromagnetic: Uses electromagneticforce.

Hydraulic power system was chosen for thefolding machine because of the followingadvantages over other methods of powertransmission (Dawei, 2008):

Figure 2: V-Die Bending, Air Bending,and Bottoming

Source: Suchy (2006)

Sheet Metal Folding MachinesSheet metal folding process is performed onsheet metal folding machine. The machineconsists of a clamping beam that holds downthe sheet metal work, and a folding beam thatperforms the folding operation. The clampingbeam consists of a detachable clamping bladeand the folding beam of a hard and removablesegment, this allows for damaged segmentsto be replaced. Another feature on the foldingmachine is a back gauge that allows forhighest repeatability of work.

Two types of sheet metal folding machinesare available, one that is hydraulic powered

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• Simpler design – In most cases, a few pre-engineered components will replacecomplicated mechanical linkages.

• Flexibility – Hydraulic components can belocated with considerable flexibility. Pipesand hoses instead of mechanical elementsvirtually eliminate location problems.

• Smoothness – Hydraulic systems aresmooth and quiet in operation. Vibration iskept to a minimum.

• Control – Control of a wide range of speedand forces is easily possible.

• Cost – High efficiency with minimum frictionloss keeps the cost of a power transmissionat a minimum.

• Overload protection – Automatic valvesguard the system against a breakdown fromoverloading.

The main disadvantage of a hydraulicsystem is maintaining the precision parts whenthey are exposed to bad climates and dirtyatmospheres, hence protection against rust,corrosion, dirt, oil deterioration, and otheradverse environmental conditions is veryimportant. Disposal of the hydraulic fluid is alsoa threat to the environment.

DESIGN OF THE FOLDINGMACHINE COMPONENTSDetailed design calculations for sizing thefolding machine components are carried outin this section. The initial conditions for basingthe design on are given in Table 1.

Maximum Folding ForceThe force required to perform folding dependson the strength, thickness, and length of thesheet metal (Groover, 2010). The maximum

folding force can be estimated by means ofthe following equation:

D

wtTSKF bf2

...(1)

where,

Kbf = 0.33

TS = 248 MPa

w = 1 800 mm

t = 2 mm; and

D = 2 mm.

Therefore;

2

2180024833.0 2F

F = 294.6 kN

Clamping Beam DesignThe clamping beam exerts a force that holdsdown the sheet metal onto the folding bed.The hold down force when performing foldingoperation is 50% the required folding forcesince it is applied across two ends of themachine. Therefore the clamping force isgiven by:

Parameter Value

Maximum bending length 1800 mm

Maximum bending thickness 2 mm

Tensile strength of sheet metal(Mild steel) 248 MPa

Clearance between folding beamand clamping beam 2 mm

Maximum folding angle 1050

Frame material Structural Steel

Folding beam and clamping beammaterial Machine Steel

Table 1: Initial Conditions

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Clamping force = 0.5 x folding force

Clamping force = 0.5 x 294.6 kN

Clamping force = 147.3 kN

The clamping beam is designed such thatit is welded onto side plates that areconnected to a clamping mechanism as shownin Figure 3.

The clamping mechanisms are located onboth sides of the clamping beam, but theclamping knob is only located on one end. Theadjusting screws on the clamping mechanismmust resist the clamping force they areexposed to. Operation of the clampingmechanism is illustrated in Figure 4.

The load is shared equally on either side ofthe clamping mechanism, therefore is equalto half the clamping force that is 73.65 kN.Allowable stress levels to 75% of proof strengthare to be used in the clamping mechanismbolts. The selected material for the clampingmechanism according to the Society ofAutomotive Engineers (SAE), is grade 4 withno head marking and proof strength of 65 ksi.

Then the allowable stress is:

a = 0.75 x proof strength ...(2)

a = 0.75 x 65000 psi

a = 48759 psi

The force on each side of the clampingmechanism is 73.65 kN = 16.55 klb

Therefore the required tensile area to whichthe force should act is:

at

LoadA

...(3)

2/4875016550

inlblbAt

2339.0 inAt

Tensile stress area of 0.339 in2 requires adiameter of 7/8 inches, which is equivalent to22.22 mm.

Hence the diameter of the clampingmechanism column should be 22.22 mm witha course thread of 9 threads per inch.

Figure 3: Clamping Beam and theClamping Mechanism

Figure 4: Operation of the ClampingMechanism

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Design of the Folding BeamFigure 5 shows the front view for the foldingbeam.

length of the beam. Figure 6 represents theloading on the beam.

Figure 5: Folding Beam Front View

The weight of the folding beam is calculatedusing the formula below.

Weight of folding beam = surface area xdensity x thickness ...(4)

Surface area, A, of folding beam:

Surface area = 2.1 (0.45)

15.026.0120.021

2755.0 mareaSurface

Using machine steel of density 77 kN/m3, ifthe thickness of the folding beam is t, thereforethe weight of the folding beam is:

W = Surface area x density x thickness

W = 0.755 m2 x 77 kN/m3 x t

kNtW 135.58

The beam is supported on its two ends andthe other force (including its weight) acting onthe beam is the maximum required foldingforce of 294.6 kN acting uniformly across the

Figure 6: Loading on the Folding Beam

Total force acting on the beam = (294.6 +58.135t) kN

beamtheofLengthbeamtheonactingforceTotallengthunitperactingForce

1.2

135.586.294 kNlengthunitperactingForce

mkNtlengthunitperactingForce /6.273.140

Reaction force at the beam supports:

2beamtheonactingforceTotalsupporteachatReaction

2

135.586.294 kNtsupporteachatReaction

kNtsupporteachatReaction 293.147

Taking moments and resolving forces atdetermined points along the folding beam andfactoring a safety factor of n = 3, and anallowable stress of 350 MPa, t is found to havethe following value;

t = 0.015 or t = -0.015

Therefore the thickness of the folding beamis 15 mm.

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Design of the Hydraulic SystemFigure 7 shows the design of the hydrauliccircuit system of the metal folding machine.

22 25.065.0, xlength

mx 69.0

x90sin

65.0sin o

0.6990sin65.0sin

o1

o4.70

The axial force, Fc, on the hydraulic cylinderrequired to generate the folding force iscalculated with reference to Figure 9.

Figure 7: Circuit Diagram of HydraulicSystem

The folding force is evenly distributedbetween the two hydraulic cylinders.

kNcylinderhydrauliceachonLoad 3.147

The axial load on each hydraulic cylindercan be determined using the angle ofinclination of the cylinders. The angle ofinclination, , is determined using Figure 8.

Figure 8: Illustration for Determiningthe Angle of Inclination of the Cylinders

Figure 9: Axial Force on the HydraulicCylinder

oo 4.70sin3.147

90sinkNFc

kNFc 4.156

The load will be evenly distributed betweenthe two hydraulic cylinders hence the force oneach cylinder is:

24.156 kNcylindereachonForce

kNcylindereachonForce 2.78

For the hydraulic system an operatingpressure of 150 bars is selected. The effective

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piston area, Aeff, is calculated below usingEquation 5.

pressureOperatingcylinderhydrauliceachonforceAxialAeff

...(5)

5101502.78

kNAeff

231021.5 mAeff

The effective piston area is used to calculatethe hydraulic cylinder bore diameter, D, asfollows:

4

2DAeff ...(6)

31021.54 D

mD 081.0

Using preferred cylinder sizes and pistondiameters, we determine that the appropriatepiston diameter, d, is 56 mm.

The annulus area, Aa, is calculated usingEquation 7.

4

2dAA effa ...(7)

4

10561021.523

3

aA

231075.2 mAa

Using the annulus area, the retracting force,Fr, is calculated using Equation 8.

ar APF ...(8)

35 1075.210150 rF

kNFr 25.41

The stroke of the hydraulic cylinders isdetermined with reference to Figure 10.

Figure 10: Illustration for Determining theStroke of the Hydraulic Cylinder

When the cylinder is fully open:

The total horizontal length is = 0.25 + 0.38sin 75

The total vertical length is = 0.65 + 0.38 +0.38 sin 15

Therefore the length of the fully open cylinder,lo, is:

22 15sin38.038.065.075sin38.025.0 ol

lo = 1.28 m

The stroke of the cylinder is calculated asfollows:

Stroke = Fully open length – fully closedlength ...(9)

Stroke = 1.28 m – 0.69 m

Stroke = 0.59 m

Using the stroke obtained above thevelocity, ve, of the hydraulic cylinder duringextension of a period of 4 seconds can bedetermined as follows:

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be used to power up the hydraulic system. Thefollowing calculations will be useful in sizingthe desired pumping unit.

Taking a volumetric efficiency, nv, of 90%and speed rating of 1200 rpm, we can obtainthe theoretical pump delivery.

deliverypumplTheoraticadeliverypumpActual

v ...(14)

v

deliverypumpActualdeliverypumplTheoratica

9.0/1071.7 34 smdeliverypumplTheoratica

Theoratical pump delivery = 8.57 x 10–4 m3/s

Theoratical pump delivery = 51.4 litres/min

The pump displacement per revolution, Dp,is calculated as follows:

ratingspeedPumpdeliverypumplTheoraticaDp

60 ...(15)

rpmsmDp 1200

60/1057.8 34

Dp = 4.29 x 10–5m3

From the calculations above a hydraulicvane pump with a rating of 1200 rpm, adisplacement of 4.29×10–5 m3 and a flowcapacity of 51.4 litres/min (13.58 GPM) isselected.

Reservoir DesignThe most suitable hydraulic tank volume is twoto three times the pump capacity. Selecting afactor of 2.5, the capacity of the hydraulicreservoir is calculated as follows:

Reservoir volume = 2.5 x pump capacity

...(16)

timeextensionCylinderStrokeVe ...(10)

smVe 4

59.0

smVe /148.0

The flow rate, Qe, during extension iscalculated as:

Qe = Aeff x velocity during extension ...(11)

Qe = 5.21 x 10–3m2 x 0.148 m/s

Qe = 7.71 x 10–4m3/s

Qe = 0.77 litres/s

GPMQe 785.36077.0

Qe = 12.21 GPM

The same flow rate is used to retract thecylinder; hence the retraction velocity, vr, isdetermined as follows:

a

er A

QV ...(12)

3

4

1075.21071.7

rV

Vr = 0.28 m/s

Therefore the retraction time, tr, is

rr v

Stroket ...(13)

smmt r /28.0

59.0

tr = 2.1 s

Hydraulic Pump SizingThe hydraulic pump supplies the pressurizedfluid for cylinder actuation. A vane pump is to

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Reservoir volume = 2.5 x 51.4 litres

Reservoir volume = 128.5 litres = 0.1285 m3

A reservoir should be high and narrow ratherthan shallow and broad. Therefore, arectangular prism shaped reservoir with alength, l, of 0.8 m and width, w, of 0.4 m ischosen.

The height, h, of the reservoir is calculatedas follows:

wlvolumeReservoirh

...(17)

mmmh4.08.0

1285.0 3

h = 0.401 m

The height tank will have to be greater than0.401 m because there must be a clearancevolume above the oil, therefore the new heightof the tank chosen will be 0.45 m.

Cylinder ConnectionsThe attachment of the hydraulic cylinders to thefolding beam has to be detachable to enable

Figure 11: Connection Detailsof the Hydraulic Cylinders

switching of the machine from hydraulic tomanual mode. Figure 11 shows the details ofthe cylinder connections on the machine.

At the base fitting a clevis mounting is usedthat provides a single pivot mounting point onthe cylinder. At the end fitting a bearing eyehead is used to mount the cylinder onto thefolding beam. The cylinder will be detachedon the end fitting to disengage manual mode.A detachable connection using an M24 boltwith a shank length of 60mm will be used toconnect the cylinder onto the folding beam.

HYDRAULIC SYSTEMSIMULATION RESULTSTo simulate the operation of the hydrauliccylinder, a package called ‘Simulation X 3.5’was used. The simulation give a mathematicalimitation of the operation of each hydrauliccylinder over the period from fully closed to fullyopen which takes 4 seconds.

Figure 12 represents the simulation resultsof the piston stroke over the complete cycletime; a maximum stroke of 590 mm isattained.

Figure 12: Piston Stroke

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Figure 13 displays the flow rate from thepump to the directional control valve, from thecalculations a flow rate of 51.4 l/min wasobtained.

decreases to zero to indicate constant velocityof the folding beam.

COMPLETE FOLDINGMACHINE MODELA complete model of the designed dualoperating mode sheet folding machine isshown in Appendix A. The model shows thelocation of the clamping mechanism (on oneend of the machine) and a protractor gauge islocated on the side frame for anglemeasurement during the bending operation.

CONCLUSIONThe article has demonstrated an engineeringprocedure for design and validation by meansof simulation, of a dual operating mode sheetmetal folding machine. A non-commercialhydraulic simulation package (Simulation X 3.5)was used for simulation of the hydraulic systemfor the machine. A constant operating pressureof 150 bars will be maintained during machineoperation whilst the folding bar shows a rapidacceleration, within a fraction of a second,towards the specimen sheet metal before it

Figure 15 shows the acceleration on theload, in the first split seconds there are highaccelerations due to shock as the load isstarting is move. The acceleration then

Figure 13: Pump Flow Rate to DirectionalControl Valve

Figure 14 shows the pressure supplied bythe pump of 150 bars during the 4 seconds.

Figure 14: Pressure Supply

Figure 15: Acceleration of the Load

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reaches a steady approach during the actualbending operation. This action preventswhipping of the specimen sheet metal and alsogives good quality bending of sheet metal parts.A full scale version of the sheet metal foldingmachine can be manufactured to benefit thesmall and medium enterprises since themachine can be scheduled to operate manuallyand through hydraulic actuation.

REFERENCES1. Groover M P (2010), Fundamentals of

Modern Manufacturing: Materials,Processes and Systems, 4th Edition, JohnWiley & Sons Inc., ISBN: 978-0470-467002.

2. Manar A E E (2013), “Design,Manufacture and Simulate a Hydraulic

Bending Press”, International Journal ofMechanical Engineering and RoboticsResearch, Vol. 2, No. 1, pp. 1-9, ISSN:2278-0149.

3. Marciniak Z (2002), Mechanics of SheetMetal Forming, Butterworth-Heinemann,ISBN: 07506 53000.

4. Simons J (2006), Redesign of a PressBrake, Master’s Thesis, TechnischeUniversiteit, Eindhoven, August, availableat http://www.kelue.edu/repositoryAccessed (July 19, 2012).

5. Suchy I (2006), Handbook of Die Design,ISBN 0-07-1426271-6, McGraw Hill.

6. Timings R (2008), Fabrication andWelding Engineering, Elsevier Ltd.,ISBN: 978-0-7506-6691-6.

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Dual Operating Mode Sheet Metal Folding Machine

APPENDIX A