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Fabrication and Welding Technology for Mechanical Engineering — Title: Understand sheet metal forming processes, joining methods and cutting processes.

30FABRICATION AND WELDING TECHNOLOGY

Sheet metal forming processes

Table of contents

List of figures 3

List of Tables 5

Sheet metal forming processes 6

Task 1a: Formability of metals 6

Task 1b: Press braking and press die forming 10

Press braking 10

Stretch forming 16

Task 1c: HERF and other sheet metal forming processes 17

Task 1d: Numerical methods in sheet metal forming 19

Task 2a: Joining methods for sheet metal 20

Task 2b: Bolting procedures 22

Task 2c: Joining of structural sections 24

Task 3a: Cutting methods used in fabrication 26

29References

List of figures

Figure 1: Sheet metal cutting operations 6

Figure 2: Punch and die sizes 7

Figure 3: Sheet metal bending 8

Figure 4: Drawing of cup-shaped parts 8

Figure 5: Tensile test specimen 9

Figure 6: Deformation geometry 10

Figure 7: Process schematic of press brakes 10

Figure 8: Press brake machine- Computer numerical control (CNC) 11

Figure 9: Punch and die configurations 12

Figure 10: Edge (wipe) bend 13

Figure 11: V-bend determination 14

Figure 12: Offset and bead forming 15

Figure 13: Stretch-forming process 17

Figure 14: Explosive forming-Unconfined type 18

Figure 15: Explosive forming-Confined type 18

Figure 16: Electro-hydraulic forming 19

Figure 17: Electro-magnetic forming 19

21Figure 18: Self-secured joints

Figure 19: Sectional view of a captive fastener 22

Figure 20: Shear joint in ultrasonic welding 22

Figure 21: Bolted (left) and screwed joint (right) 23

Figure 22: Tightening sequence-star pattern 23

Figure 22: Tack bolting 24

Figure 23: Tightening of HSFG bolts 24

25Figure 24: Bend allowance for sheet metal

Figure 25: Centre-line bend allowance 25

Figure 26: Flange depth determination 26

Figure 27: Oxytylene cutting process 27

List of Tables

Table 1: Tensile strength of materials 16

Sheet metal forming processes

Task 1a: Formability of metals

. Using these sheet metal guarantees high strength, good surface finish and dimensional accuracy, low cost and economical mass production. Sheet metal processes involve cutting operations, bending operations and drawing. Cutting operations involve shearing to separate large sheets and blanking to cut out of sheet metal part perimeters. It also involves punching to create holes in the sheet metal (Khamis, 2011). On the other hand, bending strains the sheet metal around a straight axis. Finally, drawing forms the sheet into concave or convex shapes. Allwood, 2008)In many industries, sheet metal processing is a significant process that produces personal computers, toys, and electronics and home appliances. Most of these operations are cold working and carried out on sheet of metal (thickness in the range of 0.4-6mm) and a plate stock of >6mm (

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.

Figure 1: Sheet metal cutting operations

From the figure above, the first step is just prior to the punch contacts work and the second stage is plastic deformation as the punch gets into work. The third stage is when the punch causes a smooth cut surface after compression and penetration (Confederation of British Metalforming, 2014). Finally, a fracture that separates the sheet is initiated at the opposing cutting edges.

In the punching and blanking stages, the cut piece is scrap and separated from the surrounding stock which gives the punch and die sizes as shown in the figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  1

Figure 2: Punch and die sizes

) is the difference between die size and the punch size and is in the range of 4%-8% of the stock thickness. c) is determined by punch size. Clearance (hD) is determined by die size while hole-size (bDAs shown in the figure above, the blank size (

The recommended clearance is; c = at

c = clearance;

a = allowance, and

t = stock thickness

The cutting force determines the press size (tonnage) and is given by; F = S*t*L

Where: S = shear strength; L= cut edge length and t = stock thickness

Very small clearance causes double burnishing and fracture lines while large clearances lead to excessive burr results.

On sheet metal bending, a permanent bend is made around a straight axis by straining the sheet metal. The metal outside the neutral plane is stretched while the inner one of the neutral plane is compressed as shown in figure 3 below. Tensile elongation and compression occur simultaneously during bending according to BS EN 10002-1:2001 (British Standards, 2001).

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  2

Figure 3: Sheet metal bending

Common types of sheet metal being are V-bending and edge bending where the former is carried out on a V-shaped die while the latter is done with a wiping die.

. In this case, a punch pushes the metal into an opening after the sheet metal blank is positioned under die cavity. These processes help in developing automobile body panels, ammunition shells and beverage cans. European Aluminium Association, 2015)The final stage of sheet metal forming process is drawing in which it takes various shapes such as hollow shapes, complex curves, box-shaped and cup-shapes (

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  3

Figure 4: Drawing of cup-shaped parts

From the figure above, the resulting shape is a cup-shape shown in (2).

Formability testing of sheet metals involves mainly strain and stress tests. One of the tests is the measure of anisontropy test which consists of planar anisontropy and average normal anisontropy (Hu et al., 2002).

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  4

Figure 5: Tensile test specimen

As shown in the figure above, it is possible to determine average normal anisontropy (Lankford’s coefficient) by;

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  5

.0, and α=900, α=450Where; R is expressed in three directions with respect to the direction of lamination as α=0

Planar anisontropy by;

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  6

. m determines the limiting drawing ratio. Moreover, optimal drawability is achieved by combining low Δr and high rmWhile Δr correlates with the extent of earing, r

= 0) but there is strain which results in a bi-axial stress system. Given that the stress is confined to one plane, it becomes a plane stress. 3) have an associated strain in the x-y plane (Khamis, 2011). The sheet is not constrained and is free to contact in the σ3 (z) direction. In this direction, there is no stress (σ2 and σ1In the deformation process, two dimensional bi-axial stresses act on the workpiece. The principal stresses (σ

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  7

Figure 6: Deformation geometry

. Allwood, 2008)(. Since σ3 is finite in the z direction, it is constrained thus preventing deformation 2 and σ1However, in a plane strain condition strain occurs in two dimensions parallel to σ

Task 1b: Press braking and press die forming

Press braking

Press braking produces shaped workpieces through mechanical metal deformation units along a straight axis. In modern press working, there are two types of bending; bottoming and air bending. Solid workpiece retains the same composition or mass when being altered by plastic deformation (Degarmo et al., 2003). The punch and die set used can be channel-shapped, V-shapped or U-shapped. Similarly, press brakes can be hydraulic, servo-electric, computer controlled or mechanical as shown in the figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  8

Figure 7: Process schematic of press brakes

In a computerized case, it resembles the standard hydraulic press brakes but it is capable of performing multiple angle parts without the need of depth penetration of ram or manual intervention with the backgauge. On the other hand, hydraulic press brakes use fluid power and have greater control compared to other press brakes. On the C-frames that use the upper beam, hydraulic press brakes use two synchronized hydraulic cylinders. A servo-electric brake exerts tonnage on the ram using belt drive or ballscrew using a servo-motor. A mechanical press applies energy from an electric motor added to a flywheel (Hosford, 2005). The flywheel is engaged through a clutch that moves the ram vertically using a crank mechanism. They have high speed and are very accurate. While servo-electric and pneumatic press brakes apply in low tonnage applications, hydraulic brakes are safe, use little energy and are highly accurate hence giving high quality work. In response to the device safety, the ram motion is stopped at any time unlike the fly-wheel driven presses (National Occupational Standards, 2012). The bend angle of the bend in the earlier brakes relied on tooling. Setup and tooling is relatively simple in press brake forming

Press brakes such as computer numerical control (CNC) are used to form complex shapes metals. These brakes, as shown in the figure below, are capable of performing multiple angle parts thus giving higher production and reduced setup time.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  9

(Source: Hu et al., 2002)

Figure 8: Press brake machine- Computer numerical control (CNC)

. In reshaping the material, heat and friction from the press tools are adequate as no external heating device accompanies the press brakes and their tools operate. Steel is a common tool for press brakes although other materials exist such as urethane. Some of the press brakes are shown in the figure below. Hosford, 2005)In conjunction with a press brake, press brake tools operate under heavy press or boom to bend and shape sheet metal. A punch is used by the press to create a permanent crease by forcing the metal sheet into the die and reshaping the metal (

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  10

Figure 9: Punch and die configurations

. With no springback, it creates varying angles yet it is an expensive press method.Allwood, 2008) uses heavy force to give very precise bends by driving the punch to the bottom of the die (Coininga type of three point bending. With the die’s deepest point, it does not make contact. Depending on the material’s resistance, multiple bend materials and angles employ this flexible technique. However, the punch drive may result in springback if it is not accurate.
and ispunch presses the material into the dieCommon steel bending methods are air bending and coining. Air bending

In modern press working, edge die bending and V-die bending are used. V-die bending is extensively used in stamping die operations and brake die operations. V-bending can be air bending or bottoming as shown in the figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  11

V-bend types:Figure 9

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  12

Figure 10: Edge (wipe) bend

. Half width = 4T while the optimum die width = 8T. The depth is given by; and not 90αthe bend is some angle The depth of the die is different than Figure 9 if Operator judgment is used to determine V-die openings within a range of 6 to 12 times the thickness of the material and eight times the optimum width.

)0 ≠ 0α, 0 ≠ 180α/2) (αD = 4T/tan (

D = 2.3T. , 0 = 120αD = 6.9T while at , 0 = 60For example, at α

V-shaped dies appear as equilateral right triangles (Nguyen et al., 2008). The bend angle is determined by the distance the punch enters the die under the control of the machine shut height as shown in the figure below.  bends, the thickness of the metal sheet at least eight times less than the v-die opening width. The die depth is W/2 or 4T since the 90To calculate V-bend force for 90

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  13

Figure 11: V-bend determination

When the ram is fully closed, the shut height is the vertical distance measured from the bed to the ram nose.

V-bend bending time is tforming = D/V …………………………………….. (1)

The bending force: F = 4.48(S L T2 K/ W)………………………………………… (2)

Where: F = force, (N)

)2S: Ultimate tensile strength, (N/mm

D: V-die opening depth, (mm)

L: Length of the bend, (mm)

W: die opening width, (mm)

T: stock thickness, (mm)

V = Based on Machine specification, it is the punch speed (in/sec)

K = 1.33 for a die opening of 8T (Constant for die opening distance)

rubber pad used to deform metal sheets. They are adapted to the most accurate stampings for all dimensions, especially 1250 x 2000 x 300mm, block dimensions. It measures 1200 x 1950 x 20mm for sole dimensions with or without steel sole. Coming in the hardness of up to 75 ShA, this tooling mechanism enables costs savings. Other press braking options are offset forming. wan neck soft pads are In sheet metal forming processes, s

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  14

Figure 12: Offset and bead forming

ead forming in a press brake with a single die while (b) is for two dies. In the figure above, (a) represents b

In a typical wiping operation, the edge bending force calculation is given by;

/ W……………………………………………………………………………… (3) 2F = K S L T

From the table below, it is possible to determine the typical tensile strengths of materials.

F: Force, (lbf)

S: Ultimate tensile strength, (psi)

+ T)2+ r1 W: die opening width, (in) (r

T: stock thickness, (in)

: punch radius, (in)1r

L: Length of the bend, (in)

: die radius, (in)2r

K: Die opening distance constant (For high plastic working stress, the clearance is up to 0.333 for sharp die radii and 0.167 for large die)

Table 1: Tensile strength of materials

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  15

The energy required to perform the operation can be estimated using the bending force calculated in the above as;

D…………………………………………………………………………. (4) = F b E

Energy (kJ/bend): bWhere: E

D: die closed depth, (mm)

F: bending force, (kN)

Brake forming power required is;

P = F*V……………………………………………. (5)

Where: V: Punch speed (mm/sec)

Stretch forming

Stretch forming process is where a sheet metal stretched over a die or form block upon clamping along its edge. The die is made of wood, steel plastic or zinc alloys with little or no lubrication. Although it has low volume production, it is economical and versatile and comes with various accessories.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  16

Figure 13: Stretch-forming process

. By driving the form die into the sheet, there is an increase tensile forces that deforms the sheet plastically into a new shape to plastic and elastic deformation. Hosford, 2005)On a stretch press, stretch forming utilizes a piece of sheet metal secured by gripping jaws along its edges. The sheet is stretched by pneumatic or hydraulic force gripping jaws that are attached to a carriage. A form die which is a stretch form block is a common tooling used and is a solid contoured piece that presses the sheet metal (

By stretching a metal sheet complex shapes are produced using a stretch forming equipment such as extrusion or plate on a form die. Compared to rolled or drawn parts, stretch formed parts have better surface quality and shape control. Titanium parts for aerospace applications or aluminum parts for the automobile industry are produced using stretch forming equipment. Moreover, sheet metal applications such as household appliances apply these stretch formed parts.

The stretch forming equipment is either longitudinal or transverse and has die table, jaws, and hydraulic system (OCR Document, 2011). While transverse equipment stretches along its width, the workpiece stretches by longitudinal equipment along its length. Stretch forming equipment can be interfaced to an integral front panel or console or a computer numeric control (CNC) where an operator program the radius of the jaw.

Task 1c: HERF and other sheet metal forming processes

High Energy Rate Forming (HERF) Processes are affected by the strain rates used. As strain rates increase, the flow stress also increases. Adiabatic heating increases with the temperature but makes it hard to form materials like Tungsten and Titanium alloys that require deformation under high strain rates (Hosford, 2005). Compared to conventional processes, energy of deformation is a much higher and is applied for a very shorter time interval. In contrast with conventional forming process, high particle velocities are produced with corresponding large velocity of deformation.

. However, it requires careful handling of source of energy, bigger dies to withstand high energy rates, and highly skilled personnel from design to execution. HERF processes are applied in forming of large plates (up to 25 mm thick) during ship construction and in bending thick pipes or tubes of up to 25 mm thick. Allwood & Shouler, 2007)Under extra fast application of force, metals tend to deform more readily to form large parts. HERF Processes maintain tolerances, have high production rates and relatively lower die costs (

Some of HERF forming processes are explosive, electro-hydraulic and electromagnetic. Explosive forming uses gaseous mixture of explosives such as TNT, RDX, and Dynamite to replace a punch in conventional forming (Hosford, 2005). The key factors considered are cost of tooling, behavior of work material, safety considerations and the overall capital investment. Explosive forming can be the unconfined or confined type. Unconfined types have the die cavity evacuated to produce a pressure pulse of very high intensity as shown in the figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  17

Figure 14: Explosive forming-Unconfined type

A gas bubble expands spherically and then collapses as the metal is deformed into the die with a high velocity (120 m/s). As the vacuum prevents adiabatic heating water acts as energy transfer medium. The process variables are affected by stand-off distance and the type and amount of explosive. The advantage of this forming process is that energy is transmitted effectively on the work while shock wave is efficiently transmitted through water. Moreover, thick and large parts are formed easily and have less chance of damage to work (Khamis, 2011). However, there is need for careful handling of explosives and to withstand shocks, dies must be larger and thicker. These types are common in making of elliptical domes, radar dishes and ship building.

In the confined system, the shock wave or pressure pulse produced indirectly contact the work piece to directly transfer energy without the need for a water medium (Allwood, 2008). Besides, the tube collapses into the die cavity and is used for flaring and bulging operations.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  18

Figure 15: Explosive forming-Confined type

Confined type utilizes the entire shock wave front and is more efficient. On the contrary, is not only unsuitable for large and thick plates but also hazardous during die failure.

shock wave in the water medium produced between electrodes (Kalpakjian & Schmid, 2006). The work plate is deformed by the shock wave and collapses it into the die.Electro hydraulic forming applies

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  19

Figure 16: Electro-hydraulic forming

Although it is similar to explosive forming, a capacitor bank replaces the chemical explosive as a store of electrical energy. Again, it releases less energy released compared to explosive forming. This process forms thin plates, has better control of the pressure pulse and more suitable for small to medium work size (Kalpakjian & Schmid, 2006). The process is limited by the need for vacuum and its suitability only for smaller works. It is used in thinner and small works apart from smaller cone and radar dish.

Electromagnetic forming uses a capacitor bank surrounded by current carrying coils to produce opposing magnetic fields around a tubular work piece (Allwood, 2008). With the coil held firmly, the magnetic repelling force collapses the work piece into the die cavity which assumes die shape as shown in the figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  20

Figure 17: Electro-magnetic forming

Electro Magnetic Forming Process uses a coil that produces varying and opposing magnetic fields. The process parameters are strength of the current, size of the capacitor bank, work material electrical conductivity and the work piece size (Merklein & Allwood, 2012). While suitable for small tubes, the process is safer, and makes it easier for operations like crimping, bending and collapsing. The process is not suitable for large work pieces and is applicable to electrically conducting materials only. This method is used for bulging thin tubes, bending of tubes into complex shapes and crimping of wires, tubes and coils.

Task 1d: Numerical methods in sheet metal forming

  1. The initial blank diameter of the cup is given as;

+ 4dh)Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  21

Where: d = 115mm; h = 100mm

+ 4*115*100)Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  22

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  23

D = 243.36mm

  1. Calculate the safe drawing ratio for the first draw

Suggested drawing ratio based on iterative ways

First drawing: Upper limit ≤2

The thickness ratio in the first draw is: t/D * 100 = (0.9/243.36) * 100 = 0.37

Using the simple iterative method, 1.82 * (0.37/0.4) = 1.68

The safe drawing ratio for the first draw = 1.68

  1. Calculate the blank diameter of the first draw

/max draw ratio (first draw) = 243.36 / 1.68 = 144.85mm0The blank diameter of the first draw = D

  1. Calculate draw ratio for the second stage draw

= 144.85/115 = 1.262/D1Second drawing ratio: 1.2….1.52; draw ratio = D

# of operations

Height (mm)

Diameter (mm)

Drawing ratio

0

First cup drawing

Second stage drawing

  1. Approximate press capacity for first draw

Diameter: 144.85mm

Thickness (t): 0.9mm

2Ultimate Tensile Stress: 450N/mm

): 0.9 tσDrawing coefficient (

Drawing force in the first draw: Fd,max = n* π*d*t*UTS

=0.9* π*144.85*0.9*450

=165,890.76N

Task 2a: Joining methods for sheet metal

. As adhesive cure, clinching keeps parts together. Adhesives allow high-strength joining, and are quick and reliable for a broad combination of variety of materials. The processes result in very profitable production due to very high production speeds and great automation.Hosford & Cadell, 2007)The method neither requires cooling systems for electrodes nor generates heat, fumes or sparks compared to spot welding. The joining process is used with different thickness materials and can be used with dissimilar materials ( used to assemble all types of large appliances such as washing machines, refrigerators, dryers, ovens and dishwashers. Clinching is a clean cold-forming processjoins sheet metal without rivets, bolts or screws. Common sheet metal joining methods are clinching, adhesives and stud bonding. Clinching

These joints are formed, without the aid of any additional jointing process, by interlocking and folding thin sheet metal edges. Their use with light gauge sheet metal of less than 1.6 mm thick is confined to components constructed or fabrications (Allwood, 2008). Some of the self-secured joints include the knocked-up joint, paned-down joint and grooved seam as shown in the figure below. is one that does not require mechanical fixings such as clamping devices, bolts, screws or rivets. gives an attractive appearance. A self-secured joint On the other hand, stud bonding is energy-efficient in joining thin sheets for stoves and washing machines. Stud welding reaches is limited by sheet thickness since it only applies to sheet metal of at least 0.5 mm in thickness (Hu et al., 2002). Although it impairs the appearance of sheet metal, it

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  24

Figure 18: Self-secured joints

 Holes are pre-punched through panels installed in clinching dies and widened by the calibrating collar to centre the stud. Once the Torx torsion lock is installed, the metal is pressed into the ring groove. Hosford & Cadell, 2007).are captive fasteners clinched into formed sheet metal panels. Being ready-to-install sheet metal components, they are cost-effective and reliable options to conventional weld studs. The features of this range of fasteners are torque values and high push-out forces (Patented sheet metal captive fasteners

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  25

Figure 19: Sectional view of a captive fastener

In ultrasonic welding, triangular-shaped energy director is most important design feature. Initial contact between the parts is minimized in an ultrasonically welded joint as shown in the figure below.
Owing to the patented under-head geometry, the joint is able to bear high loads. The process needs fewer steps leading to cost savings and consumption of less energy. An additional advantage is that multiple studs can be installed in a single operation.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  26

Figure 20: Shear joint in ultrasonic welding

The energy director tip melts rapidly during welding as it melts the surrounding areas slightly and fills the joint with molten resin. A permanent bond is created when the melted material from both parts solidifies.

Task 2b: Bolting procedures

In sheet metals, bolted connections are achieved using nuts and threaded studs and selected depending on the possibility of access from sides and the strength (torque) required.

Screw or threaded joints are designed as blind-hole, pierced, protruding or joints as shown in the figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  27

Figure 21: Bolted (left) and screwed joint (right)

The requirement for the parts slope surfaces coming into contact with the faying surfaces and the nut or bolt head is that it should be ≤1:20 with respect to bolt axis normal to the plane. It is not possible to achieve a 100-percent mating between the two flanges. However, it is not advisable to achieve better contact compressible materials such as sheets, insulation or gaskets if the action involved placing between these flanges. The faying surfaces should be brought up ‘evenly’ during the tightening of bolts. A star tightening pattern for the flange type connections is as shown in figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  28

Figure 22: Tightening sequence-star pattern

. The finished fastener joins the 3 to 6mm thick metal stacks, 16mm long and a 450 HV10 rating. The tack displaces material as it penetrates the stack into the threads on the shank. The result is a form fit within the joint as the cylinder raises back up to its starting position after the tack has reached its final position. (Merklein & Allwood, 2012)The surfaces should to be brought into firm contact for high strength bolted joints, although is permissible to isolate areas with no contact. The bolt preload is not prevented where gaps exist in the faying surfaces being developed. A tack, as an assembly aid, undergoes a cold heading process and is made of steel. The shank is thread-rolled, heat treated and coated with zinc nickel to offer corrosion resistance

While the anchor rod is being tightened, the nuts or the head nut fastening the head should be prevented from rotating.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  29

Figure 22: Tack bolting

.Hosford & Cadell, 2007)The two methods used to prevent rotation is by jamming another nut on the head nut or tack welding the nut to the anchor rod on the bottom (unstressed) side of the nut. The ultimate or fatigue strength of the rod is not affected by tack welding nor the jam nut (

High Strength Friction Grip bolts (HSFG) work well fatigue or fluctuating load conditions or in extremely efficient connections. These bolts require hardened washers tightened to their proof loads so as to distribute the load under the bolt heads as shown in the figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  30

Figure 23: Tightening of HSFG bolts

When used on rolled steel sections, the washers need to be tapered. Under working conditions, bolt tension prevents slippage and allows for load transmission to the bolt from plate using friction. However, the friction may be overcome under ultimate load giving rise to slippage meaning that the bearing will govern the design (Merklein & Allwood, 2012). The tightening of HSFG bolts can be done by direct tension indicator, alternate design bolt installation, calibrated wrench tightening or turn-of-nut tightening. The gap produced reduces in proportion to the load as these protrusions are compressed while the bolt is being tightened.

Task 2c: Joining of structural sections

Sheet metals are bent and stretched through an angle adjacent to the outside surfaces. In the event, the inner surfaces of the bends compress. An allowance is necessary when marking out a blank sheet or when developing a template prior to bending. The importance of the ‘neutral line’ is in the enlarged cross-section of a 90° bend (Kalpakjian & Schmid, 2006). The neutral line does not lie on the centre line of the metal due to a slight difference between the amount of tensile strain and the amount of compressive strain. As a result, it lies nearer to the inside of the bend as shown in figure below.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  31

Figure 24: Bend allowance for sheet metal

The neutral line curve is regarded as the arc of a circle in calculating the allowance for a bend in sheet metal.

The radius for neutral line (R) = inside bend radius (r) + the neutral line distance (x)

A number of factors such as inside radius of the bend, the thickness of the metal, and the properties of the metal influence the precise position of the neutral line inside the bend.

The centre line bend allowance can be calculated from the length of the blank needed to form the ‘U’ clip. Where T= 12.7 mm and the position of the neutral line = 0.5 T.

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  32

Figure 25: Centre-line bend allowance

’ and the sum of straight arm length (AB and CD). bcFrom the figure above, the blank length (L) is equivalent to the sum of mean line ‘

bcL = AB + CD +

= Semi-circular arc (R = r + T/2); bcWhere;

Given the outside diameter as 102mm, then L = 49.

The inside semi-circle diameter = 102 – 27 = 76.6mm

(inside radius) = 76.6/2 = 38.3mm, while;rTherefore,

mean radius) = (0.5 × 12.7) + 38.3 = 44.65mm(R The,

Length of arms:

AB = 80-(102/2) = 29mm

CD = 100-(102/2) = 49mm

):bcBend allowance (

= Phi (R) = 44.65 × 3.142 = 140.33mm

+ CD + AB = 140.3 + 49 + 29bcTotal blank length =

=218.3mm

Flanges join vessel, valves and pipe within a system, hence sizing limitations must be taken into consideration (Merklein & Allwood, 2012). When applying the sealing components and gasket to these flanges, it is important to consider gasket placement, optimum surface finish and the available clamp load to minimize flange rotation.

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Figure 26: Flange depth determination

. This also applies to the contact between the shoulder of the socket and the end of the pipe.Barthel et al., 2008) the assembly of the joint before welding, should be withdrawn approximately 1/16″ (1.6 mm) away after inserting into the socket to the maximum depth (inpipe or tube In determining the flange depth, serrations do not exceed 1/32” width spacing and 1/64” depth. The

In a socket weld, during solidification of the weld metal, the bottoming clearance reduces the residual stress occurring at the root of the weld.

Task 3a: Cutting methods used in fabrication

. Allwood & Shouler, 2007)Portable travel carriages also cut through beveled, contoured or straight edges. In the motor-driven carriage, there is a cutting torch on a mechanized setup that adjusts for travel speed and torch height. For oxy-fuel cutting, the requirements are a minimum of gaseous reaction products, oxidation reaction, the oxide melting point and the ignition temperature (. The equipment can be used mechanized or manually and is low cost. Cutting speed and cut quality performance can be significantly enhanced using a number of nozzle design and fuel gas options. A vigorous exothermic chemical reaction is instigated through a jet of pure oxygen directed into the preheated area. The jet pierces through the material cutting it when the oxygen jet blows away the slag. Kalpakjian & Schmid, 2006) which cuts thicknesses from 0.5mm to 250mm (oxy-fuel processThe most widely used industrial thermal cutting process is the

 carriage-driven motors in numerical controls and optical and mechanical tracers.  y and xFlame-cutting machines are stationary floor-mounted using fabricators for heavy-duty and large parts operations. Cutting-path information is fed to 

Fabrication and Welding Technology for Mechanical Engineering - Title: Understand sheet metal forming processes, joining methods and cutting processes.  34

Figure 27: Oxytylene cutting process

. National Occupational Standards, 2012)cutting speed by up to 25 percent with cutting-oxygen pressure increased to 100 PSI ( to offer tapered tipProfile programs stored in the machine controller are used in computer-numerical-control machines (CNC). This method offers more user-friendly design, faster communications, better programming features, increased speed and accuracy and more storage capacity. On the other hand, mechanized cutting uses a

run on line power and leave the edges clean because it cuts steel up to 2 in. thick and is rated at 15-150-A capacity. As the process do not need secondary gas or special plasma, it utilizes dry and clean compressed air. It runs on single or three-phase on a range of primary voltage inputs like 208-575 or 110/220. Air-PAC hand torches are rated by cutting amperage come in 700, 900, and 1800 styles but the output of the machine should match that of torch (National Occupational Standards, 2012).Air-plasma Arc-cutting machines

 guide for minimum ram deflection through the entire stroke (Adetoro & Cardoso, 2016). By reciprocating motion, the hacksaws cut are driven mechanically by hydraulic power or through a pitman-cam assembly. Circular saws are toothed around the circumference and cuts with disc blades. The saw blade, in the arbor hub, fit over holding pins and is mounted on a motor-driven arbor.uillotine which is a The ram is dropped vertically by vertical guided-ram shears or g  use cam or an eccentric to bring it down under load or to power the ram. Oil-filled cylinders for this action are employed by hydraulic machines. Compared to hydraulic systems, the cycle in mechanical shears is more quickly but user inflexible. , mechanical shearsmechanical cuttingIn

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