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Monday, 22 April 2013

FABRICATION OF EDGES, JOINTS, SEAMS, AND NOTCHES


FABRICATION OF EDGES, JOINTS, SEAMS, AND NOTCHES
There are numerous types of edges, joints, seams, and notches used to join sheet-metal work. We will discuss those that are most often used.
Edges
Edges are formed to enhance the appearance of the work, to strengthen the piece, and to eliminate the cutting hazard of the raw edge. The kind of edge that you use on any job will be determined by the purpose, by the sire, and by the strength of the edge needed.
The SINGLE-HEM EDGE is shown in figure 2-54. This edge can be made in any width. In general, the heavier the metal, the wider the hem is made. The allowance for the hem is equal to its width (W in fig. 2-54).


Figure 2-53.-Traingular development of a transition piece.
The DOUBLE-HEM EDGE (fig. 2-55) is used when added strength is needed and when a smooth edge is required inside as well as outside. The allowance for the double-hem edge is twice the width of the hem.

Figure 2-54.-Single-hem edge.

Figure 2-55.-Double-hem edge
A WIRE EDGE (fig. 2-56) is often specified in the plans, Objects, such as ice-cube trays, funnels, garbage pails, and other articles, formed from sheet metal are fabricated with wire edges to strengthen and stiffen the jobs and to eliminate sharp edges, The


Figure 2-57.-Making a grooved seam joint.

Figure 2-58.-Hand groover.


Figure 2-59.-Locking a grooved seam with a hand groover.
allowance for a wire edge is 2 1/2 times the diameter of the wire used As an example, you are using wire that has a diameter of 1/8 inch. Multiply 1/8 by 2 1/2 and your answer will be 5/16 inch, which you will allow when laying out sheet metal for making the wire edge.
Joints
The GROOVED SEAM JOINT (fig. 2-57) is one of the most widely used methods for joining light- and medium-gauge sheet metal. It consists of two folded edges that are locked together with a HAND GROOVER (fig. 2-58).
When making a grooved seam on a cylinder, you fit the piece over a stake and lock it with the hand groover (fig. 2-59). The hand groover should be approximately 1/16 inch wider than the seam. Lock the seam by making prick punch indentions about 1/2 inch in from each end of the seam.
The CAP STRIP SEAM (fig. 2-60, view A) is often used to assemble air-conditioning and heating ducts. A variation of the joint, the LOCKED CORNER SEAM (fig. 2-60, view B), is widely accepted for the assembly of rectangular shapes.

Figure 2-60.-(A) Cap strip seam, (B) Locked corner seam

A DRIVE SLIP JOINT is a method of joining two flat sections of metal. Figure 2-61 is the pattern for the drive slip. End notching and dimensions vary with application and area practice on all locks, seams, and edges.
"S" joints are used to join two flat surfaces of metal. Primarily these are used to join sections of rectangular duct. These are also used to join panels in air housings and columns.
Figure 2-62 shows a flat "S" joint. View A is a pattern for the "S" cleat. View B is a perspective view of the two pieces of metal that form the flat "S" joint. In view C, note the end view of the finished "S" joint.
Figure 2-63 shows a double "S" joint. View B is the pattern for the double "S" cleat. View A is one of two pieces of metal to be joined. Note the cross section of a partially formed cleat and also the cross section of the finished double "S" joint. his is a variation of

Figure 2-61.-Drive slip pattern and connections

Figure 2-62.-"S" joint or slip pattern and connections.

Figure 2-63.-Double "S' joint (cleat) pattern.
the simple flat "S" and it does not require an overlap of metals being joined.
Figure 2-64 shows a standing "S" joint. View B is the pattern for the standing "S" cleat. View A is one of the two pieces of metal to be joined. Note the cross section of the finished standing "S" cleat and standing "S" joint.
Seams
Many kinds of seams are used to join sheet-metal sections. Several of the commonly used seams are shown in figure 2-65. When developing the pattern, ensure you add adequate material to the basic dimensions to make the seams. The folds can be made by hand; however, they are made much more easily on a bar folder or brake. The joints can be finished by soldering and/or riveting.
When developing sheet-metal patterns, ensure you add sufficient material to the base dimensions to make the seams. Several types of seams used to join sheet-metal sections are discussed in this section.
There are three types of lap seams: the PLAIN LAP seam, the OFFSET LAP seam, and the CORNER LAP seam (fig. 2-66). Lap seams can be joined by drilling and riveting, by soldering, or by both riveting and soldering. To figure the allowance for a lap seam, you must first know the diameter of the rivet that you plan to use. The center of the rivet must be set in from the edge a distance of 2 1/2 times its diameter; therefore, the allowance must be five times the diameter of the rivet that you are using. Figure 2-67 shows the procedure for laying out a plain lap and a comer lap for seaming with rivets (d represents the diameter of the rivets). For comer seams, allow an additional one sixteenth of an inch for clearance.



Figure 2-65.-Common sheet-metal seams.



Figure 2-67.-Layout of lap seams for riveting.
GROOVED SEAMS are useful in the fabrication of cylindrical shapes. There are two types of grooved seams-the outside grooved seam and the inside grooved seam (fig. 2-68). The allowance for a grooved seam is three times the width (W in fig. 2-68) of the lock, one half of this amount being added to each edge. For example, if you are to have a 1/4-inch grooved seam, 3 x 1/4 = 3/4 inch, or the total allowance; 1/2 of 3/4 inch = 3/8 inch, or the allowance that you are to add to each edge.
The PITTSBURGH LOCK SEAM (fig. 2-69) is a comer lock seam. Figure 2-69 shows a cross section of the two pieces of metal to be joined and a cross section of the finished seam. This seam is used as a lengthwise seam at comers of square and rectangular pipes and elbows as well as fittings and ducts. This seam can be made in a brake but it has proved to be so universal in use that special forming machines have been designed and are available. It appears to be quite complicated, but like lap and grooved seams, it

Figure 2-68.-Grooved seams



Figure 2-69.-Pittsburgh lock seam.
consists of only two pieces. The two parts are the flanged, or single, edge and the pocket that forms the lock The pocket is formed when the flanged edge is inserted into the pocket, and the extended edge is turned over the inserted edge to complete the lock. The method of assembling and locking a Pittsburgh seam is shown in figures 2-70 and 2-71.
The allowance for the pocket is W + W + 3/16 inch. W is the width or depth of the pocket. The width of the flanged edge must be less than W. For example, if you are laying out a 1/4-inch Pittsburgh leek seam (fig. 2-72), your total allowance should be 1/4 + 1/4 + 3/16 inch, or 11/16 inch for the edge on which you are laying out the pocket and 3/16 inch on the flanged edge.


Figure 2-70.-Assembly of a Pittsburgh lock seam
STANDING SEAMS are used for joining metals where extra stiffness is needed, such as roofs, air housing, ducts, and so forth. Figure 2-73 is a cross section of the finished standing seam. Dimensions and rivet spacing will vary with application.
Standing seams used when stiffening is required are as follows: The SPREADER DRIVE CAP, the POCKET SLIP, and the GOVERNMENT LOCK (fig. 2-74) are seams frequently used in large duct construction where stiffeners are required.
The DOVETAIL SEAM is used mainly to join a round pipe/fitting to a flat sheet or duct. This seam can be made watertight by soldering. Figure 2-75 shows the pattern for forming a dovetail seam and an example of its use.
Notches
Notching is the last but not the least important step to be considered when you are getting ready to lay out

Figure 2-71.-Closing a Pittsburgh lock seam

Figure 2-72.-Layout of a 1/4-inch Pittsburgh lock seam.

Figure 2-73.-Cross section of a standing seam.
a job. Before you can mark a notch, you will have to lay out the pattern and add the seams, the laps, or the stiffening edges. If the patterns are not properly notched, you will have trouble when you start forming, assembling, and finishing the job.
No definite rule for selecting a notch for a job can be given. But as soon as you can visualize the assembly of the job, you will not have any trouble determining the shape and size of the notch required


Figure 2-74.-Miscellaneous seam.

Figure 2-75.-Dovetail lock seam

for the job. If the notch is made too large, a hole will be left in the finished job. If the notch is too small or not the proper shape, the metal will overlap and bulge at the seam or edge. Do not concern yourself too much if your first notches do not come out as you expected-practice and experience will dictate size and shape.
A SQUARE NOTCH (fig. 2-76) is likely the first you will make. It is the kind you make in your layout of a box or drip pan and is used to eliminate surplus material This type of notch will result in butt comers. Take a look around the shop to see just how many different kinds of notches you can see in the sheet-metal shapes.
SLANT NOTCHES are cut at a 45-degree angle across the comer when a single hem is to meet at a 90-degree angle. Figure 2-77 shows the steps in forming a slant notch.
A V NOTCH is used for seaming ends of boxes. You will also use a full V notch when you have to construct a bracket with a toed-in flange or for similar construction. The full V is shown in figure 2-78.
When you are making an inside flange on an angle of less than 90 degrees, you will have to use a modification of the full V notch to get flush joints. The angle of the notch will depend upon the bend angle. A modified V notch is shown in figure 2-79.

Figure 2-77.-Slant notch.

A WIRE NOTCH is a notch used with a wire edge. Its depth from the edge of the pattern will be one wire diameter more than the depth of the allowance for the wire edge (2 1/2 d), or in other words, 3 1/2 times the diameter of the wire (3 1/2 d). Its width is equal to 1 1/2 times the width of the seam (1 1/2 w). That portion of the notch next to the wire edge will be straight. The shape of the notch on the seam will depend on the type of seam used, which, in figure 2-80, is 45 degrees for a grooved seam.
Most of your work will require more than one type of notch, as shown in figure 2-80, where a wire notch was used in the forming of a cylindrical shape joined by a grooved seam. In such a layout, you will have to notch for the wire edge and seam.

Thursday, 18 April 2013

Welding



 Introduction to Welding Technology
Welding is a fabricationprocess used to join materials, usually metals or thermoplastics, together.During welding, the pieces to be joined (the workpieces) are meltedat the joining interface and usually a filler material is added to forma pool of molten material (the weld pool) that solidifies to become astrong joint.

In contrast, Soldering andBrazing do not involve melting the workpiece but rather alower-melting-point material is melted between the workpieces to bond themtogether.

Types of Welding

There are many different typesof welding processes and in general they can be categorized as:

ArcWelding: A welding power supplyis used to create and maintain an electric arc between an electrode andthe base material to melt metals at the welding point. In such weldingprocesses the power supply could be AC or DC, the electrode could be consumableor non-consumable and a filler material may or may not be added.

The most common types of arcwelding are:

·       Shielded Metal Arc Welding (SMAW): A processthat uses a coated consumable electrode to lay the weld. As the electrodemelts, the (flux) coating disintegrates, giving off shielding gases thatprotect the weld area from atmospheric gases and provides molten slag whichcovers the filler metal as it travels from the electrode to the weld pool. Oncepart of the weld pool, the slag floats to the surface and protects the weldfrom contamination as it solidifies. Once hardened, the slag must be chippedaway to reveal the finished weld.

Introduction to Non-Destructive Testing Techniques




·        Gas Metal Arc Welding (GMAW): A process in which acontinuous and consumable wire electrode and a shielding gas (usually anargon and carbon dioxide mixture) are fed through a welding gun.
·       Gas Tungsten Arc Welding (GTAW): A process that uses anonconsumable tungsten electrode to produce the weld. The weld area isprotected from atmospheric contamination by a shielding gas, and a filler metalthat is fed manually is usually used.

Gas Welding: In this method a focusedhigh temperature flame generated by gas combustion is used to melt theworkpieces (and filler) together. The most common type of gas welding isOxy-fuel welding where acetylene is combusted in oxygen.

Resistance Welding: Resistance welding involvesthe generation of heat by passing a high current (1000–100,000 A) through theresistance caused by the contact between two or more metal surfaces where thatcauses pools of molten metal to be formed at the weld area. The most commontypes of resistance welding are Spot-welding (using pointedelectrodes) and Seam-welding (using wheel-shaped electrodes).

Energy Beam Welding: In this method a focusedhigh-energy beam (Laser beam or electron beam) is used to melt the workpiecesand thus join them together.

Solid-State Welding: In contrast to otherwelding methods, solid-state welding processes do not involve the melting of thematerials being joined. Common types of solid-state welding include; ultrasonicwelding, explosion welding, electromagnetic pulse welding, roll welding,friction welding (including friction-stir-welding), etc.


Welding Terminology

There is some special technical vocabulary (or language)that is used in welding. The basic terms of the welding language include:

FillerMaterial:When welding two pieces of metal together, we often have to leave a spacebetween the joint. The material that is added to fill this space during thewelding process is known as the filler material (or filler metal). Two types offiller metals are commonly used in welding are welding rods and weldingelectrodes.

·       Welding Rod:The term welding rod refers to a form of filler metal that does not conductan electric current during the welding process. The only purpose of a welding rod is to supply filler metal to the joint. Thistype of filler metal is often used for gas welding.


·   Electrode: In electric-arc welding, the termelectrode refers to the component that conducts the current from the electrodeholder to the metal being welded. Electrodes are classified into two groups:consumable and non-consumable.
o Consumable electrodes not only provide a path forthe current but they also supply filler metal to the joint. An example is theelectrode used in shielded metal-arc welding.
o Non-consumableelectrodesareonly used as a conductor for the electrical current, such as in gas tungsten arcwelding. The filler metal for gas tungsten arc welding is a hand fed consumablewelding rod.

Flux: Before performing anywelding process, the base metal must be cleaned form impurities such as oxides(rust). Unless these oxides are removed by using a proper flux, a faulty weldmay result. The term flux refers to a material used to dissolve oxidesand release trapped gases and slag (impurities) from the base metal such thatthe filler metal and the base metal can be fused together. Fluxes come in theform of a paste, powder, or liquid. Different types of fluxes are available andthe selection of appropriate flux is usually based on the type of welding andthe type of the base metal.


Types of Welded Joints

The weld joint is where two or more metal parts are joinedby welding. The five basic types of weld joints are the butt, corner,tee, lap, and edge.
ButtJoint: it isused to join two members aligned in the same plane. This joint is frequentlyused in plate, sheet metal, and pipe work.

Corner and Tee Joints: these joints are used tojoin two members located at right angles to each other. In cross section, thecorner joint forms an L-shape, and the tee joint has the shape of the letter T.


Lap Joint: this joint is made bylapping one piece of metal over another. This is one of the strongest types ofjoints available; however, for maximum joint efficiency, the overlap should beat least three times the thickness of thethinnest member of the joint.



Edge Joint: it is used to join theedges of two or more members lying in the same plane. In most cases, one of themembers is flanged, as seen in the figure. This type is frequently used insheet metal work for joining metals 1/4 inch or less in thickness that are notsubjected to heavy loads.

Types of Welds

There are many types of welds. The most common types arethe bead, surfacing, plug, slot, fillet, and groove.


  • ·       A weld Bead isa weld deposit produced by a single pass
with one of the welding processes.A weld bead may be either narrow or wide,depending on the amount of transverse oscillation(side-to-side movement) used by the welder. A weld bead madewithout much weaving motion is often referred to as a stringerbead. On the other hand, a weldbead made with side-to-side oscillation is called a weave bead.


  • ·       Several weld beads appliedside-by-side are usually used in Surfacingwhichis a welding process used to apply a hard, wear-resistant layer of metalto surfaces or edges of worn-outparts.


  • ·       A Filletweld is triangular in shape and this weld is used to join two surfacesthat are atapproximately right angles to each other in a lap, tee, or comer joint.
                           

  •  Plug and Slot welds are welds made throughholes or slots in one member of a lap joint. These welds are used to join thatmember to the surface of another member that has been exposed through the hole.
  •  Groove welds (also may be referred to as Butt welds)are simply welds made in the groove between two members to be joined. The weldis adaptable to a variety of butt joints, as seen in the figure.
Groove welds may be joined with one or more
weldbeads, depending on the thickness ofthe
metal.If two or more beads are deposited in the
groove,the weld is made with multiple-pass
layers, as shown in the figure. As a rule, a multiple-passlayer is made with stringer beads in manual operations.

o The buildup sequence refers to the orderin
whichthe beads of a multiple-pass weld are
depositedin the joint. Usually, before adding
thenext pass, the previous pass needs to
cooldown to a certain temperature which is
calledthe inter-pass temperature. Also,
beforeadding the next pass, the surface of
theprevious pass needs to be cleaned from
slag, especially with SMAW, using a wire brush or otherappropriate method.



Parts of Welded Joints

While there are manyvariations of joints, the parts of the joint are described by standard terms.

  • ·      The rootof a joint is that portion of the joint where the metals are closest to each other. As shown in the figure, the root may be a point, a line, or an area, when viewed in crosssection.





  • ·       A grooveis an opening or space provided between the edges of the metal parts tobe welded.
o The grooveface is that surface of a metal part included in the groove, as shownin view A.

  • ·       A givenjoint may have a root face or a root edge.
o The root face,also shown in view A, is the portion of the prepared edge of a part to bejoined by a groove weld that has not been grooved. As you can see,
the root face has
relatively small
dimensions.
o The root edge is
basically a root face of
zero width, as shown
in view B. As you can
see in views C and D
of the illustration, the
groove face and the
root face are the
same metal surfaces
in somejoints.

·       The specifiedrequirements for a particular joint are expressed in terms such as bevelangle, groove angle, groove radius, and root opening whichare illustrated in the figure.

o The bevel angle isthe angle formed
betweenthe prepared edge of a member
and aplane perpendicular to the surface
of themember.
o The grooveangle is the total angle of the
groovebetween the parts to be joined.
Forexample, if the edge of each of two
plateswere beveled to an angle of 30
degrees,the groove angle would be 60
degrees.

o The grooveradius is the radius used to form the shape of a J- or U-groove weldjoint. It is used only for special groove joint designs.
o The root opening refersto the separation between the parts to be joined at the root of the joint. Itis sometimes called the “root gap”.
? Root penetration refersto the depth that a weld
extendsinto the root of the joint. Root penetration is
measuredon the center line of the root cross section.
? Jointpenetration refers to the minimum depth that a grooveweld extends from its face into a joint,
exclusive of weld reinforcement.
?In many cases, rootpenetration and joint penetration, often refer to the same dimension.

? Weld reinforcement isa term used to describe
weldmetal in excess of the metal necessary to
fill ajoint. The reinforcement needs to be
grindedin some casesdepending on the
intended use of the joint.

                                                                                                                                                                        

Parts of Welds

It is important to be familiarwith the terms used to describe a weld. The figure shows the parts of grooveweld and fillet welds.

·       The faceis the exposed surface of a weld on the side from which the weld was made.
·       The toeis the junction between the face of the weld and the base metal.
·       The rootof a weld includes the points at which the back of the weld intersects thebase metal surfaces.
·       In a filletweld, the leg is the portion of the weld from the toe to theroot.
·       In a filletweld, the throat is the distance from the root to a point onthe face of the weld along a line perpendicular to the face of the weld.Theoretically, the face forms a straight line between the toes.
·       The sizeof a fillet weld refers to the length of the legs of the weld. The twolegs are assumed to be equal in size unless otherwise specified.

Some other terms which are usedto describe areas or zones of welds are:


  •   The fusionzone is the region of the base metal that is actually melted. Thedepth of fusion is the distance that fusion extends into the base metal orprevious weldingpass.



  •   The heat-affectedzone (HAZ) refers to that portion of the base metalthat has not been melted; however, thestructural or mechanical propertiesof the metal have been altered by thewelding heat.

Introduction to Non-DestructiveTesting Techniques Introduction to Welding Technology Page 9 of 9
Welding Symbols
Welding symbols are used ondrawings to indicate the type and specifications of the weld.

  • The figure showsthe American Welding Society (AWS) standard welding symbol.       The most important features of thewelding symbol are illustrated below:


  •   The table showsthe Basic weld symbol for the different types of welds.




  •   The figures belowshow some examples for the use of welding symbols