Monday, 22 April 2013
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 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.
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.
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.
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.
Friday, 19 April 2013
Thursday, 18 April 2013
Introduction to Welding Technology
Welding is a fabrication process used to join materials, usually metals or thermoplastics, together. During welding, the pieces to be joined (the workpieces) are melted at the joining interface and usually a filler material is added to form a pool of molten material (the weld pool) that solidifies to become a strong joint.
In contrast, Soldering and Brazing do not involve melting the workpiece but rather a lower-melting-point material is melted between the workpieces to bond them together.
Types of Welding
There are many different types of welding processes and in general they can be categorized as:
Arc Welding: A welding power supply is used to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. In such welding processes the power supply could be AC or DC, the electrode could be consumable or non-consumable and a filler material may or may not be added.
The most common types of arc welding are:
· Shielded Metal Arc Welding (SMAW): A process that uses a coated consumable electrode to lay the weld. As the electrode melts, the (flux) coating disintegrates, giving off shielding gases that protect the weld area from atmospheric gases and provides molten slag which covers the filler metal as it travels from the electrode to the weld pool. Once part of the weld pool, the slag floats to the surface and protects the weld from contamination as it solidifies. Once hardened, the slag must be chipped away to reveal the finished weld.
Introduction to Non-Destructive Testing Techniques
· Gas Metal Arc Welding (GMAW): A process in which a continuous and consumable wire electrode and a shielding gas (usually an argon and carbon dioxide mixture) are fed through a welding gun.
· Gas Tungsten Arc Welding (GTAW): A process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas, and a filler metal that is fed manually is usually used.
Gas Welding: In this method a focused high temperature flame generated by gas combustion is used to melt the workpieces (and filler) together. The most common type of gas welding is Oxy-fuel welding where acetylene is combusted in oxygen.
Resistance Welding: Resistance welding involves the generation of heat by passing a high current (1000–100,000 A) through the resistance caused by the contact between two or more metal surfaces where that causes pools of molten metal to be formed at the weld area. The most common types of resistance welding are Spot-welding (using pointed electrodes) and Seam-welding (using wheel-shaped electrodes).
Energy Beam Welding: In this method a focused high-energy beam (Laser beam or electron beam) is used to melt the workpieces and thus join them together.
Solid-State Welding: In contrast to other welding methods, solid-state welding processes do not involve the melting of the materials being joined. Common types of solid-state welding include; ultrasonic welding, explosion welding, electromagnetic pulse welding, roll welding, friction welding (including friction-stir-welding), etc.
There is some special technical vocabulary (or language) that is used in welding. The basic terms of the welding language include:
Filler Material: When welding two pieces of metal together, we often have to leave a space between the joint. The material that is added to fill this space during the welding process is known as the filler material (or filler metal). Two types of filler metals are commonly used in welding are welding rods and welding electrodes.
· Welding Rod: The term welding rod refers to a form of filler metal that does not conduct an electric current during the welding process. The only purpose of a welding rod is to supply filler metal to the joint. This type of filler metal is often used for gas welding.
· Electrode: In electric-arc welding, the term electrode refers to the component that conducts the current from the electrode holder to the metal being welded. Electrodes are classified into two groups: consumable and non-consumable.
o Consumable electrodes not only provide a path for the current but they also supply filler metal to the joint. An example is the electrode used in shielded metal-arc welding.
o Non-consumable electrodes are only used as a conductor for the electrical current, such as in gas tungsten arc welding. The filler metal for gas tungsten arc welding is a hand fed consumable welding rod.
Flux: Before performing any welding 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 weld may result. The term flux refers to a material used to dissolve oxides and release trapped gases and slag (impurities) from the base metal such that the filler metal and the base metal can be fused together. Fluxes come in the form of a paste, powder, or liquid. Different types of fluxes are available and the selection of appropriate flux is usually based on the type of welding and the type of the base metal.
Types of Welded Joints
The weld joint is where two or more metal parts are joined by welding. The five basic types of weld joints are the butt, corner, tee, lap, and edge.
Butt Joint: it is used to join two members aligned in the same plane. This joint is frequently used in plate, sheet metal, and pipe work.
Corner and Tee Joints: these joints are used to join two members located at right angles to each other. In cross section, the corner joint forms an L-shape, and the tee joint has the shape of the letter T.
Lap Joint: this joint is made by lapping one piece of metal over another. This is one of the strongest types of joints available; however, for maximum joint efficiency, the overlap should be at least three times the thickness of the thinnest member of the joint.
Edge Joint: it is used to join the edges of two or more members lying in the same plane. In most cases, one of the members is flanged, as seen in the figure. This type is frequently used in sheet metal work for joining metals 1/4 inch or less in thickness that are not subjected to heavy loads.
Types of Welds
There are many types of welds. The most common types are the bead, surfacing, plug, slot, fillet, and groove.
- · A weld Bead is a 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 made without much weaving motion is often referred to as a stringer bead. On the other hand, a weld bead made with side-to-side oscillation is called a weave bead.
- · A Fillet weld is triangular in shape and this weld is used to join two surfaces that are at approximately right angles to each other in a lap, tee, or comer joint.
- Plug and Slot welds are welds made through holes or slots in one member of a lap joint. These welds are used to join that member 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 weld is adaptable to a variety of butt joints, as seen in the figure.
weld beads, depending on the thickness of the
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-pass layer is made with stringer beads in manual operations.
o The buildup sequence refers to the order in
which the beads of a multiple-pass weld are
deposited in the joint. Usually, before adding
the next pass, the previous pass needs to
cool down to a certain temperature which is
called the inter-pass temperature. Also,
before adding the next pass, the surface of
the previous pass needs to be cleaned from
slag, especially with SMAW, using a wire brush or other appropriate method.
Parts of Welded Joints
While there are many variations of joints, the parts of the joint are described by standard terms.
- · The root of 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 cross section.
- · A groove is an opening or space provided between the edges of the metal parts to be welded.
o The groove face is that surface of a metal part included in the groove, as shown in view A.
- · A given joint 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 be joined by a groove weld that has not been grooved. As you can see,
the root face has
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 some joints.
· The specified requirements for a particular joint are expressed in terms such as bevel angle, groove angle, groove radius, and root opening which are illustrated in the figure.
o The bevel angle is the angle formed
and a plane perpendicular to the surface
of the member.
o The groove angle is the total angle of the
groove between the parts to be joined.
For example, if the edge of each of two
plates were beveled to an angle of 30
degrees, the groove angle would be 60
o The groove radius is the radius used to form the shape of a J- or U-groove weld joint. It is used only for special groove joint designs.
o The root opening refers to the separation between the parts to be joined at the root of the joint. It is sometimes called the “root gap”.
? Root penetration refers to the depth that a weld
extends into the root of the joint. Root penetration is
measured on the center line of the root cross section.
? Joint penetration refers to the minimum depth that a groove weld extends from its face into a joint,
exclusive of weld reinforcement.
? In many cases, root penetration and joint penetration, often refer to the same dimension.
weld metal in excess of the metal necessary to
fill a joint. The reinforcement needs to be
grinded in some cases depending on the
intended use of the joint.
Parts of Welds
It is important to be familiar with the terms used to describe a weld. The figure shows the parts of groove weld and fillet welds.
· The face is the exposed surface of a weld on the side from which the weld was made.
· The toe is the junction between the face of the weld and the base metal.
· The root of a weld includes the points at which the back of the weld intersects the base metal surfaces.
· In a fillet weld, the leg is the portion of the weld from the toe to the root.
· In a fillet weld, the throat is the distance from the root to a point on the 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 size of a fillet weld refers to the length of the legs of the weld. The two legs are assumed to be equal in size unless otherwise specified.
Some other terms which are used to describe areas or zones of welds are:
- The fusion zone is the region of the base metal that is actually melted. The depth of fusion is the distance that fusion extends into the base metal or previous welding pass.
- The heat-affected zone (HAZ) refers to that portion of the base metal that has not been melted; however, the structural or mechanical properties of the metal have been altered by the welding heat.
Welding symbols are used on drawings to indicate the type and specifications of the weld.
- The figure shows the American Welding Society (AWS) standard welding symbol. The most important features of the welding symbol are illustrated below: