Fabricating Techniques and Practices
INTRODUCTION
The first step in almost every welding operation is the assembly of the parts to be joined by welding. At the very basic level, this assembly can be just placing two pieces of metal flat on a table and tack welding them together for practice welding. At a higher level is the assembly of complex equipment, buildings, ships, or other large welded structures. The important thing to remember, however, is that no matter how large or complicated the welded structure, it is assembled one piece at a time. That is true for a simple project you build as part of your welding shop learning in school or for the ship, rocket engine, or building you might construct one day.
FABRICATION
The difference between a weldment and a fabrication is that a weldment is an assembly whose parts are all welded together, but a fabrication is an assembly whose parts may be joined by a combination of methods including welds, bolts, screws, adhesives, and so on. All weldments are fabrications, but not all fabrications are weldments.
In addition to straight welding, welders are often required to assemble parts together to form a weldment. The weldment may form a completed project or may only be part of a larger structure. Some weldments are composed of two or three parts; others may have hundreds or even thousands of individual parts. Even the largest weldments start by placing two parts together. The number and type of steps required to take a plan and create a completed project will vary depending on the complexity and size of the finished weldment. All welding projects start with a plan. This plan can range from a simple one that may exist only in the mind of the welder, or it can be complex and composed of a set of drawings. As a beginning welder, you must learn how to follow a set of drawings to produce a finished weldment.
Safety
As with any welding, safety is of primary concern for fabrication of weldments. Fabrication may present some potential safety problems not normally encountered in straight shop welding. Unlike most practice welding, much of the larger fabrication work may need to be performed outside an enclosed welding booth. Additionally, several welders may be working on a structure at the same time. You must let the other workers in the area know that you are going to be welding so they can protect themselves from the arc light, sparks, and other possible hazards. Tell them about the hazards because you cannot assume that they know about the hazards of welding. Extra care must be taken to ensure that burns do not occur on you or the other welders from the arc or hot sparks. When possible, you should erect portable welding curtains. Ventilation is also important because the normal shop ventilation may not extend to the fabrication area. A portable fan may be needed to help blow the welding fumes away from the work area. Be sure the fan blows the fumes away from you and others. Often you will be working in an area that has welding cables, torch hoses, extension cords, and other trip hazards lying on the floor. These must be flat on the floor and should be covered if they are in a walkway to prevent accidental tripping. Keep all of the scrap metal and other debris picked up; a neat work area is a safe work area. As the fabrication grows in size, it will become heavier. Make sure it is stable and not likely to fall. A weldment that starts out stable and well-supported can become unstable and likely to fall as it grows in size. Keeping it well-supported is important especially if you have to crawl under it to work on the bottom side welds. Check with your supervisor or shop safety officer before working under any weldment. These and other safety concerns are covered in Chapter 2, “Welding Safety.” You should also read any safety booklets supplied with the equipment before starting any project.
PARTS AND PIECES
Welded fabrications can be made from precut and preformed parts, or they can be made from parts cut and formed by hand. In most cases, weldments are made using a variety of precut and preformed parts along with handmade pieces. Preshaped pieces may be precut, bent, machined, or otherwise prepared before you receive them. This is a common practice in large shops and on large-run projects. Large shops may have an entire department dedicated to material and part preparation. When a large number of the same items are made in a largerun shop, the shop may outsource some of the parts to shops that specialize in mass producing items. When making an assembly with precut and formed parts, little or no on-the-job fitting may be required. That, of course, depends on how accurately the parts were prepared. The opposite end of the spectrum from preformed parts is custom fabrication, in which all or most of the assembly is handmade. This might include cutting, bending, grinding, drilling, or other similar processes. Almost all weldments were produced by hand until the introduction of automated equipment for cutting, bending, and machining. Today, many items, including some large machines and almost all repair work, are still custom-fabricated.
Advantages of using preformed parts include the following:
Cost—Shops that specialize in cutting out mass numbers of similar parts can do it less expensively than a shop that makes the same parts one-by-one by hand.
Speed—High-speed cutting and forming machines can produce a large number of items quickly.
Accuracy—Automated equipment can make parts more accurately than they can be made by hand.
Less waste—The wise use of materials is important to both control cost and to conserve natural resources.
Advantages of custom-fabricating parts include the following:
Originals—It is not practical to set up an automated process when there will be only one of a kind or a limited number of an item produced.
Prototypes—Often, even if there are going to be thousands or even tens of thousands of a weldment produced, the first one, the prototype, must be made by hand to be sure that everything works as it was planned.
Repairs—Seldom would it be necessary to make a large number of the same part or piece when making a repair on a damaged or worn item.
Custom jobs—Sometimes people want to have something special or unique built or modified just for them.
TACK WELDS
Tack welds are the welds, usually small in size, that are made during the assembly to hold all of the parts of a weldment together so they can be finish-welded. Making good tack welds is one of the keys to assembly work. Tack welds must also be small enough to be incorporated into the final weld without causing a discontinuity in its size or shape. They must be strong enough to hold the parts in place for welding but small enough so that they become an unseen part of the finished weld. Deciding on the number, size, and location of tack welds takes some planning. Some of the factors to consider regarding the number of tack welds include the following:
Thickness of the metal—A large number of very small tack welds should be used on thin metal sections, while a few large tack welds may be used for thicker metal parts.
Length and shape of the joint—Obviously, short joints take fewer welds, but some long, straight joints may have very few tack welds compared to a shorter joint that is very curvy.
Welding stresses—All welds create stress in the surrounding metal as they cool and shrink. Larger welds produce greater stresses that might pull tack welds loose from an adjoining part if the tack welds are not strong enough to withstand the welding stresses.
Tolerances—The more exacting the tolerance for the finished weldment, the more tack welds are required.
Fit-up—When custom bending parts during the fit-up process, it may be necessary to use a large number of small tack welds to keep the parts in alignment and make the bends more uniform.
Tack welds must be made in accordance with any welding procedure with an appropriate filler metal. They must be located well within the joint so that they can be completely remelted into the finished weld. Posttack welding cleanup is required to remove any slag or impurities that may cause finished weld flaws. Sometimes the ends of a tack weld must be ground down to a taper to improve their tie-in to the finished weld metal. Make sure that your tack welds are not going to be weld defects in the completed weldment. A good tack weld is one that does its job by holding parts in place, yet is undetectable in the finished weld. On harder metals like steel, you can often hear a tack weld break; however, for some soft metals like aluminum, the tack weld may separate quietly. Depending on the type of metal and its size and thickness, a breaking tack weld can make a small, sharp snap or a deep, resounding thump. You might hear one break while you are welding or sometime afterward. Do not assume that a broken tack weld has no effect and continue to weld. A broken tack can allow parts to shift well out of tolerance. If you continue to weld, it may be impossible to pull the loose part back into position, which could result in the weldment not meeting its specifications. Sometimes this is referred to as “making scrap metal” and not a weldment.
LOCATION AND ALIGNMENT POINTS
Locating parts is easier when the parts being assembled are lined up on an edge starting at a corner. This makes it fairly easy for the assembler to put the pieces in their proper positions. However, if the parts being assembled are to be fitted in the middle of a surface or edge, placing them accurately becomes more difficult. Sometimes there are alignment slots, notches, or points made onto the parts to aid during the assembly.
Locating parts along an edge or at the corner is the easiest way to determine where they are to be aligned. You must look at the drawing to see how the edges are fitted. This is much more important on thicker materials than it is with thin stock. Of course, even on thin material it can be important if the part is to be built to a very tight tolerance. In that case, if the joint should be assembled but is assembled; the overall length in one direction decreases by the thickness of the material, and in the other direction the dimension increases by the thickness of the material. On thicker materials it is easy to see how the overall dimensions of a weldment could change if the parts are not properly aligned during fit-up. But in addition to not being the correct size, sometimes the weld itself will not be as strong if the joint is not aligned properly. The reason the weld might not be as strong as it is designed to be is because the tensile strength of thick metal plate differs depending on the direction in which the load is placed on the plate as compared to the rolling direction of the plate. Metal plate, much like wood, will break in one direction easier than in another. In this aspect, steel plate is similar to a wooden board in that the direction of the rolled grain of a plate and the direction of the wood grain in a board affect their strength. Many common materials have a grain; for example, when you tear a newspaper down the page, it tears fairly easily and straight. However, when you try to tear it across the page, the tear is much more jagged.
OVERALL TOLERANCE
When fabricating a weldment that is made up of a number of parts all welded together, there is a potential problem that the overall size of the weldment can be wrong. Every part manufactured has a tolerance. A part’s tolerance is the amount that a part can be bigger or smaller than it should be and still be acceptable. The more exact a part’s tolerance, the more time it takes to make and, therefore, the more it costs. In most cases, the welding engineer has calculated the effect of these slight variations in size when designing a weldment. As the weldment fabricator, you must take these tolerances into consideration as you make the assembly to ensure that the overall size of the weldment is within its tolerance. As the number of parts that make up a weldment increases, the problem of compounding the errors increases. For example, if there are 8 parts and each part is 1/8 in. (3 mm) larger than its ideal size but within its ±1/8 in. (3 mm) tolerance, the overall length of the finished weldment could be 8/8ths or 1 in. (25 mm) too long. Likewise, if each of the parts was 1/8 in. (3 mm) shorter, the overall length would be 1 in. (25 mm) too short. Therefore, an assembler must be mindful of both the size of each part and the overall size of the assembly. In addition to tolerances for size, parts also have angle tolerances. For example, each of the 10 pieces that make up the star are off by only 1°. But as you can see, when they are assembled, the last corner does not fit, making the weldment unacceptable.
Ideally, all the parts for a weldment fit up perfectly; however, in reality that does not always happen. You cannot just throw out all of the parts that do not fit in order to find the ones that would make the perfect star. That is especially true if the parts are made within the correct tolerance. When parts like the ones in this star are made on the shorter side of the tolerance, you might be able to “loosen up” the joint tolerance to make the overall star work. Welded joints, like parts, have tolerances. By slightly adjusting the alignment of each of the 10 pieces, the star can be made within its overall acceptable tolerance. In this case, by making sure that all the joints stay within tolerance, the complete star can be made without having to recut any of the parts. If you can make this assembly by adjusting the joint tolerance, you can assemble it faster, and because each of the parts is exactly the same, it will look perfect.
What else could be done to make this star fit up? Well, if the height tolerance allows some adjustment, then the fit-up can be made even better. As the height is reduced, the gap at the inside edges will close. So, by slightly flattening the star, its fit-up can be improved. Whenever possible, try to get the parts to fit without having to recut or grind them; but remember, the finished weldment must be within tolerance. You want to avoid recutting and grinding because both will add time and cost to the finished project. However, you must remember that in some cases the only way that the weldment can be assembled within overall tolerance is to recut or grind some or all of the parts. If the finished weldment is not within tolerance, it may be unusable. If you must grind a part to fit, try to do as little grinding as possible to get the parts to fit up. Hand-grinding is a time-consuming operation, and as Benjamin Franklin once said, “Remember that time is money.”
Where you recut or grind a part can sometimes greatly affect the time required. For example, if the pieces used in the star need to be ground to fit, you might want to grind along the short side, which would be faster. In addition, it might be possible to grind only part of the edge to get the parts to fit up within tolerance. The required root opening tolerance of 1/4 in. (6 mm) ±1/8 in. (3 mm) will allow the part’s edge to be ground unevenly as long as the root opening tolerance is maintained. Note how the root opening varies but stays within the acceptable tolerance. The root opening is 1/4 in. (6 mm) at one end, which is acceptable; but at the point where it becomes too close (less than 1/8 in. [3 mm]), begin grinding. The result meets the part’s fit-up specifications and requires a minimum amount of grinding. If the root opening is too wide, you may be able to make the weld, but it would be too large. Larger welds result in more filler metal being added and more heat input to the base metal. Larger welds may cause greater weld distortion and have larger heataffected zones. Both can result in a weld that will not withstand the part’s designed strength specifications. Even a quick visual inspection by a welding inspector would reveal that the weld is unacceptable and must be repaired. You cannot deviate from the root opening specified in the welding procedure; to do so is wrong.
WELD DISTORTION
To make it easier to understand what is happening to the metal during welding, we must first define the terms used. Most dictionaries define the terms distortion and warp and the terms distorted and warped very similarly. However, in this textbook the terms distortion and warp will be used as active and temporary events such as “the part warps during welding” or “weld distortion affects the weldment.” In these cases, it should be understood that once the welding is over, the metal will return to nearly its prewelded shape. The terms distorted and warped as in “the weld distorted the plate” or “the weld warped the weldment” are past tense and refer to the fact that the postwelded metal, after it has cooled, has been significantly misshapen as a result of the welding process. All metals distort by expansion when heated and distort by contraction when cooled. Parts return to their original shape when cooled if the heating is uniform and their shapes are symmetrical. However, if the heating and/or the parts’ shapes are not symmetrical, the parts to some degree will be permanently distorted as the result of the heat/cooling cycle. Almost every welding process involves some heat cycling. Most welding heat cycling is not symmetrical; so as a result, the weldment will be distorted to some degree. The two factors that affect the degree to which a metal will distort and possibly remain distorted are its rate of thermal expansion and its rate of thermal conductivity. Basically, the higher the coefficiency of thermal expansion, the greater the metal distorts. We see that tungsten has the smallest coefficiency of expansion, and zinc has the largest coefficiency of expansion. What this means is that if the same size pieces of tungsten and zinc are heated to the same temperature, the zinc will expand a lot more.
LAYOUT
Parts for fabrication may require that the welder lay out lines and locate points for cutting, bending, drilling, and assembling. Lines may be marked with a soapstone, metal marker, a chalk line, scratched with a metal scribe, or punched with a center punch. Soapstone and welding metal markers, are made to withstand most welding and cutting temperatures without vanishing like a line from a felt-tip marker will. A potential problem when using markers not specifically manufactured for welding is that they may contain elements like sulfur that can contaminate welds. Use only approved materials for marking metal for welding or cutting. When marking a straight edge on a metal part, a small gap can result if the soapstone point is not sharp enough to fit tightly up against the straight edge, which can cause the parts to not fit together properly. The end of the soapstone should be sharpened to increase accuracy. You can use a grinder to sharpen it or just rub it on some scrap metal or rough concrete. A chalk line will make a long, straight line on metal and is best used on large jobs with long, straight lines. Keep the tip of the chalk line reel pointed up to prevent an excess amount of chalk in the chalkline reel from coming out as the string is pulled out. This will keep your work area cleaner and reduce the need to refill the reel’s chalk so often.
NOTE: Keeping the chalk dry is important. The powdered chalk used in a chalk-line reel will become useless if it gets too damp or wet. It is very hard to clean out the gooey chalk mess if you let your reel get rained on or drop it in a puddle.
Either a scribe or a punch can be used to lay out an accurate line, but the punched line can be easier to see when cutting. A punch can be held, with the tip just above the surface of the metal. When the punch is struck with a lightweight hammer, it will make a mark. If you move your hand along the line and rapidly strike the punch, it will leave a series of punch marks for the cut to follow. Starting your layout along an existing edge or from the corner can both help you make a more accurate layout and conserve materials. An existing edge is easier to use to hook the end of your tape measure or line up a square. In addition, the corners and edges of new plates and sheets are straight and square. So, by starting in a corner or along the edge, you can both take advantage of the preexisting cut as well as reduce wasted material. It is often easy to mistakenly cut the wrong line. With your visibility somewhat limited by the cutting goggles or shield, it is not always possible to see far enough ahead to know when to stop cutting. Stopping to look can result in a problem with restarting the cut; therefore, you want to make sure that the lines are clearly marked. This can become more of a problem if one person lays out the parts and another makes the cuts. Even if you are doing both jobs, it is easy to cut the wrong line. Sometimes there may be layout lines used to help locate parts or there may be bend lines, both of which must not be mistakenly cut. To avoid making a cutting mistake, lines should be identified as to whether they are being used for cutting, locating bends, drill centers, or assembly locations. The lines that are not to be cut may be marked with an X, or they may be identified by writing directly on the part. Mark the X at the beginning and end of the line; and if it is a long line, you may want to make several more Xs along the full length of the line. You should also mark the side of the line that is scrap so that the kerf is removed from that side, which will leave the part the proper size. When possible, erase all unneeded lines before starting the cut. Some shops will have their own shorthand methods for identifying layout lines, or you may develop your own system. Failure to develop and use a system for identifying lines will ultimately result in a mistakenly made cut. In a welding shop, there are only those who have made the wrong cut and those who will make the wrong cut. When it happens, check with the welding shop supervisor to see what corrective steps may be taken. One advantage for most welding assemblies is that many errors in cutting can be repaired by welding. There are often prequalified procedures established for just such an event, so check before deciding to scrap the part. The process of laying out a part may be affected by the following factors:
Material shape—Lists the most common shapes of metal used for fabrication. Flat stock such as sheets and plates are the easiest to lay out, and the most difficult shapes to work with are pipes and round tubing.
Part shape—Parts with square and straight cuts are easier to lay out than parts with angles, circles, curves, and irregular shapes.
Tolerance—The smaller or tighter the tolerance that must be maintained, the more difficult the layout.
Nesting—The placement of parts together in a manner that will minimize the waste created is called nesting.
Parts with square or straight edges are the easiest to lay out. Simply measure the distance and use a square or straight edge to lay out the line to be cut. Straight cuts that are to be made parallel to an edge can be drawn by using a combination square and a piece of soapstone. Set the combination square to the correct dimension, and drag it along the edge of the plate while holding the soapstone at the end of the combination square’s blade.
Nesting
Laying out parts so that the least amount of scrap is produced is important. Odd-shaped and unusualsized parts often produce the largest amount of scrap. Computers can be used to lay out nested parts with the least scrap. Some computerized cutting machines can be programmed to nest parts. Manually nesting of parts may require several tries at laying out the parts to achieve the lowest possible scrap.
Kerf Space
All cutting processes except shearing remove some metal, leaving a small gap or space. This gap or space is called the kerf. You must allow for the kerf width when parts are being laid out side by side. The width of material being removed by the cut’s kerf varies depending on the cutting process used. Of the cutting processes used in most shops, the metal saw produces one of the smallest kerfs while the handheld oxyfuel cutting torch can produce one of the widest. Saw kerfs often range around 1/16 in. (2 mm) wide while a very good oxyfuel cut’s kerf will be around 1/8 in. (3 cm) wide. When only one or two parts are being cut, the kerf width may not need to be added to the part dimension. This space may be taken up during assembly by the root gap required for a joint. However, if a large number of parts are being cut out of a single piece of stock, the kerf width can add up. This combining of kerf width from each cut can increase the stock required for cutting out the parts.
ASSEMBLY
The assembling process, bringing together all of the parts of the weldment, requires a proficiency in several areas. You must be able to read the drawing and interpret the information provided to properly locate each part. An assembly drawing has the necessary information, both graphically and dimensionally, to allow the various parts to be properly located as part of the weldment. If the assembly drawings include either pictorial or exploded views, this process is much easier for the beginning assembler; however, most assembly drawings are done as two, three, or more orthographic views. Orthographic views are more difficult to interpret until you have developed an understanding of their various elements. On very large projects such as buildings or ships, a corner or center line is established as a base line. This is the point where all measurements for all part location begins. When working with smaller weldments, a single part may be selected as such a starting point. Often, selection of the part to serve as the base is easy because all the other parts are to be joined to this central part. However, on other weldments the selection is strictly up to the assembler. To start the assembly, make a selection of the largest or most central part to be the base for your assembly. Once this is done, all other parts will be aligned to this one part. Using a base also helps prevent location and dimension errors. A slight misalignment of one part, even within tolerances, can be compounded with other misalignments, resulting in an unacceptable weldment. Using a base line or base part results in a more accurate assembly. Identify each part of the assembly, and mark each piece for future reference. If needed, you can hold the parts together and compare their orientation to the drawing. Locate points on the parts that can be easily identified on the drawing such as holes, notches, and so on. Now mark the parts as to their top, front, or other such orientation, which will help you locate them during the assembly. Layout lines and other markings can be made on the base to locate other parts. Using a consistent method of marking helps prevent mistakes. One method is to draw parallel lines on both parts where they meet. After the parts have been identified and marked, they can be either held or clamped into place. Holding the parts in alignment by hand for tack welding is fast but often leads to errors and is not recommended for beginning assemblers. Experienced assemblers recognize that clamping the parts in place before tack welding is a much more accurate method.
FITTING
Not all parts fit exactly as they were designed. There may be slight imperfections in cutting or distortion of parts due to welding, heating, or mechanical damage. Some problems can be solved by grinding away the problem area. Hand-grinders are most effective for this type of problem. Other situations may require that the parts be forced into alignment. A simple way of correcting slight alignment problems is to make a small tack weld in the joint and then use a hammer and possibly an anvil to pound the part into place. Small tacks applied in this manner will become part of the finished weld. Be sure not to strike the part in a location that damages the surface, which could render the finished part unsightly or unusable.
More aligning force can be applied using cleats or dogs with wedges or jacks. Cleats or dogs are pieces of metal that are temporarily attached to the weldment’s parts to enable them to be forced into place. Jacks will do a better job if the parts must be moved more than about 1/2 in. (13 mm). Any time that cleats or dogs are used, they must be removed and the area ground smooth. Some codes and standards will not allow cleats or dogs to be welded to the base metal. In these cases, more expensive and time-consuming fixtures must be constructed to help align the parts if needed.