CHAPTER 2 LAYOUT AND FABRICATION OF SHEET-METAL AND FIBER-GLASS DUCT

As a Steelworker you are required to operate sheet-metal tools and to apply basic sheet-metal layout techniques. In many Naval Construction Force (NCF) projects, sheet metal is used to protect the exterior of buildings by using flashing, gutters, and at times, complete sheet-metal roofing systems. Other items made from sheet metal are dust collection systems, machinery guards, lockers, and shelving.

Although many of the parts and fittings used in sheet-metal work are stock items, which are simply installed or assembled, Steelworkers are required to fabricate parts and fittings frequently in the shop or to modify them to fit irregularities in the project design. Therefore, you must have knowledge not only in laying out patterns but also have the skills required to cut, bend, shape, assemble, and install the finished sheet-metal products. This chapter describes some of the methods of measuring, marking, cutting, forming, and joining as well as installing sheet-metal sections, duct systems, and fiber-glass ducts. In addition, the use of various hand tools and power tools required in sheet-metal layout and fabrication is provided.

SHEET-METAL LAYOUT AND CUTTING TOOLS AND EQUIPMENT

Numerous types of layout tools, cutting tools, and forming equipment are used when working with sheet metal. This section will describe the uses of the layout and cutting tools and the operation of the forming equipment.

LAYOUT TOOLS

The LAYOUT of metal is the procedure of measuring and marking material for cutting, drilling, or welding. Accuracy is essential in layout work. Using erroneous measurements results in a part being fabricated that does not fit the overall job. This is a waste of both time and material. In most cases, you should use shop drawings, sketches, and blueprints to obtain the measurements required to fabricate the job being laid out. Your ability to read and work from blueprints and sketches is paramount in layout work.

If you require information on blueprints, you will find chapters 1-3 and 8 of Blueprint Reading and Sketching, NAVEDTRA 10077-F1, an excellent reference.

Layout tools are used for laying out fabrication jobs on metal. Some of the more common layout tools that you will use in performing layout duties are as follows: scriber, flat steel square, combination square, protractor, prick punch, dividers, trammel points, and circumference rule.

Scriber

Lines are scribed on sheet metal with a SCRATCH AWL, coupled with a STEEL SCALE or a STRAIGHTEDGE. To obtain the best results in scribing, hold the scale or straightedge firmly in place, and set the point of the scriber as close to the edge of the scale as possible by tilting the scriber outward. Then exert pressure on the point and draw the line, tilting the tool slightly in the direction of movement (fig. 2-1). For short lines, use the steel scale as a guide. For longer lines, use a circumference rule or a straightedge. When you have to draw a line between two points, prick punch each point. Start from one prick punch mark and scribe toward the center.

Figure 2-1-Scribing a line.

Complete the line by scribing from the other prick punch mark in the opposite direction.

Flat Steel Square

The FLAT STEEL SQUARE is a desirable tool for constructing perpendicular or parallel lines. In the method of layout, known as parallel line development, the flat steel square is used to construct lines that are parallel to each other as well as perpendicular to the base line. This procedure is shown in figure 2-2. Simply clamp the straightedge firmly to the base line. Slide the body of the square along the straightedge, and then draw perpendicular lines through the desired points.

Before using the flat steel square or at least at periodic intervals, depending on usage, see that you check it for accuracy, as shown in figure 2-3. When the square is off, your work will be off correspondingly no matter how careful you are.

Combination Square

The COMBINATION SQUARE can be used to draw a similar set of lines, as shown in figure 2-4. An edge of the metal upon which you are working is used as the base line, as shown in the figure. One edge of the head of the combination square is 90 degrees and the other edge is 45 degrees. Combination squares are

Figure 2-2.-Using a square to cinstruct perpendicular and parallel lines.

Figure 2-3.-Checking a square for accuracy.

Figure 2-4.-Using the combination square

delicate instruments and are of little value if you handle them roughly. Store your squares properly when you have finished using them. Keep them clean and in tiptop shape, and you will be able to construct 90-degree angles, 45-degree angles, and parallel lines without error.

Protractor

To construct angles other than 45 degrees or 90 degrees, you will need a PROTRACTOR. Mark the vertex of the angle of your base line with a prick punch. Set the vertex of your protractor on the mark and then scribe a V at the desired angle (assume 700). Scribe the line between the vertex and the point located by the V, and you have constructed an angle of 70 degrees.

Prick Punch

When you locate a point and mark it with the PRICK PUNCH, be sure to use alight tap with a small ball peen hammer, ensuring it is on the precise spot intended to mark. The smaller the mark you make (so long as it is visible), the more accurate that mark becomes.

Dividers

You should use DIVIDERS to scribe arcs and circles, to transfer measurements from a scale to your layout, and to transfer measurements from one part of the layout to another. Careful setting of the dividers is of utmost importance. When you transfer a measurement from a scale to the work, set one point of the dividers on the mark and carefully adjust the other leg to the required length, as shown in figure 2-5.

To scribe a circle, or an arc, grasp the dividers between the fingers and the thumb, as shown in figure 2-6. Place the point of one leg on the center, and swing the arc. Exert enough pressure to hold the point on center, slightly inclining the dividers in the direction in which they are being rotated.

TrammelPoints

To scribe a circle with a radius larger than your dividers, you should select TRAMMEL POINTS. The method of adjusting the points, as shown in figure 2-7, is to set the left-hand point on one mark, slide the right-hand point to the required distance, and tighten the thumbscrew. The arc, or circle, is then scribed in the same manner as with the dividers.

Constructing a 90-degree, or right, angle is not difficult if you have a true, steel square. Suppose that you have no square or that your square is off and you

Figure 2-5.-Setting the dividers

Figure 2-6.-Scribing an acr/circle with dividers

need a right angle for a layout. Breakout your dividers, a scriber, and a straightedge. Draw a base line like the one labeled AB in figure 2-8. Set the dividers for a distance greater than one-half AB; then, with A as a center, scribe arcs like those labeled C and D. Next, without changing the setting of the dividers, use B as a center, and scribe another set of arcs at C and D. Draw a line through the points where the arcs intersect and you have erected perpendiculars to line AB, forming four 90-degree, or right, angles. You have also bisected or divided line AB into two equal parts.

Constructing a right angle at a given point with a pair of dividers is a procedure you will find useful when making layouts. Figure 2-9 shows the method for constructing a right angle at a given point.

Figure 2-7.-Setting trammel points.

Figure 2-8.-Constructing a 90-degree angle by bisecting a line.

Figure 2.9.-Constructing a 90-degree angle at a given point

Imagine that you have line XY with A as a point at which you need to fabricate a perpendicular to form a right angle. Select any convenient point that lies somewhere within the proposed 90-degree angle. In figure 2-9 that point is C. Using C as the center of a circle with a radius equal to CA, scribe a semicircular are, as shown in figure 2-9. Lay a straightedge along points B and C and draw a line that will intersect the other end of the are at D. Next, draw a line connecting the points D and A and you have fabricated a 90-degree angle. This procedure may be used to form 90-degree comers in stretch-outs that are square or rectangular, like a drip pan or a box.

Laying out a drip pan with a pair of dividers is no more difficult than fabricating a perpendicular. You will need dividers, a scriber, a straightedge, and a sheet of template paper. You have the dimensions of the pan to be fabricated: the length, the width, and the height or depth. Draw a base line (fig. 2-10). Select a point on this line for one comer of the drip pan layout. Erect a perpendicular through this point, forming a 90-degree angle. Next, measure off on the base line the required length of the pan. At this point, erect another perpendicular. You now have three sides of the stretch-out. Using the required width of the pan for the other dimensions, draw the fourth side parallel to the base line, connecting the two perpendiculars that you have fabricated.

Now, set the dividers for marking off the depth of the drip pan. You can use a steel scale to measure off the correct radius on the dividers. Using each comer for a point, swing a wide are, like the one shown in the second step in figure 2-10. Extend the end and side lines as shown in the last step in figure 2-10 and complete the stretch-out by connecting the arcs with a scriber and straightedge.

Bisecting an are is another geometric construction that you should be familiar with. Angle ABC (fig. 2-11) is given. With B as a center, draw an are cutting the sides of the angle at D and E. With D and E as centers and a radius greater than half of are DE, draw arcs intersecting at F. A line drawn from B through point F bisects angle ABC.

Two methods used to divide a line into a given number of equal parts are shown in figure 2-12. When the method shown in view A is to be used, you will need a straightedge and dividers. In using this method, draw line AB to the desired length. With the dividers set at any given radius, use point A as center and scribe an arc above the line. Using the same radius and B as center, scribe an are below the line as shown. From

Figure 2-10.-Laying out a drip pan with dividers.

Figure 2-11.-Bisecting an arc.

Figure 2-12.-Two methods used to divide a line into equal parts.

point A, draw a straight line tangent to the arc that is below point B. Do the same from point B. With the dividers set at any given distance, start at point A and step off the required number of spaces along line AD using tick marks-in this case, six. Number the tick marks as shown. Do the same from point B along line BC. With the straightedge, draw lines from point 6 to point A, 5 to 1, 4 to 2, 3 to 3, 2 to 4, 1 to 5, and B to 6. You have now divided line AB into six equal parts.

When the method shown in view B of figure 2-12 is used to divide a line into a given number of equal parts, you will need a scale. In using this method, draw a line at right angles to one end of the base line. Place the scale at such an angle that the number of spaces required will divide evenly into the space covered by the scale. In the illustration (view B, fig. 2-12) the base line is 2 1/2 inches and is to be divided into six spaces. Place the scale so that the 3 inches will cover 2 1/2 inches on the base line. Since 3 inches divided by 6 spaces = 1/2 inch, draw lines from the 1/2-inch spaces on the scale perpendicular to the base line. Incidentally, you may even use a full 6 inches in the scale by increasing its angle of slope from the baseline and dropping perpendiculars from the full-inch graduation to the base line.

To divide or step off the circumference of a circle into six equal parts, just set the dividers for the radius of the circle and select a point of the circumference for a beginning point. In figure 2-13, point A is selected for a beginning point. With A as a center, swing an arc through the circumference of the circle, like the one shown at B in the illustration. Use B, then, as a point, and swing an arc through the circumference at C.

Continue to step off in this manner until you have divided the circle into six equal parts. If the points of intersection between the arcs and the circumference are connected as shown in figure 2-13, the lines will intersect at the center of the circle, forming angles of 60 degrees.

If you need an angle of 30 degrees, all you have to do is to bisect one of these 60-degree angles by the method described earlier in this chapter. Bisect the 30-degree angle and you have a 15-degree angle. You can construct a 45-degree angle in the same manner by bisecting a 90-degree angle. In all probability, you will have a protractor to lay out these and other angles. But just in case you do not have a steel square or protractor, it is a good idea to know how to construct angles of various sizes and to erect perpendiculars.

Many times when laying out or working with circles or arcs, it is necessary to determine the circumference of a circle or arc. For the applicable mathematical formula, refer to appendix II of this text.

Circumference Rule

Another method of determining circumference is by use of the circumference rule. The upper edge of the circumference rule is graduated in inches in the same manner as a regular layout scale, but the lower edge is graduated, as shown in figure 2-14. The lower edge gives you the approximate circumference of any circle within the range of the rule. You will notice in figure 2-14 that the reading on the lower edge directly below the 3-inch mark is a little over 9 3/8 inches. This

Figure 2-13.-Dividing a circle into six equal parts

Figure 2-14.-Circumference rule.

reading would be the circumference of a circle with a diameter of 3 inches and would be the length of a stretch-out for a cylinder of that diameter. The dimensions for the stretch-out of a cylindrical object, then, are the height of the cylinder and the circumference.

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