Sheet metal work is a fundamental skill across many professional fields in Kenya, including manufacturing, construction, healthcare equipment maintenance, and agricultural tool fabrication. Mastery of pattern development and understanding various sheet metal products enables technicians and artisans to produce precise components and assemblies that meet specific functional and aesthetic requirements. This chapter explores the technical processes involved in pattern development and the diverse types of sheet metal products, emphasizing practical applications across sectors such as county hospitals, educational institutions, retail businesses, and cooperatives.

2.1 Pattern development

Pattern development is the process of creating templates or layouts that guide the cutting, shaping and assembling of sheet metal components. It is critical for ensuring that fabricated parts fit together accurately and perform their intended functions. Example: in a sheet metal workshop making ventilation ducts for a commercial kitchen, or storage bins for a grain store, precise pattern development reduces material waste and improves quality.

2.1.1 Methods

Pattern development methods vary depending on the shape of the component, the sheet metal thickness and the fabrication process. Three geometric methods form the foundation of sheet metal layout: the parallel line method, triangulation and the radial line method. Each one unfolds a three-dimensional surface into a flat shape, and the choice between them depends on how the surface of the object is formed. Other practical ways of producing and checking patterns are listed at the end of the procedure and applications section, under "Other ways of producing patterns".

2.1.1.1 Parallel line method

The parallel line method is used for objects whose surfaces are made up of straight lines that run parallel to each other, such as prisms, cylinders, elbows, tees and rectangular or round ducts. The surface is divided into a series of parallel strips, and the true length of each strip is projected from the elevation onto a flat stretch-out line. Example: a workshop making a round extraction duct for a commercial kitchen needs a mitred elbow, and the parallel line method gives the exact curved cutting line for each piece.

Key features of the method:

• Parallel lines of the surface: The surface is represented by lines (elements) that all run in the same direction. On a cylinder these are the lines along its length.
• Stretch-out line: A straight line drawn at right angles to the elements. Its length equals the perimeter of the object, for example π × D for a cylinder.
• Equal divisions: The perimeter is divided into equal parts, usually 12 for a round section. More divisions give a smoother curve and a more accurate pattern.
• True lengths from the elevation: Because the elements are parallel to the plane of the elevation, their lengths are seen true in that view and are transferred directly to the pattern.
• Smooth curve or straight lines: Joining the ends of the transferred lengths gives the shape of the cut edge. This is a smooth curve for round sections and straight lines for prisms.

Procedure for developing a mitred cylindrical elbow piece:

1. Draw the elevation: Draw the side view of the piece to full size, showing the diameter, the axis and the mitre (angled) cut at one end.
2. Draw the half plan: Draw a semicircle on the square end of the elevation. Divide it into six equal parts and number the points 1 to 7. These points represent twelve equal divisions of the full circle.
3. Project the elements: From each numbered point, draw a line parallel to the axis until it meets the mitre line. These lines are the elements of the cylinder.
4. Draw the stretch-out line: Extend the base line of the elevation and mark off the length π × D. Divide it into twelve equal parts and number them to match the plan, starting with the seam position.
5. Erect the ordinates: At each division on the stretch-out line, draw a perpendicular line at right angles to it. The numbered lines must match the numbered elements.
6. Transfer the lengths: Measure each element from the base line to the mitre line on the elevation, and mark the same length on the matching perpendicular. Use dividers to avoid reading errors.
7. Draw the cut edge: Join the marked points with a smooth curve, using a flexible curve or a thin lath. Complete the pattern with straight lines on the other three sides.
8. Add allowances: Add a lap or a grooved-seam allowance on one long edge, and any edge allowance needed to join the pieces. Mark the seam position on the shortest element so that it is easy to close and finish.

Worked example: a kitchen duct has a diameter of 200 mm and a 45° mitre cut.

Stretch-out length = π × D
Stretch-out length = 3.142 × 200
Stretch-out length = 628.3 mm

Width of each of the 12 divisions = 628.3 ÷ 12
Width of each division = 52.4 mm

Difference between the longest and shortest element = D × tan 45°
Difference = 200 × 1
Difference = 200 mm

The pattern is therefore a strip 628.3 mm long, with 52.4 mm divisions, and its mitred edge rises and falls by 200 mm across the width.

2.1.1.2 Triangulation

Triangulation is used to develop surfaces that have no parallel lines and cannot be unfolded by the radial line method. These are mainly transition pieces that join two different shapes, such as square to round, rectangular to round or offset connections. The surface is divided into a series of small triangles. The true length of each side is found, and the triangles are then built one after another on the flat sheet. Example: a workshop making a flour hopper fits a transition that changes from a 400 mm square inlet to a 250 mm round outlet. Triangulation gives the flat pattern for it.

Key features of the method:

• Surface divided into triangles: Each triangle has its three sides fixed by true lengths, so it can be drawn accurately with a rule and compasses.
• Plan and elevation needed: The plan shows the horizontal length of each line, and the elevation shows the vertical height of the piece.
• True length of each line: Lines that slope in space are not seen true in either view. Their true length is the hypotenuse of a right-angled triangle whose base is the plan length and whose height is the vertical height.
• Symmetry saves work: Where the piece is symmetrical, only one quarter or one half needs its true lengths found, and the results are repeated.
• Approximate result: A round section is treated as a series of short straight chords, so the pattern is accurate only as far as the number of divisions allows. Twelve divisions are normal for small pieces and more are used for large ones.

Procedure for developing a square to round transition piece:

1. Draw the plan and elevation: Draw the square base and the circular top in plan, and the side view showing the vertical height. The centres of the square and the circle must be on the same axis.
2. Divide the circle: Divide the circle in plan into twelve equal parts, then number the points. Each quarter of the circle then has three divisions.
3. Join to the corners: Draw lines in the plan from each point on the circle to the nearest corner of the square. These lines are the sides of the triangles, and they are not yet true lengths.
4. Draw the true length diagram: Draw a base line, mark the vertical height at right angles to it, and mark each plan length along the base line. Join the top of the height line to each mark. Each hypotenuse is a true length.
5. Find the chord length: Measure the length of one division of the circle in plan. This is the third side of every triangle on the circle.
6. Start the pattern: Draw the first line of the pattern at its true length. This is usually the line from a corner to the circle point at the centre of a side.
7. Build the triangles: From each end of the first line, strike arcs equal to the next true length and the chord length. Where they meet gives the next point. Repeat around the whole piece, adding the side lengths of the square between corners.
8. Complete the pattern: Join the points with straight lines and a smooth curve on the circular edge. Then add seam and joint allowances and mark the seam on the shortest line.

Worked example: a transition from a 400 mm square to a 250 mm diameter circle with a vertical height of 300 mm.

Take the circle point on the centreline of one side, and the square corner next to it. The corner is 200 mm from the centre in each direction, and the circle point is 125 mm from the centre.

Plan length = √(75² + 200²)
Plan length = √(5625 + 40000)
Plan length = √45625
Plan length = 213.6 mm

True length = √(plan length² + height²)
True length = √(45625 + 300²)
True length = √(45625 + 90000)
True length = √135625
True length = 368.3 mm

This line is drawn at 368.3 mm on the pattern, not at the 213.6 mm seen in the plan.

2.1.1.3 Radial line

The radial line method is used for objects whose surfaces taper to a point, or would taper to a point if they were extended, such as cones, pyramids and their frustums. All the lines of the surface radiate from the apex, so the pattern is drawn as a sector of a circle for a cone, or as a set of triangles that share a common point for a pyramid. Example: a workshop making a conical cover for a grain hopper uses the radial line method to find the sector that rolls into the cone.

Key features of the method:

• Common apex: Every line of the surface starts at one point. The pattern is drawn by swinging arcs from that point.
• Slant length as the radius: For a cone, the sector radius is the slant height, measured along the surface, and not the vertical height.
• Arc length equals the base circumference: The curved edge of the sector must be exactly as long as the circumference of the base, π × D.
• Frustums are cut from a full cone: The pattern of a frustum is the sector of the full cone minus the sector of the missing small cone. The missing apex is found by extending the sides in the elevation.
• Pyramids use true edge lengths: The edges of a pyramid slope in space, so their true length is found with a right-angled triangle before the faces are drawn.

The sector angle of a cone pattern is found from the formula:

Sector angle = (D ÷ (2 × L)) × 360° = 180° × D ÷ L

where D is the base diameter and L is the slant height.

Procedure for developing a frustum of a cone:

1. Draw the elevation: Draw the frustum as a trapezium showing the base diameter, the top diameter and the vertical height.
2. Extend to the apex: Extend the two sloping sides upward until they meet. This point is the apex of the full cone.
3. Measure the radii: Measure the slant length of the full cone (apex to the base) as the large radius, and the slant length of the small cone (apex to the top edge) as the small radius.
4. Find the sector angle: Use the formula with the base diameter and the full slant length. Alternatively, divide the arc length π × D by the large radius, which gives the angle in radians, then multiply by 57.3 to convert it to degrees.
5. Draw the arcs: From a point marked as the apex, draw the large arc and the small arc with the two radii. Both arcs are swung from the same centre.
6. Mark the sector: Draw a straight line from the apex to the large arc. Set the sector angle with a protractor, or step off the arc length π × D using equal chords, and draw the second line from the apex.
7. Trim the pattern: The part of the sector between the two arcs and the two radial lines is the pattern.
8. Add allowances: Add a lap along one radial edge and a folded or wired edge allowance on the arcs if required. Mark the seam clearly.

Worked example: a frustum with a base diameter of 400 mm, a top diameter of 200 mm and a vertical height of 300 mm.

Difference in radii = (400 − 200) ÷ 2 = 100 mm

Slant length of the frustum = √(300² + 100²)
Slant length = √(90000 + 10000)
Slant length = √100000
Slant length = 316.23 mm

The base radius (200 mm) is twice the difference in radii (100 mm), so the full cone is twice as long along the slant as the frustum.

Slant length of the full cone = 316.23 × 2
Full cone slant length = 632.46 mm, taken as 632.5 mm

Slant length of the small cone = 632.46 − 316.23
Small cone slant length = 316.23 mm, taken as 316.2 mm

Sector angle = 180° × D ÷ L
Sector angle = 180 × 400 ÷ 632.46
Sector angle = 113.8°

The pattern is a sector of 113.8°, with a large radius of 632.5 mm and a small radius of 316.2 mm.

2.1.2 Procedure and applications

The procedure for pattern development follows a systematic sequence to ensure accuracy and functionality. This sequence applies whether the workshop is producing ducts, trays or hoppers. The same steps apply whichever of the three geometric methods is chosen, and only step 2 changes the way the drawing is carried out.

Choosing between the geometric methods

The method is chosen from the way the surface of the component is formed. The table compares the three methods.

Tools and equipment for layout

Steps in pattern development

1. Analyse the final shape: Understand the three-dimensional form of the component, including all bends, curves and joints. For instance, a workshop making a cylindrical storage bin must consider the round body, the joining seam and the base.
2. Select the pattern development method: Choose the most appropriate method based on the shape and available tools: the parallel line method for ducts and cylinders, triangulation for transition pieces, or the radial line method for cones and funnels.
3. Take accurate measurements: Measure all dimensions including lengths, widths, heights and angles. A workshop making stainless steel trays for a commercial kitchen needs precise dimensions so that the trays fit their racks and can be cleaned easily.
4. Draw the pattern layout: Transfer measurements onto the sheet metal or a pattern material. This step demands precision to avoid material wastage, and a workshop cutting several ducts from one sheet should nest the patterns closely.
5. Add allowances: Include allowances for bending, joining and finishing processes like welding or riveting. For example, a workshop making a hopper adds a lap for the seam and a folded edge for stiffness.
6. Cut and test the pattern: Cut the pattern in paper or thin card, or in sheet metal, and assemble a prototype if needed. Adjustments are made based on fit and function before the full batch is cut from costly sheet.

Applications of pattern development

Pattern development is used wherever flat sheet is formed into a three-dimensional product. Accurate patterns reduce errors, save costs and enhance production efficiency.

• Ventilation and extraction ducts: Round and rectangular ducts, elbows, tees and transition pieces for hospitals and commercial kitchens are developed by the parallel line method and by triangulation.
• Hoppers and funnels: Grain, feed and flour hoppers use radial line and triangulation patterns so that the material flows freely to the outlet.
• Storage containers: Cylindrical and rectangular bins, covers and lids are laid out with the parallel line method.
• Agricultural machinery parts: Guards, chutes and conical covers for farm equipment are laid out from patterns so that identical parts can be repeated.
• Metal fittings: Brackets, trims, shelf units and covers for shops and offices are made from patterns that give uniform shapes and neat joints.
• Roof flashings and fittings: Flashings, ridge pieces and conical caps on buildings are cut from developed patterns so that they fit tightly and shed water.

Other ways of producing patterns

Besides the three geometric methods, a workshop uses other practical ways of producing and checking patterns.

• Flat pattern method: The three-dimensional shape is unfolded directly into a flat layout, with bend lines marked between the faces. It suits shapes with flat faces such as rectangular ducts, trays and flashings, while the geometric methods above handle curved and tapered shapes.
• Template method: Templates are physical guides made from cardboard, plywood or thin metal, shaped to the component outline. A workshop repeating a batch of identical brackets uses a template so that every piece is the same.
• Development by measurement: The pattern is worked out from the measurements of the intended final shape, using geometric formulas. A rectangular tray, for example, is laid out from its length, width, height and the bend allowances.
• Trial and error method: Used when no standard pattern exists or when making a prototype, this method creates a rough pattern in paper or thin sheet, tests the fit and adjusts it. A workshop making a one-off cover for an odd-shaped machine often works this way.
• Computer-aided design (CAD) method: CAD software creates digital patterns that can be scaled and modified exactly. Workshops with CNC cutting machines send the digital pattern directly to the machine, which improves accuracy and speed.

Chapter Summary

This chapter explored the essential techniques and processes involved in carrying out sheet metal work, beginning with pattern development. It detailed three primary methods for creating accurate patterns: the parallel line method, triangulation, and the radial line method, each with distinct procedures and specific applications in fabricating sheet metal components. The chapter then examined various types of sheet metal products commonly manufactured, including tanks, gutters, cabinets and boxes, drums, tables and desks, guardings or enclosures, and troughs. It highlighted how these products are developed using the discussed pattern methods and explained their practical applications across different industries. Emphasis was placed on understanding the relationship between pattern development and the final product, ensuring precision and functionality in fabrication. The integration of these concepts equips learners with the skills to produce high-quality sheet metal items suited for diverse uses. Overall, the chapter provided a comprehensive overview of both theoretical and practical aspects of sheet metal work.