Draft working drawings are essential in Metal Inert Gas (MIG) welding as they provide a precise visual representation of the components to be fabricated or assembled. In Kenya’s diverse professional sectors such as bottling plants, printing works, hotel kitchens, and farm packing sheds, accurate interpretation of these drawings ensures that welding projects meet design specifications and quality standards. This chapter focuses on the fundamental skills required to read, understand, and produce draft working drawings, enabling diploma-level students to apply MIG welding techniques effectively across various industries.

1.1 Drawing interpretation

Drawing interpretation is the skill of reading a technical drawing so that the part can be made correctly before any cutting or welding starts. It covers dimensions, tolerances, symbols, views, scale, and angles, and the ability to sketch plane shapes and solids to explain or check a design. Reading the drawing correctly avoids wrong cuts, poor fit-up, and rework in the welding workshop.

1.1.1 Dimensions

Dimensions on a drawing give the exact size and position of each feature of the workpiece. Correct reading of dimensions makes sure that the welded parts fit together as intended. Example: a welding workshop making a steel frame reads the length of each bar and the position of each hole from the drawing before it marks out the metal.

1.1.1.1 Linear

Linear dimensions give length, width, height, thickness, and distance between features, using a straight dimension line with a numerical value. Unless the drawing states otherwise, the values are in millimetres. Example: a drawing of a 5 mm flat bar shows its length as 450, which means 450 mm.

• Dimension line: A thin line with arrowheads at both ends, drawn parallel to the feature being measured.
• Extension lines: Thin lines that extend from the edges of the feature, without touching it, to carry the dimension line clear of the outline.
• Value placement: The number is placed above the dimension line or in a break in it.
• Units: Millimetres are assumed, and the unit symbol is not repeated on every dimension.
• No repeats: Each dimension is given once, and a repeated dimension can cause conflicts.

1.1.1.2 Angular

Angular dimensions give the angle between two lines or surfaces, in degrees. They are used for bevels, mitres, and bent parts. Example: a drawing of a plate edge for welding may show a bevel angle of 30°, which the welder prepares by grinding.

• Arc dimension line: The dimension line is an arc centred on the point where the two lines meet.
• Value: The angle is written in degrees, for example 45°, near the arc.
• Bevel angles: These show the edge preparation of plates for butt joints.
• Included angle: This is the total angle between the two prepared faces, which is the sum of the two bevels.
• Checking: The angle is checked with a protractor or bevel gauge after preparation.

Reading dimension lines

Dimensions are read from the dimension lines, extension lines, and numbers together.

• Arrowheads: They end on the extension lines.
• Breaks: A break in a long line shows that the part continues, with the full length written on the dimension.
• Positions: Dimensions are placed outside the view where possible, and the smallest are nearest the part.
• Chain and baseline dimensions: Chain dimensions add up along a line, and baseline dimensions are measured from one common datum, which avoids a build-up of error.
• Radial dimensions: Arcs and circles are given by radius (R) or diameter (the symbol Ø).

A technical drawing illustrating the use of extension lines and dimens (Source: 59d646f026634d9a)

Importance of correct dimension reading

• Accurate size: Parts are made to the right size and rework is avoided.
• Compatibility: Parts fit the mating parts in an assembly.
• Material estimate: Material and cost are estimated from the dimensions.
• Fewer errors: Mistakes during welding and fitting are reduced.
• Communication: The designer and the welder understand the same information.

1.1.2 Tolerances

A tolerance is the permitted variation in a dimension. No part can be made to an exact size, so the tolerance sets the upper and lower limits within which the part still fits and works. Example: a hole in a bracket given as 12 ± 0.2 mm may be anything from 11.8 to 12.2 mm.

Meaning of tolerances

• Allowable deviation: The amount by which the actual size may differ from the nominal size.
• Variation: It allows for the variation in manufacture and in the material.
• Interchangeability: Parts made within tolerance can be interchanged.
• Assembly: It prevents parts that are too tight or too loose.
• Reliability: It supports consistent quality of the finished product.

Types of tolerances

• Limit tolerances: The upper and lower limits are given directly, for example 50.2 and 49.8.
• Bilateral tolerances: The size may vary on both sides of the nominal size, for example 50 ± 0.2.
• Unilateral tolerances: The size may vary on one side only, for example 50 +0.4 and 0.
• Geometric tolerances: They control form, orientation, and position, such as flatness or squareness.
• Fit tolerances: They set the clearance or interference between mating parts.

Reading tolerance notations

• Beside the dimension: The tolerance is written next to the value or in a general tolerance block.
• Plus and minus signs: These show the permitted variation.
• General tolerance: A note in the title block applies to all dimensions without their own tolerance.
• Geometric symbols: They are shown in a frame that gives the symbol, the value, and the datum.

Worked example: a bar is dimensioned 120 ± 0.5 mm.

Formula: upper limit = nominal + tolerance, and lower limit = nominal − tolerance

Substitution: upper = 120 + 0.5, and lower = 120 − 0.5

Working: 120.5 and 119.5

Result: the bar is acceptable if its length is between 119.5 mm and 120.5 mm, and the total tolerance is 1.0 mm.

Importance of tolerances in MIG welding

• Joint gaps: They prevent gaps that are too wide or too tight for good fusion.
• Weld quality: Correct fit-up supports correct penetration.
• Distortion: Good fit-up reduces distortion and residual stress.
• Inspection: Tolerances give clear pass or fail limits.
• Strength: Accurate joints give stronger and longer-lasting welds.

1.1.3 Symbols and notations

Symbols and notations give short, standard information on the drawing about weld type, joint, and finish. They save space and avoid long written instructions. Example: a fillet weld symbol with a size beside it tells the welder the type and the leg length of the weld at the joint.

Common welding symbols

• Fillet weld: Shows the weld in a corner or T-joint, with the size beside it.
• Groove weld: Shows the groove shape and the preparation, such as a V or a square groove.
• Spot and seam weld: Shows the type of resistance weld and its spacing.
• Weld all round: A circle at the arrow junction shows that the weld goes all the way round the joint.
• Field weld: A flag shows that the weld is made on site.

Supplementary symbols

• Contour symbols: They show the shape of the weld surface, flush, convex, or concave.
• Finish symbols: They show how the weld is finished, such as by grinding.
• Arrow side and other side: They show on which side of the joint the weld is made.
• Backing and spacer: They show additional parts in the joint.
• Length and pitch: They give the length of each weld and the spacing between welds.

Reading a welding notation

• Arrow: It points to the joint.
• Reference line: The symbol on the line below the arrow shows a weld on the arrow side, and the symbol above shows a weld on the other side.
• Size: The dimension is written to the left of the symbol, and the length to the right.
• Tail: The tail holds notes such as the process or the specification.
• Check: Read the symbol with the views of the drawing, to find the joint.

1.1.4 Views/Elevations

Views, or elevations, show the object as seen from different directions, so that the whole shape can be understood and made. One view alone seldom shows all the features of a welded part. Example: a welded stand may need a front view for its height, a top view for the base plate, and a side view for the position of the braces.

1.1.4.1 First angle

In first angle projection the object is placed in the first quadrant, between the viewer and the projection plane. The front view is drawn first, the view from above is placed below the front view, and the view from the left is placed on the right of the front view. The symbol for first angle shows a truncated cone with the circular end on the right. Example: a drawing of a steel bracket in first angle puts its plan under its front view.

• Order of views: The view seen from the left is on the right of the front view, and the view from above is under it.
• Position of the object: The object is between the observer and the plane.
• Symbol: The projection symbol is shown in the title block.
• Alignment: Features are projected straight across and down from one view to the next.
• Checking: Always check the symbol before reading the views, because third angle swaps the positions.

1.1.4.2 Third angle

In third angle projection the projection plane is placed between the viewer and the object. Each view is drawn on the side nearest to where it is seen, so the view from above is placed above the front view, and the view from the right is on the right. The symbol for third angle shows a truncated cone with the circular end on the left. Example: the same bracket drawn in third angle puts its plan above its front view.

• Order of views: The view from above is over the front view, and the right-hand view is on the right.
• Position of the plane: The plane is between the observer and the object.
• Symbol: The projection symbol must be shown in the title block.
• Comparison: The views are the same as in first angle but in swapped positions.
• Checking: Misreading the projection method puts the views the wrong way round, so check the symbol.

Other types of views

• Front view: The main face of the object.
• Top view: The object as seen from above.
• Side views: The profile seen from the left or the right.
• Isometric view: A pictorial three-dimensional view with the main axes at 30° to the horizontal.

A 3D isometric view of a stepped block (Source: 697c72e40165f702)

• Sectional view: The object is cut by an imaginary plane, to show the inside features.

Importance of multiple views

• Complete shape: They show complex shapes fully.
• Hidden features: They show features that cannot be seen in one view.
• Measurement: They help in measuring and laying out the work.
• Welding plan: They guide the sequence of welds and the preparation of joints.
• Inspection: They help in checking the finished product.

1.1.5 Scale

Scale is the ratio between the size of the drawing and the actual size of the object. It allows a large object to fit on a sheet of paper, and a small part to be drawn large enough to show its detail. Example: a welding workshop draws a large frame at 1:10 and a small bracket at 2:1.

Meaning and types of scale

• Full size (1:1): The drawing is the same size as the object.
• Reduction (for example 1:5): The drawing is smaller than the object.
• Enlargement (for example 5:1): The drawing is larger than the object.
• Scale notation: The first number is the drawing, and the second is the object.
• Position: The scale is given in the title block, and a different scale for a detail is written beside that detail.

Selecting and using a scale

• Object size: Choose the scale from the size of the object and the sheet.
• Detail: Small parts need enlargement, and large structures need reduction.
• Dimensions: Dimensions on the drawing always give the real size, whatever the scale.
• Instruments: A scale rule gives the ratio directly, so no calculation is needed.
• Verification: Check the scale before cutting, and do not measure from a copy that has been resized.

Worked example: a length measures 40 mm on a drawing at scale 1:5.

Formula: actual size = drawing size × scale factor

Substitution: actual = 40 × 5

Working: 40 × 5 = 200

Result: the actual length is 200 mm.

1.1.6 Measurement of angles

Angles are measured to prepare joints and to set parts in position for welding. They are measured on drawings and on the workpiece with simple instruments. Example: a welder checks the 60° included angle of a plate preparation with a bevel gauge and protractor before welding.

Types of angles in drawings

• External angles: Formed outside the part.
• Internal angles: Formed inside corners and hollows.
• Included angles: The angle between two surfaces forming a joint.
• Reference angles: Used to position tools or parts.
• Compound angles: Formed by combining two or more angular measurements.

Procedure for measuring an angle

1. Place the centre hole of the protractor on the vertex of the angle.
2. Align the baseline of the protractor with one leg of the angle.
3. Read the scale where the other leg crosses it.
4. Check which of the two scales, inner or outer, starts at zero on the baseline.
5. Write down the angle value.
6. Transfer the angle to the workpiece, and confirm it with a bevel gauge.

Care: check the instruments before use, hold them firmly against the edges, and align the zero mark correctly.

1.1.7 Sketching of plane geometric forms

Sketching plane shapes lets the welder show the layout of a part before a full drawing is made. A sketch is made quickly by hand, with light guidelines first and firm outlines after. Example: a welder sketches a triangular gusset on a plate, with its dimensions, to agree its size with the supervisor.

1.1.7.1 Triangles

A triangle has three sides and three angles that add up to 180°. Triangles are used in gussets and bracing. Example: a triangular gusset joins a post to a base plate.

• Equilateral: Three equal sides and three angles of 60°.
• Isosceles: Two equal sides and two equal angles.
• Scalene: Three unequal sides.
• Right-angled: One angle of 90°, with the longest side opposite it.
• Sketching: Draw the base, mark the angles or lengths, and join the points.

1.1.7.2 Quadrilaterals

A quadrilateral has four sides, and its angles add up to 360°. Plates, frames, and panels are quadrilaterals. Example: a rectangular base plate has four right angles and opposite sides equal.

• Square: Four equal sides and four right angles.
• Rectangle: Opposite sides equal and four right angles.
• Parallelogram: Opposite sides parallel and equal, with no right angles.
• Rhombus: Four equal sides with opposite angles equal.
• Trapezium: One pair of parallel sides.

1.1.7.3 Polygons

A polygon is a closed shape with straight sides, and a regular polygon has equal sides and equal angles. Hexagonal and octagonal shapes occur in nuts and in plate profiles. Example: a hexagon of side 30 mm is marked out for a base plate.

• Sum of interior angles: (n − 2) × 180°, where n is the number of sides.
• Interior angle of a regular polygon: (n − 2) × 180° ÷ n.
• Names: Pentagon has five sides, hexagon six, and octagon eight.
• Sketching: Draw a circle, and divide it into equal parts to mark the corners.
• Check: The sides must be equal in a regular polygon.

Worked example: the interior angle of a regular hexagon.

Formula: interior angle = (n − 2) × 180° ÷ n

Substitution: (6 − 2) × 180° ÷ 6

Working: 4 × 180 ÷ 6 = 120

Result: each interior angle is 120°.

1.1.7.4 Circles and tangents

A circle is the set of points at a fixed distance, the radius, from the centre. A tangent is a straight line that touches the circle at one point and is at right angles to the radius at that point. Tangents are used where a straight edge runs smoothly into a curve, for example in a bent bar. Example: a flat bar bent round a pipe of radius 50 mm leaves the bend along a tangent.

• Radius and diameter: The diameter is twice the radius.
• Circumference: π × diameter.
• Tangent: Touches the circle at one point and is perpendicular to the radius.
• Sketching a tangent: Draw the radius to the point of contact, then draw a line at 90° to it.
• Two circles: A common tangent touches both circles.

1.1.8 Solids sketches

Solid sketches show three-dimensional objects, so that the welder can see the spatial relationship of the parts. They are made with isometric or oblique lines, starting from the base shape. Example: a welding workshop sketches a cylindrical tank on a rectangular stand before making a detailed drawing.

1.1.8.1 Prisms

A prism has two equal parallel ends, and its sides are flat faces. The cross-section is the same along its length. Beams and structural members are prisms. Example: a triangular prism is a roof-shaped support.

• Volume: Area of the end × length.
• Faces: The side faces are rectangles.
• Types: Named from the shape of the end, such as triangular or hexagonal.
• Sketching: Draw the end shape, draw parallel lines for the length, and then draw the second end.
• Use: Beams, bars, and angle sections.

1.1.8.2 Cones

A cone has a circular base and a curved surface that tapers to a point, the apex. Hoppers and funnels have conical shapes. Example: a hopper for grain has a cone at the bottom.

• Volume: ⅓ × π × r² × h.
• Slant height: √(r² + h²).
• Sketching: Draw an ellipse for the base, mark the apex above its centre, and join it to the sides of the ellipse.
• Frustum: A cone with its top cut off, as in a bucket.
• Use: Funnels, hoppers, and reducers.

1.1.8.3 Cubes

A cube has six equal square faces and twelve equal edges. It is the simplest solid to sketch. Example: a cubic box frame with edges of 300 mm is made from welded bar.

Concrete cube molds for testing (Source: Wikimedia Commons)

• Volume: edge³.
• Surface area: 6 × edge².
• Sketching: Draw a square, draw a second square shifted along 30°, and join the corners.
• Hidden edges: Draw them as dashed lines.
• Use: Blocks, boxes, and frames.

1.1.8.4 Cuboids

A cuboid has six rectangular faces, and its length, width, and height may all be different. Tanks, boxes, and plates are cuboids. Example: a rectangular steel tank is a cuboid.

• Volume: length × width × height.
• Surface area: 2 × (lw + lh + wh).
• Sketching: Draw a rectangle for the front, shift a copy along the 30° direction, and join the corners.
• Edges: Opposite edges are equal and parallel.
• Use: Tanks, boxes, and plates.

1.1.8.5 Cylinders

A cylinder has two equal circular ends joined by a curved surface. Pipes, tanks, and drums are cylinders. Example: a pipe or a storage tank has a cylindrical shape.

• Volume: π × r² × h.
• Curved surface area: 2 × π × r × h.
• Sketching: Draw two equal ellipses, one above the other, and join them with two vertical lines.
• Hidden edge: The back of the lower ellipse is dashed.
• Use: Pipes, tanks, and shafts.

Worked example: the volume of a cylindrical tank of radius 50 mm and height 300 mm.

Formula: V = π × r² × h

Substitution: V = 3.142 × 50² × 300

Working: 3.142 × 2500 × 300 = 2,356,500

Result: the volume is 2,356,500 mm³, which is about 2.36 litres.

Chapter Summary

This chapter covers the essential aspects of draft working drawings relevant to metal inert gas welding. It begins with drawing interpretation, focusing on understanding linear and angular dimensions, the application of tolerances, and the use of standard symbols and notations. The chapter explains the different views and elevations, specifically the first angle and third angle projections, and discusses the importance of scale and accurate measurement of angles. It then guides the sketching of plane geometric forms such as triangles, quadrilaterals, polygons, circles, and tangents, followed by the sketching of solid shapes including prisms, cones, cubes, cuboids, and cylinders. Moving on to draft work drawings, the chapter highlights the significance of proper drawing dimensions and the correct use of welding symbols and notations. Finally, the operation procedure section outlines the development of the drawings and their practical use in the welding process, ensuring clear communication and precise fabrication.