Tungsten Inert Gas (TIG) welding requires precise interpretation of working drawings to ensure accurate fabrication and assembly of welded joints. Draft working drawings act as a universal language between designers, engineers, and welders, providing critical information on dimensions, tolerances, and welding specifications. In Kenya’s diverse industrial sectors such as manufacturing, construction, and automotive repair, mastery of drawing interpretation enhances quality control and reduces costly errors during the welding process. This chapter focuses on developing competence in reading and sketching technical drawings specific to TIG welding applications.

1.1 Drawing interpretation

Drawing interpretation is fundamental for TIG welders to understand the exact requirements of a welding project. It involves decoding graphical representations, dimensions, and symbols that define the geometry, size, and quality standards of the weldment. In Kenya's manufacturing workshops and fabrication yards, clear comprehension of drawings ensures effective communication and accurate execution of welding tasks.

1.1.1 Dimensions

Dimensions specify the physical sizes and locations of features on the drawing and are critical for fabricating components to fit precisely. A welder reading a drawing of a stainless steel bracket uses dimensions to cut, bevel, and position each part before any arc is struck.

Dimensioning standards:

• Dimensions follow BS 8888 or ISO 129-1, which ensures uniformity in presentation.
• Units are normally millimetres (mm).
• Dimension lines end with arrowheads pointing to extension lines that mark the feature limits.
• Numbers are placed above or within dimension lines for clarity.
• Proper spacing avoids clutter and overlapping, which aids legibility.

Placement and reading of dimensions:

• Dimensions are placed outside the object outline to maintain clarity.
• Reading a dimension requires understanding the views (front, side, top) so that the exact feature being measured is identified.
• Critical dimensions related to weld preparation and fit-up are usually highlighted.
• Welders cross-check dimensions between views to avoid misinterpretation.
• Consistent reading of dimensions reduces rework and material waste.

Radial and diameter dimensions describe curves and circles, which are crucial for round pipes, tubes, and cylindrical components:

• Diameter symbol: The symbol Ø before a number means a diameter, for example Ø50 for a 50 mm round tube or bar.
• Radius symbol: The letter R before a number means a radius, for example R10 for a rounded plate corner.
• Centrelines: Chain-dotted centrelines mark the centre of holes and pipes, and dimensions are measured from them.
• Leader lines: A thin angled line with an arrowhead points to a circle or arc and carries the dimension and any note, such as the number of holes.
• Pipe size: A pipe may be given as outside diameter and wall thickness, which the welder needs to choose the joint preparation.

Common dimensioning errors:

• Omitting units or scale information leads to fabrication mistakes.
• Overlapping dimension lines cause confusion.
• Incorrect placement of dimension arrows results in inaccurate measurements.
• Mixing chain and baseline dimensions without clear notation can cause errors.
• Failure to dimension key weld features such as groove angles or root gaps compromises weld quality.

1.1.1.1 Linear

Linear dimensions indicate straight-line measurements such as length, width, height, thickness, and the distance between two features. Example: a TIG-welded stainless steel table frame is dimensioned 1200 mm long, 600 mm wide and 750 mm high, and the welder cuts the tubes to these lengths before tacking.

Key points about linear dimensions:

• Straight-line distance: They give the exact distance between two points or two extension lines, for example the length of a plate edge to be welded.
• Units: Values are normally in millimetres, with the unit stated once in the title block or notes rather than on every figure.
• Chain dimensions: A series of connected linear dimensions, each starting where the previous one ends. Errors accumulate along the chain, so the overall length may drift from the intended value.
• Baseline dimensions: All measurements start from one reference edge or point, which reduces cumulative error and suits the marking out of hole or joint positions.
• Overall dimensions: The total length, width, or height of the weldment is usually shown, so the welder can check that the sum of the parts agrees with it.
• Weld-related distances: Root gap, joint spacing, and weld length are also linear dimensions and must be read carefully before fit-up.

1.1.1.2 Angular

Angular dimensions specify the angle between two lines or surfaces and are essential for positioning weld joints correctly. Example: a 6 mm stainless steel plate butt joint is shown with a 60° included groove angle and a 2 mm root gap, so each plate is bevelled to 30°.

Key points about angular dimensions:

• Arc dimension line: The angle is shown by an arc drawn about the vertex, with arrowheads touching the two lines that form the angle.
• Units: Angles are written in degrees, using the ° sign, and sometimes in degrees and minutes for fine work.
• Included angle and bevel angle: The included groove angle is the full angle between the two prepared faces. The bevel angle is measured from the vertical or square edge of one plate, so a 60° included angle in a symmetrical joint means a 30° bevel on each plate.
• Mitre joints: Corner and mitred tube joints show the cutting angle, for example 45° on each tube for a 90° corner.
• Angular tolerance: Angles may carry a tolerance such as ±2°, which sets how far the prepared edge may differ from the drawn value.
• Checking: The tools and method for measuring angles on the workpiece are covered under Measurement of angles.

1.1.2 Tolerances

Tolerances define permissible variations in dimensions, ensuring parts fit and function correctly despite minor deviations. Example: a stainless steel tube cut to 300 mm with a tolerance of ±1 mm is accepted if it measures anywhere from 299 mm to 301 mm.

Purpose of tolerances

• Allow manufacturing flexibility while maintaining interchangeability.
• Control fit between welded components to prevent assembly issues.
• Ensure structural integrity by limiting dimensional deviations.
• Facilitate quality control inspections.
• Reduce material wastage by avoiding overly tight specifications.

Types of tolerances

• Limit tolerances: These specify upper and lower dimension limits, for example 49.5 to 50.5.
• Bilateral tolerances: These permit variation in both positive and negative directions, for example 50 ± 0.5.
• Unilateral tolerances: These allow variation in only one direction, for example 20 +0.4 / 0.
• Geometric tolerances: These control form, orientation, and position, which is important for weld joint alignment.
• Welding-specific tolerances: These cover joint preparation dimensions such as groove angle and root gap.

Worked example of a bilateral tolerance on a plate width:

Upper limit = nominal + tolerance = 50 + 0.5 = 50.5 mm

Lower limit = nominal - tolerance = 50 - 0.5 = 49.5 mm

Total tolerance = upper limit - lower limit = 50.5 - 49.5 = 1.0 mm

Reading and applying tolerances

• Tolerances are indicated adjacent to dimensions, often in parentheses or a tolerance block.
• Welders must interpret tolerances to adjust fit-up and welding parameters.
• Understanding tolerance stacking helps avoid assembly interference.
• Visual inspection and measuring tools verify adherence during fabrication.
• Knowledge of weld code standards like AWS D1.1 complements tolerance interpretation.

Consequences of ignoring tolerances

• Excessive gaps or overlaps cause weak welds or distortion.
• Parts may not assemble correctly, leading to costly rework.
• Structural failure risks increase due to improper joint fit.
• Inspection failures delay project timelines.
• Increased scrap rates raise production costs.

1.1.3 Symbols and notations

Welding drawings use standardised symbols and notations to convey welding process requirements clearly and efficiently. Example: a fillet weld symbol with a size figure on the arrow side tells the welder which side of a stainless steel T-joint to weld and how large the weld must be.

Welding symbols

• Symbols indicate weld type (for example fillet, groove, plug), size, length, and contour.
• The reference line and arrow point to the joint location.
• Supplementary symbols specify weld finish, weld-all-around, or field weld.
• Common symbols conform to ISO 2553 and AWS A2.4 standards.
• Correct interpretation avoids miscommunication between designers and welders.

Other technical symbols

• Surface texture symbols indicate machining or finishing requirements.
• Dimensioning symbols clarify units, angle types, and datum points.
• Material specification symbols identify base metal and filler requirements.
• Thread and fastener symbols may appear in assemblies involving welded components.
• Safety symbols caution about hazards during welding.

Notations and abbreviations

• Abbreviations such as "TIG" for Tungsten Inert Gas welding specify the welding process.
• Notes often clarify welding sequence or special instructions.
• Finish symbols show grinding or hammering requirements.
• Heat treatment or preheating requirements may be noted.
• Weld procedure specification (WPS) references ensure compliance with quality standards.

Common misinterpretations

• Confusing weld symbols with dimension symbols causes fabrication errors.
• Ignoring supplementary symbols leads to improper weld contouring.
• Misreading the arrow side (arrow side versus other side) affects weld placement.
• Overlooking notes on sequence or special conditions reduces weld quality.
• Failure to update symbols after design changes causes rework.

1.1.4 Views/Elevations

Multiple views or elevations provide comprehensive visualisation of the component from different directions to guide welding. Example: a TIG-welded stainless steel hopper is shown in front, side, and top views so the welder can find every seam and its length.

1.1.4.1 First angle

In first-angle projection the object is imagined to lie between the viewer and the projection plane, so each view is projected away from the viewer onto the plane behind the object. The result is that each view is placed on the side opposite to where the viewer looked from.

Key points about first-angle projection:

• Front view: The front elevation is the main view and is drawn in the centre of the layout.
• Top view: The plan, seen from above, is placed below the front view.
• Side view: The view of the left side is placed to the right of the front view, and the view of the right side to the left.
• Projection symbol: A small symbol of a truncated cone in the title block states that first angle is used. It is the mirror image of the third-angle symbol.
• Reading caution: A welder who assumes the wrong system will read the side and top views the wrong way round and may weld the wrong end of a part, so always check the symbol first.
• Origin of drawings: Drawings from different sources may use either system, so never assume.

1.1.4.2 Third angle

In third-angle projection the projection plane is imagined to lie between the viewer and the object, like a glass box around the part. Each view is drawn on the plane nearest to the side from which it is seen, so views sit where the viewer would expect them.

Key points about third-angle projection:

• Front view: The front elevation is again the main view in the centre.
• Top view: The plan is placed above the front view.
• Side view: The right side view is placed to the right of the front view, and the left side view to the left.
• Projection symbol: The truncated cone symbol in the title block is the mirror image of the first-angle symbol.
• Ease of reading: Features line up naturally with the views next to them, so a welder can follow a seam from the front view across to the side view.
• Mixed drawings: When a job package contains drawings in both systems, check the symbol on each sheet before cutting material.

Orthographic and sectional views

• Front, top, and side views represent the object's shape and features in 2D.
• Each view aligns with others to give a complete dimensional picture.
• Welders use these views to locate joints and understand part geometry.
• Sectional views cut through parts to show internal weld joint details.
• Elevations help visualise vertical positioning of welds.

Auxiliary views

• Used to display features not parallel to primary planes.
• Clarify complex geometries such as angled weld joints.
• Aid in dimensioning inclined or curved weld preparations.
• Improve accuracy in weld placement and fit-up.
• Critical in fabricating components like pipe elbows and brackets.

Isometric and pictorial views

• Provide 3D representations for better spatial understanding.
• Useful for initial visualisation and communication with non-technical stakeholders.
• Not used for dimensioning but supplement orthographic views.
• Help identify potential welding access issues.
• Commonly included in fabrication drawings for complex assemblies.

Reading multiple views

• Cross-reference dimensions and symbols across views for consistency.
• Identify weld joints that appear differently in various views.
• Understand the orientation of welds relative to the whole assembly.
• Detect hidden weld joints shown with dashed lines.
• Plan welding sequence based on joint accessibility shown.

1.1.5 Scale

Scale determines the proportional size relationship between the drawing and the actual object, critical for accurate fabrication. Example: a long stainless steel tank frame is drawn at 1:10 so that it fits on an A3 sheet, and every measurement taken from the drawing is multiplied by 10 to get the real size.

Meaning of scale

• Scale is the ratio of the drawing size to the real-world size.
• Common scales are full size (1:1), reduced (1:2, 1:5), or enlarged (2:1).
• Helps fit large objects on standard drawing sheets.
• Ensures dimensions on the drawing translate correctly to fabrication.
• Scale is usually indicated in the title block or near the drawing.

Worked example for a reduced scale of 1:5:

Actual size = drawn size × scale factor

Actual length = 120 mm × 5

Actual length = 600 mm

Worked example for an enlarged scale of 2:1:

Actual size = drawn size ÷ 2

Actual size = 40 mm ÷ 2

Actual size = 20 mm

Importance of scale in welding drawings

• Accurate scaling avoids dimensional errors during fabrication.
• Allows welders to estimate sizes and prepare materials.
• Facilitates checking of fit-up and weld joint dimensions.
• Enables proper use of measuring tools calibrated to scale.
• Ensures compatibility with other engineering drawings.

Common scale types and usage

• Full scale (1:1): Used for small components requiring exact detail.
• Reduced scale: Used for large structures like frames or tanks.
• Enlarged scale: Used for small weld details or complex joints.
• Multiple scales: These may appear on the same drawing for different views.
• Selection: The choice depends on component size and detail complexity.

Consequences of ignoring scale

• Incorrectly sized parts cause assembly failure.
• Weld sizes or joint configurations are misinterpreted.
• Materials are wasted and fabrication time increases.
• Verifying dimensions during inspection becomes difficult.
• Improperly fitted weldments increase safety risks.

1.1.6 Measurement of angles

Accurate angle measurement is essential for preparing and verifying weld joint geometry, especially groove and bevel angles. Example: before welding a 6 mm stainless steel butt joint with a 60° included groove, the welder checks each bevel at 30° with a bevel gauge.

Types of angles in welding

• Groove angle: Determines the bevel preparation for penetration.
• Fillet weld angle: The angle between the two fusion faces, which affects leg length and throat thickness.
• Mitre angle: Used in corner joints.
• Chamfer angle: Influences the weld bead profile.
• Rotation angle: Important in pipe welding for alignment.

Tools for measuring angles

• Use tools to compare drawn angles with actual workpiece preparation.
• Regular calibration ensures measurement accuracy.
• Proper handling avoids damage to sensitive measuring instruments.

Procedure to measure and mark angles

1. Select the appropriate measuring tool based on the angle type.
2. Align the tool's base with the reference edge or surface.
3. Adjust the blade or arm to match the angle.
4. Read the angle value from the graduated scale.
5. Mark the angle on the workpiece using a scriber or chalk.
6. Double-check measurements before cutting or welding.

Impact of incorrect angle measurement

• Poor weld penetration leads to weak joints.
• Improper fit-up increases distortion.
• Weld defects such as undercut or lack of fusion appear.
• Rework and material wastage increase production costs.
• Compromised structural integrity creates safety hazards.

1.1.7 Sketching of plane geometric forms

Sketching flat geometric shapes is a foundational skill for welders to visualise and plan weld joints. Example: before cutting a stainless steel gusset, the welder sketches its triangular outline and marks the lengths of the edges to be welded.

1.1.7.1 Triangles

A triangle has three sides and three angles that add up to 180°. In welding, a triangle is the cross-section of a fillet weld, and triangular gussets stiffen corners of frames and brackets.

Key points about triangles:

• Right-angled triangle: One angle is 90°. A fillet weld on a 90° joint has this shape, with the two legs lying on the plates.
• Equilateral triangle: All sides are equal and each angle is 60°.
• Isosceles triangle: Two sides and two angles are equal, as in a symmetrical gusset.
• Scalene triangle: All sides and angles differ, as in some irregular brackets.
• Squareness check: A triangle with sides 300 mm, 400 mm, and 500 mm has a right angle between the two shorter sides, which is used to check that a frame corner is square.
• Fillet weld throat: For an equal-leg fillet weld, the throat equals the leg multiplied by 0.707.

Worked example for a 6 mm equal-leg fillet weld:

Throat = leg × 0.707

Throat = 6 × 0.707

Throat = 4.24 mm

1.1.7.2 Quadrilaterals

A quadrilateral has four sides. The square, rectangle, parallelogram, and trapezium are all met in plate cutting, and the rectangle is the commonest shape of flat plate and sheet. Example: a 400 mm by 250 mm stainless steel panel is sketched before the edges are marked for welding.

Procedure to sketch a rectangle:

1. Draw a horizontal base line with a ruler.
2. Mark the desired length along the base.
3. Draw perpendicular lines at both ends to the required height.
4. Connect the top ends with a horizontal line.
5. Verify dimensions with a scale.
6. Label the sketch with dimensions and notes.

A square is sketched the same way with equal sides. For a parallelogram, the opposite sides are parallel and equal but the angles are not 90°, and a trapezium has one pair of parallel sides, as in a tapered plate.

1.1.7.3 Polygons

A polygon is a closed flat shape with straight sides, and a regular polygon has equal sides and equal angles. Example: a hexagonal stainless steel plate is cut and its six edges are welded to a base, so the welder sketches it first to see the six corners.

Key points about polygons:

• Pentagon, hexagon, and octagon: These have five, six, and eight sides and are used for covers, nuts, and decorative frames.
• Interior angles: The sum of the interior angles is (n - 2) × 180°, where n is the number of sides. For a hexagon this is (6 - 2) × 180° = 720°, so each angle is 120°.
• Octagon angles: Each interior angle of a regular octagon is 135°.
• Sketching a hexagon: Draw a circle, then step the radius around the circumference six times. The six points joined by straight lines give a regular hexagon, because its side equals the radius.
• Corner welds: Every corner of a polygon is a possible weld start or stop, so the sketch should show the corners clearly.
• Complex shapes: Irregular polygons are sketched by dividing them into triangles and rectangles.

1.1.7.4 Circles and tangents

A circle is the set of points at a fixed distance, the radius, from a centre. A tangent is a straight line that touches a circle at one point and is perpendicular to the radius at that point. Example: a pipe end is sketched as a circle, and a flat strap running along it is a tangent to the outer circle.

Key points about circles and tangents:

• Circle parts: The centre, radius, diameter, circumference, and chord are the terms used on weld drawings of pipes and holes.
• Circumference: The length of weld around a pipe is found from the diameter.
• Tangent line: To sketch a tangent, draw the radius to the contact point and then draw a line at 90° to it through that point.
• Blended corners: An arc joined smoothly to a straight edge, as in a plate with rounded corners, is built from tangents. The arc centre lies on a line perpendicular to the edge at the point where it starts.
• Ellipse: A circle seen at an angle, such as a pipe branch cut obliquely, appears as an ellipse.
• Tools: A compass gives accurate circles and arcs, while freehand circles are drawn within a light square construction box.

Worked example for the weld length around a 50 mm diameter pipe butt joint:

Circumference = π × d

Circumference = 3.1416 × 50

Circumference = 157.1 mm

Sketching techniques and tools

• Use pencils and rulers for accuracy.
• Freehand sketching aids quick visualisation.
• Employ compasses for circles and arcs.
• Maintain correct proportions using scale.
• Practise consistent line weights for clarity.

Importance in welding preparation

• Helps communicate joint design during planning.
• Facilitates understanding of weld bead placement.
• Assists in estimating material requirements.
• Enables quick checks before detailed drawing production.
• Improves spatial awareness of weld geometry.

1.1.8 Solids sketches

Sketching three-dimensional solids enhances comprehension of complex welded assemblies and joint interfaces. Example: a welder sketches a stainless steel water tank as a cuboid with a pipe connection to see where each seam will be welded.

1.1.8.1 Prisms

A prism is a solid with two identical parallel end faces joined by flat rectangular sides. Example: a fillet weld bead along a 500 mm joint is a triangular prism, and a hexagonal bar is a hexagonal prism.

Procedure to sketch a triangular prism:

1. Draw the triangular end face with a ruler, using light lines.
2. From each corner, draw a parallel line of equal length at the same angle to show the length of the prism.
3. Draw the second triangle by joining the ends of these lines, parallel to the first.
4. Join the corresponding corners with straight lines to complete the prism.
5. Show hidden edges as dashed lines.
6. Darken the visible edges and add the length and weld location notes.

1.1.8.2 Cones

A cone has a circular base and a curved surface tapering to a point. A cone with the top cut off is called a frustum. Example: a concentric pipe reducer or a funnel-shaped hopper outlet in stainless steel is a frustum, and its seam is welded along the slant line.

Procedure to sketch a cone:

1. Draw a vertical centreline for the axis.
2. Sketch an ellipse at the bottom to represent the circular base.
3. Mark the apex on the centreline at the required height.
4. Draw straight lines from the apex to the widest points of the ellipse.
5. For a frustum, draw a smaller ellipse near the top and join it to the base with straight lines.
6. Show the seam with a line along the slant, and add diameters and height.

1.1.8.3 Cubes

A cube has six equal square faces and twelve equal edges. Example: a 100 mm cube made from six stainless steel plates has twelve edge welds, each 100 mm long.

Procedure to sketch a cube in isometric view:

1. Draw a vertical line for the front vertical edge, equal to the edge length.
2. From the bottom end of this line, draw two lines of equal length at 30° above the horizontal, one to the left and one to the right.
3. From the top end, draw two parallel lines of the same length at 30°, to the left and right.
4. Draw a vertical line of the edge length at the outer end of each lower line, so that it meets the outer end of the matching upper line. This completes the left and right faces.
5. From the top ends of these two vertical lines, draw lines at 30° running back towards the centre until they meet, which completes the top face.
6. Show the three hidden back edges with dashed lines.
7. Mark the edge length and the joints to be welded.

1.1.8.4 Cuboids

A cuboid has six rectangular faces, as in a plate, a tank, or a box section. Example: a stainless steel tank 800 mm long, 500 mm wide and 600 mm high is a cuboid, and its twelve edges are possible weld seams.

A cuboid is sketched like a cube but with the length, width, and height drawn at their own sizes. The length is drawn along one 30° direction, the width along the other, and the height vertically, so the three sets of parallel edges show clearly which seams are long and which are short.

1.1.8.5 Cylinders

A cylinder has two parallel circular ends joined by a curved surface, as in a pipe or round bar. Example: the outlet pipe welded to a stainless steel tank is a cylinder, and the circular seam where it meets the tank wall is welded all around.

Procedure to sketch a cylinder in isometric view:

1. Draw a vertical axis line representing cylinder height.
2. Sketch two ellipses at top and bottom for the circular faces.
3. Connect the ellipses with straight lines at their widest points to form the sides.
4. Add centrelines to show axis symmetry.
5. Show hidden edges with dashed lines and mark the weld joints with a note or weld symbol.
6. Annotate dimensions and material notes.

Other solid forms and complex assemblies

• Pyramids: These appear in tapered parts such as hoppers.
• Spheres: These appear in ball joints or specialised components.
• Complex assemblies: These combine multiple solids, such as a tank with branch pipes and a support frame.
• Breaking down: Break any assembly into basic solids before sketching.
• Order of sketching: Sketch the main body first, then add the smaller parts.

Techniques for solid sketching

• Begin with basic shapes in isometric projection.
• Use light construction lines for proportions.
• Add depth with overlapping lines and light shading.
• Shade or hatch surfaces to indicate orientation.
• Label key dimensions and weld locations.

Benefits for TIG welders

• Visualise complex weld joints in 3D space.
• Plan welding sequences and access points.
• Communicate design intent clearly to fabricators.
• Identify potential interference or fit issues.
• Improve accuracy in joint preparation and welding.

Chapter Summary

This chapter begins with an in-depth exploration of drawing interpretation, focusing on the accurate understanding of linear and angular dimensions, tolerances, and the essential symbols and notations used in technical drawings. It further explains the different views and elevations, distinguishing between first angle and third angle projection methods, while emphasizing the importance of scale and precise measurement of angles. The chapter also covers the sketching of plane geometric forms, including triangles, quadrilaterals, polygons, circles, and tangents, before advancing to three-dimensional solid sketches such as prisms, cones, cubes, cuboids, and cylinders. Moving into draft work drawings, it highlights the critical role of drawing dimensions and welding symbols and notations in preparing clear and functional technical documents. The operation procedure section details the development of draft working drawings, the preparation of operation plans, and the practical steps for lubricating moving parts, including cleaning external surfaces and inspecting cables, connectors, and power sources. Throughout, the chapter integrates theory with practical drafting and maintenance procedures essential for effective tungsten inert gas welding operations.