Draft working drawings are essential in Tungsten Inert Gas (TIG) welding, guiding professionals through the precise fabrication of components. These drawings communicate detailed information about dimensions, tolerances, and welding symbols that ensure quality and accuracy in weld joints. Mastery of interpreting and creating these drawings is critical for welders, fabricators, and inspectors in Kenya’s manufacturing and construction industries to meet both client specifications and regulatory standards.

1.1 Drawing interpretation

Drawing interpretation is the skill of reading a technical drawing so that a welded part can be made correctly before any cutting or welding starts. It covers dimensions, tolerances, welding symbols, views, scale, and angles, together with the ability to sketch plane shapes and solids. Reading the drawing correctly reduces errors and waste in TIG welding work and gives joints that fit up properly.

1.1.1 Dimensions

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

1.1.1.1 Linear

Linear dimensions give straight-line distances such as length, width, height, thickness, and the distance between features. They are shown with a dimension line and a number, in millimetres unless the drawing states otherwise. Example: a drawing of a 3 mm stainless plate 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 written 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, because repeated dimensions can conflict.

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 a TIG butt weld 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 angle: The angle of the prepared edge measured from the surface of the plate.
• Included angle: The total angle between the two prepared faces of a joint, which is the sum of the two bevels.
• Checking: The angle is checked with a protractor or bevel gauge after preparation.

Types of dimensions

• Radial dimensions: Give the radius of arcs, written with the letter R.
• Diameter dimensions: Give the size of holes and round parts, written with the symbol Ø.
• Chain dimensions: A series of dimensions added one after another, which risks a build-up of error.
• Baseline dimensions: All measured from one common reference line, which avoids a build-up of error.
• Coordinate dimensions: Positions given from a fixed origin, which suits complex parts.

Reading dimension lines and extension lines

• Dimension lines end with arrowheads that touch the extension lines, showing the exact points being measured.

Engineering drawing with dimension lines shown (Source: Wikimedia Commons)

• Extension lines start just clear of the feature edges and mark the boundaries of the dimension.
• Units are millimetres unless the drawing says otherwise.
• Arrow styles must be the same throughout the drawing, to avoid confusion.
• Placement of dimensions must avoid clutter and overlap with other lines of the drawing.

Importance of clear dimensioning

• Fabrication errors: Clear dimensions prevent parts being made to the wrong size.
• Material use: Correct sizes reduce waste and cost.
• Inspection: Precise values make quality checks possible.
• Communication: The designer and the welder read the same information.
• Fit-up: Parts fit together at the joint, which is vital for TIG welds with small gaps.

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 dimensioned 12 ± 0.2 mm may be anything from 11.8 mm 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 variation in manufacture and in the material.
• Interchangeability: Parts made within tolerance can be interchanged.
• Function: It keeps parts fitting and strong enough for their use.
• Quality limits: It sets clear pass or fail limits, which reduces rejects.

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.
• Welding distortion allowance: Shrinkage and distortion caused by heat input must be allowed for when the parts are cut.

Reading and applying tolerances

• Beside the dimension: The tolerance is written next to the value, or in a general tolerance note in the title block.
• Plus and minus signs: These show the permitted variation.
• General tolerance: A note in the title block applies to all dimensions that have no tolerance of their own.
• Geometric symbols: They are shown in a frame that gives the symbol, the value, and the datum.

Worked example: a tube 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 tube is acceptable if its length is between 119.5 mm and 120.5 mm, and the total tolerance is 1.0 mm.

Impact of tolerances on TIG welding quality

• Joint gap: TIG welding needs a small, even gap, so a part outside tolerance can leave the joint too open or too tight.
• Penetration: Correct fit-up supports correct penetration and fusion.
• Distortion: Good fit-up reduces distortion and residual stress.
• Inspection: Tolerances give clear limits for checking the finished product.
• Rework: Staying within tolerance reduces rework and delay.

1.1.3 Symbols and notations

Welding symbols and notations give short, standard information on the drawing about the weld type, size, finish, and other requirements. 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.

Basic welding symbols

• Arrow line: Points to the joint to be welded.
• Reference line: Carries the symbols that describe the weld.
• Tail: Holds additional notes, such as the process or the specification.
• Fillet weld symbol: A triangle, showing a fillet weld.
• Groove weld symbols: Show the groove shape, such as square, V, U, or J.

Supplementary symbols

• Contour symbols: Show the shape of the weld surface: flush, convex, or concave.
• Weld all round: A circle at the arrow and reference line junction shows that the weld goes all the way round the joint.
• Field weld: A flag shows that the weld is made on site.
• Backing and spacer symbols: Show additional parts in the joint.
• Process note: The welding process, for example TIG, is written in the tail.

Reading welding symbols on drawings

• Arrow side: A symbol on the lower side of the reference line shows a weld on the arrow side of the joint.
• Other side: A symbol on the upper side of the reference line shows a weld on the other side.
• Size: The weld size is written to the left of the symbol, and the length to the right.
• Tail notes: They carry the process, the specification, or inspection requirements.
• Cross-check: Read the symbol together with the views of the drawing and the welding procedure.

Importance of standardised symbols

• Clear communication: Everyone who reads the drawing gets the same instruction.
• Fewer defects: It reduces misunderstandings that lead to wrong welds.
• Training: It makes training and assessment easier.
• Quality assurance: It supports documentation and inspection.
• Consistency: It keeps the practice the same from one drawing to the next.

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 every feature 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 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 projection symbol 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 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 placed on the right. The projection symbol 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 view from the right 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 method puts the views the wrong way round, so check the symbol.

Other types of views

• Auxiliary views: Show inclined surfaces that are not clear in the principal views, such as bevel angles on pipe joints.
• Sectional views: An imaginary cut exposes the inside features, such as weld penetration and fit-up, and section lines show where the cut is made.
• Isometric views: A pictorial three-dimensional view with the main axes drawn at 30° to the horizontal.

Importance of multiple views

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

1.1.5 Scale

Scale is the ratio between the size of the drawing and the actual size of the object. It lets a large object fit on a sheet of paper, and a small part be drawn large enough to show its detail. Example: a welding workshop draws a large frame at 1:10 and a small TIG-welded fitting 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, as for frames and tanks.
• Enlargement (for example 5:1): The drawing is larger than the object, to show small weld details.
• 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.

Indicating and using scale

• Dimensions: Written dimensions always give the real size, whatever the scale.
• Consistency: The same scale applies to all the views unless a view is marked otherwise.
• Choice of scale: It depends on the size of the object, the sheet, and the detail needed.
• Instruments: A scale rule gives the ratio directly.
• 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. Accurate angles give correct penetration and joint strength. Example: a welder checks the 60° included angle of a plate preparation with a bevel gauge and protractor before welding.

Types of angles in welding

• Bevel angle: The angle of the prepared edge measured from the surface of the plate.
• Included angle: The total angle between two bevelled edges.
• Fillet weld angle: Related to the leg lengths of the weld.
• Chamfer angle: The angle of a bevelled edge on a weld or base metal.
• Rotation angle: The orientation of weld passes in pipe welding.

Tools for measuring angles

• Calibration: Check the tools before use.
• Clean surfaces: Dirt gives wrong readings.
• Digital tools: They improve speed and accuracy.
• Bevel gauges: They help in joint preparation.
• Welding gauges: They verify that the weld agrees with the specification.

Procedure for measuring bevel angles

1. Clean the bevel surface so that no debris affects the measurement.
2. Place the arm of the bevel gauge along the bevelled edge.
3. Lock the arm in position to capture the angle.
4. Transfer the angle to a protractor, or read it from the digital display.
5. Compare the measured angle with the value on the drawing.
6. Correct the bevel if necessary before welding.

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 construction lines 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.

Techniques for sketching

• Instruments: Use a ruler and compass for accurate lines and arcs.
• Construction lines: Start with light lines to set the proportions.
• Dimensions: Mark the key dimensions clearly on the sketch.
• Notes: Annotate the sketch to remove doubt.
• Grid paper: Use grid paper to keep proportions.

1.1.7.1 Triangles

A triangle has three sides and three angles that add up to 180°. Triangles are used in gussets, bracing, and fillet weld profiles. 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. Flanges, nuts, and brackets often have polygon profiles. Example: a hexagonal plate is marked out for a base.

• 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: A pentagon has five sides, a hexagon six, and an octagon eight.
• Sketching: Draw a circle and divide it into equal parts to mark the corners.
• Check: The sides of a regular polygon must be equal.

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. 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.
• Holes and pipes: Circles represent holes, pipes, and round parts in views.

Importance of plane sketches in welding

• Communication: They help the fabricator and the designer to understand each other.
• Planning: They help to plan the welding sequence and access to the joint.
• Material estimate: They help in estimating material quantities.
• Visualisation: They allow quick visualisation of joints.
• Drawings: They form the basis for detailed working drawings.

1.1.8 Solids sketches

Solid sketches show three-dimensional objects, so that the welder can see how the parts fit together. They are made with isometric lines, starting from the base shape, with visible edges drawn firm and hidden edges dashed. 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 side faces are flat. 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 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.

• 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 and cylinders

A cuboid has six rectangular faces, and its length, width, and height may differ. A cylinder has two equal circular ends joined by a curved surface. Tanks, boxes, and plates are cuboids, and pipes, drums, and shafts are cylinders. Example: a rectangular steel tank is a cuboid, and a pipe is a cylinder.

Cuboids

• 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.

Cylinders

• 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.

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.

Importance of solid sketches in TIG welding

• Joint preparation: They give a spatial understanding of the joint.
• Fit-up and access: They show how the parts fit and where the torch can reach.
• Interference: They help to find clashes between parts.
• Communication: They help the designer and the fabricator.
• Planning: They support planning of fabrication and welding sequence.

Chapter Summary

This chapter provided a comprehensive overview of draft working drawings essential for Tungsten Inert Gas welding. It began with drawing interpretation, covering how to read and apply linear and angular dimensions, understand tolerances, and recognize standard symbols and notations. The chapter explained different views and elevations, focusing on first angle and third angle projections, as well as the importance of scale and accurate measurement of angles. It then guided students through sketching various plane geometric forms such as triangles, quadrilaterals, polygons, circles, and tangents, before advancing to solid sketches including prisms, cones, cubes, cuboids, and cylinders. The section on draft work drawings emphasized correct dimensioning and the use of welding symbols and notations to communicate welding requirements clearly. Finally, the chapter discussed the operation procedure for developing and using these drawings in practical welding contexts to ensure precision and quality in fabrication.