Technical drawings are the universal language of engineering. They communicate precise information about the design, manufacture, assembly, and installation of components and systems. The type of drawing used depends on its intended purpose and the audience it serves : from machinists on the shop floor to project managers overseeing installation.
This article covers the principal drawing types encountered in mechanical and structural engineering practice.
General Assembly Drawings
An assembly drawing (see Figure 1) represents the relative position, orientation, and shape of a group of assembled parts. It provides a clear visual representation of how individual components fit together to form a complete unit or sub-assembly.
Parts List (Bill of Materials)
A list of parts is typically provided in a separate document : commonly referred to as a parts list or bill of materials (BOM) : but can be included within the drawing itself if space permits. The parts list identifies each component by item number, description, material specification, quantity, and, where relevant, the source or part number of purchased items.
Individual components on the drawing are annotated with ballooned numbers that correspond to the item numbers in the parts list. Balloons are connected to their respective parts by leader lines, ensuring unambiguous identification even in complex assemblies.
Additional Information
Beyond the basic geometry and parts identification, assembly drawings may include the following supplementary information:
- Torque settings : These can be displayed adjacent to the relevant item balloon or consolidated into a dedicated torque table for clarity.
- Flag notes : Specific instructions such as lubricating grease type, thread adhesive, sealant application, or surface treatment requirements are shown alongside corresponding flag notes positioned near the relevant features on the drawing.
- Overall and key dimensions : Critical dimensions such as overall envelope size, mounting hole locations, and interface dimensions are included where appropriate for installation and fitment verification.
- Mass and centre of gravity : This information is particularly important for assemblies that require handling, transportation, or dynamic balancing considerations.
Sectioning
Assembly drawings frequently employ section views to reveal internal detail that would otherwise be hidden. The example in Figure 1 uses a half section, which is permissible when the assembly is symmetrical around a central axis. This approach shows internal components on one side while preserving the external profile on the other, providing an efficient combination of interior and exterior information.
If the assembly is asymmetric, a full section may be required to adequately convey internal detail. In cases where multiple internal features must be shown across different planes, broken-out sections, removed sections, or aligned sections may also be employed.
Exploded Assembly Drawings
A variant of the assembly drawing is the exploded view, in which components are separated along a common axis or axes to illustrate the spatial relationship and order of assembly. Exploded drawings are especially useful for:
- Assembly and disassembly instructions
- Maintenance and repair manuals
- Customer-facing documentation where clarity of part relationships is paramount
Components are connected by exploded lines (often dashed) that indicate the path along which each part moves during assembly.
Detailed Parts / Manufacturing drawings
A part drawing (see Figure 2) includes all the information required for the complete and unambiguous definition of a single component. Unlike assembly drawings, which focus on relationships between parts, the detail drawing must convey everything necessary for an independent manufacturer to produce the part to specification without additional instruction.
Essential Content
A well-prepared manufacturing drawing typically includes:
- Geometry: The complete shape of the part, defined by sufficient orthographic views (typically front, top, and side), section views, and auxiliary views to remove any ambiguity.
- Dimensions : All linear and angular dimensions necessary to define the size and location of every feature, applied in accordance with a recognized dimensioning standard (e.g., ISO 129, ASME Y14.5).
- Tolerances : Dimensional tolerances (bilateral, unilateral, or limit) and geometric tolerances (form, orientation, location, and runout) that define the permissible variation from nominal geometry.
- Surface texture : Surface roughness symbols and values indicating the required finish on each relevant surface. This is critical for functional surfaces such as bearing journals, sealing faces, and mating interfaces.
- Material specification : The material from which the part is to be manufactured, including grade, condition, and any applicable standards (e.g., EN AW-6082 T6, AISI 316L, S355J2).
- Heat treatment and surface finish : Any required heat treatment processes (hardening, tempering, annealing) or surface coatings (anodizing, plating, painting) specified for the part.
- General tolerances : Where no specific tolerance is stated on a dimension, the drawing should reference a standard for general tolerances (e.g., ISO 2768) that applies by default.
- Drawing border information : Title block details including part number, revision, scale, units, projection method, drawing author, checker, and approval.
Drawing Standards
Detail drawings should be prepared in accordance with recognised national or international standards to ensure consistency and universal interpretation. Common standards include:
| Standard | Scope |
| ISO 128 / ISO 1101 | Line types, geometric tolerancing |
| ISO 5456 | Projection methods |
| ASME Y14.5 | Dimensioning and tolerancing (US) |
| BS 8888 | UK technical product documentation |
Fabrication drawing
A fabrication drawing (see Figure 3) depicts a workpiece that is permanently joined together by means of welding, brazing, adhesion, mechanical fastening, or another permanent joining method. Unlike a casting or machined part : which begins as a single piece of material : a fabricated assembly is built up from multiple constituent parts (often called sub-components or parent parts).
Constituent Parts
Each constituent of the fabrication must be fully specified, either by direct reference to an existing detail drawing or by including sufficient information on the fabrication drawing itself. A parts list may be included on the drawing if space allows, along with:
- Profile cut lengths : Simple stock lengths or blank sizes required for each sub-component
- Material specifications : For each individual constituent
- Quantity : The number of each sub-component required per assembly
Welding Information
Since welding is the most common joining method in fabrication, the drawing must communicate welding requirements clearly. This is typically achieved using standard weld symbols applied in accordance with ISO 2553 or AWS A2.4. Key information conveyed includes:
- Weld type : Fillet, butt (groove), plug, slot, spot, seam, etc.
- Weld size : Leg length for fillet welds; penetration depth or groove size for butt welds
- Weld length and pitch : For intermittent welds
- Weld finish : Whether the weld is to be ground flush, left as-welded, or machined
- Welding process : If specified (e.g., MIG, TIG, MMA, submerged arc)
- Pre- and post-weld treatments : Pre-heating requirements, stress relief, non-destructive testing (NDT)
A welding procedure specification (WPS) may be referenced on the drawing for critical applications, particularly where compliance with structural codes or pressure vessel standards is required.
Fabrication Tolerances
Fabricated assemblies accumulate tolerance from multiple sources : cutting, bending, fit-up, and welding distortion. The drawing should specify fabrication tolerances that account for this, and may reference standards such as:
EN 1090 : Execution of steel and aluminium structures
ISO 13920 : General tolerances for welded constructions
AWS D1.1 : Structural welding code (steel)
Installation / Site Drawings
An installation drawing provides information required for the delivery, positioning, and connection of equipment or assemblies on site. These drawings are aimed at installation teams, riggers, and site engineers rather than manufacturing personnel.
Typical Content
- Overall dimensions and weight : For handling, transportation, and craneage planning
- Lifting points and centre of gravity : To ensure safe and balanced handling
- Foundation and anchor bolt requirements : Bolt patterns, embedment depths, grouting specifications
- Interface connections : Pipe connections, electrical terminations, ductwork interfaces
- Access and clearance requirements : Maintenance access, removal envelopes, safe working zones
- Alignment and levelling requirements : Datum references, shimming instructions, alignment tolerances
Installation drawings typically show less manufacturing detail than part or fabrication drawings and focus instead on spatial relationships between the equipment and the surrounding environment.
Schematic / Diagrammatic Drawings
While not strictly “drawings” in the geometric sense, schematic diagrams are an essential category of technical documentation. They represent the functional relationships between components rather than their physical form.
Common Types
- Piping and Instrumentation Diagrams (P&IDs) : Show process piping, instrumentation, and control systems
- Electrical schematics : Represent circuit logic, wiring, and component interconnections
- Hydraulic and pneumatic diagrams : Depict fluid power circuits and control logic
- Block diagrams : Provide high-level system architecture and signal flow
Schematics use standardised symbols and conventions (e.g., ISO 14617, IEC 60617, BS 1553) to ensure consistent interpretation across disciplines and organisations.
Summary
| Drawing Type | Primary Purpose | Typical Audience |
| General Assembly | Show how parts fit together | Designers, assemblers, inspectors |
| Detail/Manufacturing | Define a single part completely | Machinists, fabricators, QA |
| Fabrication | Define permanently joined assemblies | Welders, fabricators, inspectors |
| Installation | Guide on-site placement and connection | Site engineers, rigging teams |
| Schematic | Represent functional relationships | Designers, commissioning engineers |
Each drawing type serves a distinct purpose within the product lifecycle. Selecting the appropriate type : and preparing it to the correct standard : ensures that information is communicated efficiently and that parts, assemblies, and installations are produced and verified to the required quality.
