Design Engineering

Design Schemes/Layout Drawings & 3D CAD

The importance of design schemes cannot be over emphasised. Incomplete scheming can be a major source of assembly issues and schedule delay.

What is a Design Scheme?

A Design Scheme, or Layout drawing, sits separate to the General Arrangement (GA), Assembly and Detailed manufacturing drawings and is used to communicate the original concept for a design, presenting pictorial, notational and dimensional data to the extent necessary to convey the solution.

Historically, when designers worked with drawing boards they would layout, at scale, the various constituents of a design and annotate this drawing with notes to communicate fit, form and function. This scheme would communicate the design intent to sufficient detail that it could be used for authorising a solution and a drafting team then could use it as a basis for creating the detail definition of all the sub-components within the system, sub-system, or assembly.

The British standard, BS 88881, no longer appears to recognise Design Layouts or Schemes, however the American standard ASME Y14.24 defines the following requirements for Layout drawings in section 16:

16.2 Application Guidelines
A layout drawing may be prepared for a complete end product or any portion thereof and is prepared as one of the following:

  1. a conceptual design layout to present one or more solutions for meeting the basic design parameters and provide a basis for evaluation and selection of an optimum design approach
  2. a design approval layout to present sufficient detail of the design approach for cost estimating and design approval
  3. a detailed design layout depicting the final development of the design in sufficient detail to facilitate preparation of detail and assembly drawings
  4. a geometric study to develop movement of mechanical linkages, clearances, or arrangements

A layout is not normally used to fabricate equipment; however, a detailed design layout is sometimes used as an interim assembly drawing for development equipment.

16.3 Requirements
A layout drawing shall include, as applicable, the following:

  1. location of primary components
  2. interface and envelope dimensions including a cross-reference to applicable interface documentation
  3. paths of motion
  4. operating positions
  5. critical fits and alignments
  6. selected materials, finishes, and processes
  7. wire, pneumatic, and hydraulic routing and sizes
  8. adjustments
  9. critical assembly details and sequence
  10. identification for selected purchased items and new design items
  11. identification for the assembly depicted (when the layout is used as an interim assembly drawing)

A layout shall be drawn to scale with sufficient accuracy and completeness for its intended use.

Reference: ASME Y14.24-2020 – Types and Applications of Engineering Drawings
Do Design Schemes still have a place today?

To understand how a design scheme might integrate in a modern design development workflow lets start by considering a couple of the fundamental approaches when modelling in 3D Computer Aided Design (CAD) software. Top-down assembly design is a methodology where you start by creating the top-level assembly and then design and define individual parts or sub-assemblies, “in-context”, allowing you to control the overall structure and relationships between components from the beginning. Bottom-up assembly design starts by creating individual parts or sub-assemblies independently. These components are then brought together to form the final assembly, an approach which provides more flexibility at the component level.

In regard to outputs from the design team into manufacture, as well as traditional 2D definitions we have Model Based Definition (MBD) (also referred to as Digital Product Definition) which involves associating Product Manufacturing Information (PMI), such as fit and form tolerances, with the 3D model, eliminating the need for a separate drawing.

These approaches focus respectively on the evolution of the design solution and the outputs from the design team into manufacture, but what about the handover within the design team itself? This is a critical consideration especially where we are designing complex systems with a large team, or interfacing with separate sub-contract design organisations. In this scenario having a clear point of reference to a baseline definition of the solution can be invaluable.

How could this work using CAD and PLM?

In a structured design process it is common for organisations to differentiate between preliminary and production intent design data2. Adopting a top-down modelling approach during preliminary design leads to the creation of child parts, within the assembly, that will ultimately be evolved into the production definition. By mandating the creation of a design scheme, which references this Digital Mock Up (DMU), prior to authorising the detail definition of sub-components, this can be assigned a status (or frozen) and used as an input into a Preliminary Design Review.

Digital Mock-Up (DMU). Digital specification given to a complete mechanical product or sub-system with an independent function, not only of the geometric properties but also of its function, performance or both in a particular field.

Note 1 to entry: The digital mock-up of the product is built at a design stage and is applicable to the whole life cycle of the product, including design, manufacture, marketing and aftermarket. The digital mock-up can realise, for example, interference check, motion analysis, simulation of performance and manufacturing, technical training, advertising or maintenance planning.

Reference: ISO 10209:2022, Technical product documentation — Vocabulary — Terms relating to technical drawings, product definition and related documentation

Following this approach the project team has an authorised design baseline that future change can be managed against, along with sufficient definition of the design to make programme organisation decisions. This also allows consideration as to how the product will be structured, referred to as the Product Breakdown Structure (PBS), which can be used to drive the organisation structure of the design team, project cost models, suite of manufacturing definitions and procurement strategy.

Summary and Recommendations

I would always advocate the use of a Design Scheme to define a technical baseline for a complex system, or sub-system design. I would also note, from experience, that there is a common misconception where a product is considered less complex or novel, and we have a small team working on the project, that the time spent creating a preliminary scheme is disproportionate to the benefit it would provide.

The major benefit is the provision of a single source of reference for the design intent, communicating all Critical Conformance Features (CCFs), materials, surface finishes and interface constraints early in the design process. Its importance cannot be over stressed, incomplete scheming is a major cause of assembly and interference issues, as well as schedule delays, due to late design changes, or ambiguity in the detail definition from a lack of continuity, or knowledge transfer in the design team. So rather than not create a scheme, agree the level of detail required in advance, and tailor the output to be appropriate and proportionate to the job.

I hope this blog has been of interest and if you feel this may be an area where you need support, or a fresh perspective, please get in touch. I have witnessed first hand the benefits of the approach outlined above and am always happy to have a conversation that could be of help.

  1. BS 8888 – Technical product documentation and specification, refers to BS ISO 29845:2011 for definitions of Technical Product Documentation. This standard does appear to have carried forward the definition in section 3.2 of Bs308 (the standard which it superseded). ↩︎
  2. This may be through alpha numeric revision control, Product Lifecycle Management (PLM) systems, or part number designations. ↩︎