Design engineer’s task is to describe geometry of the future product. For a long time, this task could only be accomplished by the means of the hand-made 2D drawing. The drawing created in this way is an approximation of the geometry which exists in the design engineer’s imagination. Thus, the exact shape of the geometry if finalised only during the manufacturing process.
Until recently, the traditional 2D drafting technique was the only one available. As a result, design organisations developed a strong habit towards the limitations which derive from the manual drafting process.
Limitation #1: Incompleteness
Shape of components and their relative positions in assemblies can only be described by a limited number of views and sections on the 2D drawing. As the complexity of products increases, this is becoming a serious issue and a source of errors downstream.
Limitation #2: Non-associativity
Every drawing sheet, every single drawing view, dimension and annotation symbol are absolutely static from the moment of creation. Thus, any design change can be achieved only by erasing part of the graphic information and manually re-drawing it from scratch.
Limitation #3: Inconsistency
Assembly drawings, while being the source of the conceptual information about interaction of components, have no direct physical impact on detail drawings. Parts and subassemblies, which have to be correctly aligned with each other, do so only if their individual drawings are made in a correct way.
The above-mentioned habit of some organizations towards the limitations of the 2D drafting led to the situation when 3D CAD programs were met with lowered expectations and the potential of the 3D modelling was not fully recognised. In many cases, 3D modelling replaced 2D drafting in unchanged workflows, duplicating its traditional limitations.
Duplication of limitation #1
Models are actually being built only for the purpose of generation of 2D drawing views, rather than accurate description of entire geometry. As a result, they end up getting significantly simplified and unsuitable for the manufacturing and calculation departments, which need accurate 3D models and therefore are forced to re-build models from scratch.
Duplication of limitation #2
Models are built with insufficient parameterisation. As a result, design changes made mostly manually, and this greatly increases time required to act on change request, and makes what-if analysis of design alternatives very difficult. Again, 3D models are used only as a source of drafting view which are later exported and annotated in 2D CAD software without associative link to the parent 3D geometry.
Duplication of limitation #3
Products are designed with the bottom-up strategy, when all parts are designed separately and their relative positions and shapes are finalised manually in the review assemblies. The situation is aggravated by an insufficient use of parameterisation. Thus, parts have to be manually re-adjusted each time a design change occurs.
Described limitations can be efficiently removed through the complex implementation of 3D modelling based on the top-down design strategy. With this strategy, product design begins at the highest level where conceptual parameters are defined, then general layout is made, and after that subassemblies and piece parts are designed. Obviously, traditional technologies also base on the top-down workflow, but the key advantage of top-down CAD modelling is that links between top-level conceptual parameters and bottom-level detail design are present physically in the 3D models, rather than only in the designer’s imagination. Components are created with mutually adjusted geometry right from the beginning, which greatly reduces the need for extensive manual adjustments and checks, and makes assembly errors far less probable. Parameterisation makes possible swift and dramatic design changes and evaluation of different ‘what-if’ scenarios.
Different implementations of top-down strategy can contain different parts or stages. Implementation typical for NX contains source part, work part, and result part. Control structure is the auxiliary assembly which hold all parts together and form basis for creation of good-quality geometry links.
This is a conceptual model which contains basic geometry and primary parameters which relate to the whole product. These properties are associatively applied to all components which are supposed to match them. If source part parameters are changed, all parts linked to them are also changed. Thus, designers receive unitary source of key product information which they can use directly, and which always remains up to date.
This is a single model or assembly of models which contain all necessary environment. Design of the product components’ geometry is made in the context of this environment and thus they are made mutually compatible by default. If any of the components which contribute to the environment are changed, these changes are associatively transferred to the work part, and reflected in the component which is being designed.
This part contains digital mock-up of the product. Because all work on geometry is made in the work part, and all inter-part geometry links are also contained there, product mock-up is free of unnecessary information and has correct physical properties.
Top-down strategy is seamlessly integrated in the existing organisational structures of the industrial enterprises. Long-standing product development stages and distribution of tasks between departments is reflected in the product control structure, and as a result established business workflows do not change.
Implementation on NX CAD platform
NX allows to implement top-down design strategy with maximum efficiency because of the two key advantages:
- Unified .PRT file format
- WAVE geometry linker
PRT file format
In the unified .PRT file format there is no distinction between assembly and piece part. Any part can be turned into assembly simply by attaching another part to it, and nothing changes in the software operating mode and available toolbars. The main advantage of this architecture is absence of boundaries within the assembly. As far as interpart geometry linking is concerned, NX assembly tree is fully transparent. This allows to copy geometry associatively between any assembly levels and in any direction.
WAVE geometry linker
WAVE is a set of tools which allows to create associative interpart copies of geometry. Any geometric objects can be copied – solid and sheet bodies, user-defined coordinate systems, datum geometry, sketches, curves and points, as well as parameter expressions. All changes to the parent geometry are by default reflected in all copies of it. It is possible to create a link at a particular timestamp in the feature tree. It is also possible to temporarily suppress update of links, or to create non-associative copies. Creation, maintenance and update of WAVE links within the control structure is fully controlled by designers. Suppression of links update is employed to enable precise control over the entire design process. Thus, changes at the top level are propagated to the bottom levels only at the strictly defined moment.
Combination of .PRT file format and WAVE geometry linker provides maximum flexibility for creation of control structures and subsequent work with them. Control structures created in NX are easily scaled according to the requirements of particular projects, meanwhile overall methodology remains the same.
Removing the limitations
Top-down design strategy implemented on NX platform enables efficient elimination of all described limitations of the traditional workflows of product design and drawing release.
Elimination of limitation #1
Within the top-down design strategy, 3D model is the key source of geometric information. It is created in a context and is itself being the context for the other models. Information contained in a model is used directly by all other departments – CAM, CAE and the likes. Top-down strategy requires maximum accuracy of such information, and provides all conditions for this. Thus, the traditional limitation of non-complete description of a design by a drawing is eliminated. The design is fully and accurately described by a model.
Elimination of limitation #2
Non-associativity of documentation is removed by moving the creation of drawings to the same NX platform which is used for 3D modeling. Top-down modeling is made with the widest use of parameterization, interpart linking and reusable features. Parametric models are updated automatically upon changes in the source numeric expressions or geometric links on which they are based. Thus, the need form manual update of models or drawings is minimized.
Elimination of limitation #3
Models created with a top-down strategy are completely mutually adjusted because all parts and subassemblies are designed in the context of their respective environments, and under the rule of the control structures. All changes made to a particular part of the product are associatively propagated to the rest of the parts which are related to it. Top-level conceptual parameters, layout schemes, assembly diagrams, etc. are directly connected via interpart geometry links to the components which constitute them, and control them in a predictable fashion. Thus, the gap between conceptual and detail design is eliminated.
The potential of NX CAD parametric modeling, .PRT file format and WAVE geometry linker is fully utilized when top-down design strategy is used. This strategy, implemented in connection with NX CAM and NX CAE, is able to dramatically reduce design lead times and number of errors of the development stage. Work in control structures is similar to the everyday work of experienced NX CAD users. Implementation of top-down design strategy is easy to achieve by any design organization which has mastered NX CAD.
Did you try to implement top-down strategy for the design of your product? Did you have difficulties with it? Which impovements did this strategy bring to your design process?