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Industrial Processes

CAD support for assembly designers

   
 
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PICASSO represents a collaboration between a market-leading specialist CAD/CAM software house, one of its largest customers, and expert technical and academic advisers.
The project developed and tested a prototype system to support the design of assemblies built primarily from standard components. It will dramatically improve the speed and accuracy of assembly design, and provides a simple and reliable means of optimising manufacturing tolerances. The result will be cheaper manufacture and shorter lead times.
Refined and extended since the completion of the project, the PICASSO software promises to revolutionise the assembly design process in the mould tool industry and in other fields.

Computer aided design (CAD) systems can dramatically reduce the time and cost of bringing new products to market. Why then is CAD rarely used for the design of assemblies built from standard components - one of the areas in which it should confer the greatest benefits?
Even within industries such as mould tool manufacturing, where extensive and sophisticated use is made of CAD for more complex tasks, assembly designs are generally still prepared by hand, on old-fashioned drawing-boards.
To date, CAD systems have simply not offered support tools with the right mix of flexibility and dedication to a specific assembly type. Without such support, their benefits have not been great enough to persuade companies to abandon traditional manual or semi-manual design methods.
The PICASSO project has overcome this barrier - 1996 saw the first release of the resulting new CAD tool, which will assist assembly designers to select appropriate components and add them to finished designs.

Precision assemblies from standard parts

Every moulded plastic product, from a car dashboard to a milk crate, needs its own mould tool. This incorporates not only the milled steel plates forming the cavity into which the plastic is injected, but also the mechanism for releasing and ejecting the hardened product.
The mould tool industry uses advanced three dimensional CAD technology (3D-CAD) to define the complex surfaces of the mould plates, passing these definitions to Computer-Aided Manufacturing (CAM) software, which controls the milling process.
However, the tools which house these plates are themselves complicated pieces of equipment. Mounted on pillars, the plates must slide freely and fit snugly. To facilitate the release of items with undercut or rebated faces, they may require many sliding sections. Channels and pockets must be accurately machined.
The specification of correct tolerances is also critical. If they are too small, manufacturing costs are unnecessarily high; if they are too large, the tool will not function properly.

Costly errors

Mould tools are built almost exclusively from standard components, ordered from specialist suppliers whose catalogues contain tens of thousands of items. Current design methods depend entirely on the skill of experienced draughtsmen for the selection of the right parts, which are drawn by hand or, at best, using conventional 2D-CAD.
As they choose each component, draughtsmen must also add its part number to the list of materials to be ordered, either by referring to the supplier's catalogue or, as very often happens, relying on memory.
Some component suppliers provide CAD definitions of their parts, but selection still depends upon the expertise of the individual designer, and the process remains laborious, requiring extensive user input and manual positioning of the new part. Most draughtsmen find it quicker to draw each part from scratch.
The complete design process for a large and complex mould tool may take as long as a year, and errors are frequent and costly. Each 'misremembered' or mistyped part number means a costly delay while the correct part is ordered.
Worse, incorrect positioning on the plan of a hole in the mould plate may not become apparent until the tool has been assembled. Too expensive to discard, the plate must be removed, welded, redrilled and refitted.

Cutting design time in half

Delcam International, one of the two world market leaders in specialist CAD/CAM systems for mechanical parts with complex three-dimensional shapes, has produced an entirely new design support tool. Developed in close collaboration with academic and industrial end-user partners, PICASSO promises to revolutionise the design of assemblies built from standard components, both in the mould tool industry and in other fields.
Following evaluation of a prototype by the leading Spanish mould tool manufacturer Marés, PICASSO is expected to reduce design time by 50%. It will also dramatically reduce the incidence of the costly ordering and manufacturing errors so often generated by traditional design methods.
Operating as an add-on to Delcam's established CAD/CAM package, PICASSO enables the designer to select components from a simple list. A data library, mirroring suppliers' catalogues, contains representations of each part. Starting with the required dimensions for the mould plate, each choice constrains the options available for the next component. The system only presents parts which are compatible with the designer's previous choices.
A single mouse-click on a component type, and a second click on one of the components offered by the system, is all that is needed to draw and position a part on the plan. The software records each part number, and can automatically generate an accurate bill of materials, as well as conventional plan and three-dimensional views of the assembly.

Academic support

As a specialist supplier of CAD/CAM software, principally to the mould tool industry, Delcam knew that its customers were failing to exploit the full potential of CAD/CAM. The aim of this BRITE-EURAM project was to help these companies to standardise and streamline their assembly design processes, and to integrate them with the design of mould plates.
Marés, an established customer, was already looking for ways to reduce design times and standardise assembly tolerances around best-practice models. With support from the Technical University of Clausthal, the partners have built into PICASSO a functional tolerancing module based on Marés' expertise. A good example are the sliding-sealing fits found in mould tool components such as ejector pins. The pin must slide easily through a plate, without allowing leakage. With the function and its required tolerances defined, PICASSO can automatically apply the appropriate tolerances whenever that particular fit between two parts is called for.

Cross-industry flexibility

Like all PICASSO's features, the functional tolerancing module can be overridden. The system is highly configurable, allowing end-users to define their own non-standard components and assembly rules to supplement or supersede the supplied defaults.
This flexibility was important to mould tool manufacturers such as Marés, but Delcam was also keen to generalise the software so that it could be tailored for use in the design of quite different types of assembly. Its broader compatibility was checked by experts in press tool design at the University of Liverpool. In consultation with a number of manufacturers, the University also built libraries of press tool components, as Marés did for mould tools.
Now the system can be configured to support the design of mould tools, press tools, or even bicycles or gearboxes. The application is determined by the component, layout, and assembly rule definitions loaded into its databases.

Solid modelling

Delcam has continued the development of the project's prototype, giving it a standard MS-Windows interface and adding to its component libraries. PICASSO's first commercial release was at the end of 1996. Delcam expects the majority of its existing customers to want the new system immediately. In the longer term, it will be marketed outside the mould tool industry, following a further upgrade from wireframe to solid assembly modelling, planned for 1997.
PICASSO already provides a complete 3D model, containing enough information to control the machining and assembly process. The next stage of development, which Delcam is hoping to undertake in a follow-up project, will be its full integration with CAM systems, making possible optimised assembly manufacture at the press of a button.

 

 

Project Title:  
Practical and intelligent CAD for assembly objects

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-5693

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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