Computer Aided Systems
Ivan Kuric
Prof. Ivan Kuric , University of Zilina, Faculty of Mechanical Engineering, Department of Machining and Automation, Slovak republic, [email protected]
1.1 Introduction
Sum of all time between part development and part expedition represents time to market. Time to market direct influences on the total production cost. It is sufficient that time to market and the production costs is minimum. The computer aided (CA) systems influence on the factors of competitive advantage.
10 6 5 IDEA IDEA REALITY DESIGN MODEL DESIGN MODEL PRODUCT MODEL PRODUCT MODEL to be manufactured part MANUFACTURING STAGE MANUFACTURED PART
Engineering drawing
Development Process plannig Manufacturing Assembly Expe dition Time to market Engineer. drawing Develop ment Process
plannig Manufacturing Assembly Expedi
tion CAx
CAE CAD CAPP CAM CAA CAL
Time to market
Figure.B: Time to market without and with CA systems
1.2 CAD - Computer Aided Design
A computer support during the design of the product is utilising of the computing machine during the product design process. The computer design support is called the Computer Aided Design (CAD). The CAD system is a computer system based on the mathematical product description and application analyses.
The computer aided design is the interactive modelling of physical systems on computers, allowing various analysis of the design variants. The aim is to prepare the model of the product to be suitable for manufacturing. The CAD systems are not in the first place characteristic by design description. The significant mark of the CAD systems is modelling and simulation and it is the significant difference considering a computer graphics. Simulation according the computer science lexicon is a specific kind of modeling of systems. Simulation has a great meaning, it is not necessary after drafting stage to make a prototype and then to verify it. Verification and inspection are done parallel with the other design activities.
The principle – at first to model, to simulate and to verify and at last to manufacture – appears as a very good economical fundamental.
The modern CAD system does not look like as the first computer system dedicated to 2D drafting. The present systems enable modelling according the 3D primitives. The product is not drawn but it is modelled and virtual completed.
1.3 Background of CAD development
Before 1960 the computer application for design does not exist. Between 1960 and 1970 the most computer programs in engineering were limited to calculation programs which were processed in batch. Thus, there was no interaction possible between the user and the program. In the seventies the first mainly academic CAD systems aim an interactively supporting design as a 2D sketching.
After 1970, the meaningful step was development of graphical terminals that make possible to work interactively. However, the available hardware was very expensive and therefore the using was restricted to a limited number of companies and people. Accordingly the CAD applications occurred in small amount.
From 1980, rapid developments of computer equipment lead to more powerful computers hardware and computer systems. Workstations and personal computers became widely available at reasonable prices. This development enabled the introduction of CAD on a wider scale. Knowledge based engineering system for design was introduce in the mid-eighties. These systems employ artificial intelligence technology for representing expert design and manufacturing knowledge. The advantage of these systems is that similar designs with different specifications and geometry can be generated much faster. However, the realisation of truly intelligent CAD systems still is an academic research issue.
1.3.1 The basic design activities
in CAD systems
The CAD system provides generally the following design activities: • geometric modeling,
• engineering analysis,
• design review and evaluation, • computer drafting.
Above mentioned activities cover the all phases of general design process.
CAD SYSTEM
Graphical User Interface Programming Language Input / Output Internal CAD Database Engineering analysis Computer drafting Geometric modeling Design review and evaluation
Figure.C: CAD system
Geometric modeling is concerned with the mathematical description of the geometry of an object. This is the
first and synthesis phase of product design. The basic operations of the modeling stage are generating basic geometric elements such as points, lines, circles, primitives such as cubes and functions such as scaling, rotation, transformation, joining, wire frame representation and solid modeling.
Engineering analysis involves stress-strain calculations, heat-transfer computations and various analyses.
Analysis of mass properties and finite-element analysis is very requisite tool for providing of a solid object properties (surface area, weight, volume, centre of gravity, moment of inertia, etc.) and stress-strain and heat transfer performance. Engineer oftentimes applies differential equations to describe the dynamic behaviour of the system.
Design review and evaluation provides for engineer the tools of design evaluation. For example kinematics is
very important design review. The available kinematics tools enable to animate the motion of designed mechanisms. Afterwards there is a higher imagination. This capability enhances the designer's view and supports to avoid collision of designed objects.
Computer drafting provides realising of the routine activities such as scaling of the drawing,
autodimensioning, autohatching, various views of the drawings.
1.3.2 CAD database
All design activities such as geometric modeling, various analysis, review, evaluation and drafting are realised in data situated in some database. This database is called internal CAD database and data as CAD data,. Study of structure of the CAD database is a key to understanding of internal operations in the CAD system. There are programming tools to the access to CAD data. The database includes all inserted, modified and deleted objects. Therefore it is possible to return to the previous designed and deleted features and to renew and updated the feature properties.
Other very important database is the external database. The external database usually contains the features widely used in specifying application branch (architecture, engineering, etc.). Standardised features such as screws, female screws, washers and other subparts are typical objects included in external database.
The internal database contains not only geometric data but also non-geometric data such as material and the technological properties of the features. The database includes all the data on the product generated during design - geometry data, the bill of materials and part lists, material specifications- as well as additional data required for manufacturing. In the CAD/CAM systems the database presents integrating element CAD and CAM section of integrated system.
External representation (IGES, VDA, DXF, ...) External database of standardized objects Internal representation (B-rep, CSG)
Figure.D: Internal and external CAD database
1.3.3 CAD modeller
The purpose of CAD modellers is to be able to describe models in a precise way for engineering analysis and manufacturing purposes. The main part of CAD system is modeller determining a method how internal to describe objects. There are two known representations of the CAD data. The modern CAD systems normally use Boundary representation (B-Rep) to describe objects. The Constructed Solid Geometry (CSG) is second known descriptive methods.
Conversions from B-Rep to CSG can lead to ambiguities and thus are generally not possible to realise. However, conversion from CSG to B-Rep requires very large computer memories.
1.3.4 STEP data structure
Other very important describing is STEP (STandard for the Exchange of Product model data) method. The STEP is external format to save data of the objects.
The STEP is a new and important international standard for the product model data transfer between the CAD systems. The format is widely supported by industry. It separates the representation of product information from the implementation methods and provides a system-independent way of implementation. The STEP specifies the implementation methods used for data exchange and also defines the geometric and topological representation.
1.3.5 Boundary representation
The B-Rep models represent a solid indirectly by a representation of its bounding surface. A solid is represented as a volume contained in a set of faces together with topological information which defines the relationships between the faces. The boundary of a solid separates points inside from points outside of the solid. B-rep models can represent a wide class of objects but the data structure is complex. Therefore it can require a large memory space for a complex object.
Normally a face is a bounded region of a planar, quadratic, toroidal or sculptured surface. The bounded region of the surface that forms the face is represented by a closed curve that lies on the surface. A face can have several bounding curves to represent holes in a solid. The bounding curves of faces are represented by edges. Two vertices represent the portion of the curve that forms the edge.
1.3.6 B-Spline representation
The description and modelling of sculptured faces is a crucible of modern CAD system. The description of the faces is different. For example a planar face can be represented in many different ways: using an analytical equation, a parametric equation, a normal vector and a point on the surface, etc. In addition, a closed 3D boundary which lies on the surface is needed to define the face. In conventional CAD systems, the use of faces is usually restricted to the quadrics like cones, cylinders, spheres, etc. The modern CAD systems use a variety of formats to represent sculptured faces. One of the most common is the B-Spline representation. It is a category of surfaces employing parametric polynomials.
The B-Spline surface is a collection of B-Spline curves, i.e. the tensor product of two curves defined by two parameters. The surface is defined as the set of points obtained by evaluating of an equation.
CAD systems most often offer the possibility to assemble the solid parts already detailed into an assembly. Subsequent kinematics or dynamic analysis is sometimes possible. Links with Finite Element Method packages also exist, sometimes even associative, i.e. the features on the CAD side and FEM side are the same. Some CAD systems offer a programming interface in order to allow customers to develop their own applications. Some CAD systems offer catalogues of standard components which can be incorporated in the design.
1.4 CAPP – Computer Aided Process Planning
The computer aided process planning (CAPP) represents the implemented methodology of process planning in the software package. The CAPP includes all process planning activities needful to realise of the design of the process plan. The CAPP system has to solve the planning activities such as selection of machining operations, selection of machine and cutting tools etc.
The CAPP is concerned with the preparation of a route sheet for the engineering drawing. The part must be interpreted in terms of manufacturing process to be used. The route sheet is a listing of the sequence of operations. Closely related to the process planning are the functions of determining appropriate cutting conditions for the machining operations and setting the time standards for the operations.
The CAPP is often a heart of the CA systems in the engineering enterprise. Input for the CAPP system is output from the CAD system. Output from the CAPP system is at the same time especially the input for the CAM system. One can often to meet the integrated the CAD/CAPP and the CAPP/CAM.
process planning machine routing manufacturing planning material processing process engineering Figure.E:CAPP
1.5 CAM- Computer Aided Manufacturing
Manufacturing as the design stage is a set of activities assigned to the producing of the designed part. The manufacturing is one of other activities after design stage. The problem consist in transformation of the CAD data to the manufacturing data. The manufacturing data are sometimes called as CAM data.
Computer support in manufacturing stage - Computer Aided Manufacturing (CAM) may include the following stages:
• Tool and fixture design for manufacturing, • Numerical Control part programming, • computer aided process planning, • assembly list for production, • computer aided inspection, • robotics planning,
• scheduling.
Tool and Fixture Design is a stage that creates all the necessary equipment to produce a designed product. The
tool and the fixture design depend of the amount of the parts being produced.
NC Part Programming is planning the process for the portions of the job to be accomplished by the NC.
Usually the part programmer is responsible for planning the machine steps to be performed by NC. There are two common ways to program for NC, manually and computer assisted. The available CNC machines can be programmed either interactively by the human operator or could be computer generated.
CNC production requires the ordering of the raw work parts, specifying and preparing the tooling and any
special fixturing that may be required and setting up the machine for the job. The CNC machine then takes it over and machines the part accordingly.
Manufacturing Planning and Control includes the preparation of schedules, requirement shortage lists,
inventory lists and similar documents. The principal functions involved in discrete manufacturing are: • demand forecasting,
• capacity requirements planning, • aggregate production planning, • material requirements planning, • material procurement,
• finite capacity scheduling and production line balancing, • statistical process and quality control,
• inventory management
Simulation can be particularly useful in evaluating the feasibility and effectiveness of any one of the production planning functions as well as looking at a unified production planning and control scenario.
Manufacturing stage Post Manufacturing stage Manufacturing processes Machining Forming Pouring Welding Engineering drawings, BOM Engineering activities CAD Process plannig Removal Assembly Storage Expedition etc. CAM CAPP
Figure.F: Three stages for production of part
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