Sculptured Surface Machining: Theory and applications

by Byoung K. Choi, Robert B. Jerard

Overview

Sculptured Surface Machining (SSM) plays a vital role in the process of bringing new products to the market place. A great variety of products, from automotive body-panels to mobile phones, rely on this technology for the machining of their dies and moulds. This book documents the latest research progress and key issues affecting SSM software development. With a particular focus on the CAD/CAM environment, this book provides a rich source of reference, and covers such topics as fundamental mathematical tools, SSM-process models, process planning models and key tool-path generator (TPG), in addition to discussing more advanced theory and technology such as the new ‘C-space'- based TPG methods and ‘cloud-of-points' data machining.
Sculptured Surface Machining:

• Is the first book to discuss all aspects of SSM, from SSM-process models to SSM- software design methodologies
• Presents a feature-based CAPP (computer-automated process planning) methodology for sculptured surface machining
• Includes practical applications for NC machining technologies.

This book is essential reading for manufacturing engineers producing products with sculptured surfaces, software engineers working with CAD/CAM software, and researchers in the fields of mechanical, production and industrial engineering, and computer science.

Product Details

ISBN-13:	9780412780202
Publisher:	Springer US
Publication date:	01/31/1999
Edition description:	1998
Pages:	368
Product dimensions:	6.10(w) x 9.25(h) x 0.36(d)

Read an Excerpt

1 Introduction to sculpturedsurface machining

Many products are designed with aesthetic sculptured surfaces to enhance their aesthetic appeal, an important factor in customer satisfaction, especially for automotive and consumer-electronics products. In other cases, products have sculptured shapes to meet functional requirements. Examples of functional sculptured surfaces are:

• aerodynamic: airfoil (jet engine), impeller (compressor), marine propeller, etc.;

• optical: lamp reflector (automobile), shadow mask (TV-monitor), radar-dish, etc.;

• medical: parts for anatomical reproduction;

• structural: structural frames (aircraft), sporting goods, etc.;

• manufacturing surface: parting surface (moulding die), die face (stamping die), etc.

This introductory chapter presents the role of the sculptured surface machining technology in modern manufacturing as well as its functional and technical requirements.

1.1 Industrial Impacts

After the Second World War, the increased need for precision-machining of aircraft parts led to the development of NC milling machines in the 1950s. As an ever-increasing variety of products are being designed with sculptured surfaces, efficient machining of these surfaces has become more important in many manufacturing industries including: automobile, consumer-electronics, aerospace, ship-building, die-making, sporting-equipment, and toy-making industries. As a result, the sculptured surface machining technology has become a strategic technology in those industries.

In this book, the term 'sculptured surface machining technology' is used to denote an 'information processing technology concerned with efficient machining of sculptured surfaces by using NC machines'. abbreviation 'SSM' is used for 'sculptured surface machining' along with the following definitions:

• SSM-technology: sculptured surface machining technology as defined above;

• SSM-operation: a 'unit' machining operation;

• SSM-process: a collection of SSM-operations employed in making a sculptured part;

• SSM-system: a CAM system or NC module to generate NC data for SSM.

Since an SSM-process is applied mainly to the manufacture of dies and moulds, it often becomes a vital part of other non-machining processes, such as sheet-metal stamping and plastic injection moulding. Furthermore, since the quality of a sculptured part is no better than the quality of its dies, it is a key aspect for maintaining or improving the quality of the product. The production of dies and moulds is often the critical path in introducing a new product. For example, it is known that about 400-600 sets of dies are required to start manufacturing a new model passenger car. According to a recent survey (Fallbohmer, 1995a), the average lead-time for injection moulding dies was found to be 10 to 20 weeks, and over of the stamping dies surveyed took more than 30 weeks to deliver.

In automobile and consumer-electronics industries, the time required to introduce a new product is often limited by the production lead-times of its dies and moulds. As shown in Figure 1.1, a typical product development cycle consists of styling, part-design, die-making, tryout, and production. The die-making process in turn consists of die design, NCprogramming, machining, and polishing and assembly. The SSM-system may become a vital part of the concurrent engineering system.

In summary, the SSM-technology is regarded as a strategic technology in modern manufacturing industries because: (1) both the product quality and development time are dependent on it; and (2) it plays a vital role in product design by supporting the concurrent engineering function.

Unfortunately, its importance has not been fully recognized by many of the parties concerned, and potential improvements in productivity have not been realized.

1.2 Achievable Productivity Goals

As mentioned earlier, the SSM-technology is largely an information processing technology that has evolved from its component technologies: machine tools, NC, sculptured surfaces, and information processing technologies. Traditionally, these technologies have been developed within the separate disciplines of mechanical engineering, electrical engineering, applied mathematics, and computer science. This may be one of the reasons why the progress made in each of the component technologies is not fully reflected in today's SSM-technology. Moreover, a majority of the 'endusers' do not seem to know how to utilize fully the potential of the SSM-technology. As a result, there is still substantial room for improvement in the productivity of SSM-operations, perhaps as much as tenfold. Two sculptured surface machining examples are shown in Figures 1.2 and 1.3.

Usually, the SSM-process is characterized as time-consuming, information-intensive, and prone to costly errors. It is very time-consuming because the surface quality is obtained by 'point-milling', i.e. the final surface is generated by many thousands of straight-line movements between closely spaced points. For example, it can take a hundred hours to machine a large stamping die. It is an information-intensive process because a large amount of information is processed in obtaining NC data for machining. The SSM-process for making an automotive stamping-die may require a few hundred megabytes of NC data, and more than a week of programming effort. Since a large volume of information is processed, it is easy to make an error. Moreover, it is often very costly to correct such errors.

The application of modern SSM-technology, as presented in this book, should make it possible to achieve productivity improvements of a factor of ten. In order to explain how this improvement goal might be achieved, some new definitions are introduced:

• spindle-on hours: number of hours the spindle of the machine is on;

• spindle-on efficiency: spindle-on hours divided by the available working hours;

• programming/ machining ratio (P/M-ratio): ratio of the NC programming time to the machining time for a given sculptured part;

• cutting-efficiency: the amount of machined 'output' for a fixed amount of 'input' machining time.

The NC-programming time is defined as the time spent by the CAM programmer to develop an NC program for a particular die or mould.

Now, consider a typical shop-floor consisting of five NC machines on which one-of-a-kind sculptured parts (e.g. dies) are machined. The shopfloor is operated as follows:

1. Each machine is continuously tended by an NC operator because of quality concerns.

2. Each machine is operated during the day shift only, at 85% spindle-on efficiency.

3. The machines require five CAM programmers because the P/M-ratio is equal to one.

Since there are 2000 working hours per year (40 hours a week, 50 weeks a year), the shop-floor provides about 8,500 spindle-on hours per year (2000 x 5 x 0.85) by utilizing five NC machines and a staff of ten (five NC operators and five CAM programmers).

Now let's assume that we have made the following changes:

1. Run the machines three shifts a day, with one NC operator assigned to each shift.

2. The shop floor is operational all year around, at 75% spindle-on efficiency.

3. The NC programs for the five machines are created by one CAM programmer.

4. The cutting-efficiency has been increased by 25%.

A simple calculation shows that the yearly spindle-on hours are increased to 32 850 with a staff of four (three NC operators and one CAM programmer). Taking into account the 25% increase in cutting efficiency, the yearly throughput is increased by about four times, and the labour productivity is increased more than ten times. In order to achieve the stated productivity increase, the following 'functional improvements' are needed:

1. Unmanned operation of NC machines (from 100% tended operation).

2. The P/M-ratio is reduced to 1:15 (from 1:1).

3. The yearly spindle-on efficiency is 75% (from daily spindle-on efficiency of 85%).

4. The cutting efficiency is increased by 25%.

This book intends to show that the above functional improvements can be made with current SSM-technology. However, it should be remembered that 'technology' alone cannot make such an improvement. Organization and culture changes are also needed. More details about the above functional improvements will be given in section 1.4 where a set of functional requirements are presented...

Table of Contents

One: Fundamentals of Sculptured Surface Machining.- 1 Introduction to sculptured surface machining.- 2 Mathematical background.- 3 Sculptured surface machining processes.- 4 Process planning for sculptured surface machining.- 5 Fundamentals of tool-path generation and validation.- Two: Tool-Path Generation Methods for Sculptured Surface Machining.- 6 Introduction to tool-path generation methods.- 7 Tool-path generation for roughing.- 8 Tool-path generation for finishing.- 9 Tool-path generation for clean-up machining.- 10 Cloud-of-points data machining for reverse.- 11 Five axis machining.- Three: Controlling Accuracy and Quality in Sculptured Surface Machining.- 12 Geometric cutting simulation and verification.- 13 Cutting force modelling and physical cutting.- Four: Systems Integration for Sculptured Surface Machining.- 14 A unified CAM-system architecture.- 15 Development of integrated SSM-software.- 16 CAPP for sculptured surface machining.- References.