1.Introduction

In the process of mechanical engineering, to process a surface that meets the needs of technical requirements in a certain part of a work piece, it is necessary to fix the work piece in machine tools or fixtures before processing. The technical equipment used to fix work piece and to ensure processing precision is called machine tool fixture, often referred to as fixture[1].
Computer-Aided Fixture Designing(CAFD) is to employ computer technology in the design of fixtures to realize the semi-automation and automation of fixture design. Combined with the examples of the fixture-design software development based on the AutoCAD platform, the present research explores and discusses the standardization of the development of fixture design software[2].
Fixture design is necessary in mechanical processing. The standardization of fixtures has been applied for many years. However, due to the differences in the shapes of processed parts, a large amount of specialized parts requiring special design are needed in fixture design, which makes difficulties in meeting the needs of standardization and harmonization. Besides, it is a wasting of energy and time to choose and draft the large amount of standardized parts in fixture design.
The employment of computer software platform to develop fixture design software meeting the needs of designers can facilitate the standardization and provide guidance for beginners. This is a subject studied by many researchers and the present research explores this subject relying on the researches done before[3~6].
2.  The Requirements and Procession of Fixture Software Design
The author’s idea of the development of computer-aided fixture design was inspired from the machine tool fixture design in college mechanical majors. Students were beginners and they felt the fixture design rather abstract. Therefore, the software not only includes many animated emulations of the fixing procedure, but also the databases for various standard parts, which is more direct for beginners, especially for students.
The CAFD design software described in this paper is mainly based on AutoCAD software platform, written in Visual C++ language and AutoLISP, an embedded language of AutoCAD, or VBA,or software embedded with three-dimensional modeling using other three-dimensional software (UG, pro/E). In this way, fixture design is realized.
User-friendliness is the key to good design software, which means that designer will finish the design step by step under the guidance of the software and also learn knowledge simultaneously.
To fulfill this purpose, according to production and the rules in national standards, the software in present research includes five parts.
Part one mainly includes typical gallery. The first type of the typical fixture in the database can be  chosen according to machine tool. The commonly-seen fixtures of milling machine, drilling machine, lathe and boring machine are introduced. The second type can be chosen according to the requirements of work pieces, introducing typical fixtures in the processing of different planes, holes or circular arc. Through this window, designers can search for fixtures similar to the work piece under design, directly make modifications and use them, besides, they can also grasp the key points of design and get inspiration.
Part two is the design and selection of fixtures’ typical setting, including the commonly-used centering device, indexing device, copying device and transmission mode.
Through the window, designers will know the usage of these devices and select according to their needs.
Part three is the design and calculation of clamping device, introducing the clamping theories and the calculation of clamping forces of various clamping devices and their applications. Designers can select   suitable clamping devices and calculate clamping forces.

Part four can be divided into the databases of general parts and of standard parts. Some standard parts in fixture design are included in this part, such as brace, elements for aligning tool, guide plate, locations, clamping piece, localizer etc. Designers can choose fixtures of suitable size from the database.
Part five is other fixture designs, including modular fixture design, group fixture, accompanying fixture, etc. For example, the modular fixture part introduces the applications and standard pieces database for the hole series.and the groove series of modular fixture. This can be considered a learning platform and a designing selection platform to designers. With the knowledge of fixture design, the applications of various fixtures are much easier.

Under the CAFD software design platform, designers can design easily according to the procedure illustrated in Figure 1. For example, is it according to the shape of the work piece? Should it be in the form of a pole or a box? Is it plane processing or drilling? What is the size and what are the precision  requirements? Firstly we search for similar fixtures in typical fixture gallery, then grasp key points in designing and design according to the standard piece database provided by CAFD design software. The design using this software has the advantages of a fast understanding of the rudiments, fast design, and fast formation of a chart.
3. Design Examples

Two design examples are introduced. One is the design of typical fixture gallery, and the other is the design of standard piece database.

3.1 The Design of Typical Fixture Gallery
Figure 2 shows the design procedure of typical fixture gallery. Take the fixture design of milling machines for example, as illustrated in Figure 3, when fixtures of milling plane are chosen, various milling fixtures are shown in the table for users to choose. When users choose one fixture, they will know the whole structure of the fixture and its means of clamping through animation presentation. If the user believes that the fixture meets his needs, he can also get the two-dimensional setting chart of the fixture to understand all the standard pieces and the general pieces of the fixture, and modifications to certain parts can be made to make the piece up to the design requirement.
During the software design, one thing we should pay attention to is that the interface should provide as much information as possible. As shown in Figure 3, the left of the chart is for the selection of various types of fixtures;
when the choice is made, the application of the chosen fixture can be observed from the illustration frame in the upper right corner of the chart, and the textbox in the lower right corner provides text tips. In this way, users can get all needed information in one interface. The other thing we should pay attention to is the link with AutoCAD. As shown in Figure 4, when the needed fixture is chosen, users can retrieve the two-dimensional or the three-dimensional chart and put it directly in CAD environment for modifications and transferring.
3.2  The Construction and Application of Standard Part Database
Standard parts are used widely in the design of special fixtures, and users only need to insert a standard part provided by the software in the setting chart. For the convenience of users, it is necessary to install a large gallery and database in the software; the installation of database and the drawing must rely on the latest national and professional standards. The following issues should be emphasized in software design.
(1)user-friendly and be excellent in both illustrations and texts. The design of the interface should be interactive and the content of the interface should be manifest at a glance. There should be clear  arrangements of different types of standard part galleries. In the design of galleries, the categorization and the relationships of subordination should be noticed. For example, hinge base and eccentric wheel belong to clamping devices, while locating tab and V-shape block belong to locating devices. Dialogue boxes are used to help users search needed components and select graphs from gallery.
(2)the independence of data. Data are independent from procedure. Database, data files and internal memory variables are used. Figure 5 is the design of pressure-plate, written directly by Access. As shown in Figure 6, database can realize the independence between data and procedure, which is helpful for the development of a large fixture CAD system that can retrieve data rapidly.
(3) the possibility of modification. Although different requirements are put into consideration in software design, such as enlarging, reducing, rotating pictures, concealing central lines, users may still feel unsatisfied. Therefore, the software should allow users to modify the graphs in gallery, for example to change some lines into dotted lines or erase extra segments. Usually graphs cannot be inserted into design  as a whole block. In order not to affect the previous-drawn graphs, inserted ones are often shown in a new window.
4. Conclusion
The fixture design software of the present research is based on the secondary development of AutoCAD. Over many years’ research and application, a large number of databases, two-dimensional and three-dimensional galleries have been constructed. The goal of fast design of machine tool fixtures has been achieved. The efficiency and the quality of design have been improved. However, with the ever development in computer technology, the design of the software also needs improving to meet the needs of engineering design.