Preface Multi-spindle heads in modular machine tools typically employ a single power shaft driving multiple spindles. Because each drive shaft must be appropriately distributed within a limited space within the machine housing while avoiding interference, and because the design of each shaft must guarantee its speed, direction of rotation, strength, and rigidity, multi-spindle head design is challenging and time-consuming, often becoming a bottleneck in modular machine tool design. A solution to this problem is computer-aided modular design. Modular design is a standardized and modular design approach. It is not geared towards a specific product but rather towards a class of product systems or even adjacent product systems with similar functions. It does not require separate design for each product but rather carefully designs multiple modules, combining them in a clever, flexible, and varied manner to create diverse products. To this end, we have developed a modular machine tool CAD system that supports modular design. Applying CAD technology and modular design methods to the design of multi-spindle heads not only shortens the design and manufacturing cycle and reduces costs but also helps ensure the quality of the multi-spindle heads, enhancing the company's competitiveness. 1. System Development Environment and Module Division Currently, the R&D of domestic mechanical products is gradually shifting from 2D drafting software represented by AutoCAD to 3D design software represented by UG, Pro/E, and SolidWorks. The 3D CAD system for the multi-axis headstock of the modular machine tool selects Unigaphics (UG) as its software development platform, employing the UG/OPEN API interface, Visual Studio .NET compilation environment, and Windows 2000/XP operating system. Utilizing UG's fast, flexible, and convenient 3D parametric modeling capabilities ensures smooth system design. A module is a basic unit with specific functions within a product, natural object, or mixture thereof, characterized by standardization, serialization, and interchangeability. Modular design, based on functional analysis of products with different functions or the same function but different performance and specifications within a certain range, divides and designs a series of functional modules. Different products can be constructed through the selection and combination of modules to meet market demands. When designing a multi-spindle box in a modular fashion, the first step is to divide it into several modules. Since the specific structure of the multi-spindle box depends on the specific requirements of the parts being machined, such as the number, shape, and distribution of holes, as well as the number of parts being machined, it is not a universal component. However, by classifying the components of the multi-spindle box, its parts can be generalized. For example, the box body and front and rear covers are classified according to their outline dimensions and shape; the spindles are classified according to their purpose, such as drilling, boring, and tapping; and gears are classified according to their module, number of teeth, and hole diameter. Using these generalized parts, various multi-spindle boxes with different structural forms can be configured. The design of the multi-spindle box first uses an assembly-based feature modeling method to establish an assembly model. Then, the components are designed within this model. The design of the multi-spindle box mainly includes the design of the multi-spindle box body and the transmission system, which consists of several spindles, drive shafts, and gears. Based on the assembly hierarchy of the multi-spindle box, the design module is divided into three sub-modules, as shown in Figure 1. The multi-spindle box design module, based on the overall assembly model of the multi-spindle box, accepts user-input design data through an interactive interface. Figure 1 shows the multi-spindle box design module. 2. Multi-spindle box main module design 2.1 Multi-spindle box body design module The general body of the multi-spindle box is divided into the body, front, rear, and side covers. The basic dimensions of the multi-spindle box are specified in the standard series, with nominal dimensions expressed by the width of the corresponding slide saddle. The width and height of the multi-spindle box body are selected according to the specifications of the matching slide according to the specified series dimensions. The connecting screw holes, locating pins, and their positions on the mating surfaces are adapted to the dimensions of the power box. The multi-spindle box design module includes a parametric model of the multi-spindle box, a database storing all parameters of the multi-spindle box, and a main control program. The specifications of the multi-spindle box body are width and height. These two dimensions are set as assembly layer control parameters. When designing the parametric model of the multi-spindle box, UG/WAVE technology is used to maintain the correlation between the parts of the multi-spindle box body, front, rear, and side covers. First, a model of the multi-spindle housing is created. Then, when designing the rear cover, front cover, and side cover, the edges and screw holes of the housing are referenced and linked via WAVE. This way, when the housing parameters are changed, other components will be updated accordingly. When designing the multi-spindle housing, the user is required to select the multi-spindle housing type. There are two types: horizontal and vertical, the difference being the thickness of the front cover. Then, the housing size is selected. The main control program provides the housing sizes of all multi-spindle housings, avoiding table lookups and facilitating the selection of standardized multi-spindle housings. The housing size of the multi-spindle housing is used as a query condition to extract all design parameters from the housing parameter database and pass them to the 3D housing model in the graphics library. After the model is updated according to the provided parameters, the user can save it. When needed, it can be assembled into the assembly file. The multi-spindle housing design process is shown in Figure 2. Figure 2 Housing Design Flowchart 2.2 Gear Design Module The gear design module provides three gear design models: a single gear 3D design model, a gear model and a combined basic transmission model when designing the shaft. The creation of the gear design module is introduced using the combined basic transmission model as an example. Based on the different meshing methods between gears in the transmission system, a commonly used gear transmission assembly model library is established using the interactive modeling method of the system. There are three basic transmission models in the multi-axis transmission system of the modular machine tool: (1) one-to-one, one-to-two, and one-to-three in the same row; (2) one-to-two in different rows; (3) one-to-three in different rows. The transmission system of the modular machine tool can be composed of the above four basic transmission models. The basic transmission model module includes three parts: graphics library, database, and main control program. The graphics library includes fully parametric gear models. The database stores the coordinates of all transmission shafts and spindles for loading during calculation. The main control program completes coordinate calculation, data access, and model update operations. When designing the modular machine tool system, first, each transmission shaft and spindle is numbered, and the known shaft coordinates are stored in the database; then, the required basic transmission model is selected, the unknown shaft coordinates are calculated and stored in the database, and finally, the model is established. The main interface of the main control program is shown in Figure 3; the program flowchart is shown in Figure 4. Figure 3: Gear transmission design interface. Figure 4: Main control program flowchart. The calculation process is illustrated using a one-to-two transmission model with different gears on a single transmission shaft as an example. A one-to-two transmission model uses two pairs of gears to drive two known shafts on a single transmission shaft, as shown in Figure 5. Given the coordinates of the O-axis and A-axis, as well as the number of teeth and module of the transmission gears, the calculation process for the B-axis coordinate is shown in Figure 5: One-to-two transmission model calculation. The interface of the one-to-two transmission calculation program is shown in Figure 6. Figure 6: One-to-two transmission calculation program interface. 3. Conclusion Based on the overall scheme and design requirements of the multi-axis box of a modular machine tool, this paper divides and designs the functional modules of the multi-axis box. Using the powerful 3D modeling and assembly functions of the UG 3D software platform, a 3D parametric model of the multi-axis box module is established. The adoption of 3D design methods instead of traditional 2D design methods makes the design process more visual and the design results more intuitive and clear, which is conducive to improving design quality. The use of modular design and a 3D CAD system can greatly improve the design efficiency of multi-axis boxes of modular machine tools, improve product quality and lifespan, and ensure the success of product development on the first attempt.