Abstract: Using the Siemens 840D CNC system as the development platform, this paper describes the development of a control interface for a bevel gear CNC gear grinding machine that meets design requirements using OEM software. A dynamic link library (DLL) for communication with the Siemens CNC system was created using VC++. The user-specific interface and machining methods were embedded into the Siemens system using the development environment and VB/VC++ interfaces provided by the Siemens CNC system. The interface development under the OEM environment is introduced. Keywords: Human-machine interface; Siemens 840D; Dynamic link library; Bevel gears are widely used in the automotive and machinery industries due to their smooth and reliable transmission and high load-bearing capacity. As a partially conjugate gear pair with partial contact, its meshing quality is crucial. Gear grinding is performed to reduce tooth surface roughness, improve tooth surface contact quality, and achieve high smoothness and low noise during gear transmission. To meet the needs of the personalized market, the finishing of bevel gears requires the use of advanced CNC systems. However, the SINUMERIK 840D CNC system provided by Siemens does not have a dedicated control interface for bevel gear grinding machines; it only provides OEM development software, allowing users to integrate their own functional modules into the system through secondary development. 1. OEM Embedding Process This study uses the SINUMERIK 840D system as the development platform and a bevel gear CNC grinding machine as the research object. Under the Siemens standard interface, programs for machining are written using the standard programming language provided by the Siemens 840D CNC system and stored in the 840D CNC system. The OEM development software is installed on a PC. In the OEM environment, the human-machine interface is designed using VB, and a language dynamic link library is created using VC++. Following the OEM instructions, the designed interface is embedded into the 840D system. The role of the language dynamic link library is to enable the calling of software text. Software content can be read from the language dynamic link library. Finally, according to the OEM instructions, communication between the human-machine interface and the CNC system is achieved. The PC is connected to the Siemens system, which is then connected to the gear grinding machine for debugging, ultimately achieving control of the gear grinding machine. The designed interface not only allows for changing various parameters but also displays the machine tool's operating status for monitoring. The OEM software provides a sequential control structure, which offers a framework for SINUMERIK standard programs and compatible OEM programs. Sequential control includes: managing program (sequential) control (menu tree composed of states), querying software (vertical and horizontal), querying NC-specific keys, displaying softkey text, managing interactive prompts, and providing language support. Users need to embed the pre-designed interface in the VB environment into the sequential structure framework. The core element describing sequential control is the state; each state has a unique state number. Designed forms can be added to their respective areas via VB, and software text is installed into the language dynamic link library. The content of horizontal and vertical software is read from the language dynamic link library. Each interface in the developed operating system corresponds to a state. The sequential control application OEMSBSP20 includes six files: REGIE, INI, and MMC. OEMBSP20.INI, OEMBSP20.MDI, OEMBSP20.ZUS, LANGUAGE.DLL, REGIE.INI are program manager files, and MMC.INI is the global initialization file. OEMBSP20.INI is the initialization file for this project, edited with Notepad. Its content includes: MDIList is a list of child windows, all of which are listed in the file OEMBSP20.MDI; ControlFile represents the control file, such as status and actions, which are located in the file OEMBSP20.ZUS. OEMBSP20.MDI is the file containing the child window list, edited with Notepad. Its main content is: Each interface of the developed operating system contains only one form. Due to the large number of forms, only a portion is listed here. OEM1 is the first form, and the subsequent number 1 indicates a form type. When other software is clicked, this type of form can be deleted, and its data is deleted. Upon restarting, the previous data is invalid. The load event of this form is called again each time it starts. The OEMBSP20.ZUS file contains all the state matrices. It is also edited with Notepad. The state matrix mainly describes how the software allocates tasks in the state. The following is an explanation of one of the states. The state matrix corresponding to this state is: This is the state matrix of the second state. In the first row, [2] represents the state number, 16 represents the starting address of the horizontal soft key. If you want to write text to the soft key on each interface, you need to know which key to write text to. This requires assigning a number to each soft key; 17 represents the starting address of the vertical soft key; "OEM2" represents the state and which interface is displayed; "0" represents the return value; 0 represents the identifier. In the second row, 0 represents the first horizontal soft key, 0~7 represent the 8 horizontal soft keys, and 8~15 represent the 8 vertical soft keys; the parameter 0 in the last column means that the 0th state will be displayed when the first horizontal soft key is pressed. The parameters of the following rows are the same as those in the first row. LANGUAGE.DLL is a language dynamic link library file, edited using VC++. Language dynamic link library files are primarily used to store software text. Sequence control provides many standard modules and forms. Users can only embed the forms and modules implementing their own programs into the standard sequence control provided by Siemens to achieve special functions. 2. Interface Design of Bevel Gear CNC Grinding Machine Figure 1 shows the main interface of the gear grinding machine based on the Siemens 840D CNC system. Various buttons on the main interface lead to the specific machining interfaces required by the gear grinding machine. Figure 2 shows the parameter input interface for bevel gear grinding, which includes the helix direction of the pinion, the number of teeth on both the pinion and the gear, and the diameter of both the pinion and the gear. Based on a more user-friendly and operable design, the unit can also be selected; the operator can choose to use imperial (inches) or US (millimeters) units. The main interface clearly displays the customizable buttons for the horizontal and vertical soft keys (the vertical software content is not yet defined due to the current good number of interfaces). Pressing the third horizontal soft key, "Grinding Interface," will take you to the bevel gear grinding settings interface, as shown in Figure 3. Currently, there are two grinding modes to choose from: a single-track grinding mode from the bevel gear tooth tip to the tooth root (Figure 3a), which is the simplest grinding mode and is relatively easy to design and write for CNC programs; and an elliptical track grinding mode for the tooth surface (Figure 3b), which combines V/H adjustments from the tooth root to the tooth tip and from the large end to the small end of the bevel gear. Both grinding modes can be operated by manually inputting data, allowing you to set the grinding time for specific tooth surface positions and perform targeted grinding on areas with poor contact areas to achieve better grinding results. 3 Conclusion Using the sequential control provided by the OEM software, the designed human-machine interface was embedded into the Siemens system using VB, and the software text content was read using the language dynamic link library established in the VC++ environment. Finally, the unique functional modules were integrated into the Siemens system, forming an operating system suitable for bevel gear grinding machines. Operators can easily select the machining program and edit the parameters without modifying the workpiece machining program. They can also obtain various status information during the machining process at any time, realizing the monitoring and operation of CNC machine tools. 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