Principle and Steps of Automatic Tool Change in CNC Machining Centers

Abstract: This paper elaborates in detail on the importance of the automatic tool change device in CNC machining centers, the principle of automatic tool change, and specific steps, including aspects such as tool loading, tool selection, and tool change. It aims to deeply analyze the automatic tool change technology, provide theoretical support and practical guidance for improving the processing efficiency and accuracy of CNC machining centers, help operators better understand and master this key technology, and then enhance production efficiency and product quality.

I. Introduction

As key equipment in modern manufacturing, CNC machining centers play a crucial role with their automatic tool change devices, cutting tool systems, and automatic pallet changer devices. The application of these devices enables machining centers to complete the processing of multiple different parts of a workpiece after one installation, greatly reducing non-fault downtime, effectively shortening the product manufacturing cycle, and also having significant significance for improving the processing accuracy of products. As the core part among them, the performance of the automatic tool change device is directly related to the level of processing efficiency. Therefore, in-depth research on its principle and steps has important practical value.

II. Principle of Automatic Tool Change in CNC Machining Centers

(I) Basic Process of Tool Change

Although there are various types of tool magazines in CNC machining centers, such as disc-type tool magazines and chain-type tool magazines, the basic process of tool change is consistent. When the automatic tool change device receives a tool change instruction, the entire system quickly starts the tool change program. Firstly, the spindle will immediately stop rotating and accurately stop at the preset tool change position through a high-precision positioning system. Subsequently, the tool unclamping mechanism is activated to make the tool on the spindle in a replaceable state. Meanwhile, according to the instructions of the control system, the tool magazine drives the corresponding transmission devices to quickly and accurately move the new tool to the tool change position and also performs the tool unclamping operation. Then, the double-arm manipulator quickly acts to accurately grab both the new and old tools at the same time. After the tool exchange table rotates to the correct position, the manipulator installs the new tool on the spindle and places the old tool in the empty position of the tool magazine. Finally, the spindle performs the clamping action to firmly hold the new tool and returns to the initial processing position under the instructions of the control system, thus completing the entire tool change process.

(II) Analysis of Tool Movement

During the tool change process in the machining center, the movement of the tool mainly consists of four key parts:

• Tool Stops with the Spindle and Moves to the Tool Change Position: This process requires the spindle to stop rotating quickly and accurately and move to the specific tool change position through the moving system of the machine tool’s coordinate axes. Usually, this movement is achieved by the transmission mechanism such as the screw-nut pair driven by the motor to ensure that the positioning accuracy of the spindle meets the processing requirements.
• Movement of the Tool in the Tool Magazine: The movement mode of the tool in the tool magazine depends on the type of the tool magazine. For example, in a chain-type tool magazine, the tool moves to the specified position along with the rotation of the chain. This process requires the driving motor of the tool magazine to precisely control the rotation angle and speed of the chain to ensure that the tool can reach the tool change position accurately. In a disc-type tool magazine, the positioning of the tool is achieved through the rotating mechanism of the tool magazine.
• Transfer Movement of the Tool with the Tool Change Manipulator: The movement of the tool change manipulator is relatively complex as it needs to achieve both rotational and linear movements. During the tool gripping and tool release stages, the manipulator needs to approach and leave the tool through precise linear movement. Usually, this is achieved by the rack and pinion mechanism driven by a hydraulic cylinder or an air cylinder, which then drives the mechanical arm to achieve linear movement. During the tool withdrawal and tool insertion stages, in addition to linear movement, the manipulator also needs to perform a certain angle of rotation to ensure that the tool can be smoothly withdrawn from and inserted into the spindle or the tool magazine. This rotational movement is achieved through the cooperation between the mechanical arm and the gear shaft, involving the conversion of kinematic pairs.
• Movement of the Tool Returning to the Processing Position with the 主轴: After the tool change is completed, the spindle needs to quickly return to the original processing position with the new tool to continue the subsequent processing operations. This process is similar to the movement of the tool moving to the tool change position but in the opposite direction. It also requires high-precision positioning and a quick response to reduce the downtime during the processing process and improve processing efficiency.

III. Steps of Automatic Tool Change in CNC Machining Centers

(I) Tool Loading

• Random Tool Holder Loading MethodThis tool loading method has relatively high flexibility. Operators can place tools in any tool holder in the tool magazine. However, it should be noted that after the tool installation is completed, the number of the tool holder where the tool is located must be accurately recorded so that the control system can accurately find and call the tool according to the program instructions in the subsequent processing process. For example, in some complex mold processing, tools may need to be frequently changed according to different processing procedures. In this case, the random tool holder loading method can conveniently arrange the storage positions of tools according to the actual situation and improve the tool loading efficiency.
• Fixed Tool Holder Loading MethodDifferent from the random tool holder loading method, the fixed tool holder loading method requires that tools must be placed in preset specific tool holders. The advantage of this method is that the storage positions of tools are fixed, which is convenient for operators to remember and manage, and is also conducive to the quick positioning and calling of tools by the control system. In some batch production processing tasks, if the processing process is relatively fixed, adopting the fixed tool holder loading method can improve the stability and reliability of processing and reduce processing accidents caused by incorrect tool storage positions.

(II) Tool Selection

Tool selection is a key link in the automatic tool change process, and its purpose is to quickly and accurately select the specified tool from the tool magazine to meet the needs of different processing procedures. Currently, there are mainly the following two common tool selection methods:

• Sequential Tool SelectionThe sequential tool selection method requires operators to place tools in the tool holders in strict accordance with the sequence of the technological process when loading tools. During the processing process, the control system will take the tools one by one according to the placement sequence of the tools and put them back into the original tool holders after use. The advantage of this tool selection method is that it is simple to operate and has a low cost, and it is suitable for some processing tasks with relatively simple processing processes and fixed tool usage sequences. For example, in the processing of some simple shaft parts, only a few tools in a fixed sequence may be needed. In this case, the sequential tool selection method can meet the processing requirements and can reduce the cost and complexity of the equipment.
• Random Tool Selection
• Tool Holder Coding Tool SelectionThis tool selection method involves coding each tool holder in the tool magazine and then putting the tools corresponding to the tool holder codes into the specified tool holders one by one. When programming, operators use the address T to specify the tool holder code where the tool is located. The control system drives the tool magazine to move the corresponding tool to the tool change position according to this coding information. The advantage of the tool holder coding tool selection method is that the tool selection is more flexible and can adapt to some processing tasks with relatively complex processing processes and unfixed tool usage sequences. For example, in the processing of some complex aviation parts, tools may need to be frequently changed according to different processing parts and process requirements, and the tool usage sequence is unfixed. In this case, the tool holder coding tool selection method can conveniently realize the quick selection and replacement of tools and improve processing efficiency.
• Computer Memory Tool SelectionComputer memory tool selection is a more advanced and intelligent tool selection method. Under this method, the tool numbers and their storage positions or tool holder numbers are correspondingly memorized in the memory of the computer or the memory of the programmable logic controller. When it is necessary to change tools during the processing process, the control system will directly obtain the position information of the tools from the memory according to the program instructions and drive the tool magazine to quickly and accurately move the tools to the tool change position. Moreover, since the change of the tool storage address can be remembered by the computer in real time, tools can be taken out and sent back randomly in the tool magazine, greatly improving the management efficiency and usage flexibility of tools. This tool selection method is widely used in modern high-precision and high-efficiency CNC machining centers, especially suitable for processing tasks with complex processing processes and numerous types of tools, such as the processing of parts like automobile engine blocks and cylinder heads.

(III) Tool Change

The tool change process can be divided into the following situations according to the types of tool holders of the tool on the spindle and the tool to be replaced in the tool magazine:

• Both the Tool on the Spindle and the Tool to be Replaced in the Tool Magazine are in Random Tool HoldersIn this case, the tool change process is as follows: Firstly, the tool magazine performs the tool selection operation according to the instructions of the control system to quickly move the tool to be replaced to the tool change position. Then, the double-arm manipulator extends to accurately grab the new tool in the tool magazine and the old tool on the spindle. Next, the tool exchange table rotates to rotate the new tool and the old tool to the corresponding positions of the spindle and the tool magazine respectively. Finally, the manipulator inserts the new tool into the spindle and clamps it, and at the same time, places the old tool in the empty position of the tool magazine to complete the tool change operation. This tool change method has relatively high flexibility and can adapt to various different processing processes and tool combinations, but it has higher requirements for the accuracy of the manipulator and the response speed of the control system.
• The Tool on the Spindle is Placed in a Fixed Tool Holder, and the Tool to be Replaced is in a Random Tool Holder or a Fixed Tool HolderThe tool selection process is similar to the above random tool holder tool selection method. When changing the tool, after taking the tool from the spindle, the tool magazine needs to be rotated in advance to the specific position for receiving the spindle tool so that the old tool can be accurately sent back to the tool magazine. This tool change method is more common in some processing tasks with relatively fixed processing processes and high usage frequencies of the spindle tool. For example, in some batch production hole processing procedures, specific drills or reamers may be used on the spindle for a long time. In this case, placing the spindle tool in a fixed tool holder can improve the stability and efficiency of processing.
• The Tool on the Spindle is in a Random Tool Holder, and the Tool to be Replaced is in a Fixed Tool HolderThe tool selection process is also to select the specified tool from the tool magazine according to the processing process requirements. When changing the tool, the tool taken from the spindle will be sent to the nearest empty tool position for subsequent use. This tool change method, to a certain extent, takes into account the flexibility of tool storage and the convenience of tool magazine management. It is suitable for some processing tasks with relatively complex processing processes, numerous types of tools, and relatively low usage frequencies of some tools. For example, in some mold processing, multiple tools of different specifications may be used, but some special tools are used less frequently. In this case, placing these tools in fixed tool holders and storing the used tools on the spindle nearby can improve the space utilization rate of the tool magazine and the tool change efficiency.

IV. Conclusion

The principle and steps of automatic tool change in CNC machining centers are a complex and precise system engineering, involving technical knowledge in multiple fields such as mechanical structure, electrical control, and software programming. In-depth understanding and mastery of the automatic tool change technology are of great significance for improving the processing efficiency, processing accuracy, and equipment reliability of CNC machining centers. With the continuous development of the manufacturing industry and technological progress, the automatic tool change devices of CNC machining centers will also continue to innovate and improve, moving towards higher speed, higher accuracy, and stronger intelligence to meet the growing demand for processing complex parts and provide strong support for promoting the transformation and upgrading of the manufacturing industry. In practical applications, operators should reasonably choose tool loading methods, tool selection methods, and tool change strategies according to the characteristics and requirements of processing tasks to fully utilize the advantages of CNC machining centers, improve production efficiency and product quality. Meanwhile, equipment manufacturers should also continuously optimize the design and manufacturing processes of automatic tool change devices to improve the performance and stability of equipment and provide users with higher-quality and more efficient CNC machining solutions.

Media Contact
Company Name: Qingdao Taizheng Precision Machinery Co., Ltd.
Email: Send Email
Phone: 13605328581
Address:C-101, Building 31, Zhongnan High tech Jimo Technology Innovation Industrial Park, No. 76 Dazhong 1st Road, Jimo District
City: Qingdao
Country: China
Website: https://www.ncmillingmachine.com/