Rapid automatic tool change technology for high-speed machining centers

High-speed machining centers are typical products of high-speed machine tools, and the development and application of high-speed functional components such as electric spindles, high-speed ball screws, and linear motors have significantly improved cutting efficiency. To match the high efficiency of the machine tools, the high-speed performance of the automatic tool changer (ATC), which is one of the important components of a machining center, has also become a crucial technical aspect of high-speed machining centers.

With the increase in cutting speed and the continuous reduction in cutting time, the requirements for tool change time are also gradually increasing. The speed of tool changing has become an important indicator of high-level machining centers.

Due to the need for reliable and accurate automatic tool changing in machining centers, and the relatively complex structure, it is technically challenging to increase the tool change speed. Currently, high-speed machining centers produced by advanced foreign machine tool manufacturers are mostly equipped with rapid automatic tool changers to adapt to high-speed machining, and many adopt new technologies and methods. This article will introduce some technical methods for improving tool change speed and the rapid automatic tool changing technologies of advanced foreign high-speed machining centers, hoping to contribute to the development of high-speed machining centers in China.

1. Rapid Automatic Tool Changing Technology

Rapid automatic tool changing technology aims to reduce auxiliary processing time as the primary objective, comprehensively considering various aspects of the machine tool, and completing tool exchange within the shortest possible time. This technology includes the setup of the tool magazine, tool change methods, tool change actuators, and adaptation to the structural characteristics of high-speed machine tools.

(1) Tool Change Speed Indicators

There are mainly three methods to measure tool change speed: ① tool-to-tool change time; ② cut-to-cut change time; ③ chip-to-chip change time. Since chip-to-chip change time basically represents the time between two cutting operations in a machining center, it reflects the auxiliary time occupied by tool changing in the machining center. Therefore, chip-to-chip change time should be the most direct indicator for measuring the efficiency of a machining center. Tool-to-tool change time mainly reflects the performance of the automatic tool changer itself and is more suitable as a performance indicator for the machine tool's automatic tool changer. These two methods are commonly used to evaluate tool change speed. There is no definite indicator for how fast the tool change time should be for a rapid automatic tool changer in a high-speed machine tool. Whenever technically feasible, the tool change speed should be increased as much as possible.

(2) Basic Principles for Improving Tool Change Speed

The basic starting point for automatic tool exchange in a machining center is to reduce auxiliary processing time through automatic tool changing in machining processes involving multiple tools. On high-speed machining centers, due to the significant increase in cutting speed, the configuration of the automatic tool changer and tool magazine should consider shortening the tool change time as much as possible to match the high-speed cutting machine tool.

The tool change device of a machining center typically consists of a tool magazine and a tool exchange mechanism, commonly using methods such as robotic arms or non-robotic arms. The form and position of the tool magazine also vary. To suit the needs of high-speed operation, high-speed machining centers differ structurally from traditional machining centers, with tool feed movement being the primary focus, and reducing the mass of moving parts has become a mainstream design approach for high-speed machining centers. Therefore, when designing the tool change device, the new structural characteristics of high-speed machine tools should be fully considered.

When setting up the tool change device on a high-speed machining center, time is not a critical consideration. Firstly, tool change speed should be improved on the basis of accurate and reliable tool change actions. Especially since the ATC is a relatively high-failure-rate component among the functional parts of a machining center, this point is particularly important. Secondly, the ATC should be selected based on the application and cost-performance ratio. In applications where tool change time significantly impacts the production process, tool change speed should be increased as much as possible. For example, in automotive production lines, tool change time and frequency are factored into the production cycle time of parts. In other areas, such as mold cavity processing, the selection of tool change speed can be more flexible.

(3) Main Technical Methods for Improving Tool Change Speed

The rapid automatic tool changing technologies suitable for high-speed machining centers mainly include the following aspects:

Improving the operating speed based on traditional automatic tool changers or adopting mechanisms and drive components with faster operating speeds. For example, mechanical cam structures have significantly higher tool change speeds than hydraulic and pneumatic structures. The rapid tool change device with a mechanical cam structure produced by SODIC's MC450 vertical machining center has a tool-to-tool change time of only 0.6 seconds.

Designing the form and position of the tool magazine and tool change device based on the new structural characteristics of high-speed machine tools. For example, traditional vertical machining centers often have the tool magazine and tool change device mounted on one side of the column. However, high-speed machining centers mostly use column-moving feed methods, and to reduce the mass of moving parts, it is not advisable to mount the tool magazine and tool change device on the column.

Adopting new methods for rapid tool exchange, such as using alternative methods instead of tool magazines and robotic arms. For example, changing the spindle instead of the tool.

Utilizing the characteristic of the newly developed machining center's spindle component, which can perform high-speed motion with 6 degrees of freedom, to allow the spindle to directly participate in the tool change process. This not only provides flexibility in the placement of the tool magazine but also reduces the degrees of freedom of the tool magazine's motion, significantly simplifying the structure of the tool magazine and tool change device.

Tool holders suitable for high-speed machining centers. HSK tool holders are lightweight and have a short tool insertion and removal stroke, which can increase the speed of the automatic tool changer. The adoption of HSK hollow short taper shank tools is a trend in the development of rapid automatic tool changers.

1. Some Structural Methods for Rapid Tool Changing

In addition to improving the operating speed based on traditional tool change devices, some new methods and structures for tool change devices have emerged.

The following introduces some of them for reference.

(1) Multi-spindle Tool Changing

This type of machine does not have a traditional tool magazine and tool change device but adopts multiple spindles fixed side by side on a spindle rack, typically 3 to 18 spindles. Each spindle is directly driven by its respective motor, and each spindle is equipped with a different tool. Instead of exchanging tools on the spindle, the workpiece mounted on the fixture is quickly moved from the processing position of one spindle to another spindle equipped with a different tool to achieve tool changing and immediate processing. This movement time is the tool change time and is very short. Tool changing is accomplished by the rapid movement of the fixture, eliminating the need for a complex tool change mechanism. Machines of this structure produced by Austrian company ANGERG achieve a chip-to-chip change time of only 0.4 seconds, which is currently the shortest chip-to-chip change time in the world. This machine structure is significantly different from conventional machining centers. It can not only be used for processing that requires rapid tool changing but also allows simultaneous multi-axis processing, making it suitable for use in high-efficiency production lines.

(2) Dual-spindle Tool Changing

The machining center has two working spindles, but they are not used for cutting simultaneously. One spindle is used for processing, while the other spindle changes tools during this period. When a tool change is needed, the processing spindle quickly retracts, and the spindle with the new tool immediately moves into position for processing. Since these two processes can occur simultaneously, the tool change time is effectively the time it takes for the two spindles, already equipped with tools, to switch positions, minimizing auxiliary time and achieving the shortest chip-to-chip change time. With two spindles, this machine's tool magazine and tool change robot can be one set or two sets. For example, the machining center produced by German company Alfing-Kessler uses a dual-spindle system with one set of tool magazine and tool change robot. On the other hand, German company Hornsberg-Lamb's HSC-500, HSC-630, and HFC-630 machining centers have two spindles and two sets of tool change systems. The two spindles can move to the processing position and accelerate to the processing speed within 1.0 to 1.5 seconds. The specific exchange time depends on the size of the machine.

(3) Turret-style Tool Magazine Arranged Around the Spindle

In this method, the tool magazine itself acts as a robotic arm, and sequential tool changing is achieved through tool insertion and removal from the tool magazine, resulting in a relatively short chip-to-chip change time. If arbitrary tool changing is required in this method, the chip-to-chip change time may vary depending on the position of the selected tool in the tool magazine, with the farthest tool potentially having a longer chip-to-chip change time. Therefore, this method can only adopt sequential tool selection as a high-speed automatic tool changing device.

(4) Multi-robotic Arm Method

Similarly, the tool magazine is arranged around the spindle, but each tool has its robotic arm, allowing almost no time loss during tool changing and enabling arbitrary tool selection. Machines of this structure produced by German company CHIRON achieve a chip-to-chip change time of