The biggest difference between machining centers and CNC milling machines lies in their tool magazines and tool changing mechanisms. Common types of tool magazines in machining centers include disc-type, dome-type, and chain-type. Tool changing methods in machining centers include robotic tool changing and spindle tool changing. Disc-type and chain-type tool magazines generally use robotic tool changing, while dome-type tool magazines generally use spindle tool changing.
Tool selection methods in the tool magazine include sequential tool selection and arbitrary tool selection. The former involves sequentially installing the required tools in the tool holders according to a predetermined machining sequence. During tool changing, the tool magazine rotates sequentially. This method is simple to control and the tool magazine's movement, but it lacks flexibility and is rarely used in recent years. The latter mainly involves encoding the tool holders in the tool magazine, and the CNC machine tool selects the tool based on the encoded address. This method is widely used.
Arbitrary tool selection tool magazines are further divided into fixed-address tool magazines and random-address tool magazines based on the different tool encoding methods. Fixed-address tool magazines store tools in positions with the same and fixed tool holder numbers. Tools retrieved from the magazine are returned to their original tool holders after use, ensuring the tool number and tool holder number remain consistent. Operators can monitor the tool loading status at any time and directly write T-instructions in machining programs based on the tool distribution. Hat-type tool magazines often use fixed-address tool selection. Random-address tool magazines store tool numbers and tool holder numbers, so the system maintains a tool table that stores the relationship between tool numbers and their corresponding tool holder numbers. After each tool change, the tool number-tool holder number relationship changes, but this change is remembered in the tool table. Tool selection commands can select tools based on the tool holder number corresponding to the tool in the tool table (note that tool selection commands in the program still specify the tool number). Furthermore, operators can query the tool table at any time and edit and modify it when necessary. Disc-type and chain-type tool magazines often use random-address tool selection.
Hat-type tool magazines are generally smaller, allowing tools to be manually loaded directly into their corresponding tool holders. However, most tool magazines are manually loaded onto the spindle, and then loaded into the magazine using a tool change command. The tool loading operation is as follows:
1) First execute TxxM06, where x is the tool number to be changed.
2) Manually load tool number x onto the spindle.
Repeat the above operation to continue loading tools.
The tool changing process in a machining center generally includes two actions-tool selection and tool change, corresponding to the tool selection command Txx and the tool change command M06, respectively. Tool selection and tool change can be completed in one program segment or distributed across different program segments; the latter can improve work efficiency.
It should be noted that tool magazines in machining centers are generally manufactured by specialized companies, and their tool selection and tool change methods and command formats depend on the design of both the tool magazine manufacturer and the machine tool manufacturer. Therefore, the tool selection and tool change methods of the tool magazine must be based on the machine tool manufacturer's operation manual.
The tool change position in a machining center is generally relatively fixed. It may be set at a reference point on the Z-axis (which can be returned using the G28 command), or it may be set at a dedicated position (generally returned using the G30 command).
