The machining center is a highly automated technological equipment developed on the basis of CNC milling machines. It is a CNC machine with a tool magazine and an automatic tool changer (ATC), also known as a CNC machine with automatic tool change or a multi-process CNC machine.
After fixing the workpiece once, the CNC system can control the machine to automatically select and change the tool according to different processes; automatically change the spindle speed, feed rate and tool path relative to the workpiece, and other auxiliary functions; sequentially perform multi-process machining on multiple surfaces of the workpiece.
To increase the functionality of machining centers, machining centers with automatic tool magazine change, automatic spindle head change and automatic spindle box change appeared.
Automatic tool change mechanisms are an important unit that determines the technological capabilities of a CNC machine and allows minimizing auxiliary time. Multi-position turret heads are most widely used on CNC milling machines, in which the presence of a large number of tools allows for complete milling of parts or blanks. About 70% of blanks can be processed with only eight tools, and more than 95% of blanks – with 13 tools. Turret heads are used four-, six-, eight-position and with a larger number of positions.
According to the design and layout, the existing automatic tool change mechanisms are divided into:
1. with replacement of the entire spindle device (revolver spindle heads);
2. with tool change in one spindle (tool magazines).
Revolver spindle heads
Revolver spindle heads are the most compact and simple in design and are located, as a rule, on the spindle head of a metalworking machine. The turret head is used with low accuracy requirements and when the required quantity is up to 8 units. The tool change time is 1-3 s. After moving to the working position, one of the spindles becomes driven, receives rotation from the main drive. The device is used on lathes, milling machines, and drilling machines.
Tool shop
In the drilling and milling machines of the boring type, the use of tool magazines with placement of up to 200 tools was the most widely used.
Depending on the layout of the machine, tool stores of disc, drum and chain types can be located on the spindle head, stand, frame or next to the machine. Disk magazines are made with vertical, horizontal and inclined axes of rotation, they are discs with nests (cells) made on the end surface for tool mandrels. Disk stores can accommodate a relatively small number of tools, usually no more than 30.
Drum tool magazines are available with a vertical axis of rotation and a horizontal one, installed in cells with tool mandrels. Tool mandrels are installed according to the form. The capacity of such stores is from 16 to 72 tools, they can be multi-tiered, which significantly increases the capacity.
Chain tool stores are cells for storing mandrels, tools connected to each other by a transport chain moving along guides. Capacity from 16 to 360 instruments. Chain tool stores can be horizontal, vertical, inclined, rectangular, triangular, square and more complex in shape. This provides the possibility of easy access to the store for loading tools and the compactness of the design. They are made in an arbitrary configuration, they can be located near the working area of the equipment, but preferably they are located outside the machine. Multi-tiered drum and disk magazines of large capacity, which have a complex design and significant dimensions, are also used.
The automatic tool change system is one of the most important components of CNC machining centers, providing autonomous, fast and accurate tool change without operator intervention. Failure of this system can lead to serious delays in the production process, increased scrap and, consequently, unforeseen costs, and even damage to the equipment itself.
The sequential selection of the required tool in the tool magazine of the machine during the processing of the workpiece, its automatic installation and fixing on the working element of the machine, and the subsequent return of the tool to the magazine are carried out by the automatic tool change system (ATСS). In general, the ATСS system includes:
- a tool magazine for accumulating tools (turret);
- a device for selecting the required
- tool in the tool magazine or turret;
- an automatic operator (automatic arm) for changing the tool;
- devices that deliver the tool from the magazine to the automatic operator, and
- tilters designed to change the orientation of the tool when it is delivered to the change position (if necessary).
- a mechanism for clamping the shank or tool holder with the tool on the working element of the machine.
The process of using cutting tools on CNC machines includes two main stages:
- completing and assembling cutting tools taken from the warehouse by installing them in shanks and holders; their subsequent dimensional adjustment on special devices within the framework of developed unified tool systems (respectively for CNC lathes and multi-purpose machines) and coding;
- transporting a set of adjusted tools, installing it in the tool magazine on the machine; sequential selection of the required tool during the processing of the workpiece, its automatic change with installation and fixing on the working element of the machine; unfastening, removal and return of the tool to the magazine; return of the tool after processing a batch of workpieces to the warehouse, and, if necessary, its disassembly for re-sharpening or replacement.
The most common case of system failure, as a result of which the tool change process stops, is the accumulation of chips and lubricants in the tool gripper mechanism. This problem, as a rule, leads to a slowdown in the gripper operation and a decrease in the accuracy of the positioning of the machining tool, which, in turn, causes overload and stoppage of the system.
In order to prevent this problem from occurring, it is necessary to:
1) timely clean the grippers;
2) renew the lubricant;
3) check for damaged parts;
4) regular maintenance of the systems by certified specialists.
