Detailed explanation of the function, composition and tool changing process of CNC automatic tool changer

The CNC automatic tool changer (ATC) is the core functional module of modern CNC machine tools. It can automatically change tools during the processing without manual intervention, thereby greatly improving production efficiency, processing accuracy and automation.

ATC is widely used in high-end CNC equipment such as machining centers, turning and milling machine tools, and five-axis linkage machine tools, and is suitable for industries such as automobile manufacturing, aerospace, and precision molds. The following will systematically analyze the structural principle, type, workflow, and key technical parameters of the CNC automatic tool changer:

1. Definition and core role of CNC automatic tool changer

(1) Definition

The CNC automatic tool changer (ATC) is a mechatronic device controlled by a numerical control system (CNC) or a programmable logic controller (PLC), which can automatically store, identify, grab and replace tools. It usually consists of a tool magazine, a tool changing manipulator, a spindle tool grabbing mechanism and an auxiliary control system. It can complete tool switching within a few seconds to more than ten seconds, realizing multi-process continuous processing.

(2) Core role

1) Improve processing efficiency:

Traditional manual tool change usually takes 2 to 5 minutes, while ATC tool change time can be shortened to 0.8 to 15 seconds, greatly reducing non-cutting time.

2) Support complex parts processing:

By automatically switching different tools, multiple processes such as milling, drilling, tapping, boring, etc. can be completed on one machine tool, reducing repeated clamping of workpieces.

3) Ensure processing accuracy:

Avoid errors caused by manual tool change and ensure tool positioning accuracy (usually ≤0.002mm).

4) Realize unmanned production:

Combined with automated production lines (such as FMS flexible manufacturing systems), 24-hour continuous processing can be achieved.

2. Typical structure and composition of CNC automatic tool changer

The core structure of CNC automatic tool changer includes tool magazine, tool change manipulator, spindle tool gripper mechanism and auxiliary control system. Each part works together to ensure that the tool change process is fast, accurate and reliable.

(1) Tool magazine

The tool magazine is used to store and manage tools. Its capacity and structural form directly affect the processing capacity of the machine tool. The following is a detailed introduction to the types of tool magazines:

1) Disc tool magazine (hat-shaped)

Compact structure, suitable for small and medium-sized vertical machining centers, with a capacity of 12 to 50 tools.

Tool changing method: The tool magazine moves as a whole to the spindle position and directly exchanges tools (tool changing time is about 3 to 8 seconds).

2) Chain tool magazine

Using chain drive, the capacity can reach 30 to 400 tools, suitable for large horizontal machining centers or heavy cutting machine tools.

Tool changing method: The servo motor drives the chain to send the target tool to the tool changing position, and then the robot grabs it.

3) Linear tool magazine

Simple structure, small space occupation, mostly used for turning and milling machine tools or small precision machining equipment.

(2) Tool changing robot

The robot is responsible for grabbing tools from the tool magazine and installing them on the spindle. Its structure affects the tool changing speed and reliability. The following is an introduction to the common types of manipulators:

1) Single-arm rotary type

The manipulator performs a 180° rotation movement, first removes the old tool from the spindle, and then installs the new tool. The tool change time is about 3 to 8 seconds.

It is suitable for small and medium-sized machining centers, with a simple structure and easy maintenance.

2) Double-arm cross type (synchronous tool change)

The dual-arm structure can grab the new and old tools at the same time, and the tool change time can be shortened to less than 1 second.

It is suitable for high-speed machining centers (such as the DMG HSC series).

(3) Spindle tool gripping mechanism

The spindle tool gripping mechanism is responsible for fixing the tool to ensure rigidity and accuracy during machining.

1) Tool pulling system

Butterfly spring tensioning: The tool handle is tightened by spring force (usually 5 to 15 kN) to keep it stable during machining.

Hydraulic tool release: When changing the tool, the hydraulic cylinder pushes the pull rod to release the tool.

2) Tool handle type

BT tool handle: It adopts a 7:24 taper and is suitable for ordinary machining centers.

HSK toolholder: 1:10 short taper + end face contact, higher rigidity, suitable for high-speed cutting.

CAPTO toolholder: polygonal interface, strong torsion resistance, suitable for turning and milling machine tools.

(4) Auxiliary system

1) Tool coolant flushing: 

Automatically clean the spindle taper hole when changing tools to prevent chips from affecting accuracy.

2) Tool length measurement: 

Automatically compensate for tool length deviation through contact probe or laser measurement.

3) Tool breakage detection:

 Use force sensor or visual system to monitor tool status to prevent tool breakage.

3. Detailed explanation of the workflow of CNC automatic tool changer (taking chain tool magazine + robot as an example)

The workflow of CNC automatic tool changer is a highly coordinated and precise process involving the precise coordination of multiple subsystems. The following is a typical chain tool magazine with a robot tool change system as an example to analyze its complete workflow in detail:

(1) Tool change command trigger stage

When the CNC program executes the tool change command (such as M06 T02), the system starts the following actions:

1) Tool preselection:

The CNC controller sends a signal to the tool magazine servo system to drive the chain of the chain tool magazine to rotate and move the target tool (such as T02) to the tool change waiting position (usually at the front end of the tool magazine).

2) Spindle deceleration and braking:

The spindle motor quickly decelerates from the high-speed rotation state, and the spindle orientation and accurate stop system is started at the same time. The high-precision encoder controls the spindle to stop at the preset angle (usually 0° or 180°), with an accuracy of up to ±0.001°.

(2) Tool change preparation stage

1) Spindle positioning confirmation:

The photoelectric sensor detects whether the spindle stops accurately at the orientation position. If it does not, error compensation is triggered.

2) Tool in-position detection:

The proximity switch or RFID reader on the tool magazine side confirms that the target tool has reached the tool change position and verifies the tool parameters (such as length compensation value).

3) Robot initialization:

The tool change robot servo motor starts and drives the cam mechanism to move the robot to the standby position (usually between the spindle and the tool magazine).

(3) Old tool removal stage

1) Spindle tool release:

The hydraulic system starts, and the cylinder pushes the pull rod to overcome the butterfly spring force (about 10kN) to release the tool clamping force in the spindle taper.

2) Robot grabs the old tool:

One of the two claws of the robot closes and accurately grabs the tool handle groove of the spindle tool (the grab force is usually set to 200-500N).

3) Tool separation:

The robot moves back about 10-20mm along the linear guide rail to completely pull the tool out of the spindle taper. At the same time, the high-pressure air valve opens (0.6-0.8MPa) to blow away the residual chips on the spindle taper surface.

(4) Tool exchange stage

1) Robot rotation:

The servo motor drives the robot to rotate 180° (taking about 0.3-0.5 seconds) so that the new tool carried by the other side of the clamp is aligned with the spindle.

2) New tool insertion:

The robot pushes forward and accurately inserts the new tool into the spindle taper hole. At this time, the hydraulic system is depressurized and the butterfly spring re-tightens the tool handle (the tightening process is accompanied by a "click" mechanical locking sound).

3) Clamp release:

The robot clamp opens and the sensor detects and confirms that the tool is firmly installed (by detecting the displacement of the pull rod or the pressure change).

(5) Reset and verification stage

1) Robot return:

The robot returns to the origin position to prepare for the next tool change.

2) Tool parameter loading:

The CNC system automatically loads the compensation parameters of the tool (such as length offset H02, radius compensation D02).

3) Safety verification:

The spindle load is detected by the force sensor. After confirming that there is no abnormal looseness of the tool, the CNC resumes the processing program.

(6) Exception handling mechanism

1) Tool drop protection:

If the pressure sensor detects insufficient tension, it will immediately trigger an emergency stop and alarm.

2) Tool change timeout monitoring:

If the entire process exceeds the preset time (such as 15 seconds), the system will be judged as a fault and return to a safe state.

3) Interference avoidance:

Real-time monitoring of the machine tool coordinate system ensures that the tool change path does not collide with the fixture or workpiece.

Analysis of typical time nodes:

Tool magazine pre-selection time: 1.5-3 seconds (depending on the tool spacing)

Manipulator tool change action: 2-4 seconds

Tool parameter loading: 0.1-0.3 seconds

Total process time: 3.8-7.3 seconds (high-speed models can be compressed to within 3 seconds)

4. Summary

The CNC automatic tool changer achieves improved processing efficiency and guaranteed process quality through fast and accurate tool switching. Its core functions are to shorten non-production time, support multi-process processing, ensure positioning accuracy, and ultimately achieve automated continuous production.