CNC turret punch presses are an important category of CNC machine. They mainly consist of a control system, main drive system, feeding system, turntable, molds, and peripheral programming software. They are high-speed, precision, and flexible machining equipment integrating mechanics, electronics, hydraulics, and pneumatics, capable of punching and other forming processes on sheet metal. As a key piece of equipment in the field of sheet metal processing, CNC turret punch presses have a history of application and development spanning over seventy years, becoming one of the most important process equipment in industries such as automobile manufacturing, construction machinery, hardware, home appliances, computers, instrumentation, electronic information, and textile machinery.

Classification and Characteristics of CNC Turret Punch Presses

Based on the power source of the main drive, CNC turret punch presses can be divided into the following three categories:

(1) Mechanical turret punch presses. This type of structure has simple motion characteristics, a narrow processing range, low punching frequency, high noise, and the clutch and brake are prone to wear and have a short lifespan. It has been largely phased out of the market.

(2) Hydraulic turret punch presses. These are driven by hydraulic cylinders, with punching controlled by electro-hydraulic servo valves. This type of product was the mainstream product in the market for a considerable period of time. Compared to mechanical presses, the advantages are expanded stamping processability, increased stamping frequency, and reduced noise. However, it also has disadvantages such as oil pollution, high power consumption, poor environmental and temperature tolerance, low reliability, and difficult maintenance.

⑶ All-electric servo turret press. With the improvement and promotion of high-torque servo motors and drive systems in recent years, all-electric servo turret presses have made great progress and have gradually replaced hydraulic structures, becoming the mainstream product in the industry.

Compared with traditional mechanical and hydraulic presses, the all-electric servo main drive has many advantages:

1) Good versatility and high level of intelligence. Due to the servo function, the punch motion curve is no longer just a sine curve, but an arbitrary curve that can be optimized according to process requirements, greatly improving processing performance and expanding the processing range.

2) High precision. The encoder on the servo motor feeds back the crank rotation angle information to the CNC system. By processing the data through the functional relationship between the punch displacement and the crank angle, real-time punch position data is obtained, forming a semi-closed-loop control system. This enables precise control of the punch movement, with the bottom dead center accuracy controlled within ±0.05mm.

3) High production efficiency. Servo punch presses can adjust the punch stroke according to different processing techniques. In a work cycle, a 360° rotation is unnecessary; a certain angle of oscillation is sufficient to complete the processing, further shortening cycle time and significantly improving production efficiency.

4) Energy saving and environmental protection. Servo motor-driven CNC turret punch presses rely entirely on the motor's torque. The motor only rotates when the punch is moving, resulting in significant energy savings.

5) Low vibration and low noise. By setting the punch speed characteristic curve, the punch speed can be appropriately reduced at the moment the upper die contacts the sheet metal. Compared to traditional mechanical and hydraulic CNC turret punch presses, noise can be reduced by 8–12 dB.

Currently, there are three main types of servo main drive structures. These three mechanisms are technologically mature, each with its own characteristics, and all hold a significant market share.

⑴ Dual-motor driven crank-slider mechanism, represented by the EM2510NT CNC turret punch press from AMADA Corporation of Japan. The two motors are symmetrically arranged on both sides of the machine body, resulting in a compact and small structure that ensures sufficient torque output from the crankshaft while achieving a high punching frequency.

⑵ Crank-toggle mechanism, represented by the MOTORUM-2048LT CNC turret punch press from MURATEC Corporation of Japan. Although this structure is relatively complex, it utilizes the unique force-multiplying characteristics of the crank-toggle mechanism to reduce the load torque on the servo motor. Furthermore, with the crank rotating one revolution and the slider moving up and down twice, a higher punching frequency can be achieved.

⑶ Single water-cooled high-torque servo motor driven crank-slider mechanism, represented by the EP series and H series all-electric servo CNC turret punch presses from Jiangsu Yangli CNC Machine Tool Co., Ltd. Because it uses a single water-cooled high-torque servo motor as its power source, it boasts strong continuous workload capability and eliminates the synchronization control issues associated with dual-motor drives. This results in higher mechanical efficiency, better stability and reliability, and a high cost-performance ratio.

Typical Machining Processes The typical machining processes for CNC turret punch presses mainly include blanking and forming.

Blanking Process Blanking, also known as material separation, refers to the process where the workpiece is sheared and separated along a closed or open contour under external force. Blanking can directly produce finished parts or prepare blanks for subsequent processes such as bending, drawing, and forming.

Generally, blanking processes are mainly divided into the following four modes:

⑴ Single punch. Punching is completed in a single pass, including straight-line, arc-shaped, circumferential, and grid-shaped holes.

⑵ Continuous punching in the same direction. Using a partially overlapping rectangular die, it can process elongated holes, trimmed edges, etc. When using this punching mode, the overlapping of dies during the punching process will cause the punching load to deviate from the die center (off-center loading), which will cause torsional deformation of the machine tool structure. In severe cases, it may damage the die. Therefore, when punching large-diameter, thick-plate, high-load holes, the overlap of dies should be minimized as much as possible to reduce off-center loading.

(3) Multi-directional continuous punching. This is a processing method that uses small dies to process large holes, which can increase the flexibility of the machine tool and expand the processing range of the die. Using this punching mode, it is usually necessary to rotate the punch at different angles through a rotary station, in conjunction with the positioning movement of the feeding mechanism in the XOY plane.

(4) Inching punching. This is a common processing mode that uses a small circular die to continuously punch arcs or spline curves with a small step distance. Because a small circle is used for fitting, the contour accuracy and cross-sectional quality are usually not high, and the punching efficiency is low. When the punching size is large and the number is large, it is recommended to use a special punching die.

Forming Processes

Forming processes refer to the processing methods in which sheet metal, under external force and stress exceeding the material's yield point, undergoes plastic deformation to obtain a specific shape and size. These mainly include rolling, shallow drawing, flanging, and bending, as well as the main forming characteristics and matching molds for some typical parts.

(1) Single-pass forming: A processing method where shallow drawing is performed in one step according to the mold shape. In this forming mode, the shape and size of the mold must correspond one-to-one with the feature being processed, identical to the processing mode of ordinary presses; it is a completely "rigid" processing mode.

(2) Continuous forming: Forming methods where the forming size is larger than the mold size, such as large-size louvers, rolling, and step forming.

The forming process of sheet metal is comprehensively affected by many factors, including material mechanical properties, sheet thickness, forming temperature, and forming speed. It involves many aspects such as elastic deformation, plastic deformation, work hardening, and material anisotropy, exhibiting a high degree of nonlinearity. Its forming mechanism is extremely complex and difficult to analyze using traditional mechanical formulas in actual production. In a computer virtual environment, the finite element method (FEM) is used to simulate the material forming process, enabling the prediction and evaluation of elastic rebound, cracks, and wrinkles during the forming process. This facilitates in-depth research into the forming mechanism. In industrial production, FEM analysis can guide mold design and optimize forming process parameters, possessing immeasurable practical application value. Currently, commonly used software includes ANSYS, ABAQUS, MAC, LS-DYNA, and DYNAFORM.

Future Development Trends

With the development of CNC technology, network technology, servo control technology, and high-torque servo motors, CNC turret punch presses are showing the following development trends:

⑴ High speed and high precision.

High speed and high precision are the eternal pursuit of the machine tool industry. Improving the speed and precision of CNC turret punch presses mainly includes the following aspects: 1) Improving the dynamic performance of the feeding mechanism. Currently, the feeding mechanism mainly uses two traditional mechanical transmission methods: ball screws and rack and pinion gears, and adopts semi-closed-loop control, resulting in generally low positioning speed, acceleration, and positioning accuracy. With the development and widespread application of linear motor drive technology in recent years, using linear motor drive technology and real-time motion position feedback through grating rulers to form closed-loop control, it is possible to improve the positioning speed, acceleration, and positioning accuracy of the feeding mechanism of CNC turret punch presses to an unprecedented level; 2) Adopting a fully electric servo main drive, on the one hand, its bottom dead center accuracy is high, which can be controlled within ±0.05mm; on the other hand, this transmission mechanism has rich stamping modes, such as using a high-speed crankshaft forward and reverse oscillation processing mode, the time required to complete a 4mm stroke can be controlled within 40ms; 3) Adopting new technologies to minimize downtime, such as automatic mold wear detection technology, automatic mold replacement technology, and automatic loading and unloading technology.

⑵ Real-time intelligence.

With the development of science and technology today, the market's pursuit of CNC turret punch presses is no longer limited to precision and reliability, but has begun to focus on intelligent solutions. Key technologies to be promoted: 1) Online workpiece quality monitoring technology; 2) Online mold wear monitoring technology; 3) Automatic mold selection and replacement technology; 4) Remote intelligent fault diagnosis and troubleshooting; 5) Establishment and optimization technology of process expert database.

(⑶) Energy saving and environmental protection.

CNC turret punch presses mainly involve the following three aspects: 1) Noise pollution, which can be addressed by adopting silent processing modes, optimizing machine tool structure, and using sound insulation materials for vibration and noise reduction; 2) Oil pollution, with all-electric servo main drive replacing the traditional hydraulic main drive as an inevitable trend in industry development; 3) Green design concept based on lifespan, employing advanced design concepts and methods to strive for the most rational material application, making the machine tool less bulky and cumbersome.

(⑷) Single-machine flexible machining unit (FMC) and functional integration.

Small single-machine flexible machining units, with CNC turret punch presses as the main body, have auxiliary functions such as deburring and tapping, and are equipped with automated warehouses and automatic loading and unloading systems, possessing extremely high processing flexibility and efficiency, and can even achieve unmanned operation. Compared to large-scale flexible manufacturing systems, it occupies less space, has lower equipment purchase costs, and offers better reliability and stability, making it more readily accepted by customers, especially suitable for small and medium-sized professional sheet metal processing enterprises. The processing unit features two symmetrically arranged CNC turret punch presses, equipped with an automated warehouse and automatic loading/unloading mechanisms, significantly improving automation. Only periodic monitoring by operators is required. However, achieving truly unmanned operation still requires breakthroughs in several key technologies, such as online mold wear monitoring technology, automatic mold selection and replacement technology, online workpiece quality monitoring technology, and intelligent fault diagnosis and troubleshooting technology.