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Requirements of CNC machine tools for spindle drive system

Posted by: Chris Na 2021-05-26 Comments Off on Requirements of CNC machine tools for spindle drive system

The main shaft drive system is also called the main drive system, which is the part of the power unit that completes the main movement in the system. The spindle drive system is transformed into the cutting torque and cutting speed of the tool or workpiece installed on the spindle through the transmission mechanism, and cooperates with the feed movement to process ideal parts. It is one of the forming motions of parts processing, and its accuracy has a great influence on the processing accuracy of parts.

The spindle drive system of a cnc machine tool is also the main drive system. Its performance directly determines the surface quality of the processed workpiece. Therefore, the spindle drive system is very important in the repair and maintenance of the cnc machine tool.

There is a big difference between the spindle drive and the feed drive of the machine tool. The working motion of the machine tool spindle is usually a rotary motion, unlike the feed drive that requires a lead screw or other linear motion device for reciprocating motion. CNC machine tools usually realize the rapid relative cutting movement of the tool and the workpiece through the rotation of the main shaft and the feed of the feed axis. In the 1960s and 1970s, the spindle of CNC machine tools generally used a three-phase induction motor coupled with a multi-stage gear box to achieve a step-by-step speed change drive. With the continuous development of tool technology, production technology, processing technology and production efficiency, the above-mentioned traditional spindle drive can no longer meet the needs of production. Modern CNC machine tools put forward higher requirements for spindle transmission:

(1) Wide range of speed regulation and stepless speed regulation

In order to ensure that the appropriate cutting amount is selected during processing, to obtain the best productivity, processing accuracy and surface quality. Especially for Cnc Machining centers with automatic tool change function, in order to meet the processing requirements of various tools, procedures and various materials, the speed range of the spindle is required to be higher, and the spindle is required to be able to operate in a wide range of speed according to the CNC system The command automatically realizes stepless speed regulation, reduces intermediate transmission links, and simplifies the spindle box.

At present, the constant torque speed adjustment range of the spindle drive device can reach 1:100, and the constant power speed adjustment range can also reach 1:30. Generally, it can work continuously for 30 minutes when the overload is 1.5 times.

The spindle speed change is divided into three forms: stepwise speed change, stepless speed change and segmented stepless speed change. The stepped speed change is only used for economical CNC machine tools. Most CNC machine tools adopt stepless speed change or stepless speed change. In the stepless speed change, the frequency conversion speed control spindle is generally used for popular CNC machine tools, and the AC servo spindle is used for medium and high-end CNC machine tools.

(2) The constant power range should be wide

The spindle can provide the power required for cutting in the full speed range, and provide the maximum power of the spindle motor as much as possible in the full speed range. Due to the limitation of the spindle motor and the driving device, the spindle has a constant torque output in the low speed section. In order to meet the needs of low-speed and powerful cutting of CNC machine tools, the stepless variable speed method is often used (that is, the mechanical deceleration device is used in the low-speed section) to expand the output torque.

(3) With 4 quadrant drive capability

It is required that the spindle can perform automatic acceleration and deceleration control when it rotates in the forward and reverse directions, and the acceleration and deceleration time should be short. At present, the general servo spindle can accelerate from standstill to 6000r/min within 1 second.

(4) Possess position control ability

That is, the feed function (C axis function) and the orientation function (exact stop function) to meet the needs of certain machining processes of the machining center for automatic tool change, rigid tapping, thread cutting, and turning centers.

(5) It has high precision and rigidity, stable transmission and low noise.

The improvement of the machining accuracy of CNC machine tools is closely related to the accuracy of the spindle system. In order to improve the manufacturing precision and rigidity of the transmission parts, the gear tooth surface should adopt high-frequency induction heating and quenching process when adopting gear transmission to increase the wear resistance. The last stage is generally driven by helical gears to make the transmission stable. Toothed belts should be used when belt transmission is used. High-precision bearings and reasonable support spans should be used to improve the rigidity of the spindle components. If the structure permits, the width of the gear should be appropriately increased to increase the overlap coefficient of the gear. Variable speed sliding gears are generally driven by splines and centered by inner diameters. The side-centered spline is more beneficial to reduce noise, because this centering method has a small transmission gap and a large contact surface, but the processing requires a special tool and a spline grinder.

(6) Good vibration resistance and thermal stability.

When CNC machine tools are processed, impact and alternating forces may be caused by continuous cutting, uneven machining allowance, unbalanced moving parts, and natural vibration during cutting, which will cause the spindle to vibrate and affect the machining accuracy and surface roughness. In severe cases, it may even damage the tool and parts in the spindle system, making it impossible to work. The heating of the spindle system causes thermal deformation of the components, which reduces the transmission efficiency, affects the relative position accuracy and movement accuracy between the components, and causes machining errors. Therefore, the main shaft assembly must have a higher natural frequency, better dynamic balance, and maintain a proper fit clearance, and cycle lubrication.

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