Abstract : This paper analyzes the structure of a newly designed CNC spindle component, combining theory and practice to explain the characteristics of this new spindle structure from aspects such as the rational application of bearings, assembly angles, and thermal expansion and contraction. It elucidates the structure and principles behind the high precision, high speed, convenient maintenance and repair, long service life, and good waterproof performance of this spindle component.
Introduction : With the rapid development of science and technology, the requirements for machining precision in mechanical equipment are becoming increasingly stringent, leading to higher precision and mass production of machined parts. CNC machine tools are widely used in fields such as electronics, automobiles, medical devices, national defense, and aerospace due to their advantages in high efficiency, high precision, and increased productivity.
From its initial understanding of CNC machine tools in the 1980s to the current massive demand for CNC machine tools across various machining industries, my country is arguably the world's largest manufacturer and user of CNC machine tools. The demand for medium and small-sized CNC machine tools is particularly high, and the number of companies producing them is increasing dramatically. Achieving CNC-based and flexible machining tools and production processes has become a major direction for the development of the machining industry. However, some domestically produced CNC machine tools today suffer from high failure rates and poor machining accuracy, failing to meet the requirements for machining high-precision mechanical parts. This results in a situation where many domestically produced machine tools are unsold and stockpiled, while many high-precision CNC machine tools still rely on imports. This phenomenon seriously affects and restricts the independent development of CNC machine tools and the improvement and development of CNC technology in my country.
Currently, apart from well-known international brands such as FANUC, SIEMENS, and Mitsubishi, the CNC machine tool control systems independently developed by Chinese companies like Guangzhou GSK and Beijing KND are generally stable and reliable, with fast processing speeds and relatively high control precision. These systems have gained widespread recognition and trust from users. The main reasons for the low machining precision, poor stability, and high failure rate of some CNC machine tools, especially those produced by smaller companies, are unreasonable overall and local structural designs, inferior machining precision of machine tool components, and factors such as assembly processes and heat treatment. Undeniably, there is a significant gap compared to similar machine tools from some developed countries.
CNC lathes constitute the majority of CNC machine tools. Small and medium-sized CNC lathes with a turning diameter of φ360 or less are the most numerous. These types of CNC lathes mostly use frequency converters to control variable frequency motors for stepless speed regulation, directly driving the lathe spindle for turning operations. To improve lathe machining accuracy, in addition to improving the accuracy of components such as the bed, saddle, and ball screw, the lathe spindle is a key component for improving machining accuracy and service life. After more than ten years of analysis and research on the spindle structure of small and medium-sized CNC lathes, a relatively ideal spindle structure has been designed. This spindle structure features high speed, high accuracy, long service life, simple assembly, convenient maintenance and repair, and excellent waterproof performance.
The structure of this spindle component will be analyzed below, and its characteristics will be explained.
Spindle structure diagram:
I. Spindle Structure Analysis
Figure a shows the optimized spindle structure: near the front end of the spindle, a pair of tandem angular contact bearings (5) plus a single angular contact bearing (8) are used to form an angular contact bearing pair. The single angular contact bearing is locked by a lock nut (11). Near the rear end of the spindle, a pair of back-to-back angular contact bearings are used. The bearings are locked by a lock nut (16) through a bearing pad (15). This five-bearing support structure, with a pair of tandem angular contact bearings at the front end and an additional single angular contact bearing, has the advantages of high speed, high precision, and high rigidity.
II. Main Features of Spindle Structure
1. Two water-drop grooves are designed at the front end of the spindle (1). A water-blocking groove is machined in the inner hole of the front flange (3), and a water outlet is opened at the bottom of the flange. Once the coolant enters from the gap between the spindle and the front flange, due to the high-speed rotation of the spindle, the coolant is thrown into the water-blocking groove of the front flange under the action of centrifugal force and discharged from the bottom drain hole. Another water-drop groove is designed on the bearing pad (2), achieving the purpose of secondary waterproofing. The rear bearing part is also designed with the same waterproof structure as the front end. Its waterproof structure has a very good effect, ensuring that the coolant cannot enter the bearing and cause bearing damage. It extends the service life of the spindle bearing.
2. The spindle can be installed as a single component (except for the pulley (18)). First, install the entire bearing and bearing pad. Adjust the bearing clearance and then install the whole assembly into the spindle hole of the headstock box. Tighten the front flange screws. There are inner and outer ring adjusting pads (6) and (7) between the front-end series bearings. The bearing clearance of bearings (5) and (8) can be adjusted in advance on the tooling by grinding the inner and outer ring adjusting pads. When the bearing is installed on the spindle, the front and rear locking nuts can be tightened to the maximum, so as not to damage the spindle bearing due to the locking nuts being too tight. This eliminates the problem in the ordinary spindle structure (as shown in Figure b), where the front and rear bearing clearance of the spindle is directly adjusted by adjusting the locking nut (20) at the tail of the spindle, and the bearing is adjusted too loosely or too tightly due to the lack of experience of the assembly workers and the difficulty in controlling the tightening force, which affects the accuracy of the spindle and damages the bearing. When the bearing is used for a certain period of time, bearing wear occurs and the bearing clearance increases. The bearing clearance can also be readjusted by grinding the thickness of the bearing inner pad (6) to restore the spindle rotation accuracy.
3. The inner hole of the rear bearing of the head box (4) is a straight hole without steps. When the main shaft is running and the temperature rises, the length of the main shaft (1) changes due to thermal expansion and contraction. The outer ring of the rear bearing moves slightly in the main shaft hole of the head box to ensure the rotation accuracy of the main shaft and improve the service life of the main shaft bearing.
4. The main shaft assembly is easy to disassemble. When it is necessary to disassemble the main shaft assembly for maintenance and repair, first remove the main shaft pulley (18), then remove the fastening screws on the front flange (3), and then push the main shaft and all main shaft bearings out of the headstock as a whole from the rear of the main shaft. The maintenance operation is carried out outside the headstock.
5. Because the inner ring diameter of the bearing is small, it is blocked by the spindle shoulder, making it very difficult to disassemble the bearing. The spindle bearing uses a high-precision bearing. If the outer ring of the bearing comes off during disassembly, the bearing will be scrapped. In order to make it easy to remove the inner ring of the bearing from the spindle (1) without causing the outer ring of the bearing to come off and damage the bearing, a special bearing removal pad (2) (12) was designed. In this way, the bearing can be easily removed from the spindle by using the special bearing removal pad.
In conclusion , the spindle plays a crucial role in CNC machine tools, influencing their performance, precision, and lifespan. For CNC machine tool manufacturers, the ability to create high-quality branded machine tools is key to gaining widespread user recognition and acceptance. In today's increasingly competitive CNC machine tool industry, product quality is a decisive factor in a company's survival and development. For CNC machine tool users, many entrepreneurs aspire to use high-quality CNC machine tools to generate greater profits. For the nation, the development and improvement of CNC machine tool technology can help break free from the technological blockade imposed by some developed countries in the field. It promotes the development of China's machinery industry, national defense, aerospace, and science and technology, laying the foundation for the realization of the Four Modernizations. Let us work together to advance and improve CNC technology.
