Preventive maintenance for CNC machine tools
Proper use and preventative maintenance of CNC machine tools are crucial. Effective and appropriate preventative maintenance can significantly extend the service life of CNC machine tools, stabilize machine tool accuracy, and reduce downtime due to malfunctions.
The service life cycle of preventive maintenance equipment for CNC machine tools is divided into the break-in period for new equipment, the normal use period, and the condition-based maintenance period. The focus of preventive maintenance differs at each stage. Although different manufacturers, brands, and whether CNC machine tools are imported or domestic, and their performance, quality, and functions vary, their preventive maintenance strategies and methods are basically similar. The differences lie only in the frequency of certain preventive maintenance items and in conducting targeted repairs (referred to as item repairs) based on the specific characteristics of the machine tool.
I. Preventive maintenance of equipment in the early stages
Preventive maintenance (data collection, necessary preparation) in the early stages of equipment installation and commissioning is applicable to newly installed and commissioned equipment, but it can also be applied to equipment that has undergone major repairs or purchased second-hand equipment.
After installation and commissioning, the following preventative maintenance work needs to be performed:
NO.1
Random drawings, manuals, machine tool accuracy test reports, commissioning and acceptance reports, and other materials, as well as parameter settings, programs, tool compensation and other data, are collected and included in both paper and electronic document formats.
NO.2
Thoroughly inspect, record, and save the original typical state data of the machine tool (such as the basic data of the tool magazine, tool changer, rotary table in the specified position and program, position, coordinate system, tool compensation, etc. when the machine is in the zero state at startup and in the automatic machining state).
Abnormal conditions in CNC machine tools manifest in two main ways: alarms and accuracy anomalies. Alarms can be categorized into operational error alarms and fault alarms. Operational error alarms can be easily eliminated by comparing alarm numbers within a range. For fault alarms, basic data comparison allows for faster identification of the fault location and severity, determining whether it is a mechanical or electrical fault. For machining accuracy issues, basic data comparison helps to quickly determine whether the problem stems from machine tool accuracy issues or problems with cutting tools, fixtures, materials, heat treatment, etc.
The collected data should record self-diagnostic data under no-load and certain working conditions in the specified operating mode. This includes I/O input/output status data; the load current percentage of each servo drive axis and each additional CNC axis (X, Y, Z, spindle, tool post or MG tool magazine, ATC tool changer, APC rotary table, etc.) under the servo diagnostic I/O panel; the temperature and setting of the spindle cooling thermostatic oil tank; the temperature inside the electrical control box; and the set values of oil and air pressure at various locations.
For numerous screen data points, taking photos can be a quick way to collect data. For precision CNC machine tools and large CNC machine tools, such as precision boring centers and CNC grinding machines, vibration diagnostic measuring instruments are needed to measure the vibration status data of the entire machine, spindle, or key locations. This data is essential benchmark data for future preventive maintenance of CNC machine tools, condition detection analysis and comparison, and judging the trend of machine tool condition changes. It is also an important reference data for maintenance and adjustment.
NO.3
The first oil change and oil tank cleaning 3-6 months after a new machine tool is installed is very important. This includes the hydraulic oil tank, lubricating oil tank, spindle cooling oil tank, and rotary table reduction oil tank. The purpose is to solve the problem of oil contamination caused by the large amount of wear in the early stage of machine tool installation.
NO.4
Compile equipment operating procedures and inspection cards. The operating procedures should concisely list commonly used operating methods and common fault reset methods; the inspection cards should concisely and intuitively list inspection points, methods, standards, and diagrams.
NO.5
CNC machine tool technical support contingency plan preparation: personnel training, data backup methods and means, spare parts planning, fault cause analysis and troubleshooting methods.
Data backup involves reading CNC system data, including system parameters, PMC parameters, tool compensation data, macro parameters, machining programs, electrical ladder diagrams, etc., using a CF card, disk drive, PC, or any other method, and making electronic backups and printing them for archiving.
When backing up data, data security and operational strategies must be considered first. Spare parts planning should involve a preliminary technical preparation, followed by phased purchases based on factors such as importance, estimated usage time, supply cycle and channels, and domestic alternatives, while monitoring equipment status. For non-mainstream CNC systems, the potential for software and hardware upgrades and obsolescence of older versions several years in the future should be fully considered, and spare parts should be strategically purchased in advance.
Preview fault alarm numbers and troubleshooting methods, and establish contact channels with suppliers.
II. Preventive maintenance during normal equipment use
Once equipment is put into normal use, the focus of preventative maintenance is on upkeep and safety. This requires thorough preventative maintenance in the early stages to fully leverage the combined advantages of CNC machine tools in terms of processing quality and efficiency.
Effective and long-term maintenance
It can significantly extend the service life of CNC machine tools and maintain stable accuracy.
Maintenance can be done daily, periodically, or in different sizes and levels, but the requirements are the same: cleaning, lubrication, adjustment, and safety must be ensured.
Daily maintenance mainly involves cleaning the interior and exterior, while also checking the oil level and quality. Add oil, check safety devices, and make simple adjustments.
Periodic maintenance requires disassembling relevant protective covers for thorough cleaning and lubrication of bearings, ball screws, linear guides, slides, and other electrical components. The interior of the electrical box should also be cleaned and tidied. All lubrication points should be checked and cleared, and filters cleaned. For grease-lubricated points, a grease gun should be used for injection. Targeted preventative maintenance should be carried out, including changing hydraulic oil and calibrating oil and gas pressure. Safety interlock devices should be checked for completeness and interlocking functionality.
Other aspects of maintenance
The temperature inside the electrical control box must not exceed 55℃. In workshops in southern regions without air conditioning, air conditioning units should be installed inside the electrical control boxes of CNC machine tools. CNC machine tools that are not used for a long time should be regularly powered on for warm-up and automatic lubrication to prevent bearings, lead screws, slides, and other mechanical parts from rusting. In rainy weather, attention should be paid to the possibility of condensation on the electrical control boxes and circuit boards due to high humidity, which can cause malfunctions, especially for machine tools located near workshop doors and windows.
The safety of CNC machine tools involves the quality of machine tool design and manufacturing, and whether various safety protection and interlocking devices are well-designed. These factors are determined when the equipment leaves the factory. Neglecting safety can lead to accidents ranging from minor collisions that damage workpieces, cutting tools, and machine tool components, affecting machine tool accuracy, to serious incidents that endanger personal safety.
Security in use and maintenance includes data security and operational security:
1
Data security
In addition to backing up all data before the equipment is installed, it is essential to regularly replace the backup battery for data storage after the machine is shut down. Failure to replace the battery on time, even if it alarms, can lead to the loss of the CNC machine's absolute zero point, or even complete loss of program and parameter settings, rendering the machine unusable. For alkaline batteries, replacement annually is necessary; use reputable brand-name, high-performance iron-cased batteries to prevent leakage and corrosion of the system control board. For lithium batteries, replacement every 30 years is generally recommended.
Note: The battery can only be replaced when the machine tool control system is powered on and the machine tool emergency stop switch is pressed. It is particularly important to note that if the battery is connected incorrectly or has poor contact, all stored data will be lost upon shutdown. Battery lifespan is related to system configuration and is inversely proportional to the length of time the machine tool is powered off and the ambient temperature; that is, the longer the shutdown time and the higher the ambient temperature, the shorter the battery life. Additionally, the battery's expiration date must be noted.
2
Set security
The essence of modern CNC is computer numerical control, therefore parameters must be specified and modification permissions must be defined. CNC parameter setting involves selecting various CNC control modes and function switches, and adjusting control quantities. Therefore, modifications must be made cautiously after fully understanding the meaning and function of the parameters. Incorrect parameter settings can damage the machine tool or workpiece and cause personal injury. Proper parameter selection and setting ensure the machine tool operates normally and fully utilizes its performance, and to a certain extent, supports the safe use of CNC machine tools, reducing the impact of human error.
CNC machine tool parameters are divided into ordinary parameters (user parameters) and encrypted parameters (system function parameters). Users can only modify ordinary parameters. Commonly used parameters that need to be mastered include machine tool zero point, spindle orientation, tool magazine, tool changer, and rotary table.
The parameters of the equipment at the factory represent its maximum performance settings. After purchase, some parameter settings need to be optimized based on the characteristics and materials of the parts being processed, the tooling, fixtures, and operating environment. The main parameters that need modification include: narrowing the positive and negative soft limit dimensions of the machine tool's travel stroke; the maximum spindle speed; the maximum rapid traverse speed; and changing the ATC tool change from fast to slow. This avoids or reduces operational errors, interference, and collisions, reduces wear, and contributes to the safe, long-term, stable, and accurate use of the CNC machine tool.
3
Programming safety
At the beginning of the program, instructions such as limiting the maximum speed of the multi-axis, limiting the constant linear speed of end face cutting, automatically returning to the reference origin, establishing the coordinate system, limiting the storage-type travel limit of the machining area, and checking for interference prevention during tool change need to be included.
III. Preventive Maintenance During the Condition-Based Maintenance Period
During the condition-based maintenance period, preventative maintenance (condition monitoring, phased repairs) is crucial. Under normal two-shift production, CNC machine tools enter the condition-based maintenance period after 5-6 years, while equipment with good preventative maintenance may enter this period after 8-10 years or even later. During this period, the equipment is characterized by decreased accuracy and increased malfunctions. Symptoms include frequent tool changer or ATC malfunctions, APC rotary table malfunctions, and servo drive alarm malfunctions. However, the vast majority of these issues are actually caused by mechanical failures and wear. To reduce maintenance costs, targeted repairs are necessary.
NO.1
The majority of servo drive alarms, which account for more than 75% of fault alarms, are caused by increased mechanical load due to wear or poor lubrication of bearings, ball screws, or guide rails, which in turn causes increased servo drive current and overload alarms. Over time, this can lead to damage to the servo driver or servo motor.
Therefore, it is essential to regularly check the servo drive current and load percentage on the CNC machine tool's diagnostic screen, analyze and record the changing trends, and make comprehensive judgments to perform timely repairs, such as replacing or maintaining bearings, lead screws, or guide rails, and adjusting the tightness of the slide block. This can effectively reduce servo alarms and damage.
NO.2
Another reason that can easily cause servo drives to age and be damaged is the heat dissipation problem of the electrical box. Therefore, it is necessary to regularly check and clean the electrical box cooler and fan, such as if an air conditioner is provided.
In addition to the maintenance mentioned above, it is also necessary to check the cooling effect and determine whether there is a refrigerant leak.
