In-depth analysis of CNC machine tools: from principles to failures CNC machine tools: precision machining artifacts that listen to instructions
CNC, full name Computer Numerical Control. Simply put, a CNC machine tool is a precision equipment that can understand computer instructions and complete the processing of complex parts fully automatically, saying goodbye to the manual operation of traditional machine tools.
Compared with traditional manual machine tools, the gap between the two is huge. Old-fashioned manual machine tools rely on workers to manually shake the handle to control the tool position. Not only is the efficiency low, but the labor intensity is extremely high. Also, affected by the level of manual operation and feel, it is difficult to achieve exactly the same size for each product and the accuracy can’t be guaranteed at all.
CNC machine tools, on the other hand, are following an intelligent and standardized route. Before processing, engineers first use CAD software to draw a 3D model of the part, and then use CAM software to automatically generate a dedicated Cnc Processing program, which is often referred to as G code in the industry. In form, the G code is a series of simple characters, but in fact each sentence accurately commands the movement of the machine tool. For example, a sentence of "G01 X100 Y50 F200" means moving along a straight line to the coordinate point (100, 50) at a speed of 200 mm per minute.
Hard-core closed-loop control achieves micron-level precision
CNC machine tools can achieve ultimate precision. The core depends on the closed-loop control system. This set of "instruction-execution-feedback-correction" cycle mechanism allows the machine tool to maintain accuracy and stability at all times. The complete workflow is as follows:
- 1
The CNC controller takes the lead in reading and parsing G code instructions and fully understands the processing requirements;
- 2
The controller performs interpolation operations and breaks down complex straight lines and arc paths into tens of thousands of tiny pulse signals. Usually, each millimeter of length corresponds to thousands of pulses;
- 3
The pulse signal is transmitted to the servo driver, and the weak current signal is amplified into a strong current, driving the servo motor to rotate accurately;
- 4
The motor drives the tool or worktable through mechanical components such as ball screws and linear guide rails to achieve high-precision displacement;
- 5
Feedback sensors, that is, the encoder on the motor shaft and the grating ruler on the guide rail, collect the actual position and running speed in real time, and transmit the data back to the controller;
- 6
The controller and servo system will continuously compare the command position and the actual position with a very high control cycle. Once a slight deviation occurs, a correction command will be issued immediately to nip the error in the bud.
It is this strict closed-loop mechanism that gives CNC machine tools the performance advantage over traditional equipment, which is reflected in the following points:
Ultra-high precision
High-Precision Cnc machine tools can achieve micron-level processing accuracy under good working conditions, which is approximately equivalent to one-fiftieth of the diameter of a human hair;
Extremely repeatable
the same set of procedures can continuously process thousands of parts with almost identical dimensions;
Maximum efficiency
supports 24-hour unattended operation without manual supervision, greatly increasing production capacity;
Wide adaptability
It can process complex curved surfaces and special-shaped precision parts, covering all major high-end manufacturing fields.
Dismantling CNC machine tools: five core components, all of which are indispensable
CNC machine tools are not a single mechanical device, but a complete electromechanical integration system, which consists of five core components that work together. Without any one of them, high-precision processing can’t be achieved.
CNC controller: the “brain” of the machine tool
As the command center of the entire equipment, the CNC controller controls all core operations and instruction issuance. It is responsible for reading G codes, performing interpolation operations, generating motion pulses, monitoring equipment safety and processing PLC logic. It is equivalent to the "central nervous system" of the machine tool. Common brands on the market include FANUC, Siemens, Huazhong CNC, etc., which are also important signs for identifying the quality of machine tools.
Servo drive system: precise execution unit
This system consists of a servo drive and a servo motor, which is a key component for implementing the controller's instructions. The driver is responsible for amplifying weak current signals and providing sufficient power for the motor. The motor strictly follows the instructions and accurately controls the operating speed and position. It is the core execution component to achieve micron-level positioning.
Mechanical transmission system: motion actuator
This part is the "muscle" of the machine tool, responsible for converting the rotational motion of the motor into the actual processing motion of the tool and workbench. It mainly consists of three modules:
- Spindle system: drives the tool or workpiece to rotate at high speed, usually at 8000-30000 rpm, and is equipped with precision angular contact bearings to ensure the stability of high-speed operation;
- Ball screw: Efficiently converts motion modes, with transmission efficiency as high as over 95%, reducing power loss;
- Linear guide rails: ensure smooth and high-stiffness movement of the worktable and spindle in the X, Y and Z axes, eliminating shaking and deviation.
Feedback system: accuracy monitor
It consists of an encoder on the motor shaft and a grating ruler on the guide rail. It collects actual operating data in real time throughout the entire process and accurately feeds it back to the controller to form a closed-loop control, correct motion errors in a timely manner and maintain the bottom line of accuracy.
Auxiliary system: stability guarantee officer
Although the auxiliary system doesn't directly participate in processing, it is the backing for the long-term stable operation of the machine tool, including the coolant system, automatic lubrication system, automatic tool changer, hydraulic and pneumatic system, etc. The coolant takes away the high cutting temperature, the lubrication system reduces component friction, the automatic tool changer improves processing consistency, and the hydraulic and pneumatic systems complete actions such as workpiece clamping and tool tightening to protect the operation of the equipment in all aspects.
In-Depth Analysis: Common CNC Machine Tool Failures and Their Mechanisms
CNC machine tools have precise structures and operate at high speed and heavy loads for a long time, so various types of failures are inevitable. In essence, they are all chain reactions of physical processes, mainly stemming from friction, heat, vibration, electrical, control and other issues. The following is an in-depth breakdown of the causes and consequences of various types of failures by classifying them into root causes.
Friction, wear and lubrication failures: the most common mechanical failures
When mechanical parts operate for a long time, friction and wear are unavoidable problems. Insufficient lubrication will accelerate the occurrence of failures.
- Spindle bearing fatigue spalling: When the spindle rotates at high speed, the bearing rollers and raceways are subjected to cyclic stress. After the lubricating oil film ruptures, micro-cracks appear on the surface and gradually expand into pitting corrosion and spalling, which can cause violent vibrations, temperature surgesand even spindle lock-up;
- Bearing lubrication failure: the lubricating oil evaporates at high temperature, is washed away by the coolant, the oil film disappears, and the metal directly rubs dry, the bearing will quickly burn and become stuck, forcing processing to be interrupted;
- Ball screw wear: Insufficient lubrication and chip intrusion will cause abrasive wear and fatigue spalling of the screw, resulting in an increase in backlash and a permanent decrease in positioning accuracy;
- Guide rail crawling: Chips enter the guide rail, the oil film is missing, scratches appear on the slider, the friction coefficient increases, and the motion is stuck, which directly increases the dimensional error and reduces the surface quality of the workpiece;
- Tool damage: Cutting heat and mechanical impact will cause the tool to wear, chip or even break, resulting in increased cutting force and rough workpiece surface. In serious cases, the workpiece will be scrapped directly.
Thermal deformation failure: the culprit of accuracy drift
The heat generated during processing will cause thermal expansion and contraction of machine tool components, causing accuracy drift. This is the main reason for unstable long-term operation accuracy of machine tools.
- Spindle thermal drift: Bearing friction and motor heating cause the spindle to expand axially and radially, causing the tool tip position to shift and the processing dimensions to gradually exceed the standard;
- Thermal expansion of the screw: heat accumulates in the middle of the screw, the length elongates, generating thermal stress, and linear deviations occur in the positioning coordinates;
- Thermal deformation of the bed: the temperature of various parts of the machine tool is uneven, the bed is bent and twisted, the straightness of the guide rails decreases, and multi-axis linkage errors are amplified;
- Cooling system failure: The coolant circulation fails, the heat in the cutting area cannot be discharged, the temperature is out of control, and the thermal deformation is further aggravated.
Vibration failure: the killer of processing quality
- Spindle dynamic balance imbalance: Tool eccentricity and bearing wear cause the center of mass of the spindle to deviate. High-speed rotation generates centrifugal force, causing periodic vibration and magnifying bearing losses;
- Regenerative chatter: Fluctuations in cutting force cause the tool to vibrate, resulting in uneven cutting thickness with the next cut, forming self-excited vibration, ripples appearing on the surface of the workpiece, and the tool easily chipping;
- Structural resonance: Loose bolts, material fatigue, equipment stiffness reduction, natural frequency reduction, and cutting force frequency coupling, triggering resonance and damaging the machine tool structure;
- Loose foundation: foundation settlement, insufficient bolt pre-tightening, machine bed distortion, and permanent deterioration in geometric accuracy.
Control and auxiliary system failure
- Control anomalies: parameter errors, CPU overload, communication packet loss, leading to deviations in motion trajectories and disordered device movements;
- Tool change failure: hydraulic leakage, solenoid valve stuck, and inaccurate positioning of the tool arm, causing tool change collision and jamming;
- Clamping failure: leakage of hydraulic pressure, unstable workpiece positioning, and deviation in CNC machining processing;
- Solenoid valve failure: oil rust, coil burnt, valve core can’t be switched, air hydraulic system failure.
Electrical and feedback faults
- Servo driver is damaged: mechanical resistance increases suddenly, current exceeds the standard, power module thermal breakdown, driver burns out, machine tool axis stops moving;
- Encoder failure: oil pollution and impact lead to signal loss and distortion, position feedback failure, and closed-loop control loss of control;
- Signal interference: Improper wiring arrangement and poor grounding produce electromagnetic noise and increase position and speed calculation errors;
- Unstable power supply: power grid fluctuations, abnormal grounding, unstable drive voltage, fluctuations in torque output, and deterioration in machining accuracy.
