CNC machining precision machining ultimately rely on the accuracy of the machine itself to ensure the accuracy of CNC machine tools, including geometric accuracy, positioning accuracy, repeat positioning accuracy and cutting accuracy.

Geometric accuracy: also known as static accuracy, is a comprehensive reflection of the key components of CNC machine tools after the assembly of the integrated geometry error.

Positioning accuracy: indicates that the measured machine tool movement of all parts under the control of CNC devices, the movement can achieve the accuracy. According to the measured value of the positioning accuracy, you can determine the best automatic machining process can achieve the best workpiece accuracy. Is the difference between the actual position of the part or tool and the standard position (theoretical position, ideal position), the smaller the gap, the higher the accuracy. Is the premise of parts machining accuracy to be guaranteed.

Repeatability: Refers to the consistent degree of position accuracy obtained by repeatedly running the same program code on a numerical control machine tool. Is in the same conditions (the same CNC machine tools, different methods of operation, the application of the same part program) to process a batch of parts obtained by the consistency of the results of a row.

Cutting accuracy: It is a comprehensive inspection of the geometric accuracy and positioning accuracy of the machine under the condition of cutting.

Seen from the above, the level of numerical control machine tool sub-mechanical and electrical points, mechanical aspects such as spindle accuracy, such as beating, bus, etc .; screw precision; processing fixture accuracy, machine rigidity and so on. The electrical aspects are mainly control methods such as semi-closed loop, closed loop, etc., there are feedback and compensation methods, machining interpolation accuracy. Therefore, the level of machine precision does not depend on whether the machine is fully closed loop.

First, the principle of introduction

CNC machine tool kinematic chain includes CNC device → servo encoder → servo driver → motor → lead screw → moving parts. According to different installation position of position detection device, it is divided into full-closed control, semi-closed-loop control and open-loop control.

 1, fully closed loop control feed servo system

The position detection device (such as grating ruler, linear induction synchronizer, etc.) is installed on the moving parts of the machine tool (such as workbench), and real-time feedback on the position of the moving parts, CNC machine processing to inform the state of the machine servo motor, servo The motor automatically compensates for the movement error through the system command. However, due to its high inertia of screw, nut pair and machine tool table, it is troublesome to debug the system when debugging. In addition, such as grating ruler, linear induction synchronizer such high cost of the device, the installation is complex, it may cause oscillation, so the general machine does not use full closed-loop control.

2, semi-closed loop control feed servo system

The position detection device is installed at the end of the drive motor or the end of the screw rod, used to detect the rotation angle of the screw or servo motor, indirect measurement of the actual position of the moving parts of the machine, back to the control system by the feedback. Due to the improvement of the level of machinery manufacturing and the improvement of the thread pitch accuracy of the speed detecting element and the screw rod, the semi-closed-loop CNC machine tool has been able to achieve a relatively high feed precision. Most machine tool manufacturers widely used semi-closed-loop CNC system.

Second, the practical application

1, the whole closed-loop control system

There are different accuracy levels (± 0.01mm, ± 0.005mm, ± 0.003mm, ± 0.02mm) for position detection devices (such as grating ruler, linear induction synchronizer, etc.), so there will be errors in full closed loop control, Level effect.

Position detection device thermal performance (thermal deformation), the measuring device is generally non-metallic materials, thermal expansion coefficient and machine parts are not consistent, it is the key to the accuracy of the machine tool, so must solve the machine tool processing heat problems in order to overcome due to Thermal deformation caused by temperature. High-end machine tools will use a variety of ways, such as screw hollow cooling, rail lubrication, cutting fluid thermostat cooling, etc. to reduce the thermal deformation of the machine during processing.

Position detection device installation is also very important, in theory, the closer to the drive axis (screw pair), the more accurate measurement. Due to the limitation of the structure space, there are only two ways to install the grating. One is to install on the near side of the screw and the other is to install on the outside of the guide rail. Recommended as far as possible to choose the first installation, but overhaul and maintenance inconvenient. On the contrary, the choice of high-precision grating ruler, but did not actually meet the required precision CNC machine tools. Even in the first case, the grating ruler is installed closer to the drive axis, but the mounting position is, after all, somewhat distanced from the drive axis. This, combined with the oscillation of the object when driven, brings a great control over the detection and control of the scale Big trouble. When the driving object to the mounting side of the grating ruler, the grating ruler in the detection of mistakenly believe that the lack of speed, the system gives the acceleration signal, and the driving object to the other side of the swing, the grating ruler in the test and mistaken for moving speed too Fast, the system gives a deceleration signal, so repeatedly running, actually did not improve the control of the linear coordinate axis of the CNC machine, but aggravate the vibration of the driven object, resulting in a full closed-loop semi-closed loop strange phenomenon.

 Production environment impact: General machining factory environment is harsh, dust, vibration is a common phenomenon, but the grating ruler, linear induction synchronizer is a precision components, the working principle is to rely on the light reflection to measure the relative movement position, dust, vibration is precisely The biggest factor affecting the measurement accuracy. In addition, the machine tool in the processing, cutting oil mist, water mist is more serious on the grating ruler, linear induction synchronizer is very large. So to use the full closed-loop control system, in addition to doing a good job of installing the seal, we must increase the production environment. Otherwise, this phenomenon will appear, just arrived new machine precision is good, but with less than a year, not only decreased accuracy, the machine also often alarm.

2, semi-closed-loop control system

As the measuring device is mounted on the top of the motor or screw, relatively easy to seal, so no environmental requirements. Semi-closed-loop control system accuracy error depends mainly on the positive and negative screw clearance. With the improvement of mechanical processing technology, the current level of manufacturing technology imported screw higher, with high precision lead screw basically eliminate the positive and negative clearance. In addition, the assembly process, screw pair with double row reverse ball screw pair, can completely eliminate the positive and negative clearance. In addition, many machine tool manufacturers, in the machine tool assembly, the use of pre-stretch screw way, eliminating the thermal deformation of the screw drive accuracy. Therefore, the semi-closed loop control system has been able to ensure the machine to achieve high accuracy.

Third, the conclusion

In summary, it can be seen that, in theory, full-closed-loop control may improve basic positioning accuracy than semi-closed-loop control without considering external factors. However, if you can not solve the machine tool fever, environmental pollution, temperature rise, vibration, installation and other factors, there will be more closed-loop semi-closed loop phenomenon. A short time may be effective, but over time, dust, temperature changes on the grating ruler, will seriously affect the measurement feedback data, thus losing the function. At the same time grating ruler problems, will have an alarm, resulting in the machine can not work.

Low-end machine tools, due to consider the cost of production and competitiveness in the entire closed-loop control of the package have been simplified, such as sealing, temperature control and so there is no good protection. In this condition, the cost of a large, simple configuration grating and can not improve the accuracy of machine tools.