CNC MILLING MACHINE FOR SALE

How to ensure the machining accuracy of CNC milling machine?

People engaged in the machining industry almost every day the machining accuracy of the machine is hanging in the mouth, every day to recite several times.  People in the same industry chat together also always mention the machining accuracy of CNC milling machine.  This shows the importance of CNC milling machine processing accuracy.  So what is the processing accuracy of machine tools to ensure it?

The machining accuracy of CNC milling machine ultimately depends on the accuracy of the machine tool itself to ensure that the accuracy of CNC milling machine includes positioning accuracy, geometric accuracy, cutting accuracy and repeated positioning accuracy.

The positioning accuracy of CNC milling machine can be understood as the movement accuracy of machine tools.  The position precision that each coordinate axis of machine tool can achieve under the control of numerical control device.  The smaller the difference between the actual position of the part or tool and the standard position, the higher the accuracy.  It is the premise that the machining accuracy of parts can be guaranteed.  Numerical control milling machine movement is realized by digital program instructions, so the positioning accuracy is determined by numerical control system and mechanical transmission error.  All the moving parts of machine tool movement is done under the control of the nc device, the moving parts in the program under the instruction control can achieve the precision of the direct reflection can achieve precision machining parts, according to the actual measure the positioning accuracy of numerical, can judge the automatic processing machine tool can reach the best workpiece machining accuracy.  Therefore, positioning accuracy is a very important detection content.

Geometric accuracy:  The geometric accuracy of CNC machine tool is to reflect the geometric error of key mechanical parts of CNC milling machine (such as bed, slide plate, column, spindle box, etc.) and the geometric error after assembly.  Including the flatness of workbench face, each coordinate direction moving perpendicular degrees, countertops X and Y coordinates on the direction of movement of parallelism, spindle hole shaft axial and radial circular runout, channeling, spindle box along the Z coordinate axis direction parallel move when the main axis, the main shaft in the Z axis coordinate direction straightness and the axis of spindle of the verticality of workbench face, etc.

Cutting accuracy: it is a comprehensive inspection of the geometric accuracy and positioning accuracy of CNC milling machine under the cutting conditions, including the inspection of the accuracy of single machining and the inspection of the accuracy of the sample processed (carbide cutting tool according to the standard cutting dosage).

Repeated positioning accuracy: refers to the numerical control milling machine in a direction to go back and forth a back and forth difference.  Is in the same CNC milling machine, the application of the same NC program different operation methods to process a batch of parts by the consistency of continuous results.

Therefore, we can draw a conclusion that the mechanical and electrical parts of CNC milling machine affect the accuracy of the machine.  CNC milling machine spindle accuracy, screw accuracy, machining fixture accuracy, as well as the rigidity of the machine itself will affect the accuracy of CNC milling machine processing.  CNC milling machine control mode, compensation mode, machining interpolation accuracy will also bring a very large impact on machine accuracy.  If you want to improve the accuracy should be considered from these two aspects and choose for their own use of CNC milling machine.

CNC milling machine by CNC device input instructions to the servo encoder, servo driver drive motor, drive the screw will be converted into a linear motion of rotary motion, CNC milling machine moving parts began to move.  According to the installation position of position detection device, it can be divided into full closed loop control, semi-closed loop control and open loop control.

Semi-closed-loop control is to install the position detection device at the end of the drive motor or the end of the screw, detect the displacement of the moving parts indirectly by detecting the rotation Angle of the ball screw of the servo mechanism, and then feed back to the comparator of the CNC control device.  The semi-closed loop monitors the driving link of the final execution of the whole system, and does not monitor the final execution mechanism.  With the improvement of CNC milling machine manufacturing level and the improvement of speed detection components and screw pitch accuracy, semi-closed loop CNC machine tools have been able to achieve quite high feed accuracy.  Semi – closed – loop numerical control system is widely used by most machine tool manufacturers.

In practical applications, because the measuring device is installed on the top of the motor or lead screw, it is easier to seal, so the machine environment requirements are low.  The precision error of the semi-closed loop control system mainly depends on the positive and negative clearance of the lead screw.  With the improvement of machining technology, the production and manufacturing process of lead screw has been greatly improved, and the positive and negative clearance has been basically eliminated with the cooperation of high-precision lead screw pair.  If the double row reverse ball screw pair is used in the assembly link, the positive and negative clearance can be completely eliminated.  In the assembly of our CNC milling machine, the influence of thermal deformation of CNC milling machine on the drive accuracy of the lead screw can be eliminated by using pre-stretching mode.  Therefore, the semi-closed loop control system has been able to ensure that the machine tool to achieve high accuracy, can meet the production and use requirements of most customers.

Full closed-loop control is to compare the measured value of the system output with the expected given value, thus generating a deviation signal, using the deviation signal to adjust the control, so that the output value as close as possible to the expected value.  The position detection device (such as grating ruler, linear induction synchronizer, etc.) is installed on the moving parts of the machine tool (such as the workbench), and real-time feedback on the position of the moving parts, closed-loop monitoring is the final implementation of the whole system, it can be said that any errors caused by the system can be compensated.  However, because the screw, nut pair and machine workbench are placed in the closed loop, the stable state debugging of the system is more troublesome.  Grating ruler, linear induction synchronizer and other measurement devices are higher in price, easy to increase the cost of the machine, and the installation is complex, there may be caused by CNC milling machine oscillation, so the general CNC milling machine does not use full closed-loop control.

In the actual use of CNC this bed, grating ruler, linear induction synchronizer and other position detection device accuracy level is different, CNC milling machine using detection device accuracy level of influence machine positioning accuracy is also high and low points.  The position checking device is generally non-metallic material, the thermal deformation is different from the metal parts on the CNC milling machine. If you want to ensure the accuracy of the CNC milling machine, you must solve the heating problem in the process of using the CNC milling machine.  Avoid deformation caused by high temperature.  We usually use rail lubrication, lead screw cooling, coolant and other cooling methods to reduce the machine deformation caused by high temperature in the process of CNC milling machine use.

We usually install the grating ruler on the outside of the guide rail or on the side close to the lead screw pair according to the structure of the CNC milling machine. Because the closer to the drive axis, the more accurate the measurement is, it is usually installed on one side of the lead screw pair, but the disadvantage of this installation is not conducive to the future detection and maintenance of the CNC milling machine. With this installation method, there is still a certain distance between the detection device and the lead screw pair. The combination of this distance and the swing during machine operation has a great impact on the detection of the grating ruler. When swinging to the installation side of the grating ruler, the detection result will be mistaken for insufficient moving speed, and the CNC system will transmit the acceleration signal. When swinging to the other side, it will be mistaken for too fast moving speed, and the CNC system will give the deceleration signal. Therefore, in the non-stop operation of the CNC milling machine, it is not only unable to improve the control of each coordinate axis, but also increases the vibration, Make users feel that semi closed-loop control has more advantages than closed-loop control.

If you want to ensure the accuracy of the CNC milling machine, in addition to the configuration of the machine before leaving the factory, the actual use environment of the CNC milling machine also has a great impact on the accuracy. The environment of many workshops is very poor, and there is often a lot of dust, which causes vibration when different machines are used and processed in the workshop. These detection devices belong to precision measuring instruments. Dust and vibration have a great impact on the accuracy of measurement. Therefore, in addition to the sealing work during the installation of the detection device, we should also improve the service environment of the CNC milling machine as much as possible. Otherwise, it is likely that the accuracy of CNC milling machine will be greatly reduced after using for a period of time. And cause the machine to keep alarming, affecting the use.

The precision of CNC milling machine can be improved by using full closed loop control if some external environmental effects are not considered.  But numerical control milling machine in actual use of heat, machine vibration.  The precision control effect of the full closed-loop control system is not as good as that of the semi-closed-loop control system.  CNC milling machine can not be bought because the machine is equipped with a grating ruler on the accuracy of the machine tool is relatively high.  This is because many manufacturers have made it easier to install full closed-loop control in order to save costs.  Sealing and temperature control processing is not in place but lead to CNC milling machine accuracy can not achieve the desired effect.