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  As a position detection component of the linear axis of CNC machine tools, the grating ruler acts like the "eyes" of a human being. Its job is to "monitor" whether the linear axis moves accurately to the position required by the CNC system after executing a movement command. Without a grating ruler, the accuracy of whether the linear axis can reach the position required by the CNC system after receiving the movement command relies entirely on the accuracy of the CNC system's debugging and mechanical transmission.

 

  After the CNC machine tool has been used for a period of time, due to the modification of electrical debugging parameters and the increase in mechanical errors, the linear axis may be far away from the position required by the CNC system instruction. At this time, the CNC system and the personnel who maintain and operate the machine tool are unaware of the difference. To find out this gap, maintenance personnel need to carry out precision testing on the machine tool. Therefore, if the CNC machine tool does not have a grating ruler installed, it is necessary to regularly check the accuracy of the machine tool. Once the precision of the CNC machine tool is forgotten to be checked, it may lead to the product's accuracy exceeding the tolerance or even being scrapped.

 

  If the linear axis of the CNC machine tool is equipped with a grating ruler, the above-mentioned problem no longer needs to be worried about by personnel. The grating ruler completes this mission. If the linear axis fails to reach the position accurately due to mechanical reasons, the grating ruler, as a position detection component, will send feedback instructions to the CNC system, enabling the linear axis to reach a relatively accurate position. At this time, the grating ruler acts as a supervisory function independent of the machine tool, like human eyes, continuously "monitoring" the position of the linear axis to ensure that the linear axis reaches the position required by the CNC system.

 

  The purpose of using a grating ruler in the production of new machine tools or the renovation of old machine tools is to increase the accuracy of the linear axis's operation. However, the operation accuracy does not depend entirely on the grating ruler but mainly on the geometric accuracy of the mechanical part of the linear axis. The grating ruler cannot replace the accuracy of the machine. It can only "add icing on the cake." Many people have misconceptions about this issue. If the geometric accuracy of the linear axis of the machine tool is poor, such as some lathe Z-axis using a rack for transmission, there will be a large reverse clearance. Even if the grating ruler is used, there will be oscillations around the accurate position due to the low transmission accuracy of the linear axis.

 

  The semi-closed-loop control system cannot control the transmission error generated by the machine tool transmission mechanism, the thermal deformation error generated by the transmission mechanism during high-speed operation, and the error caused by wear of the transmission system during high-speed operation. During the machining process, these errors have seriously affected the machining accuracy and stability of the CNC machine tool. The linear grating ruler provides full closed-loop control of the various linear coordinate axes of the CNC machine tool, reducing the above errors and improving the positioning accuracy, repeatability, and accuracy reliability of the machine tool. As a critical component to improve the positioning accuracy of CNC machine tools, it is increasingly favored by users.

 

Precision of CNC machine tools

 

  The precision of CNC machine tools mainly includes three aspects: geometric precision, positioning precision, and processing precision.

 

  Geometric precision, also known as mechanical precision, is a comprehensive reflection of the geometric shape errors of key parts of the machine tool after assembly. The detection tools and methods used are basically the same as those used for ordinary machine tools, but the requirements are higher. Taking the geometric precision of a common vertical machining center as an example, the following aspects are considered:

 

  (1)Flatness of the worktable surface

  (2)Perpendicularity of the movement in each coordinate direction

  (3)Parallelism of the worktable surface during movement in the X and Y coordinate directions

  (4)Rotation accuracy of the spindle

  (5)Parallelism of the spindle line when the main spindle box moves along the Z coordinate axis

  (6)Linearity of the spindle during movement in the Z coordinate direction

Positioning precision refers to the actual position accuracy that the main components of the machine tool can achieve at the end of their motion. The error between the actual position and the specified position is called positioning error. The positioning precision of a CNC machine tool can be understood as its motion precision, which depends on the CNC system and mechanical transmission errors. The motion of each moving component of the machine tool is completed under the control of the CNC device. The precision that each moving component can achieve directly reflects the precision that the machined part can achieve. Therefore, positioning precision is a very important aspect of machine tool testing.

 

Repeat positioning precision refers to the consistency of the position accuracy obtained by repeatedly running the same program code on a CNC machine tool. The repeat positioning precision is affected by factors such as servo system characteristics, clearance and rigidity of feed transmission links, and friction characteristics. In general, repeat positioning precision is a random error that follows a normal distribution, which affects the consistency of a batch of parts machining, making it a very important precision indicator.

 

Machining accuracy

 

The geometric and positioning accuracy of machine tools is usually measured when there is no cutting load and the machine is stationary or moving at a low speed. This can only reflect the machining accuracy of the machine tool to a certain extent, as there are a series of factors that can affect the machining accuracy of the machine tool in actual working conditions. For example, due to the effect of cutting forces and clamping forces, the components of the machine tool will undergo elastic deformation. Under the influence of internal heat sources in the machine tool (such as the heat generated by electric motors, hydraulic transmission devices, bearing friction, gears, etc.) and changes in ambient temperature, the components of the machine tool will undergo thermal deformation. Due to the influence of cutting forces and movement speed, the machine tool will generate vibration. When the moving parts of the machine tool move at working speed, the movement accuracy is different from the accuracy measured at low speed due to the influence of oil film between relative sliding surfaces and other factors. All of these will cause changes in the static accuracy of the machine tool, affecting the machining accuracy of the workpiece. The accuracy of the machine tool under working conditions such as external load, temperature rise, and vibration is called the dynamic accuracy of the machine tool. In addition to being closely related to static accuracy, dynamic accuracy is also largely determined by the stiffness, anti-vibration performance, and thermal stability of the machine tool.

 

Currently, in production, the comprehensive dynamic accuracy of the machine tool is evaluated by the accuracy of the workpiece produced by cutting, which is called the working accuracy of the machine tool. Working accuracy is a comprehensive reflection of the influence of various factors on machining accuracy.

 

 

Improvement of Machine Tool Precision.

 

Currently, there are two main methods for improving the machining accuracy of CNC machine tools in the parts machining industry: error prevention and error compensation.

 

Error prevention refers to measures to improve the quality level of component design, machining, and assembly, effectively control environmental factors, and achieve the goal of eliminating or reducing error sources. For example, using high-rigidity and thermally symmetrical guides and ball screws for room temperature control can effectively reduce the thermal deformation of the machine tool and the temperature rise of heat sources, thereby reducing the occurrence of errors.

 

Error prevention methods are mainly divided into three categories: dimension error and geometric error prevention, thermal deformation error prevention, and other error prevention. Error prevention methods can to some extent reduce the probability of errors occurring, but it is almost impossible to completely eliminate thermal deformation and geometric errors. In addition, the machining accuracy of the mother machine tool has a great impact on it, and the cost of improving the quality of parts is too high, so it is not common in practical applications.

 

Error compensation method involves installing precision element probes, position sensors, grating rulers, and other equipment on CNC machine tools to feedback the machining error of the machine tool to the CNC system in real-time. The machine tool automatically compensates for the machining accuracy, improving the machining accuracy of parts and greatly saving the cost of raw materials.

 

Common faults of grating rulers as position detection elements for linear axes

 

1.The linear axis cannot find the zero pulse during the return to the reference point.

In terms of performance, this means that the axis keeps running until it hits the limit switch when returning to the reference point.

The cause of this fault is generally due to the dirty reading head or grating ruler. To solve the problem, remove the reading head and clean it with anhydrous ethanol. Clean the part with the scale using a silk cloth dipped in anhydrous ethanol.

 

2.The linear axis of the CNC machine tool reports an alarm during operation.

If Siemens 840D or Fanuc CNC systems report the alarm "hardware encoder error", or if the Heidenhain CNC system reports the alarm "feedback error", the cause may be:

① Due to vibration or other reasons, the distance between the reading head and the grating ruler on the machine tool increases during use, and the CNC system mistakenly thinks that the grating ruler is damaged. The method to solve this problem is to adjust the distance between the reading head and the grating ruler according to the requirements of the grating ruler manual. The distance between the reading head and the grating ruler body is about 1-1.5mm, and it is best not to exceed 2mm.

② The installation position of the grating ruler is not suitable, such as installing it near the oil tank, where oil and gas may contaminate the grating ruler. In this case, the "fixed ruler" and "moving ruler" of the grating ruler should be cleaned separately, and then the grating ruler should be adjusted after installation before it can be used.

③ The installation position of the reading head is not suitable, which may cause damage to the reading head. Moreover, aluminum alloy debris may appear inside the grating ruler's fixed ruler, causing damage to the grating lines and resulting in the complete scrap of the grating ruler's fixed ruler.

 

3.Runaway of linear axis in CNC machine

When the linear axis of a CNC machine runs away, it is generally because the position detection component, such as the grating ruler of the linear axis, is contaminated. To eliminate the fault, the grating or reading head of the grating ruler needs to be cleaned.

 

4.Other fault situations

In years of CNC machine maintenance, it has been found that the grating ruler, as the position detection component of the CNC system, can improve the positioning accuracy of the linear axis of the machine tool when the mechanical part of the machine tool is in good condition. In addition, the grating ruler can also detect potential or existing problems in the mechanical part of the machine tool.

 

After the C61200 lathe produced by Wuzhong was transformed into a CNC machine using the FAGOR 8055TC CNC system, when machining the rolls, the X-axis moved away from the rolls on its own when the tool touched the larger diameter parts of the elliptical roll body without an X-axis movement command. When the tool contacted the smaller diameter parts of the roll body, the X-axis moved towards the rolls on its own, causing the X-axis to move back and forth.

 

After checking the CNC system of the machine tool, it was found that when the X-axis was enabled, the AC servo motor was locked with self-locking force. When the position detection component of the X-axis was shielded and changed to semi-closed-loop, the X-axis movement phenomenon disappeared during machining. Some people think that this phenomenon is caused by a problem with the grating ruler, but it was found through inspection that the back cap of the X-axis ball screw was loose. Therefore, when the tool contacts the larger diameter parts of the roll body, the roll body exerts a "top" force on the X-axis, pushing it away from the diameter of the roll body. At this time, the X-axis movement is not caused by the CNC machine tool instruction. The grating ruler used to detect the position of the X-axis showed that the X-axis moved towards the "+X" direction (away from the diameter of the roll body) without any CNC system instructions. The function of the grating ruler is to detect whether the linear axis moves accurately under the action of the CNC instruction. If the movement is not accurate, the CNC system intervenes to position the linear axis to the correct position. Therefore, when the tool contacts the smaller diameter parts of the roll body, there is a certain gap between the tool and the roll body. Through the function of the grating ruler, the X-axis moves towards the diameter of the roll body to position it at the X-axis coordinate position issued by the CNC system. In this way, when the roll body rotates one revolution without any data instruction to move the X-axis, the X-axis alternates between moving away from and towards the diameter of the roll body, causing back-and-forth movement during roll machining due to the loose back cap of the ball screw.

 

  In a CNC machine where a certain linear axis adopts a fully closed loop, motor shaking and shaft oscillation occur. When the position detection component is shielded, this abnormal phenomenon disappears. Generally, the position detection component, such as the grating ruler and reading head, should be checked first to see if they are clean and whether the installation position of the reading head is reasonable, and the position detection component should be excluded.