Estimated reading time: 11 minute
The platform feed system in the mechanical structure is an important operating mechanism that determines the accuracy of the axis direction. With the improvement of the precision of bending machine tools and the development of semiconductor manufacturing equipment, information-related equipment, and ultra-precision processing, the market continues to put forward high-precision requirements.
Here, the important mechanical elements in the mechanism ball screw and linear guide device will be the center, and the influence of each element on positioning accuracy will be summarized.
Influencing Factors of Ball Screw Positioning Accuracy
- Lead accuracy
The lead accuracy of the ball screw is in accordance with the JIS standard established in 1980 and was partially revised in 1987. The regulations are as follows:
- Cumulative representative movement error
- The maximum amplitude relative to the effective length of the screw part
- The maximum amplitude of any 300mm
- The maximum amplitude of any rotation
Although the required function of the ball screw of the bending machine is the feed accuracy of the nut, the lead accuracy of the single shaft is generally measured in many cases. The lead accuracy is divided into a wide range and a narrow range. Relatively speaking, the wide-range accuracy seldom causes problems due to the good processing accuracy and easy correction. In the future, in order to meet the need for higher accuracy, narrow range accuracy will become more and more important.
The figure is an example of the lead error of the highest precision C0 level. The cumulative representative movement error is 1μm/208mm, the maximum range relative to the effective length of the screw part is 2μm, and the maximum range of the nut assembled state does not exceed 0.9μm, which is less than 1/2 of the specification value.
The characteristic component of the error variance is the component (12mm period) of one rotation of the shaft of the channel grinder. Although the single unit is 1 to 1.5μm, the averaging effect after assembly of the nut will be reduced to 0.5μm or less. Generally, compared with the narrow range error of the single body, the nut has a reduction of 1/2 to 1/3 after assembly.
- Non-contemporaneous ingredients
Even if the cycle of the above-mentioned lead accuracy is different, the axis rotation synchronization component will appear. Asynchronous components caused by the rotation/revolution of the rolling elements will appear in the rolling elements, which will become a major problem in the field of ultra-precision. The reason lies in the roughness of the raceway surface, the roundness, the difference between the outer diameters of the rolling elements, and the poor sphericity, etc., and the vibration problem caused by the rolling elements during the in and out process caused by the circulation mechanism. According to the report of Tsukada et al. 1), on a platform with a movement error of 0.4 to 0.5 μm, using a cage or synthetic resin spacer steel balls for the originally used ball screw will reduce the movement error to less than 0.1 μm. The parsing component of the steel ball will not only appear in the form of vibration, but also in the form of torque variation, which easily causes movement errors of the motor output torque. This type of error is different from the contemporaneous component, and it is difficult to correct by control compensation, and when the ball screw is used in the ultra-precision field of bending machine, there are few correct evaluation and research cases, which is one of the important topics in the future.
- Installation accuracy and installation error
Even if the lead accuracy is high-precision, if it is installed incorrectly, the positioning accuracy will deteriorate. In the 1987 revision of the JIS, the importance of the accuracy of the ball screw mounting part was recognized. Therefore, the specification value is more stringent than the previous change, and there will be a more stringent trend in the future.
If the concentricity of the ball screw, the bearing, and the guiding mechanism are poor, the error of rotation runout is easy to occur, and it is easy to be confused with the lead error. According to Yoneda’s case report, if the concentricity of the nut and the support bearing is controlled to a fraction of the original, the lead error characteristic during ultra-precision cutting will disappear.
The figure shows an example of position change measurement when the C0 ball screw is connected by a coupling under the guidance of the aerostatic guide rail. Although the rotational vibration component appears, the amplitude is small. And, when it is necessary to further reduce, the nut and the platform are fixed only in the axial direction, and the installation methods that are free in other directions have also been applied in practice.
If the rigidity of the ball screw as a whole is weak, a lost motion will occur. The overall stiffness of the ball screw is not only the internal stiffness of the nut (the stiffness between the steel ball and the raceway) but also the expansion and contraction of the screw shaft and the stiffness of the support bearing, which need to be considered as a whole. Generally, the expansion and contraction of the screw shaft account for the largest proportion, and the stiffness is greatly affected by the supporting conditions.
As shown in the figure, when fixing-supporting (free axis direction), the rigidity changes greatly with the position of the nut, and when fixing-fixing, the rigidity is higher and the change is smaller.
Fixed-Fixed conditions are more beneficial to high accuracy, but there are also problems such as overloading of the support bearing due to the thermal expansion of the screw shaft. In this case, there are more fixed-semi-fixed structures.
- Friction torque
The friction of the ball screw is inherently small, and it feels that the pre-tightening force can be increased in order to increase the rigidity. However, in terms of precision positioning, changes in the accumulated pulses of the control system caused by changes in the torque (especially friction torque fluctuations) of the ball screw and support bearing will cause errors in positioning accuracy. Therefore, the stability of the friction torque is more and more important. In the JIS revision in 1987, the specifications of the friction torque were defined, and the standard example sentence Tp and the relative standard example sentence change specifications were stipulated.
The friction torque also has a wide range and a narrow range. The reason for the wide range of changes is mainly due to the change in the amount of preload caused by the error of the screw shaft channel diameter (in the case of positioning preload). If there is no problem with stiffness, this change will become very small when using constant pressure preload. The narrow range variation is caused by factors such as the accuracy of the channel surface of the screw shaft, the accuracy of the shape, and the design/processing accuracy of the circulation path. The narrow range change is especially obvious at low speed and shaking. The use of spacer steel balls has a good effect on improving the narrow range change.
Compared with the sliding screw, the ball screw is considered to have a small frictional torque but large fluctuations, but as shown in the figure, it has now been greatly improved.
As will be discussed later, at high speeds, the influence of lubricant viscosity will increase the friction torque. On the contrary, when running at an ultra-low speed, the characteristics of the lubricant will affect the friction torque.
Influence of Linear Guide
As mentioned earlier, there are various linear guides. Because there are many use cases of rolling guide sliders in recent years, this article mainly explains the problems related to positioning accuracy caused by cross roller guides (non-recirculating roller type) and linear guides (recirculating ball type).
- Installation and installation accuracy
This linear guide is assembled by the guide rail and the base, and the corresponding performance is obtained after adjusting the gap. Therefore, high-precision guidance cannot be obtained without correct installation, and great attention must be paid to the installation method.
Although the roughness and curvature of the channel surface of the guide rail are already small, the verticality and shape of the channel are also very important. Because the mounting surface of the slider is finally obtained by installing the guide rail without gaps, the precision of the guide rail must reach the same level of accuracy in order to obtain high-precision running accuracy. The compression force after assembly is usually adjusted by horizontally pressing the screw, but when the screw is tightened, the guide rail will be deformed, thereby deteriorating the operation accuracy. Therefore, a horizontal pressing plate is placed between the guide rail and the horizontal pressing screw to equalize the screw force.
- Running accuracy
This type of guidance, if properly assembled, will result in good operating accuracy.
The cross roller guide rail (the same applies to other non-circulating guide rails) changes with the position of the platform, and the position of the supported rolling elements moves relatively. Therefore, the preload and rigidity are changing, and the running accuracy will also deteriorate. Therefore, it is desirable that the total length of the rolling element part is longer than the maximum stroke. Because the difference of the roller diameter will cause the variation of the running accuracy in a narrow range and the generation of non-synchronous components, according to the author’s experience, if the above-mentioned roller part becomes longer, the averaging effect can be obtained, and the running accuracy is also affected.
According to such countermeasures, the straightness of the horizontal/vertical direction of the platform with a stroke of 350 mm can reach 0.6 μm, the vertical steering can reach 1.9 seconds, and the deflection steering can reach 0.5 seconds.
- Small sliding
Such a non-circulating type linear block has a small outer shape, and has no non-synchronous fluctuation components caused by circulation compared with the cyclic type, and is suitable for high-precision use.
However, non-circulating linear sliders also have disadvantages, one of which is the phenomenon of micro-slip. This phenomenon is that during the reciprocating movement of the platform, the relative position of the cage relative to the guide rail will deviate little by little, and eventually the cage will leave the guide rail. Although stop devices are installed at both ends of the guide rail, the cage will hit the stop devices, resulting in poor operation accuracy and damage to the cage.
Because the linear guide rail is very long, the accuracy of the guide rail is finally determined by the shape of the guide rail mounting surface. Therefore, a high-precision guide surface must be manufactured so that the straightness of the guide surface and the parallelism between the mounting surfaces are highly accurate. In this case, the roughness of the mounting surface is not a problem, but the shape accuracy is important, so it is not necessary to use grinding.
Although the accuracy of the guide surface will affect the accuracy of the mounting surface under basic conditions, in reality, the effect of the interference between the guide rail and the slider, the deformation of the contact part, and other averaging effects, the accuracy of the mounting surface will be reduced to 1/2 ~1/10. The figure is a measurement example from the accuracy of the mounting surface to the accuracy of the mounting body. The above phenomenon can be clearly seen.
One of the reasons for the narrow range change is the error caused by the screw locking. This is caused by the deformation of the guide rail and the guide rail channel due to the locking force of the screw fixing. The elimination of this error can be solved by making the screw locking force during processing and installation equal to the locking force during actual use and then grinding the channel.
Another reason is the passing composition of the steel ball caused by the steel ball entering and leaving the loading circle. This component is caused by the tilt error, so it will increase when the processing point is at the overhanging point of the platform, which is a narrow range of flatness variation.
This change is caused when the steel ball enters and exits the load ring. In order to make the steel ball enter and exit the load ring smoothly, the end of the slider channel can be designed into a slowly inclined shape (Crowning) to make this change reduce. This kind of change will increase when the preload is large, and compared with the slider alone, the change will be very small after the platform is installed due to the effect of averaging.