Technical Information

F-2 Non-Circulation Type V-V Roller Guidway

F-2 Non-Circulation Type V-V Roller Guidway

Toshiba Machine has developed a non-circulation type V-V roller guideway which has a simple construction and assures high rigidity, and applies it to various high-precision machines including aspheric & free-form surface turning/grinding machines. We have established the high accuracy, high rigidity and compact guideway technology by improving the finishing method of guiding surface, roller bearing accuracy and profile of the area where the rollers are to be inserted and removed to and from the moving unit to minimize micro waviness, a defect of the roller bearing.

[Platform]
F. Slide and rotation
[Applications]
Various high-precision machine tools

[Technical points]

  • Highly accurate, highly rigid and compact element for high-precision machines.
  • Insertion of a roller bearing between the V-V guideway and moving unit to reduce friction coefficient.
    --- The Service life of about two million hours.
  • This guideway combined with linear motors is employed in the precision machine tools.
    --- Programming resolution 1nm, maximum feed rate 15m/min.

1. Introduction

The machining technique such as turning and grinding is based on the principle of transferring a tool path of the machine tool on the workpiece. For the machine tool for lens molds requiring high machining accuracy, therefore, various measures are taken to realize high accuracy of machine movements. Toshiba Machine developed the non-circulation type V-V roller guideway satisfying customers' requirements for high speed, high accuracy and high rigidity as the guide element technology supporting the basic performance of high-precision machines, and is evolving it to a diversity of products. In this report, we describe the development chronology and features of this guideway technology, and its application cases.

2. Development chronology

Fig. 1 V-V hydrodynamic guide

Fig. 1 V-V hydrodynamic guide

To assure positioning accuracy to the order of nanometer and high responsiveness to the NC command friction coefficient of the bearing should be minimized, where possible. A re-circulation type ball/roller bearing or crossed roller type bearing, which is commercially available and widely used in general machine tools, is employed in some precision machines due to low friction coefficient and high rigidity. They cannot serve, however, as the linear guideway of the machine tool producing extremely precise profiles due to generation of micro waviness when the rolling element is inserted or removed to and from the moving unit. For this reason, a V-V hydrodynamic guide, aerostatic or hydrostatic guide are mostly used as the linear guideway for high-precision machining.
Fig. 1 shows the V-V dynamic guide. Its construction is simple, and after careful scraping, smooth motion without causing micro waviness and straightness of 0.3μm per 400mm travel, or less can be assured. This performance is enough to serve as the guideway of the high-precision machine, and the V-V hydrodynamic guide is used as the feed guideway of the high-precision slicing machine or the polygon mirror generator which are not requiring precise positioning. A hydrodynamic guideway, however, the friction coefficient is as large as 0.2 to 0.3 and stick-slip is caused in the area where the federate is 0.1mm/min around. It is rather difficult to use it for the guideway requiring precise positioning to the order of nanometer or synchronization with other axes.
The hydrostatic and aerostatic guideways are said to be ideal in terms of geometric accuracy of motion, positioning accuracy, responsiveness and micro waviness. On the other hand, oil film and air film thickness will change easily with unbalanced load.
For the hydrostatic guide, control of flow ripple in supply pressure oil is difficult, and to maintain thermal stability, temperature control accuracy of supply oil should be ±0.01°, which is extremely strict. This is why application of these guideways is very limited.
As already stated above, the V-V hydrodynamic guideway can be used for high-precision machines also in terms of rigidity, accuracy and compactness. A hydrodynamic bearing, however, it is not suited for fine positioning and feed at extremely low speed. To solve this problem, we have developed the non-circulation type V-V roller guideway incorporating the rollers between the V-V guideway and moving unit, whose roundness, cylindricity and diameter are precisely controlled by making the most of the V-V hydrodynamic guideway features and reducing friction coefficient as in the liquid bearing.

3. Features of non-circulation type V-V roller guideway

Fig. 2 Straightness of non-circulation type V-V roller guideway

Fig. 2 Straightness of non-circulation type V-V roller guideway

For the non-circulation type V-V roller guideway (see the arrow-marks in the figure shown at the beginning), both the fixed side (bed) and moving unit (table) are finished by scraping with a scraper, taking the V-V master as the reference as in the manufacture of the V-V hydrodynamic guideway. The scraped surface has unevenness of 2μm to 3μm. For the hydrodynamic guideway, such unevenness constitutes an oil accumulation and dynamic pressure is generated by the wedge effect to allow the smooth table motion. For the non-circulation type V-V roller guideway, the accuracy is secured first by scraping, then unevenness on the scraped surface is eliminated by lapping with free abrasive lapping to prevent influence of the scraped surface unevenness on accuracy of rolling motion of the rollers. Additionally, micro waviness was minimized by improving the accuracy, introducing the rollers to the moving unit and improving the profile of the ejection part. Meanwhile, the bearing life was figured out to make sure that it is enough, that is, about 2 million hours under normal running conditions.
Fig. 2 shows the results of straightness measurement of this guideway, where you can find that micro waviness and straightness equivalent to those of the hydrostatic and aerostatic guideways were obtained.

Fig. 3 1nm-step feed responsiveness

Fig. 3 1nm-step feed responsiveness

Fig. 3 shows the step feed accuracy of the high precision aspheric & free-form surface turning/ grinding machine where the non-circulation type V-V roller guideway is combined with linear motors with cores serving as the drive source. It was confirmed that response of 1nm-step NC command is possible.

4. Application to various high-precision machine tools

Fig. 4 High-precision aspheric & free-form surface turning / grinding machine (ULG-100D)

Fig. 4 High-precision aspheric & free-form surface turning / grinding machine (ULG-100D)

When the non-circulation type V-V roller guideway is combined with the linear motors with cores, programming resolution of 1nm and maximum feed rate of 15m/min are realized, and they are employed in a variety of high- precision machine tools. Fig. 4 shows the high-precision aspheric & free-form surface turning/grinding machine used for machining molds for aspheric lens, diffractive optical element, etc.

Fig. 5 High-precision grooving lathe (ULR-628B(H))

Fig. 5 High-precision grooving lathe (ULR-628B(H))

Fig. 5 shows the precision grooving lathe designed for machining rolls for manufacturing liquid crystal display optical films such as prism sheet. This technology is widely utilized, centering around the optical field. Also, a compact and high-precision guideway, it is used as the positioning stage of the high-precision measuring instrument.

5. Conclusion

We have introduced the non-circulation type V-V roller guideway with high rigidity and high accuracy as the element technology supporting the basic performance of high precision machines. We will continue pursuing the performance and plan to expand the application fields in addition to the machine tool market as the highly accurate and highly rigid compact element for precision machines.