Technical Information

F-1 Aerostatic Bearing Technology

F-1 Aerostatic Bearing Technology

Toshiba Machine has specialized in the development of the aerostatic bearing technology which is the key element for high precision and high speed machining. We have studied thoroughly respective features of both annual restriction and porous restriction types and apply them to the spindle, turn table, etc. of various high precision machines for precision finishing of lens mold, high speed milling of general use mold, dicing of semiconductor substrate, etc.

[Platform]
F. Slide and rotation
[Applications]
High-precision machines

[Technical points]

  • System supporting the rotator without contact.
  • Use of two restriction types; annual restriction and porous restriction.
  • Coefficient of friction of the bearing is extremely small and vibration caused at the time of rotation can be minimized. (Spindle run out is 0.1μm or less at as per the SPAM method.)
  • Addressed to various motions ranging from extremely low speed to ultra high-speed rotation.
  • Semi-permanent service life.

1. Introduction

The mass production technology of optical parts including aspheric lens has expanded largely, supported by the increase and demand for consumer devices such as digital cameras, cellular phones, CD, DVD and liquid crystal displays. The machining technology supporting mold fabrication has also advanced, responding to the needs of the time. With the growth of the element technology for machine tools and cutting tools, the traditional machining techniques such as cutting and grinding has evolved with new innovative machining technologies as precision cutting and precision grinding, which is now essential for precision machining of parts. High precision of the machining techniques have been achieved since 1980 with the progress of precision machines incorporating an aerostatic bearing in the spindle.
Toshiba Machine, a pioneer of precision machines in Japan, has been engaged in development of the aerostatic bearing since 1977 to ever pursue its high performance.
Now, we explain the features, recent developments and application cases of the aerostatic bearing.

2. Features of aerostatic bearing

For the bearing of the spindle of high-precision machine, a aerostatic or hydrostatic system using air or liquid is employed. This is the system supporting the rotor without contact, where vibration caused at the time of rotation can be minimized due to extremely small friction coefficient of the bearing. The aerostatic bearing has the following excellent features which cannot be obtained by any other bearing system.

  • Friction coefficient of the bearing is extremely small.
  • Stick-slip (vibration caused on the hydrodynamic guideway) is not present.
  • Low heat generation.
  • Rotational accuracy is very high.
  • Addressed to various motions covering from very slow-speed to very fast-speed rotation.
  • Semi-permanent service life.
  • Silent involving no noise.
  • Clean environment is maintained.
Fig. 1 Comparison of vibration at spindle nose

Fig. 1 Comparison of vibration at spindle nose

On the other hand, the aerostatic bearing is less rigid and can handle smaller load capacities, compared with the ball or roller bearing. With the optimization of bearing design, and improvement of machining and assembling accuracies of components, however, use of the aerostatic bearing for machine tools becomes possible and advantageous.
There are several types of restriction for supplying air to the clearance of the bearing. Generally, the annual restriction type, porous restriction type, orifice restriction type and surface restriction type are widely used, which are selected according to various purposes such as rotational speed and rigidity. We mainly use the annual restriction type and porous restriction type. Table 1 shows the comparison between the annual restriction and porous restriction type. These aerostatic bearings are used in various high-precision machines according to respective features. Fig .1 shows the results and comparison of vibration caused at the spindle nose between the annual restriction type spindle and porous restriction type spindle. The porous restriction type can lower the vibration level, compared with the annual restriction type.

Table 1 Comparison of restriction technique
Type of restrictionAnnual restrictionPorous restriction
ConstructionSelf-generating restrictionPorous restriction
Bearing rigidity SmallLarge
Air consumption LargeSmall
Heat generation SmallLarge
Main application Tool spindleWork spindle Rotary table

As the annual restriction type aerostatic bearing can make the bearing clearance relatively large (15μm - 20μm) and the air with low viscosity controls heat generation to the low level during high-speed rotation, it is mainly used for the small diameter end mill, small-diameter drill and high-speed spindle (10000min-1 - 100000min-1) intended for super abrasive grinding wheel. The porous restriction type can control vibration to the low level and the bearing rigidity is higher, resulting in remarkable machining repeatability when it is applied for a machine tool spindle. It is used for the work spindle of turning/grinding machine (i.e., turning spindle) mainly intended for machining lens molds requiring high accuracy, and for the rotary table. Also, air consumption of the porous restriction type is reduced to one-sixth (1/6; compared with the annual restriction type). When the aerostatic bearing is used for the spindle, two drive systems are available; motor drive system and air turbine drive system. The motor drive system cannot use a transmission mechanism such as belt and coupling due to its high-speed rotation, and a motor built-in system is used, which directly secures the rotor to the shaft.

3. Application cases and effects

3.1 Annual restriction type

Fig. 2 Aerostatic bearing spindle for high-speed milling

Fig. 2 Aerostatic bearing spindle for high-speed milling

Fig. 2 shows the aerostatic bearing spindle for high-speed milling (maximum speed 60000min-1), and Fig. 3 the external view of the precision vertical machine (UVM-450C) incorporating this spindle. Spindle run out accuracy is 0.1μm or less at a speed of 60000min-1 as per the SPAM (Single Point Asynchronous Error Motion) method. This spindle allows high-speed milling at tool rotational speed of 60000min-1 to reduce the machining time. It also allows mirror surface machining of hardened steel used for extremely hard (HRC60) molds, using a small-diameter end mill commercially available, and is a leading development and contribution to the new evolution of polish less machining and is a leading development and contribution to the new evolution of eliminating polishing of precision dies and molds polish less machining technology of precision molds.

Fig.3 Precision vertical machine (UVM-450C)

Fig.3 Precision vertical machine (UVM-450C)

The annual restriction type aerostatic bearing is also used in the slicing machine (i.e., grooving and cutting machine) intended for dicing semiconductor substrates and glass substrates, and for grooving V-slot substrates for optical communication.

Fig. 4 Aerostatic spindle for slicing machine

Fig. 4 Aerostatic spindle for slicing machine

Fig. 4 shows the annual restriction type aerostatic spindle for the slicing machine, which allows machining with multi-blades (multiple simultaneous machining with multi-blades on a single spindle) due to increased bearing rigidity. Recently, development of SiC substrates has been advanced energetically in the power semiconductor field. As SiC is hard-to-machine material, however, productivity in the substrate dicing process possesses a problem to be solved in this field.

Fig. 5 Precision slicing machine (USM-6E)

Fig. 5 Precision slicing machine (USM-6E)

We have realized high-speed and high-accuracy dicing of SiC substrates with cutting speed of 30mm/sec and chipping of cutting surface of less than 10μm by the newly developed precision slicing machine (USM-6E) (Fig. 5) incorporating the aerostatic bearing spindle whose run out is 0.02μm at a rotational speed of 40000min-1. Air consumption of this spindle has reduced to below a half (compared with our previous one) due to optimized bearing design

3.2 Porous restriction type

Fig. 6 High-precision aspheric & free-form surface turning/ grinding machine (ULG-100D)

Fig. 6 High-precision aspheric & free-form surface turning/ grinding machine (ULG-100D)

Fig. 6 shows the external view of the high-precision aspheric & free-form surface turning/grinding machine (ULG-100D) which employs the porous restriction type aerostatic bearings for the work spindle and tool spindle, and combines the non-circulation type V-V roller guideways with the linear motors drive unit.
Fig. 7 shows the external view of the work spindle. The external view of the tool spindle is shown in the figure carried at the beginning. We achieved the run out of the work spindle of 3nm or less as per the SPAM method by using the porous restriction type aerostatic bearings.
Results of machining by this machine are shown in Fig. 8. It is the result of turning a lens mold, using a single crystal diamond tool (material of mold machined surface: electro less nickel plating), and surface roughness is less than 1nmRa (arithmetical mean roughness)

Fig. 7 External view of work spindle

Fig. 7 External view of work spindle

Fig. 8 Property of lens mold turned surface (Measuring instrument: New View ZYG0)

Fig. 8 Property of lens mold turned surface (Measuring instrument: New View ZYG0)

Fig.9 Rotation & indexing table

Fig.9 Rotation & indexing table

The aerostatic bearings are also used in the rotary & index table (Fig. 9). It is widely used as an attachment of diversified machines including the precision vertical precision machine as described above. We also manufacture large aerostatic bearings intended for 800mm-diameter table.
In addition to machine tools, the aerostatic bearings are also applicable for electrostatic coating equipments. To be more specific, a small-size aerostatic bearing spindle driven by an air turbine is used to uniformly atomize paints with centrifugal force which is generated by high speed rotation at a rate of a few ten thousand revolutions per minute. Use of the aerostatic bearing possesses no problem to service life even at a high-speed rotation under a solvent atmosphere and will not cause contamination by lubricant, thus well satisfying the requirements from customers.

4. Conclusion

We have described the features and application cases of the aerostatic bearing as the element technology supporting the basic performance of high-precision machines. To respond to the market's needs for high-efficiency and high-accuracy machining of molds, we will continuously pursue the performance including enhancement of accuracies and reduction in air consumption and expand the markets of this energy-saving and clean precision machine element, not limited to the machine tool market.