KR20040034948A - Aerostatic bearing device of high speed spindle for milling processing - Google Patents

Abstract

PURPOSE: An aerostatic bearing unit of a high speed spindle for a milling machine is provided to improve surface roughness of a workpiece by using air as a medium of the aerostatic bearing unit. CONSTITUTION: An aerostatic bearing unit includes a plurality of radial bearing pads(3,4,5,8), a thrust bearing disc(2), at least one pair of thrust bearing pads(13), and at least one exhaust hole(14) formed in a spindle housing(16). An air feeding passage(22) is formed in the spindle housing(16) so as to feed air in radial and axial directions of a spindle(1). The radial bearing pads(3,4,5,8) have a plurality of air feeding holes(21) communicated with the air feeding passage(22). The thrust bearing disc(2) is provided at one end of the spindle(1). The thrust bearing pads(13) are formed with a plurality of air feeding holes(21).

Description

Aerostatic bearing device of high speed spindle for milling processing

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high speed spindle applicable to a milling machine or a machining center, and more particularly to an air static pressure bearing device for supporting a high speed spindle using air as a bearing medium.

As the development of high speed machining to improve machining precision and productivity, the speed of the spindle and feed system is required in milling machines or machining centers. Typically, the spindle of a milling machine or machining center uses rolling bearings such as angular contact ball bearings or roller bearings.

An example of applying rolling bearings 100 and 101 to the high speed spindle 102 for milling is shown in FIG.

As shown in FIG. 1, the main shaft 102 to which the rolling bearings 100 and 101 are applied has a small pressure angle, a rolling element such as a ball or a roller made of ceramic material, inner and outer ring materials having excellent heat resistance, and bearing preload for high speed. Modification mechanisms, bearing lubrication techniques, etc. are being selectively applied. As a result of this effort, a spindle with a maximum speed of 25,000 revolutions per minute for spindle diameters of 65 to 70 mm and a speed of 45,000 revolutions per minute for spindle diameters of 40 mm was put into practical use. Research is ongoing.

However, spindles using rolling bearings have some problems.

First, the life of the spindle is short. Spindles, commonly called high speed spindles, have a life span of 2,000 to 3,000 hours at full speed. Assuming a machine tool life of about 10 years, it is very short compared to other devices. This increases the maintenance cost of high speed machine tools such as spindle replacement costs, and performance deterioration is inevitable depending on the service life.

Second, rolling bearings generate a lot of heat, depending on the lubrication method, pressure angle and structure. In particular, when preload is applied to improve the rigidity of the main shaft, the calorific value is further increased. Due to the heat generation of the main shaft, not only the main shaft is stretched, but also due to the performance of the cooler for cooling the bearing, the main shaft is repeatedly expanded and contracted. This property deteriorates the machining precision of the workpiece.

Third, the high speed spindle using rolling bearings generates vibrations and noises due to the rolling elements of the bearings and the rotation of the spindle. Vibration of the spindle causes vibration of the tool connected to the spindle, resulting in a deterioration of the surface quality of the machining surface and adversely affecting the life of the tool. In addition, depending on the condition of the factory and the machine tool, such noise may be loud enough to cause hearing loss.

Therefore, in order to solve the short life, heat generation, vibration and noise problems of rolling bearing high speed spindles, the present invention uses air as a bearing medium instead of rolling elements such as balls and rollers used in rolling bearings. Not only can the semi-permanent life be obtained at high speeds by contact, but also the heat generation is low and the operation is quiet, so that the surface roughness of the workpiece can be improved and the high-speed spindle of the air constant pressure bearing device for milling can be used for precise machining. The purpose is to provide.

1 is a cross-sectional view of a rolling bearing high speed spindle for milling generally used.

Figure 2 is a cross-sectional view of the air static pressure bearing high speed spindle for milling according to an embodiment of the present invention.

3 is an illustration of the air static pressure bearing clearance.

4A and 4B are front cross-sectional views of the air static radial bearing and section A-A in FIG. 4A.

5A and 5B are front sectional views of an air static thrust bearing, and a sectional view taken along line B-B in FIG. 5B.

6 is a sectional view of a tool clamp device and a tool unclamp cylinder.

7 is a graph showing the stiffness relationship according to the position of the air supply hole on the radial bearing pad side.

<Explanation of symbols for the main parts of the drawings>

1: main shaft

2: thrust bearing disc

3,4,5,8: Radial Bearing Pads

7: spindle motor

12,17: Pressure gap

13: thrust bearing pad

13a, 21: Supply air

14: exhaust hole

16: spindle housing

20: exhaust vent for heat dissipation

22: pressurized air supply passage

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 is a cross-sectional view of the air static pressure bearing high speed spindle for milling according to an embodiment of the present invention, Figure 3 is an illustration of the air static bearing gap, Figure 4a, 4b is a front sectional view of the air static radial bearing and of Figure 4a 5A and 5B are front sectional views of the air static pressure thrust bearing, and BB line sectional view of FIG. 5B.

The air static pressure bearing device of the high speed spindle for milling processing applied to the present invention has a pressure generated by introducing pressurized air introduced from the outside of the spindle housing 16 in the radial direction and the axial direction of the spindle 1 as shown in FIG. 2. The main shaft 1 is to be raised.

The main shaft housing 16 is formed with a pressurized air supply passage 22 for introducing pressurized air in the radial and axial directions of the main shaft 1. This pressurized air supply passage 22 is connected to a pneumatic source (not shown).

On the outer circumferential surface of the main shaft 1, a plurality of radial bearing pads 3, 4, 5, 8 for radially supporting the main shaft 1 are arranged in the axial direction.

Between the inner circumferential surface of the radial bearing pads 3, 4, 5, and 8 and the outer circumferential surface of the main shaft 1, a constant radial bearing pressure gap 17 is formed, and this bearing pressure gap 17 In order to inject the pressurized air, radially penetrating radially and radially around the main shaft 1 are supplied to the pressurized air at a predetermined circumferential interval of the radial bearing pads 3, 4, 5, and 8 as shown in FIG. Four air supply holes 21 communicating with the passage 22 are provided.

The bearing pressure clearance 17 of this embodiment is 15-25 micrometers, and is comprised so that the risk of ignition by mixing of a foreign material may be reduced. This is because when the bearing pressure gap 17 is 15 μm or less, the heat generation is large and the risk of sintering by foreign matter increases. When the gap 17 exceeds 25 μm, the load capacity decreases and the bearing rigidity is reduced.

In this embodiment, three radial bearing pads 3, 4, and 5 are directly installed at the front end of the main shaft 1 to improve rigidity.

As shown in FIG. 3, the plurality of air supply holes 21 are formed as conical holes (orifice passages) having a diameter of 0.3 to 0.7 mm, and throttling occurs. It may further have an enlarged passage to do so. At this time, the cross-sectional size of the enlarged passage is limited to a predetermined size in consideration of the compression of air.

In this embodiment, the radial bearing pads 3, 4, 5, and 8 have air supply holes 21 in two rows.

The position of the second row air supply hole 21 is preferably arranged at 1 L / 7 of each length L of the radial bearing pads 3, 4, 5, 8. This is to maximize the rigidity and load capacity of the bearing when the spindle (1) is rotating at high speed of 30,000 ~ 50,000 RPM. In the case of larger than this, the rigidity of the bearing was inferior. The experimental value for this is shown graphically in FIG. 7. FIG. 7 is a graph showing the stiffness relation according to the positions of the air supply holes 21 on the radial bearing pads 3, 4, 5, and 8, and the bearing stiffness is highest when the air supply positions are 1 L / 7. .

One end of the main shaft 1 is integrally provided with a metal thrust bearing disc 2.

On both sides of the thrust bearing disc 2, thrust bearing pads 13 and 13 made of metal are arranged to support the axial load of the main shaft 1. In this case, the material of the thrust bearing pads 13 and 13 is preferably made of a softer metal than the material of the thrust bearing disc 2. This is because the thrust bearing pads 13 and 13 are relatively easy to replace and advantageous in terms of cost. Therefore, for example, when the thrust bearing disc 2 is a bearing steel, the thrust bearing pads 13 and 13 may be made of copper.

The thrust bearing pads 13 and 13 each have an axial bearing pressure gap 12 between the side face and the thrust bearing disc 2. The thrust bearing pads 13 and 13 communicate with the pressurized air supply passage 22 and have four perforated air supply holes 13a at regular intervals on the side surface.

The position of the thrust bearing disc 2 which comprises a thrust bearing is important here. This is because the thermal deformation of the main shaft 1 affects the machining accuracy mainly because of the axial thermal deformation, and the axial thermal deformation occurs from the thrust bearing disc 2 as a starting point.

Therefore, in this embodiment, the thrust bearing disc 2 is installed at the front end of the main shaft 1 close to the site where the tool of the main shaft 1 is mounted, thereby minimizing the effect of the thermal deformation of the main shaft 1 on the machining precision. .

An exhaust hole 14 is formed in the main shaft housing 16 to discharge the air blown out into the pressure gaps 12 and 17.

At this time, the load load of the radial and thrust air static pressure bearing is the product of the area of the average pressure and the air pressure between the gaps 12 and 17.

On the other hand, the present invention further has a structure for cooling the spindle motor 7 by using the air discharged into the air supply hole 21 of the spindle housing 16.

That is, the spindle motor 7 is provided between the radial bearing pads 5 and 8 in order to cool the spindle motor 7 which imparts rotational force and rotational motion to the main shaft 1, and the spindle Both sides of the motor 7 are further provided with heat discharge exhaust holes 20 through which a part of the air discharged through the air supply holes 21 is discharged.

6 is a cross-sectional view of a tool clamp device and a tool unclamp cylinder installed in the hollow 15 of the main shaft 1, 31 is a tool unclamp cylinder, 32 is a push rod, and 34 is a collet for chucking a tool. .

Reference numeral 6 is a cooling jacket of the spindle motor, 9 is an encoder for detecting the rotational speed and position of the spindle.

The operation of this embodiment configured as described above will be described.

The pressurized air from the pneumatic source, not shown, passes through the air supply holes 21 of the main shaft 1 and the bearing pads 3, 4, 5, 8, 13, 13 is introduced into the bearing gaps 17 and 12 therebetween. The air supplied through the air supply hole 21 is throttled and adiabaticly expanded, whereby the temperature of the air is lowered to cool the bearing portion.

The air flowing into the bearing gaps 17 and 12 passes through the gaps 17 and 12 and exits to the outside atmosphere of the main shaft 1 along the exhaust hole 14 formed in the main shaft 1. In the gaps 17 and 12, the pressure between the supply pressure and the atmospheric pressure is distributed by the viscosity of the air, and the product of the pressure and the area acts as a supporting load.

Accordingly, the front end radial bearing pads 3, 4, 5, the rear end radial bearing pad 8, and the front end thrust bearing pad 13 are driven by the cutting load acting on the tool. Support the same external load.

As such, there is no mechanical contact such as rolling bearings in air static bearings. Therefore, there is no change of friction force due to the load load seen in rolling bearings. The resistance of the bearing medium due to the relative motion is determined by the viscosity of the air, and even if there are minute irregularities on the rotational surface of the main shaft by the film of air, the geometric error of the bearing is equalized.

On the other hand, heat generated in the spindle motor 7 is discharged out of the main shaft 1 by the cooling water circulating inside the cooling jacket 6 located outside the spindle motor 7. In addition, the air flowing out through the gaps 17 and 12 is warmed by the heat radiated from the spindle motor 7 and exits the main shaft through the main shaft exhaust hole 20.

The tool clamping device located inside the main shaft 1 retracts the collet 34 connected to the push rod 32 by the spring 33 as shown in FIG. 6 so that the tool 10 is opened in front of the main shaft 1. 11). Adhesion is maintained by the spring 33, when the tool unclamp cylinder 31 located behind the spindle 1 pushes the push rod 32, the tool holder 10 can be released from the spindle 1 . At this time, if the tool is replaced by a separate tool changer, the push rod 32 of the tool unclamp cylinder 31 is retracted, the push rod 32 is returned by the spring 33, the tool holder 10 Is tightly constrained to the spindle 1.

Inside the main shaft 1, a tool clamping device is incorporated to fix the tool 10 to the main shaft 1. The tool clamping device serves to separate the tool holder when the unclamp cylinder 31 installed at the rear is operated. Therefore, the automatic tool change function of a milling machine or a machining center can be performed.

As described above, according to the air static pressure bearing device of the high speed spindle for milling, the air constant pressure bearing high speed spindle for milling has no mechanical contact by using air having a very low viscosity as a bearing medium, and thus compared with the rolling bearing high speed spindle. Can provide long life.

In addition, there is no mechanical friction, and due to the low viscosity of the air, not only the heat generation of the bearing portion is low, but also the cooling effect by the adiabatic expansion is present, so the heat generation amount is very small. Furthermore, thrust bearing pads can be placed at the spindle front end close to the tool to minimize the effects of thermal deformation.

In addition, since a quiet operation is possible by the non-contact motion, the air film effect to reduce the machining error of the main shaft and the housing, a very good surface roughness can be obtained.

Due to these characteristics, the present invention satisfies the goal of high-speed machining to perform machining closest to the final shape only by cutting, and to reduce the post-processing time by increasing the surface quality of the machining surface.

Claims (4)

In the bearing device of the milling machining spindle for rotating support of the hollow spindle (1) driven in the spindle housing (16) driven by the spindle motor (7) at a constant air pressure, A pressurized air supply passage 22 formed in the spindle housing 16 for introducing pressurized air in the radial and axial directions of the spindle 1; Inserted into the outer circumferential surface of the main shaft 1 and aligned in the axial direction, and having a radial bearing pressure gap 17 in a constant radial direction with the outer circumferential surface, at regular intervals on a circumference in communication with the pressurized air supply passage 22. Radial bearing pads (3, 4, 5, 8) having a plurality of radially supplied air supply holes (21); A thrust bearing disc 2 provided at one end of the main shaft 1; It is disposed on both sides of the thrust bearing disk (2), respectively, and has a thrust bearing pressure gap (12), which communicates with the pressurized air supply passage (22) and has a plurality of perforated air supply holes (13a) at a predetermined interval on the side surface. One or more pairs of thrust bearing pads 13 and 13; And And at least one exhaust hole (14) provided in the spindle housing (16) for discharging the air blown out into the pressure gap (12,17). The method of claim 1, The thrust bearing disc (2) is an air static pressure bearing device of a high speed spindle for milling, characterized in that installed in the front end portion close to the site where the tool of the spindle (1) is mounted. The method of claim 1, The spindle motor 7 is interposed between the radial bearing pads 5 and 8, and both sides of the spindle motor 7 are further provided with heat exhaust vents 20. Air static pressure bearing device of high speed spindle for machining. The method of claim 1, The air supply hole (13a) is a high-speed spindle of the air constant pressure bearing device for milling, characterized in that it has a conical axial passage so that adiabatic expansion occurs.