Gear cutting – milling – or planing – Milling – Cutter spindle or spindle support
Reexamination Certificate
2000-03-16
2003-01-21
Wellington, A. L. (Department: 3722)
Gear cutting, milling, or planing
Milling
Cutter spindle or spindle support
C409S135000, C409S193000, C409S186000, C451S007000, C310S090500
Reexamination Certificate
active
06508614
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spindle device and, more particularly, to the spindle device provided with externally pressurized gas bearings or combined externally pressurized gas-magnetic bearings and also to a machining apparatus equipped with the same.
2. Description of the Prior Art
In recent years, a highly efficient, highly precise machining has drawn keen attentions in the field of a mold machining field. In order to implement such a machining, it is necessary to use a spindle device capable of accomplish a high speed rotation with high rotational precision and having a static stiffness and a dynamic stiffness, and it is also required to perform the machining under optimum machining conditions by detecting the status of machining.
To meet these requirements, the applicant has suggested a hybrid type non-contact bearing assembly in which externally pressurized gas bearings and magnetic bearings are combined together as disclosed in the Japanese Laid-open Patent Publication No. 11-013759. According to this suggestion, by utilizing an excellent dynamic stiffness and a rotational precision, both exhibited by the externally pressurized gas bearing, and an excellent static stiffness exhibited by the magnetic bearing, a compact bearing assembly making advantages of those different types of bearings can be obtained. Also, measurement of a machining load of a machine tool for detection of the machining status is generally carried out by a system in which the load during the machining is inferred from a value measured of a motor output for rotating the main shaft.
However, the system of inferring the load during the machining in reference to the motor output measured value as hereinabove described has a problem in that a measuring instrument designed exclusively for that purpose is required, resulting in increase of the system cost.
Also, the system of detecting the machining status in reference to the motor output measured value is incapable of detecting the machining status associated with the frequency of rotation of the main shaft.
Apart therefrom, in the spindle device in which the main shaft is rotatably supported by the magnetic bearings, a system has also been suggested to detect the machining status in reference to the excitation current supplied to the magnetic bearings. However, mere support of the main shaft solely by means of the magnetic bearings makes it difficult to secure a high precision of high speed rotation and a high dynamic stiffness. Also, in the system of detecting the machining status in reference to the excitation current supplied to the magnetic bearings, an attempt has been made to detect the machining status associated with the frequency by the use of a frequency filter. However, to detect the machining status with respect to a number of frequency regions requires the use of an increased number of frequency filters, resulting in complicated structure and increase of the cost.
In order to achieve a highly efficient, highly precise machining, the spindle device is required of a type capable of achieving a high speed rotation with a high rotational precision. To satisfy this requirement, a non-contact bearing is suitable. The spindle device of a non-contact bearing supported type is available in some types for example, a spindle device utilizing an externally pressurized gas bearing and a spindle device utilizing a magnetic bearing. The spindle device with the externally pressurized gas bearing has a rotational precision generally in the order of {fraction (1/100)}&mgr;m and is therefore suited for the highly precise machining, but has a problem in that the static stiffness and the load bearing capacity are small. On the other hand, the spindle device with the magnetic bearing is excellent in terms of dynamic stiffness and load bearing capacity, but has a low accuracy of rotation of the main shaft. This is because the accuracy of rotation of the main shaft exhibited by the magnetic bearing depends on the resolution of a sensor disposed for detecting the position of the main shaft.
In general, the highly efficient, highly accurate machining is carried out in two stages including rough and finish machining processes. During the rough machining process, the amount of material that is machined per unitary time is increased to achieve a highly efficient machining, but during the finish machining process, the amount of the material to be machined is conversely reduced to achieve a highly precise machining process. For this reason, during the rough machining process, the load acting on the main shaft tends to increase and, therefore, the spindle device must have such a spindle performance requiring the stiffness and the load bearing capacity while rotation with a high precision is required during the finish machining process.
The combined externally pressurized gas-magnetic bearing assembly suggested in the previously discussed Japanese Laid-open Patent Publication No. 11-013759 is of a type effective to satisfy those requirements.
However, the combined externally pressurized gas-magnetic bearing assembly has characteristics peculiar to the externally pressurized gas bearing and those peculiar to the magnetic bearing and, where the sensor for the magnetic bearing has a low resolution, since the rotational precision of the main shaft depends on this resolution, a high rotational precision exhibited by the externally pressurized gas bearing cannot be effectively utilized.
Also, although the combined externally pressurized gas-magnetic bearing assembly of the type discussed above is a non-contact bearing, there is the possibility that the main shaft may contact a bearing surface in the event that an excessive load acts. To avoid such a contact of the main shaft, a protective bearing such as a rolling bearing has hitherto been employed in the prior art spindle device equipped with the magnetic bearing. However, since the combined externally pressurized gas-magnetic bearing assembly is of a design wherein the externally pressurized gas bearing is formed in the magnetic bearing unit, the gap between the main shaft in the bearing unit and a magnetic bearing stator is so small, for example, not greater than some tens microns and, therefore, the protective bearing in the form of the rolling bearing generally used in the spindle device with the magnetic bearing cannot be used. Also, since the externally pressurized gas bearing surface forms a part of an electromagnet for the magnetic bearing, material for the externally pressurized gas bearing surface is limited to a ferromagnetic substance having no lubricating capability. For this reason, in the event that an excessive load is applied to the spindle device, contact between the main shaft and the bearing surface brings about a detrimental influence on the bearing unit.
Also, in the spindle device utilizing the non-contact bearing such as the previously discussed combined externally pressurized gas-magnetic bearing assembly or externally pressurized gas bearing, as shown in
FIG. 29
, the main shaft
4
has a collar
4
a
formed therein, opposite end faces of the collar
4
a
being generally utilized to form axial bearing surfaces.
In the spindle device utilizing such a non-contact bearing, as discussed with reference to
FIG. 29
, the axial position (C dimension) of a tip of a machining tool
11
fitted to the main shaft varies depending on the dimension (B dimension) of a housing
5
as measured from the position P, at which the spindle is fitted and the collar
4
a
of the main shaft
4
and the dimension (A dimension) of the main shaft
4
as measured from the collar
4
a
of the main shaft and the tip of the machining tool
11
. The spindle fitting position P represents the position at which the housing
5
is mounted on a spindle support bench
76
that is reciprocally driven by a spindle positioning mechanism
54
.
When the spindle device of the structure described is driven at a high speed, the temperature of any of the main shaft
4
and the housing
5
Ozaki Takayoshi
Suzuki Nobuyuki
Cadugan Erica E
NTN Corporation
Sughrue & Mion, PLLC
Wellington A. L.
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