1. Field of the Invention
The present invention relates to an elastic fixture and an attachment method for mounting a length measuring apparatus on an object to be measured having a different linear expansion coefficient from that of the apparatus. In particular, the present invention relates to an elastic fixture for a length measuring apparatus which is suitably employed in a unit type linear scale fixedly attached, along with a main scale made of glass contained in a frame made of aluminum, to a machine body made predominantly of iron, and which can absorb large thermal stresses caused by the difference in linear expansion coefficients. The present invention also relates to a method of attaching a length measuring apparatus by means of the elastic fixture.
2. Description of the Related Art
A unit type linear scale (herein after referred to as a linear scale) of a multi-point fixing type, such as shown in FIG. 1, has been known as a linear scale for feed-back control employed in a relatively large machine tool or industrial machine in which the generation of vibration or shock, or the like is expected.
In general, the linear scale (a length measuring apparatus) 10 includes a frame 12 made of aluminum and a detection head 16. The frame 12 contains a main scale made of glass. The detection head 16 contains a detection unit, such as an index scale, which is relatively movable with respect to the frame 12. Both the frame 12 and the defection head 16 are securely attached to a stage sliding unit of a machine body (not shown) through, for example, screws 20.
When the ambient temperature of the linear scale 10 securely attached in the manner described above is changed, thermal stress caused by the difference in linear expansion coefficients is generated between the aluminum frame 12 of the linear scale 10 and the iron machine body. Thus, deflections and distortions are generated in the frame 12 and the glass scale contained within the frame 12. In addition, the screws 20 securely attaching the frame 12 may be loosened, and the relative position with respect to the machine body may be changed. These may result in the deterioration of the measurement accuracy. Therefore, a mechanism for reducing the thermal stress should be provided in the fixing portions of the frame 12.
In order to solve the above problems, the present applicant has proposed an example of the attachment mechanism for the frame 12 in Japanese Patent Laid-Open Publication No. 2003-97936. This attachment mechanism is illustrated in FIG. 2 (a perspective view), FIG. 3 (an enlarged front view of a III-portion of FIG. 2), and FIG. 4 (a schematic illustration of the operating state).
In this example, an intermediate-portion fixing block 30 for permanent attachment is provided on a central portion of the frame 12 for maintaining the relative alignment with respect to a mating plane 8 of the machine body. In addition, an end-portion fixing block 32 is provided on each of both the ends of the frame 12. The end-portion fixing block 32 includes a parallel plate spring mechanism 34 for reducing the thermal stress connected to the frame 12 via a plate spring mechanism 13 of the frame 12.
In the case of employing the fixing block 32 described above, even when the frame 12 is expanded due to the ambient temperature change, the thermal stress is absorbed and reduced (relieved) through the deformations of a plate spring 13A of the plate spring mechanism 13 and plate springs 34A and 34B of the parallel plate spring mechanism 34, as shown in FIG. 4. In addition, since the frame 12 is permanently attached by the intermediate-portion fixing block 30 at the center, the relative alignment with respect to the mating plane 8 is unchanged.
However, the functions, as a plate spring, of the plate spring mechanism and the parallel plate spring mechanism disclosed in Japanese Patent Laid-Open Publication No. 2003-97936 strongly depend on the materials of parts, thicknesses T1 and T2 of the plate spring, the lengths L1 and L2 of the plate spring, and the like. Therefore, the sizes of the parts and the machining accuracy of the parts are restricted, and the parts are difficult to manufacture. As a result, for example, the reduction of the sizes of the end-portion fixing block 32 in the cross-sectional direction in FIG. 3 and the reduction of the parts cost are difficult to achieve.
In addition, a unit type linear scale for a manual machine tool is required to be used in relatively severe temperature environments. In order to apply the spring mechanisms to this unit type linear scale for a manual machine tool, the temperature range and the limit of the applicable length should be improved.