Field of the invention The present invention relates to an alignment method and device for use in the measurement of the run out of a rotating spindle.

Background of the invention

It is desirable to measure the run out of a rotating spindle. Where the spindle is the main shaft of a machine tool, the measurement of run out will give a measure of the working accuracy of the machine and may be used for sales support for the machine tool. During the lifetime of the machine tool, run out measurement is useful for diagnostic, maintenance, and quality control purposes. When a spindle is running true, its axis of rotation should remain

perfectly stationary. However, this condition is not even achieved in precision machines. In practice, even when new, some inaccuracy will exist and will cause its axis to wobble as the spindle rotates about its mean centre of rotation. The run out is a measurement of the maximum radial movement of the spindle axis relative to the mean centre of rotation.

Rotary shaft encoders are used in industry to measure the angular position and speed of rotation of a spindle. A shaft encoder comprises a wheel that is coupled to the spindle and has a number of features around its periphery. A stationary sensor, which is positioned to detect the

features, generates a signal pulse as each feature passes it by. The number of pulses indicates the angular displacement of the spindle, while the frequency of the pulses is a measure of the rotational speed. As an example, if the disc has one hundred features, then fifty signal pulses would signify that the spindle had performed half a revolution. There are several types of rotary shaft encoder. The wheel may be a flat disc or a drum and the features may be slots that are sensed optically, or magnetic stripes recorded on the cylindrical surface of a drum. Still other encoders use a toothed gear with optical or magnetic sensors to detect the passage of the individual teeth.

Apparatus for measuring the lateral displacement of rotating shafts (such apparatus is hereinafter referred to as an Incremental Motion Encoder, or IME for short) is available from Gyrometric Systems Limited which uses a wheel of the same type as used in a shaft encoder. In this apparatus, instead of sensing the features at only one location around the periphery of the disc, they are sensed at three or more locations using separate sensors. The signals from the sensors are processed to indicate radial displacement of the spindle.

In order to achieve great accuracy, timing values or time stamps for the passage of each feature by each encoder are collected and processed. This apparatus is described in EP 0 608 234. The processing can be understood in principle by considering the effect of a lateral displacement of the spindle at one of the sensors. The displacement would produce a phase shift indicative of the displacement in a direction parallel to a tangent to the wheel at the position of the sensor concerned. By processing all the timing values measured at all the sensors a full plot of radial

displacement can be produced. Effectively, run out will result in fluctuations in the relative phase of the signals from the different sensors and these phase differences provide an indication of the run out of the spindle.

Currently available IME apparatus can measure run out to an accuracy of one tenth of a micron on spindles rotating at speeds ranging from 15 rpm to 20,000 rpm. However, in order to achieve such accuracy, special attention needs to be paid to the manner in which the apparatus is set up. In particular, if the run out of the spindle of a machine tool is to be measured, one needs first to mount the wheel so that its centre coincides with the axis of the spindle. The sensors must then be mounted so that they cannot move relative the machine bed but they too must be correctly centred on the disc while making contact with neither the wheel nor the spindle. Object of the invention

The aim of the present invention is to provide a device and method for simplifying the correct alignment of the wheel and the sensors of an IME apparatus relative to a spindle of which the run out is to be measured.

Summary of the invention

According to a first aspect of the present invention, there is provided a device for aligning a wheel and sensors of a run out measurement apparatus with a spindle of which the run out is to be measured, the device comprising a shaft connectable for rotation with the spindle for supporting the wheel and a stationary housing surrounding the shaft for supporting the sensors, wherein a collar is provided between the shaft and the housing that is movable axially relative to the housing and the shaft between an engagement position, in which a conically tapering surface on one of the shaft and the collar makes contact with a surface on the other to centre the collar relative to the shaft, and a release a position, in which the collar is spaced from the shaft to permit the shaft axis to run out without the shaft radially impacting either one of the collar and the housing. According to a second aspect of the invention, there is provided a method of aligning a wheel and sensors of a run out measuring apparatus, which method comprises mounting the wheel on a shaft secured for rotation with a spindle of which the run out is to be measured, mounting the sensors on a housing surrounding the shaft, providing a collar between the shaft and the housing that is movable axially relative to the housing and the shaft, providing mating surfaces on the collar and the shaft of which at least one surface is conically tapering, moving the collar relative to the housing to cause the mating surfaces to contact one another and thereby centre the housing on the shaft, clamping the housing in the centred position, and moving the collar axially in a direction to create a gap between the mating surfaces .

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which :

Figure 1 is a section through an alignment device of the invention, with the top half of the drawing showing the device in an engagement position and the lower half showing the device in a release position, and

Figure 2 is an end elevation of the alignment device as viewed from the right in Figure 1.

Detailed description of the preferred embodiment

The drawings show in dotted lines a spindle 10 of a machine tool of which the run out is to be measured. The measurement is performed by a known IME apparatus which comprises a wheel 12 in the form of a disc with radial slots and three sensors 14, as shown in Figure 2, positioned around the perimeter of the wheel 12 to detect passage of the slots. The signals generated by the three sensors 14 are processed to provide a measure of the run out. To obtain reliable readings, it is necessary to ensure that both the wheel 12 and the three sensors 14 are

accurately centred on the axis of the spindle 10. In use, the wheel 12 is required to rotate with the spindle 10 while the sensors should remain perfectly stationary.

To achieve these objectives, the wheel 12 is mounted on one end of a shaft 20 of which the other end is secured to the spindle 10. For this purpose, the opposite end 22 of the shaft 20 has a gentle taper to form a taper lock with a tapered hole in the spindle. The shaft 20 has a second conically tapering section 24 the purpose of which will be described below. The sensors 14 are mounted on one end of a housing 30 that surrounds the shaft 20. The housing 30 is formed as a hollow cylinder with a cover 32 screwed onto its opposite end from the sensors 12. Shoulders at the opposite ends of the shaft 20 retain it within the housing while allowing some relative axial movement. The cover 32 has an

externally screw threaded inwardly extending annular

projection 34 that surrounds the shaft 20 with clearance.

The projection 34 is engaged by a nut 36 that can be turned from outside the housing 30 in any convenient manner. In the illustrated embodiment, this is achieved by providing arcuate apertures 40 in the side wall of the housing 30 to enable a pin to be inserted into holes 42 in the side wall of the nut 36. The nut 36 serves to set the position of an annular collar 50.

The collar 50 has a cylindrical outer surface 51 slidably mounted in, and accurately guided for axial

movement by, a cylindrical bore 35 defined by the inner wall of the housing 30. The collar 50 is urged against the nut 36 by a spring 44 that acts between a shoulder 47 of the housing 30 and a flange 52 on the end of the collar 50. The inner surface of the collar 50 is formed with a conical section 54 of the same angle of taper as the section 24 of the shaft 20. In order to align the IME apparatus 12,14 with the spindle 10, the nut 36 is turned in a direction to move the collar 50 to the right, as viewed in Figure 1, while the housing 30 is undamped and free to move in all directions relative to the shaft 20. The mating of the two conical surfaces 24 and 54 will automatically align the collar 50 with the axis of the shaft 20. Continued rotation of the nut 36 after the conical sections 24 and 54 have mated with one another will move the shaft 20 and the housing 30 axially relative to one another until an end surface of the shaft 20 comes to bear on a thrust surface of the housing 30. At this time, the shaft 20 and the housing 30 will be in an

engagement position in which the shaft is correctly centred within the housing 30 and the wheel 12 and in a fixed axial position relative to the sensors 14. Such tightening of the nut 36 can be performed either before or after the tapered end of the shaft 20 has been inserted into the spindle.

With all the components of the IME apparatus now correctly aligned and the tapered end 22 of the shaft 20 inserted into the spindle 10, the housing 30 is firmly clamped in any suitable manner so that its position does not change during measurement of the spindle run out. The preferred manner of clamping the housing 30 is to form the housing 30 of a ferromagnetic material and to hold it in position by an electromagnet. The electromagnet preferably acts on the axial end face of the housing 30 and is itself securely attached to the machine bed. This method of

clamping offers the advantage that the housing can be moved freely until the electromagnet is activated and energising the magnet does apply any radial force which would tend to bend the shaft. With the housing clamped firmly in place, the nut 36 is then loosened. The collar 50, which follows the movement of the nut 36 on account of the spring 44, moves to the left as viewed to cause the conical sections 24 and 54 to separate, as shown in the lower half of Figure 1. The IME apparatus is now correctly set up for run out measurement to be

commenced .

It is important not to use too narrow an angle of taper as taper lock between the sections 24 and 54 is to be avoided. Furthermore, the smaller the angle of taper, the more the nut 36 has to be turned to move the collar 50 to a position in which the shaft is left free to run out. In the preferred embodiment, the angle of taper lies in the range of 8° to 12°.

The only contact that remains between the shaft 20 and the housing 30 after the nut 36 has been loosened is that occurring at the axial end of the shaft 20. Thus, when the spindle 10 is turned, the shaft 20 is free to run out to the same degree as the spindle 10, thereby enabling the run out to be measured by the IME apparatus .

It will be appreciated that various modifications may be made to the illustrated embodiment without departing from the scope of the invention as defined in the appended claims. For example, it is not essential for both of the mating surfaces of the shaft 20 and the collar 50 to be conical, and if only one conical surface is present it may be formed either on the shaft 20 or on the collar 50.