GB2152217A

GB2152217A - Arrangement for the optical measurement of screw-threads

Info

Publication number: GB2152217A
Application number: GB08428866A
Authority: GB
Inventors: Alois Erben
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1983-11-17
Filing date: 1984-11-15
Publication date: 1985-07-31
Also published as: GB8428866D0; GB2152217B; CH666548A5; DE3432057A1; DD221009A1

Abstract

An apparatus for the optical testing of screw-threads, using the silhouette or interference line technique, comprises a telecentric transillumination device (2) having a light source (3), aperture stop (4) and collimator (5), and a measuring microscope (6) for observing or optically scanning the object (1) to be measured. An aperture stop (4) is displaceable in the path of the beam of the transillumination device (2), the displacement being either actual by means of mechanical adjusting means (12) or virtual by means of optical elements which deflect the path of the beam. The magnitude (a) of the virtual or actual displacement of the aperture stop (4) is such that the slope of the path of the principle ray of the illuminating beam onto the object to be measured is equal to the pitch angle phi of the screw-thread to be tested. <IMAGE>

Description

SPECIFICATION Arrangement for the optical measurement of screw-threads The invention relates to an arrangement for the optical measurement of screw-threads for use in particular in connection with measuring apparata operating in accordance with the silhouette or interference line technique.

Optical measuring apparata, by means of which screw-threads are tested in a contactless manner by the silhouette method (Zill "Messen und Lehren in Maschinenbau und in der Feingeratetechnik'', 3rd Edition, DEB Verlag Technik, Berlin, pages 143 to 147) or by means of interference line (Feingeratetechnik 26 (1977), No. 9, pages 404-407), are known in the form of universal measuring microscopes or a two-coordinate measuring apparatus in which the object to be measured is usually telecentrically illuminated from below by means of a transillumination device.

The shadow contour of the screw-thread to be tested, or the interference line occurring, is scanned by an optical sighting device which is mounted on a swingable pillar of the measuring apparatus. In both methods, the sighting device (measuring microscope) and the transillumination device, which are interconnected and disposed on the pillar, are swung out of the zero position lying at right angles to the axis of the screw-thread through the mean pitch angle of the screw-thread to be tested, so that the optical axis of the illumination and sighting device in the centre of the flank of the screw-thread is a tangent to the surface of the screw-thread or is parallel to the tangent.

A traversing angle of opposite sign must be set upon transition from one measuring point on the screw-thread to the diametrically opposite measuring point.

This frequent swivelling when taking measurements, such as occurs in production testing, is very time-consuming and requires a relatively large amount of force, especially since the assemblies to be moved also have a relatively large mass. Moreover, in order to ensure high accuracy of the measurements, high demands are made on the precision of the swivel bearing, since any displacement of the optical axis in the direction of measurement during swivelling takes effect as a measurement error. The reasons for this are transverse and torsional forces which are effective during swivelling. Furthermore, the swivelling requires relatively large openings in the base plate and measuring table of the measuring apparatus. This results in the apparatus having a large volume since high structural stability for the apparatus must be ensured.

The requirement regarding accurate measurement in a third coordinate (Z coordinate) cannot be fulfilled with a swivellable microscope and illumination device. Conditioned by the degree of freedom of swivelling, the vertical position of the z movement of the surface of the measuring table cannot be realized as accurately as in the case of an apparatus having a rigidly disposed z guide.

An object of the present invention is to avoid the disadvantages of the prior art, to reduce the cost of the apparatus and to improve the operability and accuracy of measurement.

This is achieved by swinging the principle rays out of the zero position lying at right angles to the axis of the screw-thread in conformity with the angle of pitch at the measuring point of the screw-thread to be tested, by influencing the path of the principle rays of the illumination device without swivelling the illumination device and the measuring microscope.

In accordance with the present invention, there is provided an apparatus for the optical testing of screw-threads, using the silhouette or interference line technique, comprising a telecentric transiliumination device having a light source, aperture stop and collimator, and a measuring microscope for observing or optically scanning the silhouette of the object to be measured or the interference lines extending equidistant from the shadow contour, the illumination device and the measuring microscope having a common optical axis, the aperture stop being displaceable in the path of the beam of the transillumination device, the displacement being either actual by means of mechanical adjusting means or virtual by means of optical elements which deflect the path of the beam, and being at right angles to the optical axis and parallel to the axis of the screw-thread of the object to be measured, the magnitude (a) of the virtual or actual displacement of the aperture stop being such that the slope of the path of the principle ray of the illuminating beam onto the object to be measured is equal to the pitch angle ss of the screw-thread to be tested.

It is advantageous and a simple matter to dispose, in or on the housing of the illumination device, mechanical adjusting elements, preferably including a worm drive, which are operatively connected to the aperture stop, and adjustable, preferably mechanical, stops for limiting the displacement travel.

In order to achieve virtual displacement of the aperture stop when the latter is disposed concentrically and rigidly relative to the optical axis in the path of the beam of the illumination device, a plane-parallel plate swivellable by adjustable angular amounts, or a sliding lens, is disposed in the direction of light downstream of the aperture stop.

Thus, in measuring apparata in which the microscope and the illumination device have a common optical axis at right angles to the axis of the object to be measured (screw-thread).

the path of the principle ray, extending through the centre of the aperture stop of the illuminating beam is swung out of the zero position lying at angles to the axis of the screw-thread by the amount of the pitch angle + on the measuring point of the screw-thread to be tested. The maximum angle of traverse is limited only by the aperture of the objective of the measuring microscope and the aperture angle of the path of the illuminating beam.

Considerable advantages ensue when utilising the aforegoing arrangement. Thus, it is no longer necessary to swivel the pillar of the measuring apparatus when adjusting the total path of the measuring beam to the angle of pitch of the screw-thread to be tested. There is no need to provide an expensive pivot bearing for the pillar. Measurement errors caused by this bearing no longer occur. Openings required in the base plate and in the measuring table can be substantially reduced in size, thereby increasing the stability and rigidity of the entire measuring apparatus.

The invention will be further described hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows the path of the beam through one embodiment of a measuring apparatus in accordance with the invention; and Fig.2 shows the arrangement of a deflecting optical element in the path of the illuminating beam.

The path of the beam of a measuring apparatus is illustrated diagrammatically and greatly simplified in Fig.1. The arrangement for optically testing the screw-threads on objects 1 to be measured by the silhouette or interference line method comprises a telecentric transillumination device 2 having a light source 3, an aperture stop 4, and a collimator 5, and a microscope 6 having an objective 7, an eyepiece 8 and an adjustable reticule 9 in the image plane of the eyepiece, or having a photoelectric scanning arrangement (not illustrated) for observing or optically scanning the silhouette of the object 1 to be measured, or the interference lines extending equidistant from the shadow contour.The transillumination device 2 and the measuring microscope 6 are interconnected and have a common optical axis 1 0. During measurements, the optical axis 10 and the axis 11 of the screw-thread of the object 1 2 to be measured are at right angles to one another.

As is shown in Fig. 1, the telecentric aperture stop 4 is displaced by the distance a = f tan cp away from the optical axis 10 and parallel to the axis 11 of the screw-thread, f being the focal length of the collimator 5, and + being the pitch angle of the screw-thread to be tested on the object 1 to be measured.

Upon transition to the diametrically opposite side of the screw-thread, the aperture stop 4 is moved in the opposite direction away from the optical axis 10 by the distance a. This displacement of the aperture stop 4 is effected by means of mechanical adjusting means 12, such as a worm drive, which are operatively connected to the aperture stop 4 and which, preferably, are disposed on the housing 1 5 of the illumination device 2. The object 1 to be measured is, as it were, illuminated in the direction of the pitch of the screw-thread as a result of displacement of the aperture stop 4.

The diameter of the optical system of the illumination device (collimator 5) and the measuring microscope 6 (objective 7) must be such that, in addition to the principal illuminating ray 1 3, the aperture marginal ray 1 4 is picked up, and hence all beams which slope at an angle ~ ss + 8 relative to the optical axis 10, 6, being the aperture angle of the path of the illuminating beam. Advantageously, in order to achieve rapid displacement of the aperture stop 4 to diametrical measurements on the object 1 to be measured, adjustable mechanical stops 1 6 are provided which are adjusted in conformity with the pitch angles on the leading and trailing surfaces of the screw-thread.

Fig.2 shows a part of a transillumination device 2' having an aperture stop 1 7 disposed centrically and rigidly relative to the optical axis 1 0. An optical element 1 8 in the form of a plane-parallel plate, pivotable through adjustable angular amounts, is provided downstream of the aperture stop 1 7 in the direction of light for the purpose of deflecting the path of the illuminating beam.

Alternatively, a known sliding lens may be used instead of the plate. Adjustable stops 1 9 are provided to limit the angle of traverse of the optical element 1 8. The aperture stop 1 7 is displaceable virtually on both sides at right angles to the optical axis 10 and parallel to the axis 11 of the screw-thread by means of the optical element 18.

Claims

1. An apparatus for the optical testing of screw-threads, using the silhouette or interference line technique, comprising a telecentric transillumination device having a light source, aperture stop and collimator, and a measuring microscope for observing or optically scanning the silhouette of the object to be measured or the interference lines extending equidistant from the shadow contour, the illumination device and the measuring microscope having a common optical axis, the aperture stop being displaceable in the path of the beam of the transillumination device, the displacement being either actual by means of mechanical adjusting means or virtual by means of optical elements which deflect the path of the beam, and being at right angles to the optical axis and parallel to the axis of the screw-thread of the object to be measured, the magnitude (a) of the virtual or actual displacement of the aperture stop being such that the slope of the path of the principle ray of the illuminating beam onto the object to be measured is equal to the pitch angle (p of the screw-thread to be tested.

2. An apparatus as claimed in claim 1, wherein mechanical adjusting elements, which are operatively connected to the aperture stop, and adjustable stops for limiting the displacement travel, are disposed in or on the housing of the illumination device.

3. An apparatus as claimed in claim 2, wherein said mechanical adjusting elements comprise a worm drive.

4. An apparatus as claimed in claim 2 or 3, wherein said adjustable stops are mechanical stops.

5. An apparatus as claimed in claim 1, wherein, with the aperture stop disposed concentrically and rigidly relative to the optical axis in the path of the beam of the illumination device, a plane-parallel plate swivellable by adjustable angular amounts, or a sliding lens, is disposed in the direction of light downstream of the aperture stop.

6. An apparatus for the optical testing of screwthreads, substantially as hereinbefore described with reference to and as illustrated in Fig.1 or in Fig.1 as modified by Fig.2 of the accompanying drawings.