Optical inspection of parts of revolution with several diameters is a challenge with a standard apparatus with a vertical optical axis because it is difficult to guarantee the perpendicularity of the part to the observation axis.
Figure 1: Examples of parts of revolution with several diameters
The diagram below describes the difficulty: with a vertical optical system, the length observed by the sensor (1, for example a camera) is a projected length (l ') which can be wrong if the axis of revolution of the part (2) is not perfectly perpendicular to the optical axis.
Figure 2: "Seen" versus actual length
To ensure this perpendicularity, V-shaped workpiece supports are often used.
Figure 3: Standard V-shaped bar turning part support
The part is placed on V-blades (3) on its ends of different diameters. As a first step, the position of the blades along the axis of rotation of the part must be adjusted so that the part is carried without masking its measurement positions, that is to say its reference planes. During this positioning, the part must be handled with care so that it is neither lost nor damaged. So that the axis of rotation of the part is perpendicular to the observation axis (the optical axis), the position of the blades in height must be adjusted. To control the perpendicularity, a planing operation is applied: it consists in observing and comparing the sharpness of the images of the ends of the part and of adjusting, in an iterative manner, the position of the blades in height. The use of a standard V-shaped support is not easy, because of the complexity and the duration of these two positioning operations.
To no longer support the part on two boards, the part can be placed between two flat transparent supports (6) which create a certain angle between them. The room can naturally "tip" due to gravity. The optical path of the light emitted by the source (4) is modified by using two angle transmitters. Thus, the image seen by the sensor (1) is no longer a top view, but a side view of the part and the dimensions seen correspond to reality. It is thus no longer necessary to precisely position the part along its axis of rotation. It is also no longer necessary to carry out the alignment operation.
Figure 4: Basic principle of QMVLine. On the left the optical path and on the right the "view" by the sensor
Qualimatest has developed a measurement application compatible with QMTVLine in its QMTMesure-Base software and is currently adapting all other applications to this device. QMTMesure-Pro and QMTMesure-Expert are already compatible. To facilitate the use, improvements are made to this simple and patented concept to make it compatible with multi-piece measurement.