PROBLEMS
Grade 00 This set is kept in the standards room and is used for inspection/calibration of high precision only. It is also used to check the accuracy of the workshop and grade 1 slip gauges
6.5 MECHANICAL–OPTICAL COMPARATOR
pull on the metal strip subjects it to tensile force. In order to prevent excessive stress on the central portion of the metal strip, perforations are made in the strip, which can be noticed in Fig. 6.7. A slit washer is provided to arrest the rotation of the plunger along its axis.
6.4.3 Sigma Comparator
It is a simple but ingenious mechanical comparator deve- loped by the Sigma Instrument Company, USA. A linear dis- placement of a plunger is translated into the movement of a pointer over a calibrated scale. Figure 6.8 illustrates the working parts of a Sigma mechanical comparator.
The plunger is the sensing element that is in contact with the work part. It moves on a slit washer, which provides frictionless linear movement and also arrests rotation of the plunger about its axis. A knife edge is screwed onto the plunger, which bears upon the face of the moving member of a cross-strip hinge. This unit comprises a fixed member and a moving block, connected by thin flexible strips at right angles to each other. Whenever the plunger moves up or down, the knife edge drives the moving member of the cross-strip hinge assembly. This deflects an arm, which divides into a ‘Y’ form. The extreme ends of this Y-arm are connected to a driving drum by means of phosphor-bronze strips. The movement of the Y-arm rotates the driving drum and, in turn, the pointer spindle. This causes the movement of the pointer over a calibrated scale.
The magnification of the instrument is obtained in two stages. In the first stage, if the effective length of Y-arm is L and the distance from the hinge pivot to the knife edge is x, then magnification is L/x. The second stage of magnification is obtained with respect to the pointer length R and driving drum radius r. The magnification is given by R/r.
Therefore, overall magnification is given by (L/x) × (R/r).
Thus, the desired magnification can be obtained by adjusting the distance x by operating the two screws that hold the knife edge to the plunger. In addition, the second level of magnification can be adjusted by using driving drums of different radii (r).
COMPARATORS 149
a simple optical system wherein a pointed image is projected onto a screen to facilitate direct reading on a scale.
The plunger is spring loaded such that it is biased to exert a downward force on the work part. This bias also enables both positive and negative readings, depending on whether the plunger is moving up or down. The scale is set to zero by inserting a reference gauge below the plunger. Now, the reference gauge is taken out and the work part is introduced below the plunger. This causes a small displacement of the plunger, which is amplified by the mechanical levers.
The amplified mechanical movement is further amplified by the optical system due to the tilting of the plane reflector. A condensed beam of light passes through an index, which normally comprises a set of cross-wires. This image is projected by another lens onto the plane mirror. The mirror, in turn, reflects this image onto the inner surface of a ground glass screen, which has a scale. The difference in reading can be directly read on this calibrated screen, which provides the linear difference in millimetres or fractions of a millimetre. Optical magnifications provide a high degree of precision in measurements due to the reduction of moving members and better wear-resistance qualities.
With reference to Fig. 6.9, mechanical amplification = l2/l1 and optical amplification = 2 (l4/ l3).
The multiplication factor 2 figures in the optical amplification because if the mirror is tilted by θ°, then the image is tilted by 2θ° over the scale. Thus, the overall magnification of the system is given by 2 × (l4/l3) × (l2/l1).
6.5.1 Zeiss Ultra-optimeter
The Zeiss ultra-optimeter is another mechanical optical comparator that can provide higher magnification than the simple mechanical optical comparators explained in Section 6.4. This magnification is made possible by the use of two mirrors, which create double reflection of light. Figure 6.10 illustrates the working principle of the Zeiss ultra-optimeter.
It is preferable to have a monochromatic light source passing through a condenser lens followed by an index that carries the image of two cross-wires onto a tilting mirror (marked mirror 1 in the figure). Mirror 1 reflects the image onto mirror 2 (kept parallel to it), which is again reflected to mirror 1. After the reflection from three surfaces in succession, the light rays pass through an objective lens. The magnified image is formed at the eyepiece after passing through a transparent graticule. The graticule has a scale that enables the reading of linear displacement of the plunger.
The working of a comparator is quite similar to the one explained in Section 6.5. A movement of the plunger corresponds to the change in linear dimension of a work part with respect to a Screen/
Scale
Pivot Pivot
l1
l2
l3 l4
Lever
Light source
Projection lens Condenser
Plunger
Mirror Index
Fig. 6.9 Principle of a mechanical optical comparator
standard. The plunger movement tilts mirror 1, which moves the image of cross-wires over the scale. The scale thus directly provides the linear deviation and thereby provides a convenient means for inspection of work parts. The entire set-up is enclosed in a PVC enclosure and tubings. In order to set the instrument to zero, a screw is provided to move the projected image of the graticule over the scale. Subsequent readings are either plus or minus values, depending on whether a
dimension is larger or smaller than the set value, respectively.
6.5.2 Optical Projector
An optical projector is a versatile comparator, which is widely used for inspection purpose. It is especially used in tool room applications. It projects a two-dimensional magnified image of the workpiece onto a viewing screen to facilitate measurement. It comprises three main elements:
the projector itself comprising a light source and a set of lens housed inside the enclosure, a work table to hold the workpiece in place, and a transparent screen with or without a chart gauge for comparison or measurement of parts.
Figure 6.11 illustrates the various parts of an optical projector. The workpiece to be inspected is mounted on a table such that it is in line with the light beam coming from the light source.
The table may be either stationary or movable. In most projectors, the table can be moved in two mutually perpendicular directions in the horizontal plane. The movement is effected by operating a knob attached with a double vernier micrometer, which can provide a positional accuracy of up to 5 µm or better. The light beam originating from the lamp is condensed by means of a condenser and falls on the workpiece. The image of the workpiece is carried by the light beam, which passes through a projection lens. The projection lens magnifies the image, which falls on a highly polished mirror kept at an angle. The reflected light beam carrying the image of the workpiece now falls on a transparent screen. Selecting high-quality optical elements and a lamp, and mounting them at the right location will ensure a clear and sharp image, which, in turn, will ensure accuracy in measurement.
The most preferred light source is the tungsten filament lamp, although mercury or xenon lamps are also used sometimes. An achromatic collimator lens is placed in the path of a light beam coming from the lamp. The collimator lens will reorient the light rays into a parallel beam large enough in diameter to provide coverage of the workpiece. Mounting and adjustment of the lamp are critical to assure proper positioning of the filament with respect to the optical axis.
The collimated beam of light passes across the area the workpiece is positioned on the work table. Care should be taken to ensure that the contour of the workpiece that is of interest is directly in line with the light beam. The distance of the table from the projection lens should
Eyepiece Condenser
Mirror 2 Mirror 1
Work part Plunger
Lamp
Fig. 6.10 Zeiss ultra-optimeter Objective
lens
COMPARATORS 151
be such that it matches with the focal length of the lens, in order to ensure a sharp image. The table can be either stationary or movable. The movable tables are designed to generally travel in two mutually perpendicular directions in the horizontal plane. The table moves on anti-friction guide-ways and is controlled by the knob of a double vernier micrometer. This micrometer provides an accurate means of measuring the dimensions of the workpiece.
The light beam, after passing through the projection lens, is directed by a mirror onto the viewing screen. Screens are made of glass, with the surface facing the operator, ground to a very fine grain
size. The location of the screen should be such that it provides an accurate magnification and perfectly conforms to the measurement indicated by the micrometer. A reticle attached to the end of the projection lens provides images of two mutually perpendicular cross-wires, which can be used for the purpose of measurement. Many projector screens can also be rotated about the centre, thereby enabling measurement of angular surfaces also.
The following are the typical applications of profile projectors:
1. Inspection of elements of gears and screws
2. Measurement of pitch circle diameters of holes located on components
3. Measurement of unusual profiles on components such as involute and cycloidal, which are difficult to measure by other means
4. Measurement of tool wear (Drawing of a tool to scale is made on a tracing sheet. The tracing sheet is clamped on to the screen. Now, the used tool is fixed on the table and the image is projected to the required magnification. Using a pencil, one can easily trace the actual profile of the tool on to the tracing sheet. This image superimposed on the actual drawing is useful for measuring the tool wear.)