Engineeringdev

Electrical Comparator The principle of these comparators is to convert the linear displacement of the measuring stylus into an electric output Whetstone bridge circuit is used for this comparator When the circuit is balance no current is detected Hence the following condition will exit R1 = R3 R2 R4 Electrical Comparators are extremely sensitive and are capable of high magnifications up to 30000. If one of these resistors is varied then imbalance in the circuit will result in a current flow through the galvanometer By arranging the variable resistor a measuring head mechanism is formed The galvanometer may be calibrated to read linear displacement The whetstone bridge circuit is suitable only for D.C supply Although the operating principle is same certain modification is that the resistors are replaced by inductors Two of which are incorporated in the measuring head When the stylus moves vertically, the circuit will become imbalance This is due to the m

Optical Comparator This is the commercial measuring instrument This utilizes a plunger tilted mirror, objective lens,prism and observing eye piece to provide a high degree of magnification The mirror is mounted on a knife-edge It can be tilted about the fulcrum by any linear vertical movement of the contact plunger A beam of light passes through a graticule suitably engraved with a linear scale The movement of mirror causes this scale to move up (or) down past a translucent screen inside the observing hood of the instrument The eye placed near the eye piece views the image of a small scale engraved on scale after reflection from the plunger actuated mirror The” plan” view for the mirror is shown in fig In the focal plane of the eye piece , a fine reference line ( index 0) is provided The system of lenses is so arranged that the image of the scale is projected in the same focal plane Thus with movement of scale the image can be measured with reference to the fixe

Principle of pneumatic comparator The measurement is made by taking reading using master gauge of known accuracy and comparing with component reading It uses a controlled, pressurised jet of air to measure small dimensional variation in the size of component The working principle of pneumatic comparator depends on the flow of air between the gauge head and the component being checked Important characteristics Very high magnifications are possible It can be used to measure diameters, length, squareness, parallelisms, concentricity,taper, and other geometric conditions. Internal dimensions can be readily measured , in other words, while measuring a bore it can reveal complete details of size, taper, straightness,camber and bell mouth etc. It is independent of the operator skill High pressure air gauging can be done by cleaning the parts which helps to eliminate errors due to dirt and foreign matter. As there is no physical contact made either with the setting gauge (or)

Introduction If four people take a measurement with the same instrument,it is for sure that there will be discrepancies in their readings. Hence precision measurements with consistent accuracy, the human element should be completely eliminated This is achieved by using instruments called comparators Classification of comparators : Depending on the method by which the difference in dimensions is magnified,the comparators are classified as 1.mechanical 2.electrical 3.optical 4.pneumatic In general,mechanical comparators are limited to the lower magnification Pneumatic comparators are the most accurate type in their operation Use of comparator Comparator is an indirect type of instrument with the help of which an unknown dimension of a work piece is compared with a working standard (usually slip gauges) Micro-comparator is one of the instruments used for checking various gauges in the industry. It ensures that the wear effects do not allow the gauge dimensions t

Measurement The surface texture represents the combined effect of several causes,of which the roughness,waviness and the error of form are more predominant If only the total height is considered then idea of above three characteristics can be had by considering different sample lengths Surface Texture If only surface measurement is considered then the sampling length should be neither too big as to include the waviness, nor too small as to ignore the occasional deep scratches Determination of roughness value To compute the roughness value by taking mean results from the measurement of several sampling lengths taken consecutively along the profile So that it gives better indication and is the correct representation of the whole of surface roughness Roughness comparison specimens The roughness comparison specimens are used for qualitative assessment of the surface roughness Qualitative assessment of the roughness of the finished product is evaluated by vis

Surface roughness On any finished surface,imperfections are found and these take the form of a succession of hills and valleys Which vary both in height and in spacing and result in a kind of texture which in appearance or feel. Straight edge Straightedges are commonly used for testing the straightness and flatness of plane surfaces Flatness is checked by a light test The narrow edge of the tool is applied to the surface to be tested and kept horizontally at the eye level Watching for light between the straightedge and the surface along the straightedge Light seen at one place or another indicates deviation from straightness Surface plate Surface plate is used for testing the flatness of work itself and is also used for marking out work This is used for small pieces of work while the marking -out table is used for larger jobs It is used in conjunction with other gauges,the plate becomes a functional gauge in itself. Surface gauge Surface gauge is used in lay

Autocollimator The autocollimator is an optical instrument and is used to measure angular displacements with high accuracy of 0.2 seconds arc It can also be used to measure the flatness of a large surface Collimation of light If a point source of light is placed at the focal point of convex lens ,parallel rays of light are reflected by the lens. This is known as collimation of light A device with a lens suitable for causing light rays from a point source to become parallel is known as a collimato r Auto collimation of light Auto collimation of light occurs when reflecting surface is used in conjunction with a collimation lens system The collimation rays will be reflected back from the surface These rays pass through lens ,and unless reflecting surface is perfectly tangential to the lens axis ,will be brought to a focus along side of the light source Principle of the autocollimator From the fig. The light in the form of parallel rays , i

Interferometer Interferometer is an optical device used for measuring flatness of a surface and also for determining the thickness of slip gauges. It is an extension of the application of optical flat. Interferometers are based upon the principle of “ interference “ and employ wavelength of light as their measuring units. The interferometers make use of beam divider that splits an incoming ray into two parts. These two parts of the ray travel along different paths until they are recombined, usually in the same beam divider. An interferometer is an optical device which utilizes the effect of interference. Typically, it starts with some input beam, splits it into two separate beams with some kind of beam splitter (a partially transmissive mirror), possibly exposes some of these beams to some external influences (e.g. some length changes or refractive index changes in a transparent medium), and recombines the beams on another beam splitter. The power or the spatial shape of the resulti