Linear Measurement 596v1x

Linear Measurement • Linear measurement includes the measurement of lengths, diameters, heights and thickness including External and Internal Measurement. • The basic principle of linear measurement is that of comparison with standard dimensions on a suitably engraved instrument or device.

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Classification of Linear Measurement • Linear measuring instruments are categorized depending upon their accuracy. 1. Non-precision instruments 1. Steel rule 2. Caliper 3. Divider

2. Precision instruments 1. Micrometers 2. Vernier Caliper 3. Slip Gauge 4. Vernier height and depth Gauge

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Classification of Linear Measurement • Classification based on Method of Measurement: 1. Direct measuring Instrument 2. Indirect measuring Instrument

• Instrument also can be classified : 1. Graduated Instrument 1. 2. 3. 4.

Micrometers Vernier Caliper Steel rule Vernier height and depth Gauge

2. Non- Graduated Instrument 1. Caliper 2. Slip Gauge 3

Steel Rule • It is the simplest and most common linear measuring instruments in inspection. • It works on of comparing an unknown length to be measured with the previously calibrated length. • The rule must be graduated uniformly throughout its length. • Rules are made in 150, 300, 500 and 1000 mm length. • The degree of accuracy when measurements are made by a steel rule depends upon the quality of the rule, and the skill of the .

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Calipers • Calipers are used for measurement of the parts, which cannot be measured directly with the scale.( Circular Part) Thus, they are accessories to scales. • A caliper is used to transfer the distance between the faces of a component to a scale or micrometer. • Calipers are of two types : 1. Spring type Calipers 2. Fixed t (firm t) type Calipers

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Fixed t (firm t) type Calipers • They work on the friction created at the junction of the legs. • The two legs are identical in shape with the points equally distant from the fulcrum and are ed together by a rivet. • The legs become certain loose after certain use but can be adjusted easily. • They can be designed for outside as well as inside measurement. • These calipers are available in the nominal size of 100, 150, 200, 250 and 300 mm.

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Fixed t (firm t) type Calipers

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Spring type Calipers • Spring caliper are improved variety of ordinary friction t caliper. • The caliper are adjusted to set dimension by means of knurled nut on a threaded adjusting screw. • As the name explains, the two legs are attached with spring in this type of calipers. • The tendency of the spring is to force the legs apart and the distance between them can be adjusted by applying the spring pressure by tightening the nut. • Spring caliper are more accurate and permit accurate sense of touch in measuring. • The inside and outside calipers are available in the size of 75, 100, 150, 200, 250 and 300 mm. 8

Spring type Calipers

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Outside Calipers • These are designed to measure outside dimensions. • An outside caliper has two legs which are bent inwards as shown in fig. • It can be used for measuring or comparing diameters, thickness and other outside dimensions by transferring the reading to the steel rule or micrometer.

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Outside Calipers

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Inside Calipers • These are designed to measure inside dimensions. • An inside caliper has two legs which are bent outwards at the end as shown in fig.

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Inside Calipers

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Transfer Calipers • These are used for measuring recessed areas from which the legs of calipers can not be removed directly but must be collapsed after the dimension has been measured. • Therefore, an auxiliary arm is provided with two legs so that it can preserve the original setting after the legs are collapsed.

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Divider • A divider is similar in construction to a caliper except that both legs are straight with sharp hardened points at the end as shown in Figure. • Divider are used in the process of marking out suitable work pieces. A steel scale must be used with this instrument. • These are used for scribing arcs and circles and general layout work.

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Vernier Calipers • The vernier calipers give a direct reading of the distance measured with high accuracy and precision. • Vernier calipers can measure internal dimensions (using the uppermost jaws), external dimensions using the lower jaws and depth by the use of a depth bar that is attached to the movable head and slides along the centre of the body.

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Vernier Calipers • Principle : It uses two scales (main and auxiliary scales) or division slightly different size are used, then the difference between them can be utilized to enhanced the accuracy of measurement.

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Vernier Calipers Parts of a vernier caliper: • Outside jaws: used to measure external diameter or width of an object • Inside jaws: used to measure internal diameter of an object • Depth probe: used to measure depths of an object or a hole

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Least count of Vernier Calipers • Ratio of smallest division on main scale to total no. of division on vernier scale.

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Measurement with Vernier Calipers • To read a measurement from a vernier caliper, first the main scale reading up to the zero of the vernier scale is noted down. It will give accuracy up to the smallest division of the main scale. • Now, vernier number of vernier scale division from its zero, which coincides exactly with the main scale is noted. This number when multiplied with the Least count gives the vernier scale reading. The actual length is obtained when the vernier scale reading is added to the main scale reading.

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Measurement with Vernier Calipers • An enlarged view of the calipers in Figure shows they have an accuracy of 0.02 mm. • The 3 mm is read off from the upper (fixed) scale. The 0.58 mm is obtained from the lower (sliding) scale at the point of closest alignment between the two scales. • The reading is 3.58 mm.

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Vernier Height Gauge • It works on the principle of vernier caliper except that it has to be used in conjunction with a surface plate. • The vernier height gauge is mainly used in inspection of parts and layout work. • It may be used to measure & mark vertical distance above a reference surface. • The reading taken similar to vernier caliper.

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Vernier Height Gauge

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Vernier Height Gauge 1.base 2.beam 3.vernier scale 4. fine setting device 5. vernier plate 6. locking screws 7. scriber

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Vernier Depth Gauge • A vernier depth is very commonly used precision instrument for measuring depth of holes & slot. • Its method of reading are similar to those of a vernier caliper. • Vernier depth gauge consist of main scale, vernier scale, base and locking screw. • In vernier depth gauge graduated scale can slide through the base and vernier scale remain fix.

Pushpak Patel, Asst. Professor,CSPIT,Changa

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Vernier Depth Gauge 1. 2. 3. 4. 5.

Base Graduated beam ( Main Scale) Clamping screw Fine adjustment mechanism Vernier scale

Pushpak Patel, Asst. Professor,CSPIT,Changa

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Vernier Depth Gauge

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Micrometer

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Micrometer • WOKING PRINCIPLE: The micrometer works on the principle of screw and nut .The longitudinal movement of the spindle during one rotation is equal to the pitch of the screw. The movement of the spindle to the distance of the pitch or its fractions can be accurately measured on the barrel and thimble.

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Parts of Micrometer • Spindle : one end of the spindle is the measuring face. The other end is threaded and es through a nut. The threaded mechanism allows for the forward and backward movement of the spindle. • Spindle lock nut : The spindle lock nut is used to lock the spindle at a desired position.

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Parts of Micrometer • Thimble : On the beveled surface of the thimble also , graduation is marked. The spindle is attached to this. forward and backward movement of the spindle is controlled by rotation of Thimble. • Barrel : It has fixed graduation which are clearly engraved on it. • Anvil: The anvil is one of the measuring faces which is fitted on the micrometer frame. It is made of alloy steel and finished to a perfectly flat surface. • Ratchet : The ratchet ensures a uniform pressure between the measuring surfac e.

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Micrometer • Least count of micrometer is defined as value of smallest division on main scale ( Barrel) to the total no. of division on Thimble . • L.C. = 0.01 mm

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Micrometer Reading • barrel reading = 13.50 • thimble reading = 0.13 • total = 13.63 mm

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Types of Micrometer • • • • •

Outside micrometer Inside micrometer Depth micrometer Digital micrometer Screw Thread Micrometer

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Types of Micrometer

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Inside Micrometer • It is used to check internal dimensions such as diameter of a hole and width of a slot. • Unlike the conventional micrometer, an inside micrometer does not have any U-shape frame and spindle. • One of the jaws is held stationary at the end and second one moves by the movement of the thimble. A locknut is provided to stop the movement of the movable jaw.

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Inside Micrometer • The inside micrometer is intended for internal measurement to an accuracy of 0.01 mm. • In principle, it is similar to an external micrometer and is used for measuring holes with a diameter over 50 mm. It consists of : (a) measuring unit (b) extension rod (c) handle.

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Inside micrometer

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Inside micrometer

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Depth micrometer • Depth micrometers is used to measure the depth of holes, slots and recess area. •

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Screw Thread Micrometer • Screw Thread Micrometer is used to measure measuring pitch diameter of screw thread.

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Slip Gauge / Gauge Blocks • Slip gauges are rectangular blocks of high grade hardened steel having a cross-section of about 9 mm x 30 mm. • These are first hardened to resist wear and carefully stabilized so that they are independent of any subsequent variation in size or shape. • After being hardened, blocks are carefully finished on the measuring faces to such a fine degree of finish(Lapping), flatness and accuracy .

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Slip Gauge / Gauge Blocks

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Wringing of Slip Gauge • The combination is made by pressing the faces into and then imparting a small twisting motion while maintaining the pressure. This is called wringing. • Wringing occurs due to molecular adhesion between a liquid film and the mating surface.

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According to accuracy, the slip gauges can be graded into categories : • Reference grade : these slip gauge are used as standards by manufacturing slip gauges. • Calibration grade : these slip gauge is used where highest level of accuracy required in normal engineering practices. these gauge are used with comparator to calibrate the other slip gauge. • Grade 00 : This grade would be kept in the Standard Room and would be kept for work of the highest precision work as checking other lower grade slip gauges (Grade I & II). • Grade 0 : This is more commonly known as the inspection grade, and its use is to tool room or machine shop inspection where high precision required. Pushpak Patel, Asst. Professor,CSPIT,Changa

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According to accuracy, the slip gauges can be graded into categories : • Grade I: these slip gauge are used for general purpose manufacturing gauges in applications like tool, gauges and component productions. • Grade II : these slip gauge is used for checking of components having wide tolerances. These are also known as workshop grade.

Pushpak Patel, Asst. Professor,CSPIT,Changa

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Normal Set of Slip Gauge • Slip gauges of the following dimensions are available in this type of set.

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Special Set of Slip Gauge • Slip gauges of the following dimensions are available in this type of set.

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Application of Slip Gauge • They are used to check the accuracy of verniers, Micrometers and other measuring device. • They are used to set the comparator to a specific dimension. • They are used for direct precise measurement where the accuracy of work piece is important. • They are frequently used with sine bar to measure angle of work piece. • They can be used to check gap between parallel locations.

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COMPARATORS • A comparator is a precision instruments used for comparing the dimension of a workpiece with a working standards or master setting which represents the basic size. • A comparator works on relative measurements, it does not measure the actual dimension but indicates how much differs from the basic dimension. • The indicated difference is normally small and hence suitable magnification device is provide to measure the difference with consistent accuracy.

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Characteristics Of Good Comparator • It should be compact and easy to handle. • The comparator design must be such that it can be used for wide range of operations. • The display or indicating system should be display result in least possible time. • It should have less maintenance. • It should free from friction and wear. • The recording and measuring scale should be linear and uniform. It should be record and measure small variation in the dimension. • The instruments should be very sensitive as per requirement.

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Application of Comparator • The comparators are used to check the parts in mass production at a very fast rate. • They are used to inspect newly purchased gauges. • They are work as a laboratory standards from which working or inspection gauges are set and correlated. • They are also used as working gauges to prevent work spoilage and to maintain required tolerance at all important stage of manufacturing.

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CLASSIFICATION OF COMPARATORS (1)Mechanical comparators (2) Mechanical optical comparators (3) Electrical and electronics comparator (4) Pneumatic comparators (5) Fluid displacement comparators (6) Projection comparators (7) Multi check comparators (8) Electro mechanical comparators

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Comparator • They are employed to find out, by how much the dimensions of the given component differ from that of a known datum. If the indicated difference is small, a suitable magnification device is selected to obtain the desired accuracy of measurements. • It is an indirect type of instrument and used for linear measurement. If the dimension is less or greater, than the standard, then the difference will be shown on the dial. • It gives only the difference between actual and standard dimension of the work piece.

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Comparator • In short, Comparator is a device which (1)Picks up small variations in dimensions. (2) Magnifies it. (3)Displays it by using indicating devices, by which comparison can be made with some standard value.

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Mechanical Comparator • It is self controlled and no power or any other form of energy is required. • It employs mechanical means for magnifying the small movement of the measuring stylus. The movement is due to the difference between the standard and the actual dimension being checked. • The method for magnifying the small stylus movement in all the mechanical comparators is by means of levers, gear trains or combination of these. They are available of different make and each has it's own characteristic. • The various types of mechanical comparators are dial indicator, Reed type Comparator, sigma comparator, Johansson mikrokator. 56

Mechanical Comparator • To check the height of the job H2 ,with the standard job of height H1. • Initially, the comparator is adjusted to zero on its dial with a standard job in position as shown in Figure(a). The reading H1is taken with the help of a plunger. Then the standard job is replaced by the work-piece to be checked and the reading H2 is taken. • If H1and H2 are different, then the change of the dimension will be shown on the dial of the comparator.

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Mechanical Comparator

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Johansson Mikrokator • This comparator was developed by C.F. Johansson. • Principle: • It works on the principle that when a tension is applied to the twisted strip, this causes the strip to untwist resulting in rotation of pointer which attached at centre of twisted strip.

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Johansson Mikrokator • Construction and working : • It consist of a twisted metal strip. Any pull on the strip causes the centre of the strip to rotate. A very light pointer made of glass is attached to the centre of the twisted metal strip.

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Johansson Mikrokator • The measuring plunger is on the slit washer and transmits its motion through the bell crank lever to the twisted metal strip. The other end of the twisted metal strip is fastened to the cantilever strip. • When the plunger moves by a small distance in upward direction the bell crank lever turns to the right hand side. This exerts a force on the twisted strip and it causes a change in its length by making it further untwist. Hence the pointer at the centre rotates by some amount. • Thus small movement of plunger is magnify in to large rotation of pointer.

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Reed Type Mechanical Comparator • Fig. shows the reed type mechanical comparator. • The reed mechanism is frictionless device for magnifying small motions of spindle. • It consists of a fixed block A which is rigidly fastened to the case and floating block B, which carries the plunger and is connected horizontally to the fixed block A by reeds C. • A vertical reed D is attached to each block with upper ends ed together. • These vertical reeds are shown in the figure by letter D. Beyond this t extends a pointer or target.

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Reed Type Mechanical Comparator

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Reed Type Mechanical Comparator • A linear motion of plunger moves the free block B vertically causes rotation of pointer . • So, a small displacement produces a large angular movement.

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Advantages of Mechanical Comparator: 1. They do not require any external source of energy. 2. These are cheaper and portable. 3. These are of robust construction and compact design. 4. The simple linear scales are easy to read. 5. These are unaffected by variations due to external source of energy such air, electricity.

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Disadvantages of Mechanical Comparator: 1. Range is limited as the pointer moves over a fixed scale. 2. Pointer scale system used can cause parallax error. 3. There are number of moving parts which create problems due to friction, and ultimately the accuracy is less. 4. The instrument may become sensitive to vibration due to high inertia.

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Mechanical - Optical Comparator • In mechanical optical comparator, small variation in the plunger movement is magnified: first by mechanical system and then by optical system. • Optical comparators are based on the principle of projection of image.

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Mechanical - Optical Comparator

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Mechanical - Optical Comparator • The movement of the plunger is magnified by the mechanical system using a pivoted lever. • From the Figure the mechanical magnification = l2 / l1. • High optical magnification is possible with a small movement of the mirror. • optical magnification = 2 × l4/l3 • It is multiplied by 2, because if mirror is tilted by an angle θ, then image will be tilted by 2 × θ. • Thus overall magnification of this system = 2 × (l2/l1) (l4/l3) • These Comparators have less number of moving parts, due to this there is less wear and hence less friction. 69

Advantages of Mechanical - Optical Comparator • it have less number of moving parts, due to this there is less wear and less friction ,hence a higher accuracy. • It has very high magnification. • Optical lever is weightless.

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disadvantages of Mechanical-Optical Comparator • An electrical supply is necessary. • Projection type instruments occupy large space and they are expensive. • When the scale is projected on a screen, then it is essential to use the instrument to a dark room in order to take the readings easily.

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Electrical Comparators • Electrical comparators are also known as electromechanical measuring systems as these employ an electro- mechanical device which converts a mechanical displacement into electrical signal. • Fig. shows a block diagram of an electro- mechanical measuring system.

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Electrical Comparators • The movement at the probe tip actuates inductance transducer which is supplied with an alternating current from the oscillator. • The transducer converts this movement into an electrical signal which is then amplified and fed via an oscillator to the demodulator. The current in D.C. form, then es to the meter and the probe tip movement is displayed as a linear measurement.

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Advantages of Electrical Comparators: • Less no. of moving parts, hence it can retain accuracy over long periods. • It is possible to have a very high magnification and the same instrument may have two or more magnifications. Thus the same instrument can be used for various ranges. • The mechanism carrying the pointer is very light and not sensitive to vibrations. • The measuring unit can be made very small and it is not necessary that the indicating instrument be close to the measuring unit, rather it can be remote also.

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Disadvantages of Electrical Comparators: • It requires an external agency to operate i.e., the A.C. electrical supply. • Heating of coils in the measuring unit may cause zero drift and alter the calibration. • This is usually more expensive than mechanical instrument.

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Pneumatic Comparators • Pneumatic comparators are the widely used precision instruments which use the principle of obstructed nozzle. • The schematic diagram of a pneumatic comparator is shown in Figure.

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Pneumatic Comparators • It has two orifices O1 and O2 with diameter d1 and d2 respectively. • Through O1, air is supplied at a constant pressure, Ps , which is the pressure of the source.

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Pneumatic Comparators • Thus, the area of orifice O2 is variable and depends upon the displacement of the work piece x. • The intermediate pressure Pi between the fixed orifice and the outlet is dependent upon the source pressure Ps, and the pressure drops across the two orifices O1 and O2. Since area A2 of the orifice O2 varies with displacement x, the intermediate pressure, Pi, also changes with change in x. Thus, change of pressure is a function of displacement x and hence can be used as a measure of dimension x.

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Advantages of Pneumatic Comparators • The gauging member does not come into with the part to be measured and hence no wear takes place on the gauging member. • The method is self cleaning due to continuous flow of air through the jets and this makes the method ideal to be used on shop floor. • It has usually very small number of moving parts ,Thus the accuracy is more due to less friction and wear.

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Disadvantages of Pneumatic Comparators • The accuracy may be influenced by the surface roughness of the component being checked. • Different gauging heads are needed for different jobs. • Auxiliary equipments such as air filters, pressure gauges and regulators are needed. • The apparatus is not easily portable and is rather elaborate for many industrial applications.

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