Linear Measurements

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Introduction:

• Linear measurement applies to measurement of lengths, heights, diameters and thickness including external and internal measurement.

• The instruments used for linear measurements can be classified as:

(i) Direct measuring instruments

(ii) Indirect measuring instruments

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The direct measuring instruments are of two types:

(a) Graduated instruments:

- The graduated instruments include rules, vernier callipers, vernier height gauges, vernier depth gauges, micrometers, dial indicators etc.

(b) Non graduated instruments:

- The non- graduated instruments include callipers, surface gauges, wire gauges, slip gauges etc.

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• They can also be classified as:

(a) Non-precision instruments such as steel rule, callipers etc.

(b) Precision instruments such as venier instruments, micrometers, dial gauges etc.

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Linear Measurement instruments

• Steel Rule (Scale)• Callipers• Vernier caliper• Vernier Height gauge• Vernier Depth gauge• Micrometer• Depth Micrometer• Telescopic gauge• Slip gauge• Combination set

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ENGINEERING STEEL RULE

• An Engineer’s steel rule is also known as ‘Scale’ and is a line measuring device.

• It is precision measuring instrument and kept in a nicely polished conditions.

• It works on the basic measuring technique of comparing an unknown length to the one previously calibrated.

• The edges are accurately ground to form straight edge.

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• Material: It is made from steel or stainless steel.

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• ACCURACY: Reading accuracy of steel rule is 0.5mm in metric system.

• SIZE OF STEEL RULE: Steel rule are available in different length .The common size being 150mm,300mm,600mm.

The steel rule graduated in 10mm,5mm,1mm,0.5mm.

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• MEASURING WITH A STEEL RULE:The figure given below shows the method of using a try square and steel rule for accurate measurement. For accurate readings, it is necessary to read vertically and avoid errors arising out of parallax.

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• IMPORTANT SAFETY:

- For maintaining the accuracy of a steel rule ,it is important to see to it that its edges and surfaces are protected from damage and rust.

- Do not place a steel rule with any cutting tools. Apply a thin layer of oil when not in use.

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CALIPERS

• Caliper are simple measuring instruments used to transfer measurement from a steel rule to objects, and vice versa.

• Calipers are of different types depending on the type of the joint and the shape of the leg .

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TYPES OF CAPILERS

(1) FIRM JOINT CALIPER:

• In the case of firm joint calipers ,both legs are pivoted at one end. To take measurement of a workpiece ,the caliper is opened roughly to the required size. Fine setting is done by tapping the caliper lightly on a wooden surface.

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Continue… (2) SPRING JOINT CALIPER:

• For this type of calipers, the legs are assembled by means of a pivot loaded with a spring. For opening and closing the caliper legs, a screw and nut are provided.

• Spring joint calipers have the advantage of quick setting. The setting made will not change un less the nut is turned. The size of a caliper is specified by its length ,which is the distance between the pivot centre and the tip of the leg.

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TYPES OF LEGS OF CALIPERS:

• Out side and inside calipers are differentiated by the shape of the legs.

• Calipers used for out side measurements are known as out side calipers. The calipers used for internal measurements are known as in side caliper.

• Caliper are used along with steel rule ,and the accuracy is limited to 0.5 mm.

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VERNIER CALIPER

• A vernier caliper is a precision measuring instrument. It is used to measure up to an accuracy of 0.02 mm.

• MATIRIAL: It is made from nickel chromium steel.

• USE: Vernier caliper is use to measure outside, inside, depth of the size.

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PARTS OF A VERNIER CALIPER

1. FIXED JAWS(1 AND 2) :Fixed jaws are part of the beam scale one jaw is used for taking external measurement ,and the other for internal measurement.

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PARTS OF A VERNIER CALIPER

2. MOVABLE JAWS: Movable jaws are part of the vernier slide. One jaw is used for external measurements.

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PARTS OF A VERNIER CALIPER

3.VERNIER SLIDE: A vernier slide moves over the beam and can be set in any position by means of a spring –loaded thumb lever.

4.BEAM: The vernier slide , and the depth bar attached to it, slide over the beam. The graduation on the beam are called the main scale division.

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PARTS OF A VERNIER CALIPER

5.DEPTH BAR: The depth bar is attached to the vernier slide and is used for depth measurement.

6.THUMB LEVER: The thumb lever is a spring loaded lever which helps to set the vernier slab in any position on the beam scale.

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PARTS OF A VERNIER CALIPER

7. VERNIER SCALE: The vernier scale is the graduation marked on the vernier slide. The divisions of this scale are called vernier divisions.

8. MAIN SCALE: Main scale graduations or divisions are marked on the beam.

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Important Note:

• Never use a vernier caliper for any purpose other than measuring. Vernier calipers should be used only to measure machined or filed surfaces. They should never be mixed with any other tools. Clean the instrument after use, and store it in a box.

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Types of vernier callipers:

• There are three types of calipers.

1. The Simple vernier caliper:

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Types of vernier callipers:

2. The dial caliper:

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Types of vernier callipers:

3. The digital electronic caliper.

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Least count of vernier calliper:

• Vernier calliper have two scales, main scale and vernier scale. The main scale is fixed and vernier scale is sliding over a main scale.

• Least count (L.C.) is the difference between the value of smallest division on main scale and value of smallest division on vernier scale.

OR• Least count is the ratio of the value of min.

division on the main scale to the number of division on the vernier scale.

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DETERMINING THE LEAST COUNT

• In the vernier caliper shown in fig the main scale division (9mm) are divided in to 10 equal parts in the vernier scale.

i.e. one main scale division = 1mm

(MSD)

one vernier scale division = 9/10 mm

(VSD)

Least count = 1 - 9/10 = 1 / 10 mm = 0.1mm

The difference between one MSD and one VSD = 0.1mm

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DETERMINING THE LEAST COUNT

• VERIER CALIPER WITH 0.02MM LEAST COUNT IS GENERLY USED IM WORK SHOP.

• In this vernier caliper main scale division (49mm) are divided in to 50 equal part in the vernier scale.

i.e. 1 main scale division =1 mm (MSD) 1. vernier scale division =49\50 mm (VSD) Least count is 1mm – 49\50 =1\50 mm• THE DIFFERENCE BETWEEN 1.MSD and 1. VSD=0.02MM

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DETERMINING THE LEAST COUNT

Least count of the Vernier

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EXAMPLE OF READING

• Main scale reading = 35mm

• The vernier division coinciding with the main scale is the 20th division.

• Value = 20 × 0.02=0.20mm.

• Total reading = main scale reading + L.C. × number of division exactly coincides with a division of main scale

• Total reading = 35mm + 0.02 ×20 = 35.20mm

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EXAMPLE OF READING

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EXAMPLE OF READING

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Possible errors in vernier

• Zero line on main scale may not coincide with that on the vernier scale. This is called as zero error.

• In correct reading of the vernier scale as the scales are difficult to read even with the aid of magnifying glass.

• If the line of measurement does not coincide with the line of scale.

• Error may be introduced due to incorrect feel.• Because of wear vernier scale may not slide squarely on

main scale.

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Precautions in use of vernier

• The line of measurement must coincide with the line of scale.

• The calliper should not be tilted or twisted. While measuring the outside diameter the planes of the measuring tips of the calliper must be perpendicular to the center line of the work piece.

• Grip the instrument near or opposite to the jaws and not by over hanging.

• Move the calliper jaws on the work with light touch. Do not apply under or over pressure.

• Measuring instrument must always be properly balanced in hand and held tightly.

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Vernier Height Gauge• The Vernier Height Gauge is used for accurate measurements and marking of vertical heights above a surface plate datum.

•It can also be used to measure differences in heights by taking the vernier scale readings at each height and determining the difference by substraction. 34HDV

Vernier Height Gauge

• The main parts of a vernier height gauge and their function are given.

1.base 2. beam3.vernier slide4. fine setting device5. vernier plate6. locking screws7. scriber

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Vernier Height Gauge

• SIZE: The size of the vernier height gauge is stated by the height of the beam. The most commonly used size has a beam of 300 mm height.

• Vernier height gauges are used with surface plates or other accurate flat surface.

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Example Of ReadingInch Reading

• Refer to the left side bar graduations and the inch vernier plate. Inches are numbered in sequence over the full range of the bar. Every second graduation between the inch lines is numbered and equals .100". Each bar graduation is .050".

• The vernier plate is divided into 50 parts, each representing .001". Every fifth line is numbered - 5, 10, 15 . . . 45, 50 - for easy counting.

• To read the gage, first count how many inches and how many .050" lines lie between the zero line on the bar and the zero line on the vernier plate and add them.

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Continue…• Then count the number of graduations on the vernier plate from its zero line that coincides with a line on the bar. Multiply the number of vernier plate graduations you counted by .001" and add this figure to the number of inches and .050" lines you counted on the bar. This is your total reading.

Example:In the photo, the vernier plate zero line is five inches (5.000") plus .750" beyond the zero line on the bar, or 5.750". The 25th graduation on the vernier plate coincides with a line on the bar (as indicated by stars). 25 x .001 (.025") is therefore added to the 5.750" bar reading, and the total reading is 5.775".

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Millimeter Reading

• Refer to the right side bar graduations and millimeter vernier plate. Each bar graduation is 1.00 mm. Every tenth graduation is numbered in sequence - 10 mm, 20 mm, 30 mm, etc. - over the full range of the bar. This provides for direct reading in millimeters.• The vernier plate is divided into 50 parts, each representing 0.02 mm. Every fifth line is numbered in sequence - 0.10 mm, 0.20 mm, 0.30 . . . 0.80 mm, 0.90 mm - providing for direct reading in hundredths of a millimeter.• To read the gage, first count how many millimeters lie between the zero line on the bar and the zero line on the vernier plate.

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• Then find the graduation on the vernier plate that coincides with a line on the bar and note its value in hundredths of a millimeter. Add the vernier plate reading in hundredths of a millimeter to the number of millimeters you counted on the bar. This is your total reading.

Example• In the photo, the vernier plate zero line is 146 millimeters beyond the zero line on the bar, and the 0.68 mm graduation on the vernier plate coincides with a line on the bar (as indicated by stars). 0.68 millimeters is therefore added to the 146 millimeter bar reading, and the total reading is 146.68 millimeters.

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VERNIER DEPTH GAUGE

• A vernier depth is very commonly used precision instrument for measuring depth of holes recesses, slot and step.

• Its construction and method of reading are similar to those of a vernier caliper.

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PARTS OF VERNIER DEPTH GAUGE

Base (1)

Graduated beam (2)

Clamping screw (3)

Fine adjustment mechanism (4)

Vernier scale (5)

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CONSTRUCTION OF VERNIER DEPTH GAUGE

• The base (anvil) is the fixed unit and serves as a datum for measurement. It also carries the vernier scale and the fine adjustment mechanism.

• The beam with the main scale graduations is the sliding member or part.

• Fine adjustments for measurement are made after tightening the clamping screw and the fine adjustment mechanism.

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USES OF VERNIER DEPTH GAUGE

• While taking measurements the base should be firmly held against the reference surface. The relief given at the end of the beam of some vernier height gauges is to avoid the seating in the corners of slots and to ensure correct reading.

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IMPORTANT NOTE

• REMOVE BURRS, IF ANY, BEFORE TAKING MEASUREMENTS. EXCESSIVE PRESSURE ON THE BEAM, WHILE TAKING MEASUREMENTS WILL LIFT THE BASE FROM THE REFERENCE SURFACE AND WILL SHOW WRONG MEASUREMENTS.

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MICROMETER

• A micro meter is a precision instrument used to measure a job, generally within an accuracy of 0.01mm.

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PARTS OF MICROMETER OUTSIDE

(1) THIMBLE: On the beveled surface of the thimble also, graduation is marked. The spindle is attached to this.

(2) 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.

(3)RATCHET STOP: The ratchet stop ensures a uniform pressure between the measuring surface.

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Spindle

Lock Nut

Flame

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(4) Lock Nut: A Lock nut is provided on the micrometer spindle to lock it when the micrometer is at its correct reading.

(5) Sleeve or Barrel: The sleeve is accurately divided and clearly marked in 0.5 mm division along its length which serves as a main scale. It is chrome plated.

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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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Least Count Of Micrometer• Least count is the minimum

distance which can be measured accurately by the instrument. The micrometer has a screw of 0.5mm pitch, with a thimble graduated in 50 divisions.

• L.C. = Pitch of the screw / No. of divisions on thimble

= 0.50 / 50 = 0.01 mm

• Least count is thus the value of one division on a thimble.

Pitch = distance travelled by thimble on linear sacle in one rotation.

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Procedure for Setting Zero

• Before a micrometer is used, it is necessary to set a zero (by removing zero error if it exists).

• To do this, first clean the measuring faces and then the thimble until the two anvils are touching and ratchet slips.

• At this point reading is taken, this should read zero.

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• If the zero mark on the sleeve does not coincide with zero on the thimble there is an zero error.

• Then use the adjusting spanner to correct the error with slight rotation of the barrel. A small hole in the barrel is provided for this purpose.

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READING OF MICROMETER Reading = Linear scale reading + (number of division on thimble exactly coincides with a liner scale x Least count)

Read the value of the visible line on the left of the thimble edge. See below fig. i.e. main div.= 13.00 sub div.=0.5 mm so, total =13.50mm. Next read the thimble graduations in line with the barrel datum line, 13th div. So, Reading = 13.50 + (13 * 0.01) = 13.63 mm

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Example:

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Reading = Linear scale reading +

(number of division on thimble exactly coincides with a liner scale x Least count)

= 2.5 mm + (46 x 0.01)

= 2.96 mm

Solution:

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PRECAUTIONS WHILE USING MICROMETER

1. First clean the micrometer by wiping of oil, dirt and grit etc.

2. Clean the measuring faces of anvil and spindle with a clean piece of paper or cloth.

3. Set the zero reading of the instrument before measuring.

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4. Hold the part whose dimension is to be measured and micrometer properly. Then turn the thimble with forefinger and thumb till the measuring tip just touches the part and find adjustment should be made by ratchet so that uniform measuring pressure is applied.

5. While measuring dimensions of circular parts, the micrometer must be moved carefully over representative arc so as to note max. dimension only.

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POSSIBLE SOURCES OF ERROR

• Lack of flatness of anvil surfaces.

• Lack of parallelism of anvils at some or all parts of the scale.

• In accurate setting of the zero reading (zero error).

• In accurate readings shown by the fractional divisions on the thimble.

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• Applying to much pressure on the thimble or not using ratchet.

• Wear of the anvil surfaces threads on spindle, due to constant or incorrect use.

• Wear of ratchet stop mechanism, locking arrangement etc.

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DEPTH MICROMETER

• Depth micrometer as the name indicates is used for measuring the depth of holes, slots and recessed areas.

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USES OF DEPTH MICROMETER

Depth micrometers are special micrometers used to measure

• The depth of holes.

• The depth of grooves and recesses

• The heights of shoulders or projections.

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CONSTRUCTION

• The depth micrometer consists of a stock on which a graduated sleeve is fitted.

• The other end of the sleeve is threaded with a 0.5 mm pitch ‘v’ thread.

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• A thimble which is internally threaded to the same pitch and from, mates with the threaded sleeve and slides over it.

• The other end of the thimble cap is fitted.

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Continue…• A set of extension rod is

generally supplied. On each of them the range of sizes that can be measured with that rod, is engraved as 0-25,25-50,50-75,75-100,100-125,125-150.

• This extension rod can be inserted in side the spindle and the sleeve. The extension rods have a collar-head which helps the road to be held firmly.

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• The measuring faces of the stock and the rods are hardened, tempered and ground. The measuring face of the stock is perfectly machined flat. The extension rods may be removed and replaced according to the size of depth to be measured.

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GRADUATION

• On the sleeve a datum line is marked for a length of 25 mm.this is divided into 25 equal part and graduated,each line representing one millimeter.

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• Each Fifth line is drawn a little longer and numbered. Each line representing 1mm is further subdivided into two equal part. Hence each sub-division represents 0.5 mm. The graduation are numbered in the reverse direction to that marked on an outside micrometer.

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Continue…• The zero graduation of the

sleeve is on the top and the 25mm graduation near the stock. The bevel edge of the thimble is also graduated. The circumference is equally divided into 50 equal parts and every 5th division line is drawn a little longer and numbered. The numbering is in the reverse direction and increases from 0, 5, 10, 15, 25, 30, 35, 40, 45 and 50 (0).

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GRADUATION

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Screw thread micrometer Caliper

•These micrometers have a pointed spindle and a double V-anvil,

•Both shaped to contact the screw thread as shown in the drawing.

•Screw thread micrometer caliper . It is used for accurate measurement of pitch diameter of screw threads. 

Outside Micrometer caliper

• This is special kind of micrometer widely used in industries.

• Used to measurements large than 250mm with better accuracy and speed.

• Here spindle and anvil are not flush with each other for 0 reading.

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V- Anvil Micrometer caliper

• It is used to measured out of roundness in any machining and center less grinding process.

• Used in the measurement of fluted taps, milling cutter, reamers.

• Micrometer angle of is of 60 ۫.

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Blade type Micrometer caliper

• It is mainly used for the measurements of circular form tool, slot, key way etc.

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Micrometer for measuring thickness of cylinder walls

• Two measure wall thickness of any cylinder job such as tube, slip, bush etc.

• Micrometer anvil is made having spherical measuring surface and frame is cut so that anvil can go inside the cylinder.

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Digital Micrometer

• More accurate and precise micrometer which can take measurement with 0.001 mm accuracy without any calculation.

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VERNIER MICROMETER

• The Vernier principle may also be applied to an outside micrometer, increasing its accuracy.

• It gives reading with an accuracy of 0.001mm.

• The vernier scale is engraved on the micrometer barrel.

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• There are 10 divisions on vernier scale, and these are equal to 9 divisions on the thimble.

• Hence one division on the vernier scale is equal to 1/10 * 9 = 9/10 that of the thimble.

• But one division on the thimble is equal to 0.01 mm.

• Therefore one division on vernier scale = 9/10 * 0.01 = 0.009 mm.

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• The least count according to principle of vernier will be, value of smallest division on thimble – value of smallest division on vernier scale = 0.01 – 0.009 = 0.001 mm.

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HOW TO READ VERNIER MICROMETER

• Reading =

Reading on the sleeve of micrometer +

(L.C * Reading by thimble) +

(L.C. of vernier scale * line on the sleeve vernier scale which exactly coincides with one on the thimble)

So, Reading =

5.5 + (0.01 * 28) + (0.001 * 3)

Reading = 5.783 mm.

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INSIDE MICROMETER

• The inside micrometer is used  for  measuring  inside dimensions,  such  as  pump casing wearing rings, cylinders, bearings, and bushings. • Inside micrometers usually come in a set that includes a micrometer head, various length spindles (or extension rods) that are interchangeable, and a spacing collar that is 0.500 inch in length.

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• The spindles  (or  extension rods) usually  graduate  in1-inch increments of range; for example, 1 to 2 inches, 2 to 3 inches.

• When  the  1 to 2 inch  spindle  is  used, and the sleeve and thimble scales are set to 0.00 inch, the distance between the face of the anvil and the face of the spindle is exactly  1.00  inch.

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Depth Micrometer

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TELESCOPIC GAUGE

• The Telescopic gauge is used for measuring internal diameter, slots and grooves etc.

• It consists of a handle with two rods in a tube at one end and a Locking screw at the other end.

• The rods having Spherical contacts can slide within a tube and are forced apart by an internal spring.

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• The locking screw can lock the rods at any desired position through a spring.

• While taking measurements, the rods are pressed closer and inserted into the hole to be measured.

• The rods then open out to touch the metal surface of the hole on both sides.

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• They are then locked in position by means of a locking screw.

• The telescopic gauge is then taken out from the hole.

• The dimensions across the tips is measured by micrometer or vernier callipers.

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Hemispherical Gauge or Small Hole Gauge

• This is also an indirect measuring instrument used to measure diameters of holes in the range of 3-13mm.

• It contains spring steel arms, with wedge shaped member. This member is joined with the spindle and it slides in or out when spindle is rotated with the help of screw head.

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Conti…• With the movement spring steel arms are expanded and

contracted.

• The outer faces of the end of the arms are of sphere shapes.

• This instrument is inserted inside the hole and screw is rotated to expand the arms till the same touch the side faces of the hole.

• Now the instrument is taken out and distance between hemispherical ends are measured with micrometer or vernier caliper to know bore of the hole.

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Bore gauge• This indirect measuring is used to

measure bore of deep hole.

• Upper end of the instrument is having dial and lower end is having measuring head.

• It contains an anvil on one side which works with spring action and anvil or spindle of required size on the side.

• Measuring head is inserted inside the hole at required depth and position in such a manner that axis of the anvil remains perpendicular to axis of the hole.

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Conti…

• This instrument is generally used to measure bore diameter in range of 50-150mm.

• Least count 0.01 mm

• It is also known as Kelipart gauge.

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Error in Measurement

• We cannot get product with 100% accuracy with any process.

• There always remain some difference with nominal value and measured value of any dimension.

• Errors are classified as : -

1. Gross Errors

2. Systematic Errors

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Gross Errors

• The class of error mainly covers human mistake in reading instruments recording and calculating result.

Systematic ErrorsThere are divided as :-• Instrument• Environmental• Observational

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Conti…

• Such error are occurring due to some specific reasons and the can be know and controlled.

• Respected with each measurement.

Random Error• Such error occurs suddenly and their reasons well as

amount can not ne know and hence very difficult to control.

• Such error are not occurring systematically and continuously.

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Dial Indicator

• It is clock shape measuring instrument having circular or semi circular scale on dial and plunger.

• When plunger comes in contact with job movement is reflected by pointer on the dial.

• It is used to check minor difference in leaner measurement to set tool, Job, fixtures in alignment of machine, for sizes of finished product

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Construction of dial indicator

Bezel and bezel clamp:-

• Bezel is a part which can be rotated by hand to set zero reading of the pointer.

• To lock bezel in one particular position bezel clamp is provided.

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Conti…

• Contact Point:-

• Contact point is provided to have contact with surface of the job to be measured.

• It is made out of hardness steel and it is replaced when its shape is distorted of worm out

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Conti…

Tolerance Pointers• Tolerance pointer are provided

to have faster checking of the component.

• It is mainly used in statistical quality control.

• Tolerance pointers are adjusted as per maximum and minimum acceptable limits of the size so that defective parts cam be sorted out easily and speedily.

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Conti…

Stem:-

• It is a tube like part used to facilitate straight movement of plunger.

• While moving plunger passes through stand.

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Conti…

Plunger:-

• Plunger is having contact pointer at its end.

• It is used to transmit the movement of contact point up to the dial indicator through mechanism.

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Conti…

Back Lug:-

• It is used to set dial indicator on stand.

Body:-

• Circular shape body contains the mechanism and other parts.

Mechanism:-

• Gear train and lever mechanism.

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Principal or Mechanism of Dial Indicator

• The dial of dial indicator is having circular scale marked on it which is having hundred division when pointer moves one revolution that is by hundred divisions, linear movement of plunger is of 1 mm.

• Movement of the plunger is transmitted to pointer by rack and gear train arrangement.

• Required pressure for measurement is provided by the spring.

• To remove backlash between teeth of the gear train light hair spring is provided.

• Diamond bearing is provided on each pivot to counter the effect of friction.

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Conti…

• Bezel is rotated to set pointer on zero.

• Over all magnification

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Types of Dial indicator

Type I Dial Gauge

• Total plunger movement or lift of 3, 5 and 10 mm.

• Dial gauge has the plunger movement parallel to the plane of dial.

Type II Dial Gauge• Total measuring range of 1

mm, 2 mm, 3 mm, 5 mm, 10 mm.

• Dial gauge has the plunger movement perpendicular to the plane of dial.

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Conti…

Type I Dial Gauge Type II Dial Gauge

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Precautions in the use of dial Indicator

• Taking measurement plunger axis should match with axis of measurement.

• Plunger movement should be as less as possible.

• Dial should be firmly fitted in the stand

• Worm out contact point should be replaced.

• Plunger should come in contact with job smoothly.

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Application of Dial Indicator

• To check surface roughness

• To Check roundness of cylindrical job

• To set job on the machine

• Set fixture, tool, work- holding device etc

• Machine tool alignment

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Advantages

• Take Reading is easy.

• Semiskilled operator can also use it.

• Used for variety of measurements.

• Maintenance costs are less.

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Comparators

• The general principle of comparator is to indicate the differences in size between the standard and the work being measured by means of some pointer on a scale with sufficient magnification.

• All comparators consist of three basic features

• 1)A sensing device which faithfully senses the input signal

• 2) A magnifying or amplifying system to increase the signal to suitable magnitude. Mechanical, Optical, Pneumatic, hydraulic and electronic methods are used for this purpose.

• 3) A display system (usually a scale and pointer) which utilizes the amplified signal to provide a suitable readout.

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Types of Comparators

• Comparators can be classified as

1. Mechanical

2. Optical

3. Electrical &Electronic

4. Pneumatic

5. Fluid displacement comparators

6. Mechanical – optical comparator

7. Electro-mechanical comparator

8. Multi-check comparator.

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Principal

• Comparator work on principal of relative measurement.

• Master standard slip gauge is prepared equivalent to the basic size of the product.

• Now this standard is kept under the plunger of the comparator and zero reading is set.

• There after product is kept under the plunger one by one to know the difference in their size either on positive or negative side.

• Like this with comparator defective products can be segregated speedily in mass production.

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Mechanical Comparator

• One vertical beam is kept in between two flat springs A.

• These flat spring are attached with fixed member K which is fixed with back plate.

• Shank B at the end of vertical beam provided to have contact with the job. HDV 110

Conti…

• Stock C controls upper and lower position.

• To transfers movement of the beam/plunger, without friction, up to the pointer Y arm E is adjusted on diamond bearing.

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Conti…• Metallic ribbon F wound

one turn on spindle G is attached with the ends of Y arm.

• Pointer H is kept on spindle G and moves on marked dial.

• Trigger J is provided for the safety of contact member.

• Required magnification is achieved by arrangement of Y arm.

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Conti…

• The vertical capacity of this comparator is 150 to 600 mm and magnification is 500 to 5000. Its least count is 0.25 micron.

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Line Diagram of Mechanical Comparator

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Application

• In work shop under normal atmosphere.• For faster inspection of component.• To check the gauge and other parts.• These are also used as laboratory standards.• Comparators are used for inspecting newly

purchased gauges

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Advantage

• These are usually cheaper in comparison to other devices of amplifying.

• These do not require any external supply such as electricity or air and as such the variations in outside supplies do not affect the accuracy.

• Usually the mechanical comparators have linear scale which is easily understood.

• These are usually robust and compact and easy to handle.

• For ordinary workshop conditions, these are suitable and being portable can be issued from a store.

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Limitation

• The mechanical comparators have got more moving parts than other types. Due to more moving parts, the friction is more and ultimately the accuracy is less.

• ii)    Any slackness in moving parts reduces the accuracy considerably.

• iii)  The mechanism has more inertia and this may cause the instruments to be sensitive to vibration.

• iv)  The range of the instrument is limited as the pointer moves over a fixed scale.

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Electrical Comparator

• These comparators depend for their operation on Wheatstone bridge circuit. In d.c.circuit, a change of balance of the electrical resistance in each arm of the bridge is caused bythe displacement of an armature relative to the arm under the action of the measuring plunger.

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Conti…

• Once out of balance is caused in the bridge, it is measured by a galvanometer graduated to read in units of linear movement of plunger.

• This circuit is operated by battery. For the bridgeto balance, the ratios of the resistances in two arms must be equal.

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Advantages

• 1)Small number of moving parts

2)High range of usage

3)Not sensitive to vibrations

4) As a A.C source is used for the working of comparator the cyclic vibration generated by A.C source reduces the sliding friction

5) Measuring unit can be made very small

HDV 120

Disadvantages

1) Requires external power supply

2) Calibration may be altered due to heating elements used

3) Expensive

HDV 121

Application

• For external and internal gauging.

• To measure flatness, wall thickness of tube.

HDV 122

Pneumatic Comparator• When air is passed with

constant pressure between two orifices than air pressure at the gape of this two orifice depends on cross sectional area of two orifice.

• If cross sectional area of one orifice is kept constant than air pressure between two orifice will depend on cross sectional area of another orifice.

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Conti…

• Compressed air is gating expanded in deep tube keeping the head of water “h” constant.

• Excess air goes out in the form of bubbles.

• Thus air with constant pressure passes through control orifice and ultimately leaves out of measuring jet.

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Conti…

• Pressure of air at the space in between control orifice and measuring jet depends on the air gape between measuring jet and surface of the job.

• This gape depends upon size of the job.

• Thus depending upon different sizes of the job this gape increases or decreases.

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Conti…

• This makes changes in the air pressure between control orifice and measuring jet and due to this changes liquid head in the manometer changes.

• And by this size of the job is known with calibrated scale.

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Conti…Advantages

• No Frication and accuracy can be maintained.

• Soft surface of the job can also be measured.

• Any dirt inside the job is getting removed.

• Higher magnification can be achieved.

• Higher L/D are measured.

Disadvantages

• Accurate control is required for pressurized air.

• Measuring scale is not uniform.

Application• Measuring deep hole.

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Optical Comparators

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Principal

• In these comparators, use is made of a fundamental optical law and instead of a printer, the edge of the shadow is projected on to a curved graduated scale indicate the comparison measurement.

• If the mirror is tilted through an angle α the reflected ray of light has moved through an angle 2α

• In optical comparators, the mirror is tilted by the measuring plunger movement and the movement of the reflected light is recorded as an image on a screen. 131

Conti…• In mechanical optical comparators, small displacement

of the measuring plunger are amplified first by a mechanical system consisting of pivoted levers.

• The amplified mechanical movement is further amplified by a single optical system involving the projection of an image. – In this system, Mechanical amplification L2/L1 (by lever principal)

and

If the movement of the plunger causes the mirror to tilt by an angle α, then the image will be tilted by 2α.

– optical amplification = L4/L3 ×2

Overall magnification of this system = 2 × L4/L3 × L2/L1

– Maximum accuracy in mm – 0.00025 mm

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Application

• To check the profile of smaller parts i.e. gear tooth, Screw thread, cam, jig etc.

• Measured thin and metallic foil, needle

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Conti…

• Advantage Moving parts are less

Accuracy can be maintained.

High magnification.

Absence of sufficient lighting reading can be taken easily

• Disadvantages Cannot work in

absence of electric supply.

Bulky construction High cost Continuous reading

cannot be taken

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Selection of Instruction for Given Situation

• Measuring instruments can be selected as per the situation keeping following factors

• (1) Measuring range, accuracy and precision.• (2) Resolution and sensitivity• (3)Types of production and job.

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COMBINATION SET

• Combination set is a set of equipment combining the functions of protractor, engineer square, steel rule, Centre finder, level rule, and scriber.

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Measuring heads attached to it.

• Three measuring heads are attached to the stainless steel rule (blade), allowing versatile measurements on various types of work pieces.

(1)Square Head: used to set the rule at 90º or 45º to an edge of a work piece.

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Continue…

(2) Centre head: used to locate centers of round work pieces.

(3) Protractor Head: used to set the rule at a desired angle to an edge of a work piece. Also used for measuring angles.

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Slip gauges

• Slip gauges or gauge blocks are universally accepted end standard of length in industry.

• They are precision-round square or rectangular blocks are made of steel, chrome, or tungsten carbide.

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Slip gauges are used for

Gage blocks are used for:

- calibration and inspection of precision instruments- to set comparators/indicators- setting of sine bars- machine setup

- To check gap between parallel locations.

- The distance of plugs, spigots etc on fixtures are best measured with the slip gauges

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Procedure for wringing

• Before using slip gauges are cleaned by using a cotton cloth, a cleansing tissue.

• One slip gauge is then oscillated slightly over the other gauge with a light pressure.

• One gauge is then placed at 90° to other by using light pressure and then it is rotated until the blocks one brought in one line.

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Purpose of wringing

• In this way air is expelled out from between the gauge faces causing gauge faces to adhere.

• The gap between two wrung slips is only of the order 0.635 X 10-3 mm.

• The success of precision measurement by slip gauges depends on the phenomenon of wringing.

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Continue…

- For second decimal place of 0.05, select 1.05mm slip gauge.

Therefore the reminder = 28.55 – 1.05 = 27.5 mm.

- Now select 7.5 mm and 20 mm of slip gauges.- Thus, we have 20 + 7.5 + 1.05 +1.008 =

29.558 mm.

- Always begin wringing with the largest sizes first.

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