Linear measurements
The linear measurement includes the measurement of length, diameter, height & thickness.
The basic principle of linear measurement is that of comparison with standard dimension on a suitably engraved instrument or device.
2.1.1 Steel Rule
The steel rule is one of the most useful tools in the shop used for taking linear measurements of blanks and articles to an accuracy of 0.5 to 1mm. It consists of a strip of hardened steel having line graduations etched or engraved at interval of standard unit of length. This is usually marked both in inch and centimeter. Steel rules are made 150, 300, 500 and 1000mm long.
2.2 Calipers
These are non-graduated tools used for measuring the distance between two points of contact on the work piece. The distance must be transferred to an actual dimension by use of a graduated direct measuring instrument. Generally it consists of two movable metal legs hinged at top, and ends of the legs span the part to be inspected. Calipers can be either spring type or firm joint type.
Types of calipers are:
1. Outside caliper 2. Inside caliper
3 .Hermaphrodite caliper 4.Transfer caliper
Accuracy depends largely on the inherent skill of the user. A fine “feel”, a good eye, and patience will give the best results
2.2.1 Outside caliper:
It is a two-legged instrument with its leg bent inwards. It is used for measuring or comparing thickness, diameters and other outside dimensions. A steel rule is needed for direct reading.
2.2.2 Inside caliper:
It is also a two-legged tool with its legs bent outwards. This is used for measuring or comparing hole diameter, distance between shoulders or other parallel surface of any inside dimension. A steel rule is used for direct reading.
Outside Caliper Inside Caliper
Outside Caliper (Spring type) Inside Caliper
2.2.3 Hermaphrodite caliper: This is also called odd leg caliper .It has one pointed leg like a divider and one bent leg. The caliper is extremely useful for scribing lines parallel to the edge of the work and for finding the center of a cylindrical work.
2.2.4 Transfer caliper:
Odd leg caliper Transfer caliper
2.3 Vernier Scale
It is an auxiliary scale used along with an ordinary scale to get fractions of the smallest scale division. The least count of a vernier is equal to the difference between a scale division and a vernier division.
2.3.1 Principle of Vernier
Principle of vernier can be understood from the figure. Here length of vernier is equal to 9 main scale divisions and it is divided into 10 equal parts.
The difference between a scale division and a vernier division equals 1-9/10 =1/10 of a scale division.
In figure each scale division be equal to 1mm.Then L.C =0.1mm.
Length of object = 7 main scale div. + Fractional length AB
AB = AC - BC
= 7 m. s. d – 7 v .s. d
= 7 mm – 7 x 9 / 10 mm
= 7 x 1 / 10 mm = 7 x Least count
Total length = 7 + 7 x 0.1 = 7.7 mm
Hence when measuring with a scale and a vernier, the length of object = main scale reading + div of coincidence of vernier x least count.
2.4 Least Count : Least count of an instrument is the smallest measurement that can be made with that instrument
Metric Vernier – L.C : 0.1
9 Main scale div. = 10 Vernier scale div.
1 Main scale div. = 1 mm
1 Vernier scale div. = 9/10 mm
L.C = 1m.s.d – 1 v.s.d = 1- 9/10 = 0.1 mm
2.4.2 Metric vernier – L.C : 0.05
19 Main scale div. = 20 Vernier scale div.
1 Main scale div. = 1 mm
1 Vernier scale div. = 19/20 mm
L.C = 1m.s.d – 1 v.s.d
= 1- 19/20 = 0.05 mm
Metric vernier – L.C : 0.02
49 Main scale div. = 50 Vernier scale div.
1 Main scale div. = 1 mm
1 Vernier scale div. = 49/50 mm
L.C = 1m.s.d – 1 v.s.d
= 1- 49/50 = 0.02 mm
2.5 Vernier caliper
It is an instrument which makes use of the principle of vernier to measure the length accurately. The instrument comprises of a beam, or main scale, measuring jaws, and a vernier head with vernier scale engraved on it. Jaws are attached to the main scale and vernier scale in such away that when the two jaws are brought into contact, the zero of the vernier coincides with the zero of the vernier scale.
Usually a vernier caliper is constructed to read both mm and inches. A fine adjustment mechanism is provided on some vernier calipers for the accurate setting of the sliding jaw. Commonly used metric verniers are-0.1, 0.05, and 0.02mm
Vernier Caliper
Use of vernier caliper
When taking inside measurements care must be taken to ensure that the caliper jaws are parallel to the center line of the bore
Slanting or tilting of the caliper results in wrong reading
Actual value = Read off value + thickness of
measuring tips
Read –off value = 34.5
Actual value = 34.5 + 2 x 5
= 44.5
On a caliper with cross jaws the actual value is equal to the read - off value
2.5.1 Vernier depth gauge
This tool is used to measure the depth of blind holes, slots etc. It is designed on the same principle as the vernier caliper. It has no jaws. When measuring with the vernier depth gauge the base is made to sit on the surface from which the hole depth is to be measured and the beam is advanced until it rest against the bottom of the hole.
Vernier depth gauge
On a hooked vernier depth gauge, main scale is marked on both sides of the beam, the graduation on each side starting from the opposite ends. Because of this it is possible to use this as a plane depth gauge or hooked depth gauge.
A hooked depth gauge enables measurement of depth of location of a slot especially in applications like a 2 stroke engine as shown schematically.
2.5.2 Vernier height gauge
Vernier height gauge is an instrument used for checking the height of work pieces or scribing lines accurately in relation to a given surface. It has a base the lower face of which is accurately ground and a beam-mounted square to the base. Both the height gauge and work piece are used in conjunction with precision ground surface such as a surface plate or the worktable of a machine tool. Vernier scale is marked on a slider, which can be moved along the beam.
Slider assembly carries a removable clamp by means of which a scribing attachment or a dial indicator can be held. The scribing attachment can be used to scribe line parallel to a reference side and the dial indicator can be used to take height measurement accurately.
1) Beam 2) Beam face 3) Clamping screw 4) Vernier scale 5) Slider clamping screw 6) Slider
7) Base 8) Scriber 9) Clamp 10) Fine adjustment clamp 11) Main scale 12) Guiding edge
2.5.3 Gear Tooth Vernier Caliper
It is used to measure the thickness of gear teeth and racks at pitch line. It consists of two beams at 900 to each other and two slides which move along corresponding beams .The vertical slide has a tongue which controls the height on the gear tooth at which the measurement is taken the thickness of the tooth is measured by the jaws on the horizontal slide.
While measuring, first the tongue is set to the required addendum. Then the caliper is placed with the tongue resting on top of the tooth being measured and jaws are brought in contact with the sides of the tooth being measured. Reading is taken on the horizontal beam
Addendum: It is the radial height from the pitch circle to the top of the tooth.
Pitch circle: It is an imaginary circle corresponding to he original friction surface of gears.
2.6 Micrometer
Micrometer is one of the most useful close-tolerance measuring devices. It works on the principle of screw and nut.
2.6.1 Outside Micrometer
It is used to measure external dimensions like diameter of shaft , thickness of parts etc to an accuracy of 0.01mm. The essential parts of the instrument are frame, anvil, spindle, barrel or graduated sleeve, thimble, ratchet stop, spindle clamp, as shown in figure on page 16.
Barrel scale is marked to read mm and 0.5mm and every 5 div. are marked. The micrometer screw has a pitch of 0.5mm while the thimble has a scale of 50 divisions Marked around its circumference. Thus when the thimble is rotated by one complete turn the spindle moves forward or backward by one thread pitch of 0.5mm.
Therefore one div. Of the thimble scale is equivalent to a spindle movement of
0.5 x 1/50 = 0.01
Micrometer is fitted with a ratchet stop mechanism, which ensures that a constant measuring pressure is maintained. When the permissible pressure is reached, the ratchet will slip and the screw will not turn.
2.6.1.1 Reading examples
Eg. 1 Eg. 2
Barrel reading = 12.00 Barrel reading = 9.5
Thimble reading ; 31(31x0.01) = 0.31 Thimble reading ; 7(7x0.01) = 0.07
Total reading = 12.31 Total reading = 9.57
2.6.1.2 Error on micrometer
Positive error: When the micrometer is closed, if the zero on the thimble scale is below the index line, the error is positive and the error is to be deducted from the reading.
Eg. Reading = 12.57 and error = 0.02, Actual reading = 12.57 – 0.02 = 12.55
Negative error: When the micrometer is closed, if the zero on the thimble scale is above the index line, the error is negative and the error is to be added to the reading.
Eg. Reading = 10.36 and error = 0.02, Actual reading = 10.36 + 0.02 = 10.38
2.6.2 Inside micrometer
It is intended for precision internal measurements. In principle it is similar to an external micrometer, and is used for measuring holes with a diameter more than 2” or 50mm.
Without extension rods minimum reading obtained is 50mm and maximum 75mm. Any reading more than this can be obtained by the fitment of extension rods. Extension rods of the following sizes are in common use.
13, 25, 50, 75, 100, 150, 200, & 600.
Depth micrometer:
The principle of measurement is similar to an external micrometer, but instead of frame it has a base with measuring spindle. Depth micrometer is supplied with interchangeable spindles and thus has the measuring ranges of 0-25, 25-50, 50-75, 75-100, 100-125 & 125-150. Depth micrometers are used only on places where there is a satisfactory seating for the base of the instrument.
2.6.4 Vernier micrometer or Micron micrometer
A micron micrometer or vernier micrometer has an additional vernier scale to give more accurate reading. Accuracy of the instrument is 0.001mm. The barrel and the thimble graduations are similar to an ordinary micrometer. On the barrel the vernier scale is constructed by dividing nine thimble divisions into ten equal parts.
Barrel reading =12.500
Thimble reading [23x 0.01] = 0.230
Vernier reading [7x0.001] = 0.007
Total reading = 12.737
2.6.5 Screw thread micrometer
It is used for measuring pitch diameter of screw threads. In this type of micrometer, interchangeable inserts of thread form can be fitted to anvil and spindle. For measuring different threads, inserts are available in pairs and permit measuring threads with pitches of 1 to 1.75, 1.75 to 2.5 etc.
2.6.6 Dial indicating micrometer
It is mainly used when jobs in greater numbers are to be checked. The advantage of this instrument is that the correct deviation form the nominal size can be read on the indicator. In this type of micrometer, anvil is movable and can be relieved from the measuring pressure by a knob. The needle of the indicator is connected to the anvil with levers and gears in such a ratio that one micron can be read on the indicator. For checking large number of component micrometer is fixed on a stand dial set to zero by using slip gauges. The component can be inserted and removed by relieving the anvil and the deviation can read on the dial.The field of tolerance can be made more visible by means of two shutters, which moves along positive and negative scales of indicator.
2.6.7 ‘V’ Anvil Micrometer
‘V’ anvil micrometer is used for measuring circularity of cylindrical works. V-anvil micrometer, which has a tapered spindle and a v, shaped carbide-tipped anvil are designed for measuring the diameter of odd-fluted taps, milling cutters and reamers. This micrometer is also used for checking the roundness of work pieces.
2.6.7 Other types of micrometers are:
1. Micrometer hole gauge or tri-point micrometer used for measuring the diameter of bores.
2. Blade type micrometer in which the anvil and spindle are thinned to a blade shape used for checking root diameter of circular form tools, depth of narrow slots, keyways, recess etc.
3. Disc type micrometer used to check ribs, lands, thickness of gear tooth etc.
4. Direct reading micrometer, where the measurement can be read directly from figures appearing in small windows on the barrel.
5. Quick adjusting micrometer, which allows the spindle to be slid quickly to any point within it’s range. It is used for checking work where a variety of dimensions are involved.
6. Thread measuring micrometer used to measure outside diameter, root diameter and pitch diameter of a thread. Spindle and anvil are provided with holes to accommodate interchangeable anvils. Different anvils available are V anvil, tapered anvil and anvil with flat face.
7. Micrometer with spherical anvil used for measuring wall thickness of tubes and pipes.
8. Bench micrometer, which measures quickly, and accurately diameters and lengths on work
9. Micrometer with recessed measuring faces used to measure recesses and grooves.
10. Inside micrometer caliper used to check internal dimensions. The measuring tips are constituted by the jaws with contact surfaces.
11. Stick micrometer designed for the measurement of longer internal lengths.
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