Linear measuring instruments

Linear Measuring Instruments:

Line graduated measuring instruments incorporate graduation spacings representing

known distances. These are used for direct measurements of specific distances within their capacity range. The sensitivity of the measurement is dependent primarily on the instrument's basic design (the least distance between the individual graduations). The following instruments can be used for linear measurements.

Steel scale/rule:

It is a line measuring device. It is the simplest and most common measuring instrument used in inspection. It works on the basic measuring technique of comparing an unknown length to the one previously calibrated. It consists of a strip of hardened steel having line graduations etched or engraved at interval of fraction of a standard unit of length. Depending upon the interval at which the graduations are made, the scale can be manufactured in different sizes and styles. Various possible combinations of scales are found depending upon whether the scale is graduated either on both sides and both edges or on edges of only one side. The scale can be either 150 mm long or 300 mm or 600 mm or 1000 mm long. The scale need not be graduated uniformly throughout its length. In some part e.g. it may have 10 divisions of centimeter, in some portion 20 divisions, so that it can be used for all types of work and the particular range chosen depending upon the accuracy required.

Steel tape: 

A tape measure or measuring tape is a flexible form of ruler. It consists of a ribbon of cloth, plastic, fiber glass, or metal strip with linear-measurement markings. It is a common measuring tool. Its flexibility allows for a measure of great length to be easily carried in pocket or toolkit and permits one to measure around curves or corners. It can be used for measuring various structural steel sections in the workshop, in construction work and s use tape measures in lengths of over 100 m. For workshop a 3m length tape can be used. 

Vernier caliper: .

The meter scale enables us to measure the length to the nearest millimeter only. Engineers and scientists need to measure much smaller distances accurately. For this a special type of scale called Vernier scale is used. The Vernier Caliper is a precision instrument that can be used to measure internal and external distances extremely accurately. The vernier caliper is usually a manual caliper. Measurements are interpreted from the scale by the user. This is more difficult than using a digital vernier caliper which has an LCD digital display on which the reading appears. The manual version has both an imperial and metric scale. Manually operated vernier calipers are much cheaper than the digital version.

The Vernier calliper consists of a main scale fitted with a jaw at one end. Another jaw, containing the vernier scale, moves over the main scale. When the two jaws are in contact, the zero of the main scale and the zero of the vernier scale should coincide. If both the zeros do not coincide, there will be a positive or negative zero error

The Vernier scale consists of a main scale graduated in centimeters and millimeters. On the Vernier scale 0.9 cm is divided into ten equal parts. The least count or the smallest reading which you can get with the instrument can be calculated as under:

Least count = one main scale (MS) division - one vernier scale (VS) division.

Suppose 10 division of vernier scale = 9 division of main scale. Therefore one division of vernier scale = 9/10  = 0.9 mm of main scale division ( one division of main scale = 1 mm). Therefore the least count will be

= 1 mm - 0.9 mm

= 0.1 mm

= 0.01 cm

Reading of vernier caliper: 

In the following example 50 divisions on the vernier scale = 49 division on main scale. The value of one division on main scale is 1mm. Therefore the least count = 1-49/50 = 0.02 mm

Dial caliper: 

 Instead of using a vernier mechanism, which requires some practice to use, the dial caliper reads the final fraction of a millimeter or inch on a simple dial.

In this instrument, a small, precise gear rack drives a pointer on a circular dial, allowing direct reading without the need to read a vernier scale. Typically, the pointer rotates once every 1 millimeter. This measurement must be added to the coarse whole centimeters read from the slide. The dial is usually arranged to be rotatable beneath the pointer, allowing for "differential" measurements (the measuring of the difference in size between two objects, or the setting of the dial using a master object and subsequently being able to read directly the plus-or-minus variance in size of subsequent objects relative to the master object).

The slide of a dial caliper can usually be locked at a setting using a small lever or screw; this allows simple go/no-go checks of part sizes.

Digital caliper: 

A refinement now popular is the replacement of the analog dial with an electronic digital display on which the reading is displayed as a single value. Some digital calipers can be switched between centimeters or millimeters. All provide for zeroing the display at any point along the slide, allowing the same sort of differential measurements as with the dial caliper. Digital calipers may contain some sort of "reading hold" feature, allowing the reading of dimensions even in awkward locations where the display cannot be seen.

Ordinary 150-mm digital calipers are made of stainless steel, have a rated accuracy of 0.02mm and resolution of 0.01mm. 

Vernier height gauge: 

This is also a sort of vernier caliper, equipped with a special base block and other attachments which make the instrument suitable for height measurements. Along with the sliding jaw assembly, arrangement is provided to carry a removable clamp. The upper and lower surfaces of the measuring jaws are parallel to the base, so that it can be used for measurements over or under a surface. The vernier height gauge is mainly used in the inspection of parts and layout work. With a scribing
attachment in place of measuring jaw, this can be used to scribe lines at certain distance above surface. However dial indicators can also be attached in the clamp and many
useful measurements made as it exactly gives the indication when the dial tip is just touching the surface. For all these measurements, use of surface plates as datum surface is very essential.   

Similar to caliper, the height gauges are also available as dial height gauges and digital height gauges. The principal of working remain same.

Vernier depth gauge:  

For measuring the depth of holes, recesses and distances from a plane surface to a projection, the vernier depth gauge is employed. In vernier depth gauge, the graduated scale can slide through the base and vernier scale remains fixed. For use of vernier depth gauge, its base or anvil is rested on or against a reference surface and the scaled beam or tongue is pushed beyond the base to contact the measured point. Errors are made due to manipulation. It must be ensured that the reference surface on which the depth gauge base is rested is satisfactorily true, flat and square. The gauge, though true and square, can be imperceptibly tipped or canted.

 

Micrometer: The micrometer is a precision measuring instrument, used by engineers. Each revolution of the rachet moves the spindle face 0.5mm towards the anvil face. The 0.5 mm is the pitch of the screw to which spindle is attached. The object to be measured is placed between the anvil face and the spindle face. The rachet is turned clockwise until the object is ‘trapped’ between these two surfaces and the rachet makes a ‘clicking’ noise. This means that the rachet cannot be tightened any more and the measurement can be read.

Calculation of  least count of a micrometer. Use the given formula:

Least Count (L. C) = Pitch/No. of divisions on micrometer barrel (thimble) where,
Pitch = distance travelled by thimble on linear scale in one rotation, which is usually 0.5 mm unless stated.

In the examples below, the number of division on the barrel are 50. Therefore the least count of the micrometer will be 0.5/50 = 0.01 

1. Read the scale on the sleeve. The example clearly shows12 mm divisions. 2. Still reading the scale on the sleeve, a further ½ mm (0.5) measurement can be seen on the bottom half of the scale. The measurement now reads 12.5mm. 3. Finally, the thimble scale shows 16 full divisions (16 x 0.01 = 0.16 mm). The final measurement is 12.5mm + 0.16mm = 12.66

The digital micrometer is shown is the figure. There are many types of micrometers which depend on the type of anvil and spindle faces such as gear tooth micrometer, sheet metal micrometer etc..

Micrometer head can be part of any measuring instrument which makes the instrument known with prefix as micrometer such as micrometer depth gauge, micrometer bore gauge etc.

Indirect measuring instruments

Firm Joint Calipers: 

These are the devices for comparing measurements against known dimensions. In the case of firm joint calipers, two legs and working ends are suitably hardened and tempered to a hardness of 400 to 500 HV and measuring faces are hardened exactly identical in shape with the contact points and equally distant from the fulcrum, the legs are joined together by a rivet. The legs are set correctly so that the working ends meet evenly and closely when brought together. The capacity of the caliper is the maximum dimension which can be measured by it. The capacity of the caliper should not be less than its nominal size.The distance between the roller centre and the extreme working end of one of the legs is known as nominal size and these calipers are available in the nominal size of 100,150, 200 and 300 mm. Different calipers are shown in figure. 

Spring joint calipers: The spring joint calipers are shown in the figure. The functions these calipers are similar to firm joint calipers.

Definition:

A standard is defined as something that is set up and established by authority as a rule for the measurement of quantity, weight, extent, value or quality etc.

Any system of measurement must be related to a known standard otherwise the measurement has no meaning. Industry commerce, international trade and in fact modern civilization itself would be impossible without a good system of standards. The role of standards is to support the system which makes uniform measurement throughout the world an helps to maintain interchangeability in mass production.

Measurement: Measurement is the act , or the result of a quantitative comparison between a predetermined standard and an unknown magnitude. Measurement provides us with a means of describing various phenomena in quantitative terms. The quantities such as pressure, displacement, accoustics, temperature, fluid flow and related parameters, mass, length, time etc; can be done by mechanical measurement. These quantities can also be measured by electrical means by transducing them into an analogous electrical quantity.

System of measurement: A measuring system is based on few fundamental units e.g., length, mass, time, temperature, etc. All the physical quantities can be expressed in terms of these fundamental units. the following systems of measurement are in use in different countries such as FPS, Metric and SI system. Our country follows SI system which is an extension of metric system.

SI base unit definitions:

The metre is the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second.

The kilogram is equal to the mass of the international prototype of the kilogram.

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the

transition between the two hyperfine levels of the ground state of the caesium-133 atom.

The ampere is that constant current which, if maintained in two straight parallel

conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10^-7 newton per metre of length.

The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0,012 kg of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

The candela is the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and has a radiant intensity in that direction of 1/683 watts per steradian.

 

Subdivision of standard: 

Depending upon the importance of accuracy required for the work the standards are subdivided into four grades:-

Primary standards: 

For precise definition of the unit, there shall be one, and only one material standard, which is to be preserved under most careful conditions. It is called as primary standard. International meter is an example of primary standard. Primary standard is used only at rare intervals solely for comparison with secondary standard. It has no direct application to a measuring problem encountered in engineering.

Secondary standard: 

Secondary standards are made as nearly as possible exactly similar to primary standards as regard design, material and length. They are compared with primary standards after ling intervals and the records of deviation are noted. These stands are kept at number of places for safe custody. They are used for occasional comparison with tertiary standards whenever required.

Tertiary standards: The primary and secondary standards are applicable only as ultimate control. Tertiary standards are the first standard to be used for reference purpose in laboratories and workshops. They are made as true copy of the secondary standards. They are used for comparison at intervals with working standards.

Working standards: 

Working standards are used more frequently in laboratories and workshops. They are usually made of low grade of material as compared to primary, secondary and tertiary standards, for the sake of economy. They are derived from fundamental standards. Both line and end working standards are used. 

Line and end standards

Line Standards – When length is measured as the distance between centers of two engraved lines, it is called Line Standards. Material Standards, yard and meter are line standards E.g. Scale, Rulers, Imperial Standard Yard.

Characteristics of Line Standards :

(i) Scale can be accurately emblemed, but the engraved lines posses thickness and it is not possible to accurately measure

(ii) Scale is used over a wide range

(iii) Scale markings are subjected to wear. However the ends are subjected to wear and this leads to undersized measurements

(iv) Scale does not posses built in datum. Therefore it is not possible to align the scale with the axis of measurement

(v) Scales are subjected to parallax errors

(vi) Assistance of magnifying glass or microscope is required.

End Standards –  

When length is expressed as the distance between centers of two flat parallel faces, it is called End Standards. Slip Gauges, End Bars, Ends of micrometer Anvils.

Characteristics of End Standards:

(i) Highly accurate and used for measurement of closed tolerances in precision engineering as well as standard laboratories, tool rooms, inspection departments.

(ii) They require more time for measurement and measure only one dimension.

(iii) They wear at their measuring faces.

(iv) They are not subjected to parallax error.

Comparison between line and end standard

S.No.	Characteristics	Line standard	End standard
1	Principle	Length is expressed as distance between 2 lines	Length is expressed as distance between 2 ends
2	Accuracy	Ltd. To ± 0.2mm.	Highly accurate of closed tolerances to ±0.001mm
3	Ease	Quick and easy	Time consuming and requires skill
4	Effect of wear	Wear at only the ends	Wear at measuring surfaces
5	Alignment	Cannot be easily aligned	Easily aligned
6	Cost	low cost	High cost
7	Parallax Effect	Subjected to parallax effect	Not subjected to parallax effect
8	Examples	Scale, meter	Slip gauges, end bars, vernier caliper, micrometer etc.