Metrology # 2

METROLOGY

1 CHAP#2

MEASUREMENTS

International System of Units (SI): It is the system established in 1960 by (CGPM) General Conference of Weights and Measures and abbreviated as SI (System International) in all languages. In Pakistan, India, we switched over to metric system of Weights and Measures conforming to SI units an act of Parliament. This SI like traditional metric system is based on decimal arithmetic. The SI is a coherent_______ system, the sense that the product or quotient_______ of any two unit quantities in the system in the unit of the resultant quantity. If unit of length is meter, then unit of area will be square meter and not acres or begs etc. The seven base SI units established by the General Conference of Weights and Measures are given under.

S. #

Physical Quantity

Name of unit

Unit symbol

Base of Definition

1

Length

Meter

m

Wavelength of red light in Krypton 86

2

Time

Second

S

Cycles of radiation of cesium

3

Mass

Kilogram

Kg

Platinum cylinder prototype

4

Temperature

Kelvin

K

Absolute zero and water

5

Electric current

Ampere

A

Force between two conducting wires

6

Luminous intensity

Candela

cd

Intensity of an area of platinum

7

Quantity of

Mole

substance

mol

Amount of atoms in carbon 12

Standards: The two standard system for linear measurement used throughout the world are English and Metric (yard and meter0. the various standards now known for linear measurements are :¾ Line standard ¾ End standard ¾ Wavelength standard

Line standard: According to it, yard or meter is defined as the distance between scribed_____ lines on a bar of metal under certain conditions of temperature. These are legal standards.

End standard: For all practical measurements in workshop, we employ end standards for example:- slip gauges, gap gauges, end of micrometer anvils etc Thus the importance of end standards (which are actually used in general measurement applications) arose. Length bars and slip gauges were then made which were equal in length to the legal line standard.

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End Bars or Length Bars: These are used for the measurement of larger sizes of work. These consist of carbon steel round bar about 20 mm in diameter and made in sizes varying from 10 mm to 1200 mm. these hardened only at ends up to 800 HV an supported at ‘Airy’ points so that end surfaces are parallel to each other. Therefore, generally not found in majority of engineering work but in standardizing laboratories etc. these are also available in four grades of accuracy. a) Reference b) Calibration c) Inspection d) Workshop

Wavelength standard: Using wavelength of monochromatic____ light which is natural and invariable unit of length, the working standard is no more dependent upon the physical standard. Cadmium 114, krypton 86, and Mercury 198 are possible sources of radiation of wavelengths suitable as natural standard of length.

Slip Gauges: Slip gauges are often called Johannsen gauges also, as Johannsen them; these are rectangular blocks of steel having a cross-section of about 30 by 10 mm. There are first hardened to resist wear and carefully stabilized so that they are independent of any subsequent variation in size or shape. These may be used as reference standards for transferring the dimensions of the unit of length from the primary standard to gauge blocks of lower accuracy and for the verification and graduation of measuring apparatus, and length measure for the regulation and adjustment of indicating measuring apparatus and for direct measurement of linear dimensions of industrial component.

Slip gauges are classified according to their guaranteed accuracy: AA for master slip gauges A for reference purpose, and B for working slip gauges. AA slip gauges are accurate to plus or minus two microns per meter. Type A is guaranteed accurate up to plus or minus four microns per meter. Type B for plus or minus eight microns per meter.

As regards grades or classes of slip gauges, these could also be designed in five grades as under:

Grade 2: This is the work shop grade. Typical uses include setting up machine tools, positioning milling cutters and checking mechanical widths.

Grade 01: Used for more precise work, such as that carried out in a good-class tool room. Typical uses include setting up sine bars sine tables, checking gap gauges and setting dial test indicators to zero.

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Grade 0: This is more commonly known as the inspection grade, and its use is confined tool room or machine shop inspection. This means that it is the inspection departments only who have access to this grade of slip.

Grade 00: This grade would be kept in the standard room and would be kept for work of the highest precision only. A typical example would be the determination of any errors present in the work shop or Grade 2 slip.

Calibration grade: This is a special grade, with the actual size of the slip stated or calibrated on a special chart supplied with the set. This chart must be consulted when making up a dimension, and because these slip are not made to specific or set tolerance, they are not as expensive as the grade 00 . Except for the calibration grade, all slip gauge sets are manufactured to within specified limits; the close the limits more expensive the slip gauges.

Basic Forms of Slip Gauges: Slip gauges with three basic forms are commonly found. These are rectangular, square with centre hole, and square without centre hole.

Tolerances: .Tolerance can be defined as the magnitude of permissible variation of a dimension or measured criterion from the specified value. Tolerances have to be allowed because of the inevitable human failings and machine limitations which prevent ideal achievement during fabrication. The primary purpose of tolerance is to permit variation in dimensions without degradation of the performance beyond the limits established by the specification of the design.

Functional and Non-functional Dimensions: Functional dimensions are those which have to be machined and fit with other mating components .Non-functional dimensions are those which need not be machined to a high degree of accuracy. These have no effect on the quality performance of the component or assembly.

Why tolerances are specified? Ideal conditions would call for part without any kind of dimensional variation, but in actual practice it is impossible due to following reasons: i.

Variations in the properties of the material being machined introduce errors.

ii.

The productions machined themselves have some inherent ______inaccuracies___ built onto them and have the limitation to produce perfect parts.

iii.

It is impossible for an operator to make perfect settings. In setting up machine, i.e. in adjusting the tool and work piece on the machine, some errors are likely to creep in.