ABCQ Metrology Handbook

III. METROLOGY

CMI 1998 QUALITY COUNCIL OF INDIANAIII - 1

THERE IS MEASURE IN ALL THINGS.

HORACE SATIRES, BOOK I, 35 B.C.

III. METROLOGY UNITS OF MEASUREMENT


Introduction

Metrology is the science of measurement. The word metrology derives from twoGreek words: matron (meaning measure) and logos (meaning logic). With todayssophisticated industrial climate the measurement and control of products andprocesses are critical to the total quality effort. Metrology encompasses thefollowing key elements:

& The establishment of measurement standards that are bothinternationally accepted and definable

& The use of measuring equipment to correlate the extent that productand process data conforms to specification (expressed in recognizablemeasurement standard terms)

& The regular calibration of measuring equipment, traceable toestablished international standards

Units of MeasurementThere are three major international systems of measurement: the English, theMetric, and the System International D`unites (or SI). The U.S. has effectivelyretained the English System as a remanent of British colonial influence.

The metric and SI systems are decimal-based, the units and their multiples arerelated to each other by factors of 10. The English system, although logical to us,has numerous relic defined measurement units that make conversions difficult.Most of the world is now committed to the adoption of the SI system. The SI systemwas established in 1968 and the U.S. officially adopted it in 1975. The transition isoccurring very slowly. The final authority for standards rests with the internationallybased system of units. This system classifies measurements into seven distinctcategories:

1. Length (meter). The meter is the length of the path traveled by light invacuum during a time interval of 1/299,792,458 of a second. The speed oflight is fixed at 186,282.3976 statute miles per second, with exactly 2.540centimeters in one inch.

2. Time (second). The second is defined as the duration of 9,192,631,770periods of the radiation corresponding to the transition between the twohyperfine levels of the ground state of the cesium - 133 atom.



Units of Measurement (Continued)3. Mass (kilogram). The standard unit of mass, the kilogram is equal to the

mass of the international prototype which is a cylinder of platinum iridiumalloy kept by the International Bureau of Weights and Measures at Sevres(near Paris, France). A duplicate, in the custody of the National Institute ofStandards and Technology, serves as the standard for the United States. Thisis the only base unit still defined by an artifact.

4. Electric current (ampere). The ampere is that constant that, if maintained intwo straight parallel conductors of infinite length, of negligible circular crosssection, and placed one meter apart in vacuum, would produce between theseconductors a force equal to 2 x 10 Newtons per each meter of length. -7

5. Temperature (Kelvin). The Kelvin, unit of thermodynamic temperature, is thefraction 1/273.16 of the thermodynamic temperature of the triple point ofwater. It follows from this definition that the temperature of the triple pointof water is 273.16 K (0.01 C). The freezing point of water at standardatmospheric pressure is approximately 0.01 K below the triple point of water.The relationship of Kelvin, Celsius and Fahrenheit is shown below.

Temp F = 1.8 (Temp C) + 32Temp C = (Temp F - 32) 1.8Temp K = Temp C + 273.15

KELVIN CELSIUS FAHRENHEIT

WATER BOILS 373.15( 100( 212( WATER FREEZES 273.15( 0( 32( ABSOLUTE ZERO 0( -273.15( -459.67(

Table 3.1 Relationship of the Three Common Temperature Scales

6. Light (candela). The candela is defined as the luminous intensity, in a givendirection, of a source that emits monochromatic radiation of frequency 540x 10 hertz and that has a radiant intensity in that direction of 1/683 watt per12steradian.

7. Amount of substance (mole). The mole is the amount of substance of asystem which contains as many elementary entities as there are atoms in0.012 kilogram of carbon 12. The elementary entities must be specified andmay be atoms, molecules, ions, electrons, other particles or specified groupsof such particles.



SI System UnitsListed below is a table of the SI system units:

QUANTITY MEASURED UNIT SYMBOL FORMULA

FUNDAMENTAL UNITS

AMOUNT OF SUBSTANCE MOLE molLENGTH METER mMASS KILOGRAM kgTIME SECOND sELECTRIC CURRENT AMPERE ATEMPERATURE DEGREE KELVIN KLUMINOUS INTENSITY CANDELA cd

SUPPLEMENTARY UNITS

PLANE ANGLE RADIAN radSOLID ANGLE STERADIAN sr

DERIVED UNITS

AREA SQUARE METER mVOLUME CUBIC METER mFREQUENCY HERTZ (ONE CYCLE PER SECOND) Hz (s )DENSITY KILOGRAM PER CUBIC METER kg/mVELOCITY METER PER SECOND m/sANGULAR VELOCITY RADIAN PER SECOND rad/sACCELERATION METER PER SECOND SQUARED m/sANGULAR ACCELERATION RADIAN PER SECOND SQUARED rad/sFORCE NEWTON N (kg # m/s )PRESSURE NEWTON PER SQUARE METER N/mKINEMATIC VISCOSITY SQ METER PER SECOND m /sDYNAMIC VISCOSITY NEWTON-SECOND PER SQ METER N # s/mWORK, ENERGY, QUANTITY OF HEAT JOULE J (N # m)POWER WATT W (J/s)ELECTRIC CHARGE COULOMB C (A # s)VOLTAGE (ELECTROMOTIVE FORCE) VOLT V (W/A)ELECTRIC FIELD STRENGTH VOLT PER METER V/mELECTRIC RESISTANCE OHM 6 (V/A)ELECTRIC CAPACITANCE FARAD F (A # s/V)MAGNETIC FLUX WEBER Wb (V # s)INDUCTANCE HENRY H (V # s/A)MAGNETIC FLUX DENSITY TESLA T (Wb/m )MAGNETIC FIELD STRENGTH AMPERE PER METER A/mMAGNETOMOTIVE FORCE AMPERE ALUMINOUS FLUX LUMEN lm (cd # sr)LUMINANCE CANDELA PER SQ METER cd/mILLUMINATION LUX lx (lm/m )

2

3

3

2

2

2

2

2

2

-1

2

2

2

Table 3.2 SI System Units Symbols and Formulas

III. METROLOGY INSTRUMENTS AND TOOLS


Methods Used in Dimensional Measurement

METHOD DESCRIPTION

Direct measurement An instrument such as a micrometer provides adirect dimensional reading.

Mechanical indicator An instrument such as a dial indicatormechanically amplifies a small dimensionalreading onto a larger dial scale.

Electronic comparator The instrument electronically amplifies a smalldimension onto a larger scale.

Pneumatic gage The relative escape of air by pressure or flowregulation against a work piece is measured ona dimensionally graduated scale.

Optical comparator A beam of light is directed upon the part to beinspected, and the resulting shadow ismagnified by a lens system, and projected upona viewing screen by a mirror. The image canthen be inspected by comparing it with a mastersilhouette having tolerance limits.

Interferometer An actual measurement is accomplished by theinterference interaction of light waves that are180( out of phase.

Coordinate measuring machine (CMM) Computer controlled measurements are takenon three mutually perpendicular axes. A CMMis often used for layout before machining andinspection after machining.

Table 3.3 Description of Dimensional Measurement Methods



Measuring Instruments

Overview

The terms measuring tool and measuring instrument are used interchangeably inthis text. Some very basic tools are reviewed but other commonly used tools aredescribed in summary form only. The reader is advised to seek other sources, suchas Griffith (1992), Farago (1982) and Kennedy (1987), for a more in-depth treatmentof specific instruments. See the references at the end of this Section. In asubsequent portion of this Section, the causes of error in measurement arediscussed. It is worthwhile to study the possible errors related to each tool.

Tool Care

Measuring instruments are typically expensive and should be treated with care topreserve their accuracy and longevity. Some instruments require storage in acustomized case or controlled environment when not in use. Even sturdy handtools are susceptible to wear and damage. Hardened steel tools require a light filmof oil to prevent rusting. Care must be taken in the application of oil since dustparticles will cause buildup on the gage's functional surfaces. Measuring toolsmust be calibrated on a scheduled basis as well as after any suspected damage.Tools should be examined frequently for wear on the measuring surfaces. Themilitary standard covering the care and calibration of gages is MIL-STD-120. Thisstandard was last revised in 1950, but it is still useful as a reference for moderngaging.

Reference/Measuring Surfaces

A reference surface is the surface of a measuring tool that is fixed. The measuringsurface is movable. Both surfaces must be free from grit or damage, secure to thepart and properly aligned for an accurate measurement.

Transfer Tools

Transfer tools have no reading scale. An example, discussed later in this Section,is spring calipers. Jaws on these instruments measure the length, width or depthin question by positive contact. The dimension measurement is then transferred toanother measurement scale for direct reading. There are a number of measuringdevices that may be used to transfer measurements from a reference piece to a part



surface.



Variable GagesVariable measuring instruments provide a physical measured dimension. Examplesof variable instruments are line rules, vernier calipers, micrometers, depthindicators, runout indicators, etc... Variable information provides a measure of theextent that a product is good or bad, relative to specifications. Variable data is oftenuseful for process capability determination and may be monitored via control charts.

Attribute GagesAttribute gages are fixed gages which typically are used to make a go, no-godecision. Examples of attribute instruments are master gages, plug gages, contourgages, thread gages, limit length gages, assembly gages, etc... Attribute dataindicates only whether a product is good or bad (in most cases, it is known in whatdirection the product is good or bad). Attribute gages are quick and easy to use butprovide minimal information for production control.

The Steel RuleThe steel rule is a widely used factory measuring tool for direct length measurement.Steel rules and tapes are available in different degrees of accuracy and are typicallygraduated on both edges. See the drawing below.

Figure 3.4 A Typical Steel Rule

The fine divisions on a steel rule (thirty-seconds on the one above) establish itsdiscrimination. Steel rules commonly discriminate to one thirty-second inch, onesixty-fourth inch or one-hundredth inch.



Use of the Steel RuleThe steel rule typically has discriminations of 1/32, 1/64, or 1/100 of an inch.Obviously, measurements of .001" or greater should be performed with other tools(such as a digital vernier caliper).For maximum accuracy, the rule should measure a part with both butted firmlyagainst a rigid flat surface. The end of a rule may be worn, rounded or damagedwhich produce errors in measurement. If a flat surface is not available the 1" markmay be used as a reference point. Diagramed below are the correct and incorrectmethods of measurement.

Incorrect Correct

Figure 3.5 Use of a Flat Surface with a Steel Rule

Hook RulesSteel rules may be purchased with a moveable bar or hook on the zero end whichserves in the place of a butt plate. These rulers may be used to measure aroundrounded, chamfered or beveled part corners.

Figure 3.6 Steel Rule with Hook Attachment

The hook attachment becomes relied upon as a fixed reference. However, by itsinherent design, it may loosen or become worn. The hook should be checked oftenfor accuracy.



Spring CalipersSpring calipers are transfer tools that perform a rough measurement of wide,awkward or difficult to reach part locations. These tools usually provide ameasurement accuracy of approximately 1/16 inch.

Although these calipers are referred to as spring calipers, there are differentvarieties (spring joint, firm joint, lock joint, etc...) which describe the type ofmechanical joint that connects the two sides of the unit.

A spring caliper measurement is typically transferred to a steel rule by holding therule vertically on a flat surface. The caliper ends are placed against the rule for thefinal readings . See the diagram below.

Inside Calipers Outside Calipers

Figure 3.7 Spring Caliper Applications



Gage BlocksNear the beginning of the 20th century, Carl Johansson of Sweden, developed steelblocks to an accuracy believed impossible by many others at that time. Hisobjective was to establish a measurement standard that not only would duplicatenational standards, but also could be used in any shop. He was able to build gageblocks to an accuracy within a few millionths of an inch.

When first introduced, gage blocks or "Jo" blocks as they are popularly known inthe shop, were a great novelty. Seldom used for measurements, they were keptlocked up and were only brought out to impress visitors.

Today gage blocks are used in almost every shop manufacturing a product requiringmechanical inspection. They are used to set a length dimension for a transfermeasurement, and for calibration of a number of other tools.

The American National Standard for gage blocks, ANSI B89. 1.9 -1973, distinguishesthree basic gage block forms - rectangular, square and round. The rectangular andsquare varieties are in much wider usage. Generally, gage blocks are made fromhigh carbon or chromium alloyed steel. Tungsten carbide, chromium carbide andfused quartz are also used.

All gage blocks are manufactured with tight tolerances on flatness, parallelism andsurface smoothness. Gage blocks may be purchased in 4 standard grades:

FEDERAL ACCURACY GRADES ACCURACYIN LENGTH

NEW DESIGNATION OLD DESIGNATION0.5 AAA .000001

1 AA .000002

2 A+ + .000004- .000002

3 A & B + .000008- .000004

Table 3.8 Gage Block Grades

Master blocks are 0.5 or 1 gradeInspection blocks are 1 or 2 grade

Working blocks are 3 grade



Gage Blocks (Continued)Gage blocks should always be handled on the non-polished sides. Blocks shouldbe cleaned prior to stacking with filtered kerosene, benzene or carbon tetrachloride.A soft clean cloth or chamois should be used. A light residual oil film must remainon blocks for wringing purposes.

Block stacks are assembled by a wringing process which attaches the blocks by acombination of molecular attraction and the adhesive effect of a very thin oil film.Air between the block boundaries is squeezed out. The sequential steps for thewringing of rectangular blocks is shown below. Light pressure is used throughoutthe process.

Hold Crosswise Swivel the Slip into Finished Pieces Position Stack

Figure 3.9 Illustration of the Wringing of Gage Blocks

Wear Blocks

For the purpose of stack protection, some gage manufactures provide wear blocks.Typically, these blocks are .050-inch or .100-inch thick. They are wrung onto eachend of the gage stack and must be calculated as part of the stack height. Since wearblocks "wear" they should always be used with the same side out.

Gage Block SetsIndividual gage blocks may be purchased up to 20" in size. Naturally, the lengthtolerance of the gage blocks increases as the size increases. Typical gage blocksets vary from 8 to 81 pieces based upon the needed application.



Gage Block Set ContentsListed below are the contents of a typical 81 piece set:

Ten-thousands blocks (9)0.1001, 0.1002 . . . . . . . . . . . . . . 0.1009

One-thousands blocks (49)0.101, 0.102 . . . . . . . . . . . . . . . . 0.149

Fifty-thousands blocks (19)0.050, 0.100 . . . . . . . . . . . . . . . . 0.950

One inch blocks (4)1.000, 2.000, 3.000, 4.000

Also included in the set, are two wear blocks that are either 0.050" or 0.100" inthickness.

Minimum Stacking A minimum number of blocks in a stack lessens the chance of unevenness at theblock surfaces. Stack up 2.5834" using a minimum number of blocks:

2.5834- .1004 . . . . . (use this block)2.483

- .133 . . . . . . (use this block)2.350 . . . . . . (use 0.350 and 2.000 blocks)

This example does not consider the use of wear blocks.

Surface PlatesTo make a precise dimensional measurement, there must be a reference plane orstarting point. The ideal plane for dimensional measurement should be perfectlyflat. Since a perfectly flat reference plane does not exist, a compromise in the formof a surface plate is commonly used.



Surface Plates (Continued)Surface plates must possess the following important characteristics:

& Sufficient strength and rigidity to support the test piece& Sufficient and known accuracy for the measurements required

Surface plates require appropriate care and maintenance:

& The surface should be cleaned before use & The surface should be covered between uses& Work should be distributed to avoid concentrated wear& Move the test pieces and equipment carefully& A surface plate should not become a storage area

Cast Iron Vs. GraniteSurface plates are made of cast iron or granite. Each have merits:

Cast iron plates:

& Usually weigh less per square foot of plate area& Are not likely to chip or fracture& Are acceptable for magnetic fixtures& Can provide a degree of wringability

Granite plates:

& Are noncorrosive and require less maintenance& Do not burr or retain soft metals& Are cheaper per relative size& Have closer flatness tolerances& Have greater thermal stability& Are nonmagnetic

Surface Plate UsageSurface plates are customarily used with accessories like: a toolmaker's flat,angles, parallels, V blocks and cylindrical gage block stacks. Dimensionalmeasurements are taken from the plate up since the plate is the reference surface.

ABCQ Metrology Handbook

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