metrology.docx

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Measurement and Metrology

Measurement is extremely important Errors in measurement can lead to :

- Non-conforming parts shipped to customers- Excessive waste on products being produced oversized- All leading to inflated costs and and loss of business

Metrology is the science of measurement Measurement is the language of science

- We use it to communicate about size, quantity, position, condition and time- The ability to measure reliably is of course very important

Three Reasons Why We Need Measurement

To make things To control the way other people make things For scientific description

Measurement to Make Things

Require numerical values for things to fit together Measurement skill reduces hand fitting The better the ability to measure the faster skilled jobs are completed No crafts person can be higher skilled than their ability to measure

Measurement to Control Manufacture

An extension of the individual craftsman is the control of others doing the work e.g. inspectors and quality control personnel High speed production requires the use of techniques such as statistical process control Mistakes in measurement in high production environments are extremely costly TQM is resurrecting the concept that the production worker is also the inspector

Measurement for Science (Progress)

Experiments could not be repeated without measurement If we can t measure the present how can we get better Continuous improvement is a phrase you will hear over and over again in business A business must get better every year and to do this we must be able to benchmark ourselves as

compared to our competition

Communication about Measurement

All measurement uses completely arbitrary units Two essential terms to understand measurements are


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The last positional error results when the line of measurement, standard and axis of thecomparison instrumentare all in line

CHAPTER 4LANGUAGE OF MEASUREMENT

Linear Measurement

Express the distance separating two points Measurement have two things in common

- A unit of length- A multiplier

Standards preserve accepted unitsSignificant Figures

Begin with the first digit to the left of the decimal point that is not zero and end with the lastdigit to the right that is correct-not and approximation

Therefore precision increases as zeros are added to the right 0.25 is less than 0.250 In the above the third number to the right is significant- the zero is not an approximation Every measurement begins at a reference point(or reference end) and ends at a measured point This creates a line of measurement that has direction and is usually reversible

Dimensions

At the design stage-reference point and measured point define the perfect separation Achieving perfection is manufacturing is impossible so an acceptable range for these

measurement is created(tolerance)

Features - Male and Female ***

Must understand the material condition of the edge- Whether it is an outside edge of less than 180 degrees- Or inside edge with more than 180 degrees

The Act of Measurement

A comparison of the standard of length and an unknown feature To create a measurement you must compare the feature to the standard of length

Length Measurement

All measurement require three elements- The part or feature being measured- The measuring device- The standard


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Accuracy, Precision, and Reliability

Accuracy is hitting the target or specification Precision describes how closely grouped the shots or measurements are Reliability is obtained by having both well-centred accuracy and precision Show a normal distribution and review capability

Earliest Recorded Standard

Egyptian Cubit Egyptians were very serious about ensuring everyone was using the same standard Failure to calibrate the working cubit to the royal cubit at each full moon was punishable by

death

Two Axioms of Metrology

There must be a standard The standard must be reproducible

Metric System

In 1790, put into law in FranceMeter Standard

A metal standardMeter of the Archives was made and adopted in 1799. Some of the original metric standards were dropped110 day weeks, 10 hour days etc.

Metric System

International Conferences began in 1870 to establish a worldwide metric system In 1889, 30 prototype meters and 40 prototype kilograms were constructed of a platinum-

iridium alloy

These were distributed to participants to become national standards In 1960, the metric system was revised worldwide and its new name abbreviated SI

Comparison of Metric and Inch-Pound System

Three criteria have to be considered :- Metrological factor ie which system will yield useable results- Computational factor ie which system yields figure that we can use mathematically- Communicative factor ie which system makes it easiest for as to shared the measurement

with other people

DecimalInch System


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In about 1930, the decimal-inch system was developed to eliminate some of the computationproblems with the inch system

First breakthrough with Ford, then adopted by ASME, ASA, SAERounding Off Numerical Values

Round off to eliminate unnecessary figures in any calculation Must know the correct method of rounding off and the number off significant figures Rule of significant figures is - the answer can be no more accurate than the least accurate

measure used

In rounding off we use the following- Leave rounded to digit same if the last number is less than 5- Increase the digit if the last number is greater than 5- When the digit to be dropped is exactly 5, rounf off to the nearest even number eg 0.09375

to 0.938 , but 0.09385 to 0.0938

MODULE 3MICROMETERS

The designated micrometer size is its largest opening, not the range Micrometers are available over 400 mm in size Available in both imperial and international units Regardless of size the range is usually limited to 25 mm(1 inch) The amount of amplification increase is related to the increase in the circumference of the

thimble and a decrease in the screw thread(lateral travel of the screw in one revolution-Fig. 7.4)

Advantages of Micrometers

More accurate than rules Greater precision than verniers Better reliability than rules or verniers No parallax error Small, portable and ease to handle Relatively inexpensive

Disadvantages of Micrometers

Short measuring range


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Single purpose measurements End measurement only Limited wear area of anvil and spindle

Vernier Micrometers

Metric micrometers allow one to measure to 0.002 mm Imperial micrometers allow one to measure 0.0001 inch Note the rule of ten if one wanted to measure reliably to 0.002 mm one would require an

instrument accurate to 0.0002 mm.

Care of Micrometers

Keep clean Oil is essential on steel surfaces to prevent corrosion Never leave mike in closed position when not in use Discuss Calibration and gauge control

The Steel Rule Versus A Scale

Ascale is graduated in proportion to a unit of length A rule is a unit of length , its divisions and its multiples

Scale

One unit on a scale m. Typically narrow steel strips with one or more sets of graduated marks Marks are referred

Three Factors in Choosing the Correct Steel Rule

Which style of rule will do the best job Which measurements divisions (scale should be used)

Interpolation

They only acceptable time to interpolate is when you cannot find an instrument discriminatingenough to provide an accurate measurement

Steel Rules Sources of Error

Observational errors- Parallax is importantthe object being measured appears to shift when the observer

changes position

- Parallax does not affect measurement when the scale of the rule is directly on the line ofmeasurement(Fig. 5.12)


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Manipulative error if that produces hoped for results- Not easy to hold both the part and the rule and measure to 0.5 mm or even closer- Common manipulative errors are show in Fig. 5.13

Problem of Bias

Bias is an unconscious influence on each measurement we make A normal tendency to take the easiest route-especially if that produces hoped for results Never reject any reading, unless it is definitely known to be fallacious

Other Scaled Instruments

Show Figure 5-17 and 5-18 Figure 5-28 and 5-30 simple callipers(these are transfer instruments) Calipers are made in a number of sizes, designated by the length of their legs-not their

measurement range

Summary

Calipers are transfer instruments Measurement with all these instruments. Instruments that require contact depend upon the feel of the user for precision Cleanlines will promote reliability, but only skill will provide accuracy

Repetition

If one unskilled in the use of an instrument, measure the same part several times to ensurereliability

If in doubt re-measure is a golden ruleSignificant Figures

Numbers and their combinations by means of arithmetic give us an exact.. Every quantity can be written as a decimal number between 1 and 10 and the appropriate

power of ten(Scientific notation)

Instead of writing 6,370,000 meters as the radius of the earth, we write 6.37x10 Likewise

Using Significant Number we show the limited accuracy of our knowledge by omitting all digitsabout which we have no information

The number of digits about which we .. feel reasonably sure is called the number of significantnumber

In the example of the hair we have indicated only one significant figure This means When write four significant figures we imply a fifth would have no meaning


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If our accuracy were ten times greater than we would use another significant figures.. The mea..27.8 hrs, 1.324 hrs, 0.66 hrs Without regard to significant figures the total is 29.784 hrs In any number obtained by measurement all digit.. In this case we should round off all our measurements to the nearest tenth so that all the digits

in our answer will be significant

This gives 27.8 hrs, 1.3 hrs, o.7 hrs and total of 29.8 hrs Since the first no is only known to the nearest tenth of an hour Subtraction of measured quantities. same way. Division..

The use of verniers scale :

Increases their amplification Increases their discrimination

Advantages

Amplification is achieved by design- not on moving parts which could wear and go out ofcalibration

No interpolation is possible in reading There is no theoretical limit to the scale range(Length) Zero setting adjustment is easy

Disadvantages

Principal disadvantage lies in the instruments on which vernier are used Reliability of the reading depends more upon the observer than instruments Discrimination is limited No way to adjust for errors other than zero setting Provide long measurement 6 to 80 inches One pair can substitute for many outside and inside micrometers One can make reliable measurements but care must be taken as they deviate from Abbes Law

Vernier Calipers Accuracy

We know that error reduces accuracy Major cause of error with Vernier Calipers is manipulation errors Additional errorscould come from instrument wear(most common is jaw wear) Close the jaws tightly Hold the calliper towards a light source If you do not.

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