chapter 2 language and systems of measurement dpt 312 metrology
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OVERVIEW
“ a child reportedly asked President Lincoln how long his legs were. The President answered, ‘ long enough to reach from my body to ground’. “
A humorous response, but the clever use of language does not provide an accurate response to the child’s question.
A human hair is approximately 0.003 inch thick
A pieces of paper is approximately 0.003 inch thick
A grain of salt is approximately 0.004 inch in size
Do you know??
METROLOGY TERMS Dimensional measurement are used daily in designing
building and operating the objects that is surround us for communicating about past, present and future object
The principal dimensional measurement is length(length – the number of units of measurement that separates two points)
Secondary measurement are angle and curvature Linear measurement – the distance separating two points
that has direction (reference pt to measured pt) Measurement
* A dimension is an intended size, whereas length is the measured size
Unit of length
multiplier
cardinal
fractional
decimal
Exponential multipliers are not only more compact, they are much easier to use in
calculations. They are widely used in engineering and science.
Figure 2.3
The Act of Measurement
The act of measurement is a comparison of the standard of length or the distance to be reproduced and an unknown feature
Two method to compare unknown lengths to the standards
I. Interchange method (measurement by comparison)
II. Displacement method (measurement by translation or transfer)
Figure 2.4: All measurement consist of the comparison of the unknown with a known. The
method for comparison vary but fall into one of two groups: interchange or displacement
ACCURACY, PRECISION AND RELIABILITY
Accuracy, precision and reliability are used interchangeably
But in precision measurement for manufacturing, the use of the wrong term affects the choice of measuring instrument and the efforts of the quality control team
The incorrect use of one of these terms could mean failure to meet quality control standards, excess production, excess materials use and more
Figure 2.6 Which of these targets represents
Accurate shooting? Precise shooting?
Reliable shooting?
Figure 2.7 A change in one variable, such as wind, alters the results as shown. Does this show which
shooting was most reliable?
ACCURACY
Accuracy is a comparison : the desired result is compared with the actual measurement.
“ quality of conformity” Answer the question good enough? Indicates whether the object
complies with a standard, is too big or small, or is in or out of tolerance.
PRECISION Precision reports the dispersement of
results or degree of repeatability within the manufacturing and measurement systems.
“quality of refinement” Precision answer the question how much? It provides a fineness for the range of
sizes allowable for one part or many parts, or it defines the fineness to which an instrument can read
RELIABILITY
Reliability shows the relationship between the predicted results and actual result and whether or not we actually can predict what will happen
Reliability can refer to the manufacturing or inspection process or to the part’s expected future performance
Figure 2.8 Reducing the target size by one half shows that the accuracy requirement may dictate the
precision requirement
ORIGIN OF THE METRIC SYSTEM
1790, French Republican established the meter: the distance between the north pole and the equator passing through Paris, divided into ten million parts
1799, metal standard, the Meter of the Archives was made and adopted as official standard
1870, the first series of international conference to establish the metric system worldwide was held (48 delegates representing 25 countries)
1889, a general conference in Paris approved the work of the committee. ( the one new standard most nearly equal to the Meter of the Archives was selected as the International Prototype Meter
1960, the metric system was revised worldwide, and the system was renamed to distinguish it from the other metric system. (Le Systeme International d’Unites- SI)
Figure 2.9 These are the prefixes used in SI to show magnitude. A centimeter, for example, is one-hundredth of a
meter
THE INTERNATIONAL INCH
The Mendenhall Act defined the U.S inch as 25.4000508mm
But the British Imperial Standard Inch was 25.399978mm
1922 - the British revised the standard to 29.399956mm
1951 - Canada revised their in to exactly 25.4mm
1959 - all three governments (U.S, Britain, Canada) agreed that one “international” inch would equal 25.4mm.
FUNDAMENTAL CRITERIA
In order to evaluate possible measurement systems, we must understand as much as possible about each system and be unbiased as possible
Natural of the
system
e.g Length of a forearm The distance from pole to equator
Economic consideratio
n
In the past – machines, tools, and manuals were replaced only when the old equipment was obsoleteToday – manufacturers are competing in global economy- manufacturers must recognize international demands and adapt manufacturing process to the accepted standards of the global economy
Either / or reasoning
As long as metrologists have a thorough understanding of conversion methods and their implications to precision and quality assurance, there is no reason why we cannot accommodate the use of both systems of measurement
THE BEST SYSTEM
The best system of measurement depends on what is being measured
What use the measurement has, whether scientific, commercial or cultural
The audience must understand the results of measurement process.
Use measurement system that helps other people understand the goals that what we are trying to accomplish
PRACTICAL CRITERIA
Metrological factor Computational factor
Communicative factor
Which act of measurement will yield usable result
Which system yields figures that we can use mathematically
Communication factor- which system makes it
easiest for us to share the measurement with other
people
Rounding off- Numerical Values
When we round off, we eliminate unnecessary figures in any computation
However, you must know both the correct method of rounding off and the number of significant figures needed in order to round off properly.
Consistency is one of the most component of reliable measurement (use same method of measurement)
*then, even we make errors, the errors have a chance to cancel each other out or to be caught more easily)
The reason for rounding off is to eliminate the meaningless digits
Figure 2.12 In this group of five values the rounded up column yielded high values, the rounded down, low values. The more items involved, the greater the total error
and average error.
CONCLUSION
To understand the language of manufacturers, you need to understand how big 0.01 inch and 0.001 inch are
To understand standards, reverence surfaces and surface finishes, you need to understand how big 0.000001 inch is
Quiz
1. What is principal dimensional measurement? length2. What is secondary dimensional measurement? Angle & curve3. Name two methods to compare unknown length to the
standard. Displacement & interchange4. When measurement are repeatedly taken on the same unit,
the extent to which an instrument replicates its measurements is precision
5. How many 0.001 inches in one inch? 10006. If 1mm= 0.0394 inches, 700 mm equal to how many inches?
27.581. The ability of two or more inspectors to obtain consistent
results repeatedly when measuring the same set of parts and the same measuring instrument called precision
2. Round off 25.7853901 to two significant digits 25.793. Le Systeme International d Unites is abbreviated for SI
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