METROLOGY
Production Engineering 2
Code: MEC3202
1
MEASUREMENT AND INSPECTION
•Metrology
•Inspection Principles
•Conventional Measuring Instruments and Gages
•Measurement of Surfaces
•Advanced Measurement and Inspection Techniques
2
Measurement Defined
A procedure in which an unknown quantity is compared to a known
standard, using an accepted and consistent system of units
• The measurement may involve a simple linear rule to scale the
length of a part
• Or it may require a sophisticated measurement of force versus
deflection during a tension test
• Measurement provides a numerical value of the quantity of
interest, within certain limits of accuracy and precision
• Uncertainity, since the true value cannot be known, the error of
a measurement is also unknown. We are uncertain how well the
measured value represents the true value.
3
Inspection Defined
A procedure in which a part or product characteristic, such as a
dimension, is examined to determine whether or not it conforms
to design specification
• Many inspection procedures rely on measurement techniques,
while others use gaging methods
– Gaging determines simply whether the part characteristic
meets or does not meet the design specification
– Gaging is usually faster than measuring, but not much
information is provided about the actual value of the
characteristic of interest
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Metrology
Defined as the science of measurement
• Concerned with six fundamental quantities:
– Length
– Mass
– Time
– Electric current
– Temperature
– Light radiation
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Metrology
• From these basic quantities, most other physical quantities are
derived, such as:
– Area
– Volume
– Velocity and acceleration
– Force
– Electric voltage
– Heat energy
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Manufacturing Metrology
• In manufacturing metrology, the main concern is with measuring
the length quantity in the many ways in which it manifests itself
in a part or product
– Length and width
– Depth
– Diameter
– Straightness, flatness, and roundness, etc.
– Surface roughness
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Basic concepts
Accuracy - the degree to which the measured value agrees with
the true value of the quantity of interest
• A measurement procedure is accurate when it is absent of
systematic errors
– Systematic errors are positive or negative deviations from
the true value which are consistent from one measurement
to the next
Precision is the degree of repeatability in the measurement
process
• Good precision means that random errors in the measurement
procedure are minimized
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Accuracy versus precision in measurement:
(a) high accuracy but low precision;
(b) low accuracy but high precision; and
(c) high accuracy and high precision
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•Error = measured value – true value
•Random error, the values are randomly distributed above or
below the measured value.
•Systematic error, the values are consistently above OR below the
true value.
•Precise measurements do not imply accuracy!
•Accurate measurements do not imply precision!
•Allowance is a planned deviation between an actual dimension
and a nominal/theoretical dimension.
•A tolerance is the limit of acceptable unintended deviation from
a nominal or theoretical dimension. A pair of tolerances, upper
and lower, defines a range within which an actual dimension may
fall while still being acceptable.
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· Standards are the basis for all modern accuracy. As new
methods are found to make more accurate standards, the level
of accuracy possible in copies of the standard increase, and so
on. A well known metric standard is the metric 1m rod.
· Many standards are available for measuring, and many
techniques are available for comparison.
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Two Dominant Units Systems
• Two systems of units have evolved into predominance in the
world:
1. U.S. customary system (U.S.C.S.)
2. SI (for Systeme Internationale d'Unites) - the “metric
system”
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Inspection
• Inspection involves the use of measurement and gaging
techniques to determine whether a product, its components,
subassemblies, or starting materials conform to design
specifications
• Inspections divide into two types:
1. Inspection by variables - product or part dimensions of
interest are measured by the appropriate measuring
instruments.
2. Inspection by attributes - parts are gaged to determine
whether or not they are within tolerance limits.
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Manual Inspection
• Inspection procedures are often performed manually
• The work is boring and monotonous, yet the need for precision
and accuracy is high.
• Hours may be required to measure the important dimensions of
only one part.
• Because of the time and cost of manual inspection, statistical
sampling procedures are often used to reduce the need to
inspect every part.
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Sampling Inspection
• When sampling inspection is used, the number of parts in the
sample is usually small compared to the quantity of parts
produced
– The sample size may be 1% of the production run
• Because not all of the items in the population are measured,
there is a risk in any sampling procedure that defective parts
will slip through
– The risk can be reduced with a larger sample size
– But the fact is that less than 100% good quality must be
tolerated as the price of using sampling
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100% Inspection
• Theoretically, the only way to achieve 100% good
quality is by 100% inspection
– Thus, all defects are screened and only good quality
parts are passed
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Manual 100% Inspection
• However, when 100% inspection is done manually, two
problems are encountered:
1. The expense - the unit inspection cost is applied to every
part in the batch.
2. In 100% manual inspection, there are almost always
human errors.
• Therefore, 100% inspection using manual methods is no
guarantee of 100% good quality product
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Automated 100% Inspection
and Corrective Action
• Automated 100% inspection can be integrated with the
manufacturing process to accomplish one or both of the
following corrective actions:
1. Parts sortation - separating parts into two or more quality
levels, such as acceptable and unacceptable
2. Feedback of inspection data to the upstream
manufacturing operation to allow compensating
adjustments in the process to reduce variability and
improve quality
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Conventional Measuring Instruments
and Gages
• Precision gage blocks
• Measuring instruments for linear dimensions
• Comparative instruments
• Fixed gages
• Angular measurements
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Precision Gage Blocks
The standards against which other dimensional measuring
instruments and gages are compared
• Usually square or rectangular in shape
• Measuring surfaces are finished to be dimensionally accurate
and parallel to within several millionths of an inch and are
polished to a mirror finish
• Precision gage blocks are available in certain standard sizes
or in sets, the latter containing a variety of different sized
blocks
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Gage Blocks continued
The purpose of gauge blocks is to provide linear dimensions
known to within a given tolerance.
· The requirements of gauge blocks are:
- the actual size must be known
- the faces must be parallel
- the surface must have a smooth finish mirror polish
- the surfaces must be flat
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Gage Blocks continued
Most gauge blocks are made by normal techniques, but the high
accuracy is obtained by a process called lapping. The materials
from which gauge blocks are made from are selected for:
- hardness
- temperature stability
- corrosion resistance
- high quality finish
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Assembling a gauge block stack
1. Remove the gauge blocks required from the protective case.
2. Clean of the oil that they have been coated in using a
special cleaner. It is acceptable to handle the blocks, in fact the
oil from your hands will help them stick together.
3. One at a time, hold the blocks so that the faces just overlap,
push the blocks together, and slide them until the faces overlap
together. This will create a vacuum between the blocks that
makes them stick together (this process is known as wringing).
4. Make required measurements with the gauge blocks, being
careful not to damage the faces.
5. Take the blocks apart, and apply the protective coating oil,
and return them to their box.
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Grades of gauge blocks
They are available in various grades depending on their intended
use.
•reference (AAA): small tolerance (± 0.00005 mm or 0.000002
in) used to establish standards
•calibration (AA): (tolerance +0.00010 mm to -0.00005 mm) used
to calibrate inspection blocks and very high precision gauging
•inspection (A): (tolerance +0.00015 mm to -0.00005 mm) used
as tool room standards for setting other gauging tools
•workshop (B): large tolerance (tolerance +0.00025 mm to -
0.00015 mm) used as shop standards for precision measurement
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•When using gauge blocks, minimize the number used. Each
block will have tolerance errors, and as the stack of blocks
becomes larger, so does the error.
•Do not leave gauge blocks wrung together for long periods of
time.
•Example 1
•Example 2 -To build up 58.345 mm with a M 112 set
1st gauge 1.005 mm
2nd gauge 1.04 mm
3rd gauge 1.30 mm
4th gauge 5 mm
5th gauge 50 mm
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Measuring Instruments for
Linear Dimensions
• Divided into two types:
• Graduated measuring devices include a set of markings
(called graduations) on a linear or angular scale to which
the object's feature of interest can be compared for
measurement
• Non-graduated measuring devices possess no such scale and
are used to make comparisons between dimensions or to
transfer a dimension for measurement by a graduated
device
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Two sizes of outside calipers
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External micrometer, standard one inch size with digital
readout
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Comparative instruments
•Used to make dimensional comparisons between two objects such
as a work part and a reference surface.
•Usually not capable of providing an absolute measurement of
the quantity of interest but instead they measure the magnitude
and direction of deviation between two objects.
•Such instruments include mechanical and electronic gages.
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Mechanical Gages: Dial Indicators
• Mechanical gages are designed to mechanically magnify the
deviation to permit observation
• The most common instrument in this category is the dial
indicator, which converts and amplifies the linear movement
of a contact pointer into rotation of a dial
– The dial is graduated in small units such as 0.001 inch (or
0.01 mm)
– Applications: measuring straightness, flatness, parallelism,
squareness, roundness, and runout
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Dial indicator: top view shows dial and graduated face;
bottom view shows rear of instrument with cover plate
removed
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Dial indicator setup, as part is rotated about its center, variations
in outside surface relative to the center are indicated on the dial
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Electronic Gages
A family of measuring and gaging instruments based on
transducers capable of converting a linear displacement into an
electrical signal
• The electrical signal is then amplified and transformed into
suitable data format such as a digital readout
• Applications of electronic gages have grown rapidly in recent
years, driven by advances in microprocessor technology
• They are gradually replacing many of the conventional
measuring and gaging devices
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GO/NO-GO gages
So-called because one gage limit allows the part to be inserted
while the other limit does not
• GO limit - used to check the dimension at its maximum material
condition
– This is the minimum size for an internal feature such as a hole,
and it is the maximum size for an external feature such as an
outside diameter
• NO-GO limit - used to inspect the minimum material condition of
the dimension in question
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Fixed gages
·
•One gauge must fit inside the feature, and the second must not.
In other words the GO gauge must fit inside/outside the feature,
the NO GO gauge must not.
•If the GO gauge does not fit, the tolerance is above the
maximum metal tolerance. If the NO GO gauge goes, the
feature is below the minimum metal tolerance.
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Angular measurements
•Angles can be measured using several protractor styles such as
a simple protractor and a bevel protractor.
•High precision in angular measurements can be made using a
sine bar.
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Measurement of Surfaces
• Two parameters of interest:
– Surface texture - geometry of the surface, most commonly
measured as surface roughness
• Surface roughness - small, finely spaced deviations
from nominal surface determined by material and
process that formed the surface.
– Surface integrity - deals with the material characteristics
immediately beneath the surface and the changes to this
subsurface resulting from the manufacturing processes that
created it
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Measurement of Surface Roughness
• Three methods to assess surface roughness:
1. Subjective comparison with standard test surfaces
2. Stylus electronic instruments
3. Optical techniques
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Stylus Instruments for
Surface Texture Measurement
• Similar to the fingernail test, but more scientific
• In these electronic devices, a cone shaped diamond stylus is
traversed across test surface at a constant slow speed
• As the stylus head is traversed horizontally, it also moves
vertically to follow the surface deviations
• The vertical movement is converted into an electronic signal
that represents the topography of the surface
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Sketch illustrating the operation of stylus type instrument. Stylus head traverses
horizontally across surface, while stylus moves vertically to follow surface
profile. Vertical movement is converted into either: (1) a profile of the surface,
or (2) the average roughness value
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Advanced Measurement and Inspection
Technologies
• Substituting for manual measuring and gaging techniques in
modern manufacturing
• Include contact and non-contact sensing methods:
1. Coordinate measuring machines
2. Lasers
3. Machine vision
4. Other non-contact techniques
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Coordinate Measuring Machine (CMM)
Measuring machine consisting of a contact probe and a
mechanism to position the probe in three-dimensions relative
to surfaces and features of a workpart
• The probe is fastened to a structure that allows movement
relative to the part
• Part is fixtured on worktable connected to structure
• The location coordinates of the probe can be accurately
recorded as it contacts the part surface to obtain part
geometry data
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Coordinate measuring machine
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CMM Probes
• Modern "touch trigger" probes with sensitive electrical contact
that signals when the probe is deflected from neutral position
in the slightest amount
– On contact, the coordinate positions are recorded by the
CMM controller, adjusting for overtravel and probe size
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Applications of CMM machines
•Dimensional measurement
•Profile measurement
•Depth mapping
•Digitizing or imaging
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CMM Advantages
• Higher productivity - a CMM can perform complex
inspection procedures in much less time than traditional
manual methods
• Greater inherent accuracy and precision than conventional
methods
• Reduced human error
• Versatility - a CMM is a general purpose machine that can
be used to inspect a variety of part configurations
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Measurements with Lasers
• Laser stands for light amplification by stimulated emission of
radiation
• Lasers for measurement applications are low-power gas
lasers that emit light in the visible range
• Laser light beam is:
– Highly monochromatic - the light has a single wave length
– Highly collimated - the light rays are parallel
• These properties have motivated many applications in
measurement and inspection
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Scanning Laser Systems
Uses a laser beam deflected by a rotating mirror to produce a
beam of light that sweeps past an object
• A photo detector on the far side of the object senses the light
beam during its sweep except for the short time while it is
interrupted by the object
• This time period can be measured quickly with great
accuracy
• A microprocessor system measures the time interruption
related to the size of the object in the path of the laser, and
converts it to a linear dimension
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Machine Vision
Acquisition, processing, and interpretation of image data by
computer for some useful application
• 2-D systems view the scene as a plane
– Examples: dimensional measuring and gaging, verifying
presence of components, and checking for features on a flat
surface
• 3-D vision systems are required where contours or shapes are
involved
• The majority of current applications are 2-D
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Image Acquisition and Digitizing - Step 1
• Accomplished by a video camera connected to a digitizing
system to store the image data for subsequent processing
• With the camera focused on the subject, an image is obtained
by dividing the viewing area into a matrix of discrete picture
elements (pixels)
– Each pixel assumes a value proportional to the light
intensity of that portion of the scene
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Image Acquisition and Digitizing - continued
• In a binary vision system, the light intensity is reduced to
either of two values
– Black or white = 0 or 1
• Each set of pixel values is a frame, which is stored in
computer memory
– Reading the pixel values in a frame is performed at 30
Hz in U.S., 25 Hz in European systems
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a)Dark coloured part against a light background.
b) A 12*12 matrix of pixels imposed on the scene.
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111111111111
111111111111
111111111111
111111100011
111111011011
111110011011
111100000011
111000000011
111010000011
111100000011
111111111111
111111111111
Pixel values in a binary vision system for an image
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Image Processing and Analysis - Step 2
• The data for each frame must be analyzed within one scan
(1/30 or 1/25 s)
• Techniques to analyze image data:
1. Edge detection - determining locations of boundaries of
an object
• Done by identifying contrast in light intensity between
adjacent pixels at borders
2. Feature extraction - determining feature values of an
image, such as area, length, width, diameter, perimeter,
and aspect ratio
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Interpretation - Step 3
Concerned with recognizing the object
• Identifying the object in the image by comparing it to
predefined models or standard values
– Accomplished using extracted features
– One common technique is template matching, which refers
to methods that compare one or more features of an
image with corresponding features of a model (template)
stored in computer memory
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Machine Vision Applications
1. Inspection
2. Part identification
3. Visual guidance and control
4. Safety monitoring
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Machine Vision Inspection
• Most important category - accounts for about 90% of all
industrial machine vision applications
• Most applications are in mass production where high cost of
programming and installation can be spread over many units
• Typical tasks:
– Dimensional measurement or gaging
– Verification functions
– Identification of flaws and defects
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Thank you!
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