Metrology linear

Engineering Metrology
Linear measurements
Dr. Belal Gharaibeh

10/7/2011

Dr. B. Gharaibeh 10/7/2011 1

Introduction
• modern manufacturing can produce features that are more
accurate than we can measure by hand, therefore we need
tools to assist us.
• • These tools allow us to quantitatively evaluate physical
properties of objects.
• • EVERY industry uses these tools to some extent, for
example,
• - machine shops
• - tailors
• - dentists
• - automotive manufacturers
• - etc.


Definitions
• Assembly - the connection of two or more separate parts to
make a new single part.
• Basic Dimension - The target dimension for a part. This
typically has an associated tolerance.
• Dimension - A size of a feature, either measured, or specified.
• Dimensional Metrology - The use of instruments to determine
object sizes shapes, form, etc.
• Limits - These typically define a dimensional range that a
measurement can be expected to fall within.
• Machine Tool - Generally used to refer to a machine that performs a
manufacturing operation. This is sometimes confused with the actual
cutting tools, such as a drill bit, that do the cutting.


Definitions
• Measurement - The determination of an unknown dimension. This
requires that known standards be used directly, or indirectly for
comparison.
• Metric System - A measurement system that has been standardized
globally, and is commonly used in all modern engineering projects.
• Metrology - The science of measurement. The purpose of this discipline is
to establish means of determining physical quantities, such as dimensions,
temperature, force, etc.
• Repeatability - Imperfections in mechanical systems can mean that during
a Mechanical cycle, a process does not stop at the same location, or move
through the same spot each time. The variation range is referred to as
repeatability.
• Standards - a known set of dimensions, or ideals to compare others
against.
• Standard Sizes - a component, or a dimension that is chosen from a table
of standard sizes/forms.
• Tolerance - The allowable variation in a basic dimension before a part is
considered unacceptable

Tolerance

• Impossible to make perfect parts
• Too small tolerance, cost is high
• Boeing 747-400 has 6 million parts, measurement
of 28 features, 150 million measurements
• NIST (U.S. National Institute of Standard and
technology); tolerance shrink by a factor of 3
every 10 yearsultra-precision ion-beam
machining 0.001mm


Importance of tolerance
• Parts from the same machine can be
different
– Speed of operation
– Temperature
– Lubrication
– Variation of incoming material
– Other factors
• ISO system; definitions


Scales
• The most common tool for crude measurements
is the scale (also known as rules, or rulers)
• These are limited by the human eye
• Parallax error can be a factor when making
measurements with a scale-line of sight not being
normal to the scale of the instrument


Errors
1- Instrument limitations.
2-Geometric errors (flatness and parallelism).
3-Thickness of the grade line.
4-Least increment limitation.
5-Observation error.
6- Alignment error.
7- Parallax error (object not well aligned with scale).


Calipers
• A tool used to transfer measurements from a part to a scale, or
other instrument.
• Calipers may be difficult to use, and they require that the operator
follow a few basic rules,
– Do not force them, they will bend easily, and invalidate measurements
made
– Try to get a feel, or personal technique for using these instruments.
– If measurements are made using calipers for comparison, one
operator should make all of the measurements (this keeps the feel
factor a minimal error source).
• These instruments are very useful when dealing with hard to reach
locations that normal measuring instruments cannot reach.
• Obviously the added step in the measurement will significantly
decrease the accuracy


Construction & Use:
• They consist of two legs hinged at the top with
the ends of the legs span the parts to be
inspected.


Types of calipers.
• Calipers can be classified as : Outside, Inside.

• Calipers can be classified as : Spring, Firm
joint, Lock joint , Transfer.


firm joint calipers

Inside firm joint caliper Outside firm joint caliper

Spring caliper

Inside spring caliper outside spring caliper

• Operating principle: They are devices for
comparing measurements against known
dimensions.


• Qualities: They should be free from cracks,
seams, dirt, flaws and must have smooth
bright finish.

• Nominal Size is the distance between the
center of the rolling end and the extreme
working end of a leg.


• Caliper’s Capacity is the maximum dimension
that can be measured by the caliper. It should
not be lesser than the nominal size.

• The accuracy depends on the sense and feel
of the operator. Therefore, caliper should be
held gently and square to the work with slight
gauging pressure applied.


Instruments
• Vernier scales
• Like a normal scale with extra secondary scale subdividing major
increments
• Secondary scale is one increment shorter than a main scale
therefore indicating relative distance between two offsets of the
main scale
• On imperial sliding vernier scales the main scale divisions are 0.050”
apart, and on the vernier scale they are 0.049”, giving a reading of
0.001” per graduation.
• On metric sliding vernier scales the main scale divisions are 1mm
apart, and the vernier scale they are 0.98 mm, giving a reading of
0.02mm per graduation.


Scalar vernier


How to read a vernier caliper.
• The reading on the main scale just before the
zero of the vernier is noted. This is called Main
scale reading (M.S.R).
• The number of division on the vernier which
coincides perfectly with any one of the main scale
divisions is noted.This is called vernier
coincidence (V.C).
• The vernier coincidence (V.C) is multiplied by
least count to get the fraction of a main scale
division.This is added to the main scale reading
(M.S.R) to total reading.


Micrometers


Micrometer scales
• This is a very common method for measuring
instruments, and is based on the thread principle.
• In effect, as a thread is turned, a large motion on the
outside of the thread will result in a very small advance
in the position of the thread.
• On imperial micrometers the divisions are typically
.025” on the sleeve, and 0.001” on the thimble. The
thread used has 40 T.P.I. = a pitch of 0.025
• Metric micrometers typically have 1 and 0.5 mm
divisions on the sleeve, and 0.01mm divisions on the
thimble. The thread has a pitch of 0.5mm.
• Depth micrometers have an anvil that protrudes, out
the end, and as a result the scales are reversed to
measure extension, instead of retraction.


• The micrometers pictured below have major scales, as
well as minor scales. The major scales are read first,
and the micrometer scales are read second and the
readings added on.
• The metric micrometer below reads 13.5 = 13.5mm on
the major scale, and 31 = .31mm on the thimble, for a
total of 13.81mm
• The Imperial scale below shows a micrometer reading
of 4.5 = .45” on the main scale, and 9 =.009” on the
thimble, for a total of .459


Magnification
The operation of micrometers is based on magnification using
threads.
• A large movement on the outside of the micrometer thimble will
result in a small motion of the spindle
• There are two factors in this magnification. First, the difference in
radius between the thread, and the thimble will give a change in
sensitivity relative to the difference in radii. Second, the pitch of the
thread will provide a reduction in motion.
M = magnification from the moving head to the hand motion
C = measuring diameter of the instrument
D = diameter of the thread
pitch = the number of threads per unit length


Dial Indicator
• Converts a linear displacement into a radial movement to
measure over a small range of movement for the plungers
• The radial arm magnification principle is used here.
• These indicators are prone to errors caused by errors that are
magnified through the gear train.
• Springs can be used to take up any play/backlash in the rack
and pinion to reduce these errors.
• The gears are small, but friction can result in sticking, thus
reducing accuracy
• A spring is used on the rack to return the plunger after
depression.
• The problems mentioned earlier will result in errors in these
instruments. If the dial indicator is used to approach a
dimension from two different sides, it will experience a form
of mechanical hysteresis that will bias the readings.
• Causes of hysteresis are: bending strain, inertia, friction and
play in the instrument Dr. B. Gharaibeh 10/7/2011 29

Example of errors in dial indicators


Gauge blocks
• Blocks with knows linear dimensions within a given tolerances
• Requirements of gage blocks
– Known actual size (measured)
– Parallel faces
– Smooth finish surface
– Flat surface
• Gage blocks materials are selected for:
– Hardness
– Temperature stability
– Corrosion resistance
– High quality finish

You will have a set of blocks to arrange them by wringing to get the
desired dimension
Wringing: sliding blocks on each other with a layer of oil ( 0.2 µ-in)

Gage blocks continue
• there are four grades of blocks,
– reference (AAA) - high tolerance (± 0.00005mm or 0.000002”)
– calibration (AA) (tolerance +0.00010mm to -0.00005mm)
– inspection (A) (tolerance +0.00015mm to -0.0005mm)
– workshop (B) - low tolerance (tolerance +0.00025mm to -0.00015mm)
• Original gauge block sets had lower tolerances and had a total of
91 pieces with values, 0.010” to 0.100” in 0.001” steps
• An 81 piece set of gauge block was developed by Johansson and is
capable of covering wider ranges of dimensions.
• 0.1001” to 0.1009” in 0.0001” steps
• 0.1010” to 0.1490” in 0.0010” steps
• 0.0500” to 0.9500” in 0.0500” steps
• 1.0000”, 2.0000”, 3.0000”, 4.0000” blocks
• (2 wear blocks at 0.0500”)


Example
from the 81 piece set above, build a stack that is
2.5744”
-0.1004”
therefore the gauge blocks are,
2.4740”
0.1004”
-0.1000”
2 wear blocks @ 0.0500”
2.3740”
0.1240”
-0.1240”
0.2500”
2.2500”
2.0000
-0.2500”
2.0000”
-2.0000”
0”


• Gage block dimensions will change with temperature variation
• If the readings are taken at temperature other than the gage blocks
design temperature, then there should be temperature
compensation for the dimensional change
• Gage blocks standard temperature is 20 C (68 F)
• Temperature will add bias error, either zero offset and/or scale
error
• Linear dimensional changes caused by thermal expansion given by:

L  Lb 1  T 
   p   b
Lb  the block gage nominal length after
summing all blocks wrung together
T  Tr  standard reference temperature
 p  the temp.coeff. of expansion of the part being gaged
 b  the temp.coeff. of expansion of the gage block material (ppm/)
Tr  ambient temp.
Temperature compensation Dr. B. Gharaibeh 10/7/2011 35

Special accessories: Comparators
• Use gage blocks to calibrate a comparator
– Instead of using blocks every time to measure a
certain part, you can calibrate an indicator to the
desired length
– Useful for measuring many parts of same desired
dimensions
– Useful for small or complicated shapes


Fixed gages
• Go/no-go plug type gage
• Rule: dimension the plug gage to 10% of the tolerance of the part
• The go-no-go gage will be inserted every time in the hole
• Go-gage is calibrated using high resolution gage blocks


1.500
1.504 Example of fixed gage

• Hole diameter 1.50/1.504 (tolerance =0.004)
• Gage tolerance is 0.0004 (10% of original 0.004)
• Go side
– 1.500 -0.0001 = 1.4999
– 1.500 +0.0003= 1.5003
– Note: total tolerance is 0.0004 but we make the tolerance
towards the gage wear of the specified limit
• No-go side:
• 1.504-0.0002= 1.5038
• 1.5004+0.0002= 1.5042
• Note: tolerance is made equally for the NO-GO limit
because it will not pass through the hole and there is no
friction wear

Optical methods-linear measurement
• Areas of measurement:
1. Accurate small dimensions (less than 1 m)
2. Measurement of large dimensions (more than 1
m) by using alignment telescopes with
projection systems.
• We will discuss the first type only


Monochromatic light
• Mono = single
• Chromatic = waves, like sound or light
• You can produce monochromatic light with a prism. But
this is NOT a good way for measurement
• a better way is to use electrical excitation of atoms of
certain elements that radiate light at certain (mono)
wavelength (chromatic)

Prism diffracts light to monochromatic light beams

Lamps and lasers

• Monochromatic lights can be produced by exciting
certain material elements in the form of lamps or
lasers
• Examples of lasers:
– Red: helium-neon (HeNe), blue-green:argon-ion laser
• Gas Lasers are more accurate than lamps because:
– Extremely monochromatic with very narrow
monochromatic light
– Collimated light forming narrow and directional beam
– Highly coherent: the light stays in phase with it self for
the total length of the beam


Fringes
The Visible Light Spectrum
Color Wavelength (nm)
Red 625 - 740
Orange 590 - 625
Yellow 565 - 590
Green 520 - 565
Cyan 500 - 520
Blue 435 - 500
Violet 380 - 435

Fringes are half the wavelength distance of a certain lamp or laser, t
The distance between fringes indicates the height between the
optical flats and work
Δd=fringe interval x N = λ/2 x N
N: number of fringes

light for dimensional comparison
• Flats and monochromatic light are used as an optical comparator
• The number of fringes indicates the height depending on the angle

View
direction
From
light

A Optical flat
C

B
work


Surface plate
• Accurate reference plane- very flat surface.
• Made of machine-lapped and polished granite surface
plate
• The general advantages of these plates over cast
iron are,
– durability
• closer tolerances
• lower cost
• lower thermal expansion
– Quality
• non-rusting
• burrs do not occur, but chipping does
– ease of use
• non-magnetic
• less glare
• no oil is required, thus dust does not stick
• less wringing
44
• inserts are often provided for clamping

Metrology linear

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