mem560 topic 2_metrology & instrumentation

7/30/2019 MEM560 Topic 2_Metrology & Instrumentation

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Topic 2Topic 2

In this topic we are going to cover aspects of;2.0 Introduction to metrology

2.1 Tolerance, Limits and Fit

2.2 Linear Measurement Using Tools and such as Digital Gauges,

Micrometer and Calipers

2.3 Angular Measurement Using Profile Projector

2.4 Straightness, Flatness, and Roundness Measurement

2.5 Surface Roughness Measurement

2.6 Measurement Using GO and NO GO Concept

2



xx

xx

x

x

x

xPrecise &

accurate

Accurate

Precise, notaccurate



Define:

Science of measurement – experimental and

theoretical with certain level of uncertainties.

i.e: Length – Mass – Flatness – Profiles – taperImportance:

Topic 2 4

With a universal meaning

With a defined uncertainties

With a judicious standardization

With/using good and correct technique



StandardsRespective parties responsible to maintain standards.

Local : SIRIM - Standards and Industrial Research Institute of Malaysia (wholly-owned company of the Malaysian Government under the

Ministry of Finance Incorporated) International : ISO (International Organization for Standardization)

Doing a measurement? – With equipment/ apparatus (attributes – Yes/NoOR variables – dimensional wise)

Differentiate the Metric (SI) and Imperial System measurement

- SI: system of decimal units of weights and measures planned in France

and been adopted; it has since been adopted by most of thetechnologically developed countries of the world. It is based on a unit of length, called the meter (m), and a unit of mass, called the kilogram (kg)

- Imperial : system of measurement are based on the older English units of measurement

Topic 2 5

Why SI?

•Simplicity. The Metric system has only 7 basic measures, plus a substantial number of measuresusing various combinations of these base measures. Length quantity include: length, width,

depth, diameter, straightness, flatness, roughness

• Ease of calculation. All the units in the metric system are multiplied by 10 (to make larger units)

or divided by 10 (to make smaller units). For example a kilometer is 1000 meters (10 * 10 * 10).

• International Standard. With the exception of the USA, all major countries have converted tothe metric system (although in some countries, such as the UK, the conversion to metric is not yet

complete). Consequently, for any international communication (trade, science, etc.) the metricsystem is the most widely used and accepted.



Numerous measuring instruments and devices are used in engineeringmetrology, each of which has its own application, resolution,precision, and other features.

Terms that commonly used to describe the type and quality of aninstrument are:

1. Resolution the smallest difference in dimensions that the measuringinstrument can detect or distinguish

2. Precision the degree of repeatability in measurement process - can beachieved by selecting proper instrument technology for the application(random)

3. Accuracy the degree to which the measured value agrees with the truevalue of the quantity of interest – can be maintained by proper & regularcalibration (systematic)

4. Rule of 10 the measuring device must be 10 times more precise than thespecifies tolerance. e.g. if the tolerance to be measured is ±0.25mm, themeasuring device must have a precision of ±0.025mm



5. Repeatability (Equipment Variation): the variation inmeasurements obtained with one measurement instrument when

used several times by an appraiser while measuring the identical

characteristic on the same part.

6. Reproducibility (Appraiser Variation): the variation in the average

of measurements made by different appraisers using the same

instrument when measuring the identical characteristic on the

same part.



-Linear Measurement Using Tools and such asDigital Gauges, Micrometer and Calipers

-Angular Measurement Using Profile Projector

-Straightness, Flatness, and RoundnessMeasurement

-Surface Roughness Measurement

-Measurement Using GO and NO GO Concept

Topic 2 9



Tolerances (difference between two limit of size)

Tolerance that specify permitted variability of forms: e.g flatness,

straightness, roundness & cylindricity.

The difference between these two sizes is called the tolerance,

which can be defined as the amount of variation in size which is

tolerated.

Shaft - A member which fits into another member

Hole – A member which house or fits the shaft Nominal size - Size by which an item is designated as a matter of

convenience

Basic size - This is the size from which the limits of size are derived

by the application of the upper and lower deviations. Has the same

size for both the shaft and the hole for a given fit that usually equalto nominal size. Topic 2 10

4.25 +0.01 / - 0.01

(BILATERAL)

4.25 +0.01 / -0.00

(UNILATERAL)

4.2495 / 4.2505

LIMIT DIMENSIONS



Limits of sizeThese are the extremes of size which are allowed for a tolerance

dimension.

Two limits are possible:

a) upper limit of size

b) lower limit of size

Maximum Material Limit (MML) the max. (upper) limit of size for an external feature (shaft)

the min. (lower) limit of size for an internal feature (hole)

Least Material Limit (LML)

the min. (lower) limit of size for an external feature (shaft)

the max. (upper) limit of size for an internal feature (hole)



Allowances ( 8 Classes as per ANSI)

Topic 2 13

Clearance fit & loose fit / free fit Interference fit



Calibration

- a comparison between measurements - of known magnitude or

correctness made or set with one device and another measurement

made in as similar a way as possible with a second device.

- device with the known or assigned correctness is called the standard .

The second device is the unit under test (UUT), test instrument (TI), or

any of several other names for the device being calibrated .

-Equipment is compared to;

Primary standard

Secondary standard (with higher accuracy than the equipment itself)

Known input source

- Calibration can be called for:

• with a new instrument

• when a specified time period is elapsed• when a specified usage (operating hours) has elapsed

• when an instrument has had a shock or vibration which potentially may

have put it out of calibration

• sudden changes in weather

• whenever observations appear questionable

Topic 2 14



Methods of Measurement

1) Direct Comparison with the

Standard

In the direct comparison method of

measurement, we compare the

quantity directly with the primary

or secondary standard.

e.g. length of the rod – measured

with measuring tape or scale that

acts as the secondary standard.

Hence, comparing the quantity to

be measured directly with the

standard.

There are some difference,

between the actual value of thequantity and the measured value

of the quantity. Why??

2) Indirect Method of

Measurement

There are number of quantities

that cannot be measured directly

using some instrument.

e.g. The strain in the bar due to

applied force directly.

We may use indirect methods.

Measurements comprises of the

system that senses, converts,

and finally presents an

analogues output in the form of

a displacement or chart. It is

necessary to amplify it to read itaccurately and make the

accurate reading of the quantity

to be measured.

Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck



Selection of Inspection Equipment;

• Gage capability

• Linearity

• Repeat accuracy

• Stability / drift

• Magnification

• Resolution / sensitivity

1. Linear measurement :

• Measurement along a line (straight, curve, angle).

• It applies to area and volumes as these can be reduced to

their component lines.

• Parameters : length, width, distance, height, depth,thickness

• Instruments : ruler, tape, caliper, vernier, gauge,

micrometer



Analog & Digital Measurement



Digital Gauges, Micrometer and Calipers

Topic 2 18

Digital Caliper Dial Caliper Vernier Caliper

Point

Micrometer

Dual Point

Micrometer

Outside

Micrometer

Nibs

Anvil Spindle

Frame Thimble

SleeveRatchet stop

Jaw

http://www.tresnainstrument.com/product/ec12.html

http://www.tresnainstrument.com/product/m13.html









http://www.tresnainstrument.com/product/vc01.html

http://www.tresnainstrument.com/product/dc03.html

http://www.tresnainstrument.com/product/ec12.html



Example of vernier reading;

Step 1: Look at the zero line on the plate to see

where you start. In this case, it is not quite at 1”

and the zero line is at 0.95” (plus something)

Step 2 : Remember (write down) 0.95

Step 3: Now go to the vernier scale on the plate

to see how much to add to 0.950 to obtain the

correct reading

Step 4 : When you find the one line line that

lines up with any line on the bar, you add the

amount shown on the plate (at the line) to the

number 0.950 you already had. In this case, theline is 0.006 as shown by the small circles drawn

on the scale.

* The zero line showed that the reading starts at

0.950 plus something. The vernier scale shows

0.006 line. So the answer is 0.956”



Angular Measurement

Topic 220

Profile Projector/optical comparison ;-Diascopic (behind) / episcopic (above)

illumination

-Measuring stroke

-Contour illumination system

-Standard equipped turret

-X and Y wheel-Digital displays

-Focusing (motor-driven or manual)

-Bulbs

-Overlay Charts

-Resolutions

-Data processing unit

Angle measurement;Protractor : measuring angle via

displacement method in reading of

degrees



Bevel protractor applications

Example of reading on the equipment;

Step 1 : Use the 0 line on the main scale

and obtain the reading degrees. This is at12 degres (almost 13 degrees).

Step 2 : Look to the right of the 0 line

and see what line (between 0 and 60)

lines up precisely with any line on the

main scale. In this example it is the 50-minute line

Step 3 : Hence, the reading is 12 degrees

and 50 minutes (12°50”)

* Read the vernier scale in the same

direction from zero as the main scalereading



Straightness, Flatness, and Roundness Flatness

Tolerance zone formed by two parallel planes containing all elements on asurface

Straightnessi.e Laser beam

Flatness (having no irregularities, roughness, or indentations: Eveness of surface; the state of being plane or level)

i.e reference of surface (3 points) onto new reference plane (3 points)

i.e use of calibrated granite table

Roundness (expressed as the radius of the average radius of curvature of the edges or corners to the radius of curvature of the maximum inscribedsphere)

i.e use of dial indicator with object set on V-block, error in roundness can bedirectly known by comparing the peak height as measured by the dialgauge.

PrecisionThe ability of a measurement to be consistently reproduced.

The number of significant digits to which a value has been reliablymeasured.

Accurate

Conforming exactly to fact; errorless.

Deviating only slightly or within acceptable limits from a standard.Capable of providing a correct reading or measurementTopic 2 23



Surface Roughness Measurement

Topic 2 24

Characteristic of a surface is its structure, texture and roughness.

-Structured : microscopic level - effect of processes that the raw material had

gone through.-Texture : repetitive and/or random deviations from the nominal surface of an

object (roughness, waviness, form, lays and flaws).

{surface profile = error of form + waviness + roughness}

-Roughness : surface profile (peaks and valley), closely space, irregular deviations

on small scale expressed in heights, widths and distance along the surface.

Method I – Arithmetic mean value, Ra [Ra = a + b + c ….(absolute values)/n]Method II- Root-mean square (RMS) roughness, Rq [Rq = √a2 + b2 + c2 +…/n]

-Waviness : recurrent deviation from flat surface; measured and described in terms

of space between adjacent crests of the waves (waviness width) and height

between crests and valleys of the waves (waviness height)

-Integrity : sub-surface alterations produced due to manufacturing.

Method 1- Fingernail comparatos : running your finger across the comparatorthen, across the part – feeling the roughness. Then run your fingernails cross the

part and make decision which is rougher.

Method II – Electronic for surface finish measurement : e.g profilometer



Surface profile = Error of form + Waviness + Roughness



GOGO and NO GO Conceptand NO GO Concept

Gauges: device that measures the dimension of an object or to

measure the dimension of produced part if it is withinspecified limits.

: gauge of a numeric scale for metal thickness

GO-NO-GO and GO gauges

Gauges that gives categorical data on one or moredimension of a work piece is within specification.

Limit Gauges / Attribute gauges

Topic 2 26



Attribute gages (Go-No GO)

- Fixed gages that are designed to check single

dimension and/or tolerance limit.- It does not tell the actual size of a dimension, just

the goodness or badness of part in question.

Progressive ring gage Two ring gages (Go- No Go)

* To check outside diameters



Attribute gages (form gages)



Some types and examples;

i. Observational- Misreading the gage- Parallax errorii. Manipulative- Holding the gage incorrectly- Not locating datums properly- Mounting the part on the wrong datum- Not aligning the gage properlyiii. Bias- Rounding off (on purpose)- Gage inaccuracyiv. Gage error- Precision loss (e.g a sluggish or sticking

indicator)

- Accuracy loss (e.g incorrect gage block stack)v. Part error- Within the piece variation- Dirty parts- Poor surface finish- Flaws

Errors

With all dimensioninspected, tendency forerrors are great with 3major categories namely;

1. Systematic error

- Due to quality of equipment: causes

consistent signs (+ve or –ve)

- Due to ambientconditions : temperature,humidity, CO2 or pressureof work piece orinstrument

2. Accidental error- Due to the smallest

gradient in equipment or

- Inexperience of equipment user

3. Illegitimate error



Rachet

Micrometer caliper

Parallax

Dial gage Resolution

Vernier caliper

CMM

mem560 topic 2_metrology & instrumentation

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