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Measuring and gauging instrument

1. Measurement2. Metrology 3. Measuring and gauging Instruments4. Measurement of Surfaces

5. Inspection Principles6. Advanced Measurement and Inspection

Techniques

MeasurementProcedure in which an unknown quantity is compared to a known standard, using an accepted and consistent

system of units is known as measurement.

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

MetrologyDefined as the science of measurement.

Concerned with seven fundamental quantities (standard units shown in parentheses):

1. Length (meter)2. Mass (kilogram)3. Time (second)4. Electric current (ampere)5. Temperature (degree Kelvin)6. Light intensity (candela)7. Matter (mole)

From these basic quantities, most other physical quantities are derived, such as:

Area Volume Velocity and acceleration Force Electric voltage Heat energy

Manufacturing Metrology

In manufacturing metrology, we are usually concerned with measuring a length quantity of a part or product

Length and width Depth Diameter Straightness, flatness, and roundness, etc. Surface roughness

Key Terms in Measurement

Basic size: The theoretical absolute perfect dimension, without any consideration of limits or tolerances. Limits: The maximum and minimum allowable dimensions, above and below the basic size. Tolerance: The permissible variation in the size of the part; shows the permissible variation above and

below the basic size. Clearance: The difference in size between mating parts where the outside dimension of the shaft is

smaller than the internal dimension of the hole. Allowance: The intentional difference in the dimensions of mating parts, or the minimum clearance that

can be allowed between parts. It provides different classes of fits.

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FitsThe eight classes of fits are recognized by the ANSI;

1. Loose fit; large allowance2. Free fit; liberal allowance3. Medium fit; medium allowance4. Snug fit; zero allowance5. Wringing fit; zero to negative allowance

6. Tight fit; slight negative allowance7. Medium force fit; negative allowance8. Heavy force and shrink fit; considerable

negative allowance

Accuracy and Precision

Accuracy - degree to which a 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 - positive or negative deviations from true value that are consistent from one measurement to the next

Precision - degree of repeatability in the measurement process. Good precision means that random errors in the measurement procedure are minimized

Accuracy versus precision in measurement:

a) High accuracy but low precision;b) Low accuracy but high precision; andc) High accuracy and high precision.

Two Dominant Systems of Units

Two systems of units have evolved into predominance in the world: 1.U.S. customary system (U.S.C.S.)2.SI (for System International units) - the “metric system”

Measuring Instruments and Gages

Conventional measuring instruments and gages include: Precision gage blocks Measuring instruments for linear dimensions Comparative instruments Fixed gages Angular measurements

Precision Gage Blocks

The standards against which other dimensional measuring instruments and gages are compared Usually square or rectangular blocks Surfaces are finished to be dimensionally accurate and parallel to 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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Measurement of Linear Dimensions

Measuring instruments are divided into two types: Graduated measuring devices include a set of markings on a linear or angular scale to which the

object's feature of interest can be compared for measurement Non-graduated measuring devices have no scale and are used to compare dimensions or to transfer a

dimension for measurement by a graduated device

Micrometer Calipers

Mechanical Gages: Dial IndicatorsMechanical gages are designed to mechanically magnify the deviation to permit observation

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 0.01 mm or 0.001 inch Applications: measuring straightness, flatness, parallelism, squareness, roundness, and runout

Dial Indicator Dial Indicator to Measure Runout

Electronic GagesFamily of measuring and gaging instruments based on transducers capable of converting a linear

displacement into an electrical signal

Electrical signal is 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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Figure-1: External micrometer, standard one-inch size with digital.

Figure-2: Two sizes of outside calipers.

Figure 4: Dial indicator setup to measure runout; as part is rotated about its center, variations in outside surface relative to center are indicated

Figure 3: Dial indicator: front view shows dial and graduated face; back view shows rear of

instrument with cover plate removed.

Error = 0.1 cm

Error = 0.1 cm

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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 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

Snap Gage

Measurement of Surfaces

Two parameters of interest: Surface texture - geometry of the surface, commonly measured as surface roughness

Surface roughness - small, finely-spaced deviations from the 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 that resulted from the processes that created it

Measuring Instruments

RulerA ruler is used to measure lengths from a few cm up to 1 m. A metre rule has an accuracy of 0.1 cm (i.e. 1 mm).

Precautions to be taken when using a ruler:

a) Ensure that the object is in contact with the ruler to avoid inaccurate readings.

b) Avoid parallax errors.Parallax errors in measurement arise as a result of taking a reading, with the eye of the observer in the wrong position with respect to the scale of the ruler.

c) Avoid zero and end errors.The ends of a ruler, which may be worn out, are a source of errors in measurement. Thus it is advisable to use the division mark `1' of the scale as the zero point when taking a measurement.

Vernier CaliperLengths smaller than 1 mm can be measured with the help of an instrument called a vernier caliper. A vernier caliper is used to measure an object with dimensions up to 12 cm with an accuracy of 0.01 cm.

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Figure shows the correct position of the eye when reading the scale

Length of the block, l =3.2cm-1.0cm = 2.2 cm

Figure 5: Snap gage for measuring diameter of a part; difference in height of GO and NO-GO gage

buttons is exaggerated.

Fig: Vernier Caliper

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There are two pairs of jaws; one is designed to measure linear dimensions and external diameters while the other is to measure internal diameters.

To measure with a vernier caliper, slide the vernier scale along the main scale until the object is held firmly between the jaws of the caliper (Figure). The subsequent steps are as follows:

o The reading on the main scale is determined with reference to the `0' mark on the vernier scale. The reading to be taken on the main scale is the mark preceding the Figure 1.10 shows that the '0' mark on the vernier scale lies between 3.2 cm and 3.3 cm. The reading to be taken on the main scale is 3.2 cm (the `0' mark on the vernier scale acts as a pointer).

o The reading to be taken on the vernier scale is indicated by the mark on the vernier scale which is exactly in line or coincides with any main scale division line. Figure 1.10 shows that the fourth mark on the vernier scale is exactly in line with a mark on the main scale. Thus the second decimal reading of the measurement is:

Vernier scale reading = 4 x 0.01 cm = 0.04 cm

o The reading of the vernier caliper is the result of the addition of the reading on the main scale to the reading on the vernier scale.

Caliper reading = Main scale Reading + Vernier scale reading

Thus the reading of the vernier caliper in Figure is3.2 + 0.04 = 3.24 cm

A vernier caliper has a zero error if the `0' mark on the main scale is not in line with the '0' mark on the vernier scale when the jaws of the caliper are fully closed.

i. Positive zero errorZero error = +0.04 cm.

ii. Negative zero errorZero error = -0.02 cm.

Micrometer Screw Gauge

A micrometer screw gauge is used to

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3.2

0.04

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measure small lengths ranging between 0.10 mm and 25.00 mm.

This instrument can be used to measure diameters of wires and thicknesses of steel plates to an accuracy of 0.01 mm.The micrometer scale comprises a main scale marked on the sleeve and a scale marked on the thimble called the thimble scale.

The difference between one division on the upper scale and one division on the lower scale is 0.5 mm. The thimble scale is subdivided into 50 equal divisions. When the thimble is rotated through one complete turn, (i.e. 360) the gap between the anvil and the spindle increases by 0.50 mm.

This means that one division on the thimble scale is 5mm50

= 0.01 mm.

When taking a reading, the thimble is turned until the object is gripped very gently between the anvil and the spindle. The ratchet knob is then turned until a `click' sound is heard. The ratchet knob is used to prevent the user from exerting undue pressure. The grip on the object must not be excessive as this will affect the accuracy of the reading.

Readings on the micrometer are taken as follows:

o The last graduation showing on the main scale indicates position between 2.0 mm and 2.5 mm. Thus the reading on the main scale is read as 2.0 mm.

o The reading on the thimble scale is the point where the horizontal reference line of the main scale is in line with the graduation mark on the thimble scale Figure 1.15(b) shows this to be the 22nd mark on the thimble scale, thus giving a reading of 22 x 0.01 mm = 0.22 mm.

o The reading of the micrometer screw gauge is the sum of the main scale reading and the thimble scale reading which is:2.0 + 0.22 =2.22 mm

o Positive zero errorIn Figure, the horizontal reference line in the main scale is in line with the 4th division mark, on the positive side of the `0' mark, on the thimble scale. The error of +0.04 mm must be subtracted from all readings taken.

Zero error = +0.04 mm

o Negative zero errorIn Figure, the horizontal reference line on the main scale is in line with the 3rd division mark, below the `0' mark of the thimble scale. Zero error = -0.03 mm

Inspection

Procedure in which a part or product feature, such as a dimension, is examined to determine whether or not it conforms to design specification

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Many inspections 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 feature of interest

Types of Inspection

Inspection involves the use of measurement and gaging techniques to determine whether a product, its components, subassemblies, or 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 instruments2. Inspection by attributes – product or part dimensions are gaged to determine whether or not

they are within tolerance limits

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

Sampling Inspection

When sampling inspection is used, the number of parts in the sample is usually small compared to the quantity of parts produced

Sample size may be 1% of 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 by taking a larger sample size Fact is that less than 100% good quality must be tolerated as the price of using sampling

100% Inspection

Theoretically, the only way to achieve 100% good quality is by 100% inspection. All defects are screened and only good quality parts are passed.

Definition of Non Destructive Testing

The use of noninvasive techniques is to determine the integrity of a material, component or structure or quantitatively measure some characteristic of an object, i.e., Inspect or measure without doing harm.

When NDE Methods are used?

To assist in product development To screen or sort incoming materials To monitor, improve or control manufacturing processes To verify proper processing such as heat treating To verify proper assembly To inspect for in-service damage

Common Application of NDT

Inspection of Raw Products

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Inspection Following Secondary Processing In-Services Damage Inspection

Advanced Technologies in Inspection

Substitutes for manual measuring and gaging techniques in modern manufacturing Usually faster and more reliable than manual inspection Include contact and non-contact sensing methods:

1. Coordinate measuring machines2. Lasers3. Machine vision4. Other non-contact techniques

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 work part:

The probe is fastened to a structure that allows movement relative to the part Part is fixture 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

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 over travel and probe size

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

Measurements with Lasers Laser stands for light amplification by stimulated emission of radiation Lasers for measurement 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

Scanning Laser Systems

Laser beam deflected by a rotating mirror to sweeps a beam of light past an object Photo detector on 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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Figure: Scanning laser system for measuring diameter of cylindrical work part; time of interruption of light beam is proportional to diameter D.