Gauges & Measurements 1

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Gauges & Measurements

Chapter 1: Linear Measurements

International standard metre: The metre or meter is the basic unit of length in the SIunits (international System of units). Symbol of meter is m.Definitation: The metre is the length of the path travelled by light in vacuum during atime interval of1299 792 458 of a second.

International Prototype Meter

International Prototype meter is defined as the straight line distance, at 0c between theengraved lines of a platinum irridium alloy of 1020 mm of total length and having atresca cross-section as shown in the figure. The graduations are on the upper surface ofthe web, which coincides with the neutral axis of the section. The sectional shape givesbetter rigidity for the amount of metal involved and is therefore economic in use for anexpensive metal.

Imperial standard Yard:A Yard is a unit of length in several different systems. It is equals to 3 feet or 36 inches,although in SI units yd is equals to 0.9144 mtrs.

Standard lengths on the wall of the Royal Observatory, Greenwich, London - 1 yard (3feet),

The Yard is used as standard unit of length measurement; there are corresponding unitsof area and volume, the square yard and cubic yard respectively.


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Airy and Bessel points:

Airy and Bessel Points are used for precision measurement to support a lengthstandards (Ex: Length Gauges, scales) to minimise the bending or droop. The points(supports) are symmetrically arranged in both the sides from center of the lengthstandard and are separated by a specific distance. equal to L/3 or

or about approximately 5/9ths of the total length of the measuring device.

Airy points

The end faces of bar can be made exactly parallel by spacing the two supportssymmetrically as shown above. These points are known as Airy points. And commonlyused to ensure that the ends of length base are parallel to one another, so that thelengths is well defined.

S= about approximately 5/9ths of the total length of the measuring device

For example, a 450 mm length gauge would have an Airy point separation of 450 mmtimes 5/9 = 250 mm. A line or pair of lines would be marked onto the gauge 100 mm infrom each end. Support blocks at these points ensure that the calibrated length ispreserved. If the length gauge is not supported at the Airy points, the measurementuncertainty is increased.

Sa a

L

Center of

length std.


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Bessel Points

Change in length of a bar due to bending can be minimized by spacing the two supportsas shown above, these points are known as Bessel points and commonly used tominimize change in overall length.

Standards of Measurements

1. Material Standards

(a) Line Standard When length is measured as the distance

between centers of two engraved lines.

(b)End Standard When length is measured as the distance between to flatparallel faces.

2. Wavelength StandardThe wavelength of a selected orange radiation of Krtypton-86 isotope wasmeasured and used as the basic unit of length.

Line and End Standards and differentiate between them.

Line Standards When length is measured as the distance between centers of two

engraved lines, it is called Line Standards. Both the material Standards, yard and metreare line standardsE.g. Scale, Rulers, Imperial Standard Yard.

Characteristics of Line Standards:

(i) Scale can be accurately emblemed, but the engraved lines posses thickness andit is not possible to accurately measure

(ii) Scale is used over a wide range(iii) Scale markings are subjected to wear. However the ends are subjected to wear

and this leads to undersize measurements(iv) Scale does not posses built in datum. Therefore it is not possible to align the

scale with the axis of measurement(v) Scales are subjected to parallax errors(vi) Assistance of magnifying glass or microscope is required.

End Standards When length is expressed as the distance between centers of twoflat parallel faces, it is called End Standards. Slip Gauges, End Bars, Ends ofmicrometer Anvils.

Characteristics of End Standards


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(i) Highly accurate and used for measurement of closed tolerances in precisionengineering as well as standard laboratories, tool rooms, inspectiondepartments.

(ii) They require more time for measurement and measure only one dimension.(iii) They wear at their measuring faces(iv) They are not subjected to parallax error.

Differentiate between Line and End Standards

Sl no Characteristics Line Standard End Standard1. Principle Length is expressed

as distancebetween 2 lines

Length is expressedas distance between 2ends

2. Accuracy Ltd. To 0.2mm.

Highly accurate ofclosed tolerances to0.001mm

3. Ease Quick and easy

Time consuming andrequires skill

4. Effect of wear Wear at only theends

wear at measuringsurfaces

5. Alignment Cannot be easilyaligned

easily aligned

6. Cost low cost high cost7. Parallax Effect Subjected to

parallax effectnot subjected toparallax effect

Wavelength standards and its advantagesA major drawback with the material standards that their length changes with time.Secondly, considerable difficulty is expressed while comparing the sizes of the gauges byusing material standards.

Jacques Babinet suggested that wave length of a monochromatic light can be used as a

natural and invariable unit of length. 7th

general Conference of Weights and Measuresapproved in 1927, approved the definition of standard of length relative to meter.

Orange radiation of isotope Krypton-86 was chosen for the new definition of length in1960, by the 11th General Conference of Weigths and Measures. The committeerecommended Krypton-86 and that it should be used in hot cathode discharge lamp,maintained at a temperature of 63K.

According to this standard metre was defined as equal to 165763.73 wavelengths of thered-orange radiation of Krypton-86 isotope.

A standard can now be produced to an accuracy of about 1 part of 10^9.

Advantages:

(a)Not a material standard and hence it is not influenced by effects of variation ofenvironmental conditions like temperature, pressure

(b)It need not be preserved or stored under security and thus there is not fear of beingdestroyed.

(c)It is subjected to destruction by wear and tear.(d)It gives the unit of length which can be produced consistently at all times.(e)The standard facility can be easily available in all standard laboratories and

industries(f) Can be used for making comparative measurements of very high accuracy.


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ANGULAR MEASUREMENT

SINE BAR

Principle: The sine principal uses the ratio of length of two sides of a right angle triangleis deriving the angle.Sine bars are not complete measuring device but requires surface plate and slip gauge

set for its use. The sine bars are used either to measure angles very accurately orlocating any work to a given angle with in close limits. Sine bars are made from highcarbon, high chromium and corrosion resistant steel, hardened ground and stabilized.Two cylinders are mutually parallel to each other and also parallel to & at equal distancefrom the upper surface of the sine bar. The distance between the axes of two cylindersis exactly 5 inches or 10 inches in British system and 100,200 and 300mm in metricsystem.

All the working surfaces and the cylindrical surfaces of the rollers are finished tosurface finish of 0.2 [im Ra value or better. Depending upon the accuracy of the centredistance,sine bars are graded as of A grade or B grade. B grade of sine bars are guaranteedaccurateupto 0.02 mm/m of length and A grade sine bars are more accurate and guaranteed

upto 0.01mm/m of length. Although there are several forms of sine bars, but the one shown inFig. is most commonly used. Some holes are drilled in the body of the bar to reduce theweight and to facilitate handling.


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TYPES:


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USES:


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Another Method


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Limitations of Sine Bars.Devices operating on the sine principle are fairly reliable at angles less than 15, but becomeincreasingly inaccurate as the angle increases.Sine bars inherently become increasingly impractical and inaccurate as the angle exceeds 45because of following reasons : The sine bar is physically clumsy to hold in position.

The body of the sine bar obstructs the gauge block stack, even if relieved. Slight errors of the sine bar cause large angular errors. Long gauge stacks are not nearly as accurate as shorter gauge blocks. Temperature variation becomes more critical. A difference in deformation occurs at the point of roller contact to the support surface and to thegauge blocks, because at higher angles, the weight load is shifted more toward the fulcrum roller. The size of gauges, instruments or parts that a sine bar can inspect is limited, since it is notdesigned to support large or heavy objects.

Precautions in use of sine bars.

(i) The sine bar should not be used for angle

greater than 60 because any possible error in construction is accentuated at this limit.(ii) A compound angle should not be formed by mis-aligning of workpiece with the sinebar. This can be avoided by attaching the sine bar and work against an angle plate.(iii) Accuracy of sine bar should be ensured.(iv) As far as possible longer sine bar should be used since many errors are reduced byusing longer sine bars.


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GEOMETRIC DIMENSIONING & TOLERANCINGGD&T is a language used in mechanical engineering drawings, which consists of symbols used toefficiently communicate geometry requirements for features on components & Assemblies.The definition of a Tolerance, per ASME Y14.5.5M-1994, is the total amount a specificdimension is permitted to vary. For instance, a dimension shown as 1.498 to 1.502 means that itmay be 1.498 or 1.502

Important Terms Definition:

Maximum Material Condition (MMC) Is the condition where a feature of a finished partcontains the maximum amount of material. That is, the largest shaft or smallest hole. See Example1.

Least Material Condition (LMC) - Is the condition where a feature of a finished part contains theleast amount of material. That is, the smallest shaft or the largest hole. See Example 1.

Nominal Size Approximate size used for the purpose of identification such as stock material.

Basic Size Is the theoretical exact size from which limits of size are determined by the

application of allowances and tolerances.

Tolerance The total amount by which a given dimension may vary or the difference between thelimits.

Limits The extreme maximum and minimum sizes specified by a toleranced dimension.

ASME Y14.5M-1994 is a revision of ANSI Y14.5M-1982, which was approved as an ASME

(The American Society of Mechanical Engineers) Standard as well as an AmericanNational Standard a few years ago. Significant steps are taken in this revision to resolvesome long-standing differences between ANSI (the American National Standards

Institute) and ISO (the International Orgnization for Standardization) practices. So

nowadays we can say that the two standard systems are working in a very simular wayin the GD&T field. But the differences still exist. Regarding the GD&T symbolic definition,following defferences between the two standard systems are recognized:

Tolerancing Principle

ASME Y14.5M-1994 continues to use the "envelope principle" for geometrical

tolerancing, while ISO uses the "independentcy principle". ISO allows, nevertheless, the

option of the envelope principle by either reference to a national standard (for example,ASME Y14.5M-1994, DIN 7167), or by invoking the symbol (E) (a "E" with a circle around

it) after a dimension value. That doesn't make sense for ASME Y14.5M-1994, so thesymbol (E) is not defined there.

Projected Tolerance Zone Symbol

Both in the ASME and in the ISO standard, the projected tolerance zone symbol (P) (a

"P" with a circle around it) is placed in the feature control frame. But in the ASME

standard the projected tolerance zone symbol is followed by a dimension value, which

indicates the minimum height of the tolerance zone, in ISO standard this minimumheight of the tolerance zone is dimensioned at the geometrical element directly (see theexample in the table attached below).

'Two Single-Segment Feature Control Frame"


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The composite positional tolerancing concept is expanded in ASME Y14.5M-1994. It

distinguishes between the "composite feature control frame" and the "two single-

segment feature control frame" (the table attached below shows how the "two single-segment feature control frame" looks like). The "two single-segment feature controlframe" is not defined in ISO.

Symbols only used in ASME Y14.5M-1994

There are some symbols which are only used in ASME Y14.5M-1994, not in the ISOstandards. They are put together in the following attached table. The following table

gives also a summary of all the differences between the ASME and ISO standard forGD&T symbols mentioned above.

Table: Differences between ASME Y14.5M-1994 and ISO standards in GD&T symbols.


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GEOMETRIC DIMENSIONING & TOLERANCING

GD&T (per ASME Y14.5M-1994) is an international language that is used on engineering drawingsto accurately describe the size, form, orientation, and location of part features. It is also a design-dimensioning philosophy that encourages designers to define a part based on how it functions inthe final product or assembly.

Feature: Feature are specific portion of a part and may include one or more elements such ashole, threads, profile , faces, slots etc,,,.

Systems of GD&T:

ANSI - American National Standards InstituteASME- American Society of Mechanical EngineersISO International standard organization

ANSI was originally formed in 1918, by five engineering societies of America. (AIEE, ASME, ASCE,AIME, ASTM-America society for testing and materials).There are both ISO and ANSI standards for geometric tolerances .These two standards areslowly reconciling their differences, but still some remaining. Both standards were deigned foruse with engineering drawings, not CAD models. Consequently the intention

of the standards is to create a common language, by use of symbols, conventions, and rules sothat theperson interpreting the tolerance on an engineering drawing will follow the intentions ofthe designer.

ANSI/ASME Y14.5M-1994 - The national standard for dimensioning and tolerancing in the UnitedStates. The Y14.5 is the standard number. "M" is to indicate the standard is metric, and 1994 is thedate the standard was officially approved. The latest revision of Y14.5 is ninety percent compatiblewith the GD&T sections of the ISO standards, bringing OEMs more in step with the globalmanufacturing scene.

Feature: Feature are specific portion of a part and may include one or more elements such as

hole, threads, profile , faces, slots etc,,,.

feature of size (FOS) as "one cylindrical or spherical surface, a circular element, and a set of twoopposed parallel elements or opposed parallel surfaces, each of which is associated with a directlytoleranced dimension."

For example : Hole is a Feature and dimension links this hole with other feature is called Feature ofsize.

Definations:

Actual Local Size - The value of any individual distance at any cross section of a feature of size

Basic Dimension - A numerical value used to describe the theoretically exact size, true profile,orientation, or location of a feature or datum target.

Bilateral Tolerance - A tolerance that allows the dimension to vary in both the plus and minusdirections.


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Bonus Tolerance - An additional tolerance for a geometric control. Whenever a geometrictolerance is applied to a feature of size, and it contains an MMC (or LMC) modifier in the toleranceportion of the feature control frame, a bonus tolerance is permissible.

Circularity - A condition where all points of a surface of revolution, at any section perpendicular toa common axis, are equidistant from that axis.

Circularity Control - A geometric tolerance that limits the amount of circularity on a part surface.

Circular Runout Control - A geometric tolerance that limits the amount of circular runout of apart surface.

Concentricity Control - A geometric tolerance that limits the concentricity error of a part feature.

Cylindricity - A condition of a surface of revolution in which all points of the surface areequidistant from a common axis.

Cylindricity Control - A geometric tolerance that limits the amount of cylindricity error permittedon a part surface.

Datum - A theoretically exact plane, point or axis from which a dimensional measurement ismade.

Dimension - A numerical value expressed in appropriate units of measure and used to define thesize, location, orientation, form, or other geometric characteristics of a part.

Feature - A general term applied to a physical portion of a part, such as a surface, hole, or slot.

Feature of Size - One cylindrical or spherical surface, or a set of two opposed elements oropposed parallel surfaces, associated with a size dimension.

Flatness - The condition of a surface having all of its elements in one plane.

Flatness Control - A geometric tolerance that limits the amount of flatness error a surface isallowed.

Functional Dimensioning - A dimensioning philosophy that defines a part based on how itfunctions in the final product.

Least Material Condition - The condition in which a feature of size contains the leastamount of material everywhere within the stated limits of size.

Limit Tolerance - When a dimension has its high and low limits stated. In a limit tolerance, thehigh value is placed on top, and the low value is placed on the bottom.

Maximum Material Condition - The condition in which a feature of size contains themaximum amount of material everywhere within the stated limits of size.


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Parallelism - The condition that results when a surface, axis or centerplane is exactlyparallel to a datum.

Parallelism Control - A geometric tolerance that limits the amount a surface, axis, or centerplaneis permitted to vary from being parallel to the datum.

Perpendicularity - The condition that results when a surface, axis, or centerplane isexactly 90 to a datum.

Perpendicularity Control - A geometric tolerance that limits the amount a surface, axis, orcenterplane is permitted to vary from being perpendicular to the datum.

Profile of a Line Control - A geometric tolerance that limits the amount of error forline elements relative to their true profile.

Profile of a Surface Control - A geometric tolerance that limits the amount of error asurface can have relative to its true profile.

Projected Tolerance Zone - A tolerance zone that is projected above the part surface.

Radius - A straight line extending from the center of an arc or circle to its surface.

Regardless of Feature Size - The term that indicates a geometric tolerance applies at anyincrement of size of the feature, within its size tolerance.

Symmetry - The condition where the median points of all opposed elements of two ormore feature surfaces are congruent with the axis or centerplane of a datum feature.

Symmetry Control - A geometric tolerance that limits the symmetry error of a part feature.

3-2-1 Rule - Defines the minimum number of points of contact required for a part datum featurewith its primary, secondary, and tertiary datum planes.


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Total Runout - A composite control affecting the form, orientation, and location of allsurface elements of a diameter (or surface) relative to a datum axis.

Total Runout Control - A geometric tolerance that limits the amount of total runout of a surface.

Unilateral Tolerance - A tolerance where the allowable variation from the target value is all inone direction and zero in the other direction.


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WHY IS GD&T IMPORTANTs Saves money

For example, if large number of parts are being made GD&T can reduce oreliminate inspection of some features.

Provides bonus tolerances Ensures design, dimension, and tolerance requirements as they relate to the actual functions Ensures interchangeability of mating parts at the assemblys Provides uniformitys It is a universal understanding of the symbols instead of words


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For Example

Reads as: The position of the feature must be within a .003 diametrical tolerance zone atmaximum material condition relative to datums A, B, and C.


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Datums Datums are features reference surfaces from which other measurements are made. Used in

designing, tooling, manufacturing, inspecting, and assembling components and sub-assemblies.

s As you know, not every GD&T feature requires a datum, i.e., Flat

s Features are identified with respect to a datum.s Always start with the letter As Do not use letters I, O, or Qs May use double letters AA, BB, etc.s This information is located in the feature control frame.

s

s Datums on a drawing of a part are represented using the symbol shown below.

s

1.000


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s

s


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s

s A primary datum is selected to provide functional relationships, accessibility, andrepeatability.

s Secondary datum: All dimension may not be capable to reference from the primary datumto ensure functional relationships, accessibility, and repeatability.

s Tertiary Datums This datum is always perpendicular to both the primary and secondary

datums ensuring a fixed position from three related parts.


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Circularity ( Roundness)

In this example each circular element of the surface must lie within a tolerance zone defined by

two concentric circles separated by the specified tolerance value. All points on the surface must liewithin the limits of size and the circularity limit.


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Circularity is the condition of a surface where all points of the surface intersected by any planeperpendicular to a common axis are equidistant from that axis. The circularity tolerance must beless than the size tolerance

In this example the entire surface must lie within a tolerance zone defined by two concentriccylinders separated by the specified tolerance value. All points on the surface must lie within the

limits of size and the cylindricity limit.Cylindricity is the condition of a surface of revolution in which all points are equidistant from acommon axis. Cylindricity is a composite control of form which includes circularity (roundness),straightness, and taper of a cylindrical feature.


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Tolerances of Runout :

Means this

The tolerance zone for any individual circular element is equal to the total allowable movement of adial indicator fixed in a position normal to the true geometric shape of the feature surface when thepart is located against the datum surface and rotated 360 degrees about the datum axis. Thetolerance limit is applied independently to each individual measuring position along the featuresurface.


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Limits, Fits & Tolerances

Tolerance: Tolerance is defined as the magnitude of permissible variation of dimension from thespecified value. They constitute an engineering legality for deviation from ideal value. Primary

purpose of tolerances is to permit variation in dimensions without degradation of the performancebeyond the limits established by the specification of the design.

Why tolerances are specified ?

Ideal conditions would call for parts without anykind of dimensional variation, but in actual practice it is impossible due to following reasons:(i) Variations in the properties of the material being machined introduce errors.(ii) The production machines themselves have some inherent inaccuracies built into themand have the limitations to produce perfect parts.(iii) It is impossible for an operator to make perfect settings. In setting up the machine, i.e.in adjusting the tools and workpiece on the machine, some errors are likely to creep in.

Different ways of expressing Tolerances.

Tolerances are basically specified in twoforms, i.e. unilateral and bilateral. In unilateral tolerances, the total tolerance as related to a basicdimension is in one direction only. This form of tolerance is usually indicated when the machiningof mating parts is called for, as this greatly assists the operator. This form of tolerance is usuallyindicated when the machining of mating parts is called for, as this greatly assists the operator. Theoperator machines to the upper limit of shaft (lower limit for a hole) knowing fully well that he stillhas the whole tolerance left for machining before the parts are rejected. In case of bilateraltolerances, the total tolerance is specified on both sides (plus and minus) of the basic dimension.Bilateral tolerances usually have plus and minus tolerance of equal amount, but not necessarilyalways. This system permits operator to take full advantage of the limit system especially in

positioning a hole.

In the case of unilateral tolerances, the dimension is allowed to vary only in one direction.This system is used in machining processes like drilling in which case the dimensions are mostlikely to deviate in one direction only (in drilling, hole is always of over size rather than undersize).Bilateral tolerance system defines the theoretically desired size and the probable deviationpermitted on either side of the basic size. This system is used in mass production techniques wheremachine setting is done for the basic size. Under such conditions, if tolerances are specified asunilateral, then these should be changed into bilateral tolerances by changing the basic size. Thebasic size should be mid way between upper and lower limits.

Ex.:

Unilateral: 25.000mm, 25.002mm (dia. of hole)

24.999mm, 24.997mm (die of shaft)

OR

Bilateral 25.000 mm

25.000 + 0.002 0.000mm (dia. of hole)


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25.000 0.001 0.003mm (dia. of shaft)

Functional and Non-functional Dimensions.

Functional dimensions are those whichhave to be machined and fit with other mating components. Non-functional dimensions are thosewhich need not be machined to a high degree of accuracy. These have no effect on the qualityperformance of the component or assembly.

Basic size: The basic size is the standard size for the part and is the same for both the hole andits shaft. Ex. 50mm diameter hole and shaft.

Nominal size: the normal size of a dimension of part is the size by which it is referred to as amatter of convenience (used for purposes of general identification). Often, basic and nominal sizesof a part of dimensions are used wish the same sense.

Actual size: It is the measured size of part.

Zero line: It is the line, which represents the base size so that the deviation from the basic size iszero.

Allowances: An intentional difference between the hole dimension and shaft dimension for any typeof fit is called allowance.

Tolerance Accumulation.

If a part comprises of several steps, each step having sometolerance over its length, then overall tolerance on complete length will be sum of the toleranceson


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individual length.

It would be appreciated that this practice of specifying tolerances would result in hightolerances on the overall length. The effect of accumulation of tolerances can be minimised byadopting progressive dimensioning from a datum as shown in Fig

Interchangeability - It occurs when one part in an assembly can be replaced for a similar partwhich has been made to the same drawing.

Interchangeable parts are parts identical and they are made to specifications that ensure that theyare so nearly identical that they will fit into any device of the same type. One such part can freely

replace another, without any custom fitting (such as filing). This interchangeability allows easyassembly of new devices, and easier repair of existing devices, while minimizing both the time andskill required of the person doing the assembly or repair.

Interchangeability is possible only when certain standards are strictly followed. Ininterchangeability the mating parts are taken from two different manufacturing sources andassembled together.

This is desirable. When all parts to be assembled are made in the same manufacturing unit, thenlocal standards may be followed which is known as local interchangeability.

The concept of interchangeability was crucial at the beginning of the 20th century,Interchangeability of parts was finally achieved by combining a number of innovations andimprovements in machining operations and machine tools. These innovations included

Invention of new machine tools, Jigs for guiding the machine tools, Fixtures for holding the work in the proper position and Blocks and gauges to check the accuracy of the finished parts. Modern machines tools often have numerical control (NC). Modern cutting edges use high speed steel or materials such as tungsten carbide. Drop forging and stamped steel parts, which reduced or eliminated the amount of machining

Selective Assembly


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Today consumer not only wants Quality & trouble free products, but also he wants them inattractive prices. This is possible only by adopting automatic gauging for selective assemblieswhere parts are manufactured in different tolerances are segregated to make different qualityassemblies. This is a old idea of inspection, in which parts are identified as good or bad parts. Inselective assembly the parts are produced by machine are classified in to several groups accordingto their size.Only the matched grades of mating parts are assembled. The technique is most suitable where aclose fit of two component assemblies is required. It results in complete protection against non-conforming assemblies and reduces machining costs since close tolerances are maintained.

For Example: Parts are produced in 0.1 mm tolerance. An automatic can segregate them in toten different groups with 0.01mm limit for selective assembly of the individual parts.

Process capability of hole making machine

Process capability of shaft making machine

Fig above shows process capability of shaft and hole making machine is same and but parts areproduced in different tolerance. In such a cases parts are segregated in to different groups basedfinal quality of part required and parts in shaft region S1 are matched with parts in holes regionH1, S2 with H2 and so on.

H

1

H

2H

3

H

4H

5

H

6

S

1

S

2S

3

S

4S

5

S

6


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No of groups = Process capabilityTolerance desired


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FITS

Definition: Fit is the general term used to signify the range of tightness or looseness that mayresult from the application of a specific combination of allowances and tolerances in mating parts.

Allowances: An intentional difference between the hole dimension and shaft dimension for any

type of fit is called allowance.

Deviation: Algebraic difference between a size and corresponding basic size.

Fig

Upper deviation: It is the difference between Maximum limit of size ( of either hole or shaft) andthe corresponding basic size.

Upper deviation = Maximum limit of size basic size.

Designation: ES for hole, es for shaft

It is positive when maximum limit of size > basic size and vice versa.

Lower deviation: It is the difference between minimum limit size ( of either hole or shaft) and thecorresponding basic size.

Lower deviation = Minimum limit size basic size

Designation: EI for hole, ei for shaft

Positive when minimum limit of size > basic size and vice versa

Fundamental deviation: this is the deviation either the upper or the lower deviation, which thenearest one to the zero line (for both hole or a shaft).


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Clearance fit: In this type of fit, the largest permitted shaft diameter is smaller than the diameterof the smallest hole, so that the shaft can rotate or slide through the difference degrees accordingto purpose of mating members Ex.Bearing and shaft.

Note: Minimum clearance: In the clearance fit it is the difference between the minimum size ofthe hole and the maximum size of the shaft.

Maximum clearance: In a clearance or transition fit it is the difference between themaximum size of hole of the minimum size of the shaft.

Transition fit: In this type of fit, the diameter of the largest allowable hole is greater than that ofthe smallest shaft, but the smallest hole is smaller than the largest shaft, so that small positive ornegative clearance between the shaft and hole members employable. Location fits Ex. Spigot inmating holes, coupling rings and recesses are the examples of transition fit.


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Interference fit: In this type of fit, the minimum permitted diameter of the shaft is larger thanthe maximum allowable diameter of the hole. In this case the shaft and the hole members are

intended to be attached permanent and used as a solid component but according to the applicationof this combination, this type of fit can be varied. Ex. Bearing bushes, which are in interference fit

in their housing Ex. The small end of the connecting rod in an engine.

Minimum interference: It is the difference between maximum size of hole and the minimum sizeof shaft in an interference fit prior to assembly.

Maximum interference: In an interference fir or a transition fit it is the difference between theminimum size of hole and the maximum size of shaft prior assembly.

Hole based system: (Pl. note: Hole is kept constant) This is one which the limits one the hole orkept constant and the variations necessary to obtain the classes of fit are arranged by varyingthose on the shaft.

Ex. Assume a hole of dimensions


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1. Shaft (S1) of 28 mm Clearance fit

2. Shaft (S2) of 28 mm Transition fit

3. Shaft (S3) of 28 mm Interference fit

Shaft based system: (Pl. note: Shaft is kept constant) This is one which the limits on the shaftare kept constant and the variation necessary to obtain the classes of fit are arranged by varyingthe limits on the holes.

Note: (1) From manufacturing point of view it is preferable to use hole-based system. Becauseholes are produced with standard tooling (reamers, drills) those size not adjustable and shaft sizesare readily variable. Thus hole based system results in considerable reduction in reamers and otherprevisions tools as compared to a shaft based system.

(2) Basic shaft: A shaft whose upper deviations is zero.(I.e. Max. of size=Basic size)

(3) Basic hole: A hole whose lower deviation is zero.(I.e. Min. of size = Basic size)

International Tolerance Grade (IT):

Note: Fundamental Deviation is the deviation closest to the basic size.


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They are a set of tolerances that varies according to the basic size and provides a uniform level ofaccuracy within the grade.

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