lec# 3 & 4 1

Metrology & Statistical Quality Control

5th Term, Batch: 2010

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lec# 3 & 4 2

Standards Of Measurement (Cont..)

Characteristics of line Standards:1. Accurate engraving on the scales can be done but it

is difficult to take full advantage of this accuracy. Eg: a steel rule can be read up to ± 0.2mm of the true dimension.

2. It is easier & quicker to use a scale over a wide range.

3. The scale markings are not subject to wear although significant wear on leading end leads to undersizing

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lec# 3 & 4 3

Standards Of Measurement (Cont..)

Characteristics of line Standards (Cont..):4. There is no ‘built in’ datum in a scale which

would allow easy scale alignment with the axis of measurement, this again leads to under sizing.

5. Scales are subjected to the parallax effect, a source of both positive & negative reading error

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lec# 3 & 4 4

Standards Of Measurement (Cont..)

Characteristics of End Standards1. Except for applications where microscopes can be

used, scales are not generally convenient for the direct measurement of engineering products, where as slip gauges are in every day use in tool-rooms, workshop, & inspection departments throughout the world

2. Modern end standards consists fundamentally of a block or bar of steel generally hardened, whose end faces are lapped flat & parallel to within a few millionth of a cm.

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lec# 3 & 4 5

Standards Of Measurement (Cont..)

Characteristics of End Standards (cont..)

3. Highly accurate & well suited to close tolerance measurements4. Dimensional tolerances as small as 0.0005mm can be obtained5. Subjected to wear on their measuring faces6. To provide a given size, the groups of blocks are “wrung”

together. Faulty wringing leads to damage7. There is a “built-in” datum in end standards, because their

measuring faces are flat & parallel and can be positively located on a datum surface

8. As their use depends on “feel” they are not subject to the parallax error

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6

Standards Of Measurement (Cont..)

Relative Characteristics of Line and End Standards

Item Line Standard End StandardManufacture &cost of equipment

Simple and low Complex process & high

Accuracy in measurement

Limited to ± 0.2mm. In order to achieve high accuracy, scales have to be used in conjunction with micro scope

Very accurate for measurement of close tolerances up to ± 0.001mm

Effect of use Scale marking not subject to wear but the end of scale is worn. Thus it may be difficult to assume zero of scale as datum

Measuring faces get worn out. To take care of this end pieces can be hardened, protecting type, buil-in datum is provided

Other errors There can be parallax error Errors may get introduced due to improper wringing of slip gauges. Some errors may be caused due to change in laboratory temperature

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lec# 3 & 4 7

Limits, Fits & Tolerance

1. Tolerance: is the difference in size b/w limits, prescribed in order to allow reasonable errors in workmanship1.1. Unilateral Tolerance: one which is disposed either below or above the basic size1.2. Bilateral Tolerance: one which is disposed both above & below the basic size

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lec# 3 & 4 8

Limits, Fits & Tolerance (cont..)

2. Interference: is the difference in size b/w female feature & male mating feature when the later feature is larger

3. Clearance: is the difference in size b/w female feature & male mating feature when male feature is smaller

4. Allowance: is the specified difference b/w low limit of size of female feature & the high limit of size of male feature

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lec# 3 & 4 9

Limits, Fits & Tolerance (cont..)

5. Limits: The extreme allowable range in any manufacturing feature , Ormaximum & minimum allowable errors above the basic size

6. Fits: Literal meaning is to adjust. The fit b/w the two mating features is the relationship which results with respect to clearance or interference obtained

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lec# 3 & 4 10

Limits, Fits & Tolerance (cont..)

7. Limits System: It consists of series of tolerances & allowances arranged to suit a specific range of sizes, so that limits of size may be selected & given to pairs of mating feature to ensure specific class of fit

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lec# 3 & 4 11

Limits, Fits & Tolerance (cont..)

Definitions:1. Shaft: it refers not only the diameter of a

circular shaft but to any external dimension on a component

2. Hole: it refers not only the diameter of a circular hole but to any internal dimension on a component

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lec# 3 & 4 12

Limits, Fits & Tolerance (cont..)

Definitions (cont..):3. Actual size of a shaft: this is the measured

dimension of the part4. Basic Size: the basic size is a standard size for the

part & is the same for both the hole & its shaft5. Zero line: this is the line which represents the

basic size so that the deviation from the basic size is zero

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lec# 3 & 4 13

Limits, Fits & Tolerance (cont..)

Classes of Fits:1. Clearance Fit:

A Clearance fit could be obtained by making the lower limit on the hole equal to a larger than the upper limit on the shaft. Any hole & any shaft made to these tolerances would assemble with a clearance fit with certainity

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lec# 3 & 4 14

Limits, Fits & Tolerance (cont..)

Classes of Fits (cont..):2. Interference Fit:

An Interference fit would be obtained with equal certainty by making the lower limit on the shaft equal to or larger than the upper limit on the hole

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lec# 3 & 4 15

Limits, Fits & Tolerance (cont..)

Classes of Fits (cont..):3. Transition Fit: Transition fit, b/w these two

conditions lies a range of fits known as transition fit. These are obtained when the upper limit on the shaft is larger than the lower on the hole, & the lower limit on the shaft is smaller than the upper limit on the hole. It must be realized that transition fits exist only as a class; any actual hole & shaft must assemble with either a clearance or interference fit.

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lec# 3 & 4 16

Limits, Fits & Tolerance (cont..)

Basis of Fits (or limit) System:This system may be arranged on the followingbasis:1. Hole Basis System: Hole basis system is one

in which the limits on the hole are kept constant & the variation necessary to obtain the classes of fit are arranged by varying those on the shaft.

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lec# 3 & 4 17

Limits, Fits & Tolerance (cont..)

Basis of Fits (or limit) System (cont..):2. Shaft basis System:

Shaft basis system is one in which the limits on the shaft are kept constant & the variations necessary to obtain the classes of fit are arranged by varying the limits on the holes.

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lec# 3 & 4 18

Limits, Fits & Tolerance (cont..)

Preferred Basis of Fits (or limit) System:In present day industrial practice hole basis system is used

because a great may holes are produced by standard tooling,

for example reamers, drills etc, whose size is not adjustable.

Subsequently the shaft sizes are more readily variable about

the basic size by means of turning & grinding operations. Thus

the hole basis system results in considerable reduction in

reamers & other precision tools as compared to a shaft basis system,

because in shaft basis system due to non-adjustable nature of reamers

drills etc. great variety (of sizes)of these tools are required for

producing different classes of holes for one class of shaft for obtaining

different fits

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lec# 3 & 4 19

Limits, Fits & Tolerance (cont..)

Systems of Specifying Tolerances:For Detail refer bookDesignation of Holes, Shafts & Fits:

A Hole or Shaft is completely described if the basic size, followed by the appropriate letter & by the number of tolerance grade, is given.

Eg: - A 25mm H-hole with the tolerance grade IT8 is given as: 25mm H8 or simply 25H8 - A 25 mm f-shaft with the tolerance grade IT7 is

given as: 25mmf7 or Simply 25f7

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lec# 3 & 4 20

Limits, Fits & Tolerance (cont..)

Designation of Holes, Shafts & Fits (cont..):A ‘Fit’ is indicated by combining the designations for both the hole & shaft with the hole designation written first, regardless of the system (i.e. hole system or shaft system), eg: 25H8-f7 Or 25H8/f7

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lec# 3 & 4 21

Limits, Fits & Tolerance (cont..)

Commonly Used Holes & Shafts:In several engineering applications the fits required can be met by a quite small selection from the full range available in the standards. The holes & the shafts commonly used are as follows:

Holes: H6, H7, H8, H9, H11.Shafts: c11, d10, e9, f7, g6, h6, k6, n6, p6, s6.IS: 919 gives the most commonly used holes & shafts

up to 500mm for the purpose of general engineering

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lec# 3 & 4 22

Limits, Fits & Tolerance (cont..)

ISO System of Limits & Fits:ISO has presently been universally adopted & as a matter of fact IS:919 is almost in line with this system

Other characteristics such as fundamental deviations & tolerance unit etc are same in both the systems

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ISO IS: 919

Classes of holes 28 25

Tolerance grade 18 18

Classes of shaft 28 25

lec# 3 & 4 23

Limits, Fits & Tolerance (cont..)

Types of Fits:1. Selective Fit2. Push Fit3. Driving Fit4. Forced or Pressed Fit5. Shrinkage Fit6. Freeze Fit

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