Rd Reflex Tolerances

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MECHANICAL POWERTRANSMISSIONS (SPG8005)

NEWCASTLE UNIVERSITY

School of Mechanical & Systems Engineering

INTRODUCTION TO

TOLERANCES

1. INTRODUCTION TO SPECIFICATION OF DIMENSIONS AND FORM (SHAPE)

Much of the information in this document is derived from BS 308: Parts 1-3 (1990), from BS8888 (2000) and from other related documents, including PP7308, PP7309 and PP8888-2.However, the full British Standards should be consulted for complete explanation. BS8888: Parts1-3: 2000 have now superceded BS 308: Parts 1-3 but the concepts andsymbols remain.

1.1 Standards of measurement

Definition of the metreoriginal: 1/10 000 000 of distance from North Pole to Equator current: 1 650 763.73 wavelengths in vacuum of orange light from Krypton 86

Hierarchy of standardscomparison of international standards

national standardscalibration organisation

factory calibration standardsworkshop instruments

Typical Metrology systemcomparison of international “Master Gears”national collection of “Master Gears” (Newcastle Univ.)Newcastle testing machines checked with Master Gearsfactory testing machines checked with Master Gearsproduction gears checked on factory test machinesproduction machinery checked by checking output




1.2 Measuring uncertainties

Reading errors

accuracy of using the instrument (if it can be done wrong.......)

ease of reading

interpolation errors

beware meaning of readouts

System / fundamental errors

mounting errors: of object to be measured

of measuring scales (etc.)

temperature differences object - equipment “reversal errors”

temperature changes during measurement Abbe Error dynamic errors during movement environment

friction, dirt, shocks calibration errors

control system feedback “error” signals software errors

ABILITY TO READ ABILITY TO RESOLVE

(the scale) (be repeatable)

Lots of digits are often “fluff”.

RESOLUTION ACCURACY

Even if can resolve does not mean that every

increments reliably, increment is identical or accurate.

ACCURACY REPEATABILITY

Correct indication does not imply correct

on one day, indication the next.




2. FUNCTIONAL DIMENSIONING & TOLERANCING

Because the correct performance of any product is vital, the dimensions of eachcomponent clearly need to be controlled.

However, dimensions and their tolerances must always be related to the function of the component features they control.

Make sure that the features that control performance are dimensioned and toleranceddirectly and not by a “tolerance chain” of many toleranced dimensions. See the drawing for an example.




3. DIMENSIONAL TOLERANCINGThe ISO system of dimensional tolerances is intended to control the limits of size of adimension: is does not control the shape of the component or its features, just the value of the dimensioned distances. Whilst the information often refers only to round components,the system can be applied to any drawn dimension.

2.1 Each dimension can be allocated any one of 18 different tolerance grades: the tolerancegrades specify the different between the maximum and minimum allowable values of thedimension. The tolerance grades are known as IT grades.

3.2 Components may be required to assemble freely with some slack (a clearance fit) or maybe required to need force to assemble (an interference fit). It is convenient to say that a20mm hole is needed for a M20 bolt: however, they need (usually) to assemble easily (a

clearance fit) and neither dimension should be exactly 20mm. See the diagram for atypical clearance fit and note that none of the specified dimensions are on the “zero” linewhich represents the nominal dimensions.




3.3 Each tolerancerange (specified by the ITgrade table) can beallocated any one of 56different “fundamentaldeviations”: these specifythe value of the dimensionrelative to the nominal(drawn) dimension.

3.4 In practice, mostdimensions are specifiedfrom a much reduced set of IT grades and fundamental

deviations: see below.

These may be said to be“Hole Based” (used wherethe hole dimensions aremore difficult to vary: i.e.normally) or “Shaft Based”used where the shaftdimensions are moredifficult to control.

3.5 The “Hole Basis”simplified table is shown onthe next sheet.




Extracted from BS 4500 BRITISH STANDARD SELECTED ISO FITS - HOLE BASIS (from) Data Sheet 4500A

Larger than

nominal sizeNOMINAL SIZE

Smaller thannominal size

Clearance Fits Transition Fits Interference Fits

Nominal sizes Tolerance (0.001mm) Tolerance (0.001mm) Tolerance (0.001mm) Tolerance (0.001mm) Tolerance (0.001mm) Tolerance (0.001mm) Tolerance (0.001mm) Tolerance (0.001mm) Tolerance (0.001mm) Tolerance (0.001mm) Nominal sizes

Over (mm)

to(mm)

H11 c11 H9 d10 H9 e9 H8 f7 H7 g6 H7 h6 H7 k6 H7 n6 H7 p6 H7 s6 Over (mm)

To(mm)

- 3 +60

0

-60

-120

+25

0

-20

-60

+25

0

-14

-39

+14

0

-6

-16

+10

0

-2

-8

+10

0

0

-6

+10

0

+6

+0

+10

0

+10

+4

+10

0

+12

+6

+10

0

34 - 3

3 6 +750

-70-145

+300

-30-78

+300

-20-50

+180

-10-22

+120

-4-12

+120

0-8

+120

+9+1

+120

+16+8

+120

+20+12

+120

+27+19

3 6

6 10 +900

-80-170

+360

-40-98

+360

-25-61

+220

-13-28

+150

-5-14

+150

0-9

+150

+10+1

+150

+19+10

+150

+24+15

+150

+32+23

6 10

10 18 +1100

-95-205

+430

-50-120

+430

-32-75

+270

-1634

+180

-6-17

+180

0-11

+180

+12+1

+180

+23 +12

+180

+29+18

+180

+39+28

10 18

18 30 +1300

-110-240

+520

-65-149

+520

-40-92

+330

-20-41

+210

-7-20

+210

0-13

+210

+15+2

+210

+28+15

+210

+35+22

+210

+48+35

18 30

30 40 +1600

-120-280 +62

0-80-180

+620

-50-112

+390

-25-50

+250

-9-25

+250

0-16

+250

+18+2

+250

+33+17

+250

+42+26

+250

+59+43

30 40

40 50 +160

0

-130

-29040 50

50 65 +190

0

-140

-330 +740

-100-220

+740

-60-134

+460

-30-60

+300

-10-29

+300

0-19

+300

+21+2

+300

+39+20

+300

+51+32

+30

0

+72

+5350 65

65 80 +1900

-150-340

+300

+78+59

65 80

80 100 +2200

-170-390 +87

0-120-260

+870

-72-159

+540

-36-71

+350

-12-34

+350

0-22

+350

+25+3

+350

+45+23

+350

+59+37

+350

+93+71

80 100

100 120 +220

0

-180

-400

+35

0

+101

+79100 120

120 140 +250

0

-200

-450+100

0-145-305

+1000

-84-185

+630

-43-83

+400

-14-39

+400

0-25

+400

+28+3

+400

+52+27

+400

+68+43

+40

0

+117

+92120 140

140 160 +2500

-210-460

+400

+125+100

140 160

160 180 +2500

-230-480

+400

+133+108

160 180

180 200 +2900

-240-530

+1150

-170-355

+1150

-100-215

+720

-50-96

+460

-15-44

+460

0-29

+460

+33+4

+460

+60+31

+460

+79+50

+460

+151+122

180 200

200 225 +2900

-260-550

+460

+159+130

200 225

225 250 +2900

-280-570

+460

+169+140

225 250

250 280 +320

0

-300

-620 +1300

-190-400

+1300

-110-240

+810

-56-108

+520

-17-49

+520

0-32

+520

+36+4

+520

+66+34

+520

+88+56

+52

0

+190

+158250 280

280 315 +3200

-330-650

+520

+202+170

280 315

315 355 +3600

-360-720 +140

0

-210

-440

+140

0

-125

-265

+89

0

-62

-119

+57

0

-18

-54

+57

0

0

-36

+57

0

+40

+4

+57

0

+73

+37

+57

0

+98

+62

+570

+226+190

315 355

355 400 +3600

-400-760

+570

+244+208

355 400

400 450 +4000

-440-840 +155

0-230-480

+1550

-135-290

+970

-68-131

+630

-20-60

+630

0-40

+630

+45+5

+630

+80+40

+630

+108+68

+630

+272+232

400 450

450 500 +400

0

-480

-880

+63

0

+292

+252450 500




Refers to the axis Refers to the generatingor the median plane line or the surface

4. GEOMETRICAL TOLERANCING

Geometrical tolerancing can be used to specify:-# the size & shape of each component feature &# the relative position of component features.

4.1 Toleranced feature and datum feature indications

A

geometricaltoleranceapplies tothe wholeextent of atolerancedfeatureunlessotherwisespecified, as for example 0.02/50 which indicates that a tolerance of 0.02 is permitted for an extent of 50 at any place on the toleranced feature.

4.2 Identification of toleranced features and datums

When ageometricaltoleranceapplies to anaxis or a medianplane, thearrowhead of

the leader lineterminates at the dimension line as onthe left.

When ageometricaltolerance appliesto a surface or lineon a surface, theleader line with itsarrowhead

terminating on thecontour of the feature has to be clearlyseparated from the dimension line as on theright.

4.3 Symbols, indications and interpretations

Form tolerances limit the deviations of an individual feature from its ideal geometricalform.

Orientation, location and run-out tolerances limit the deviations of the mutual orientationand/or location of two or more features.

For functional reasons one or more features may be indicated as a datum. If necessary, ageometrical tolerance is applied to the datum feature in order to ensure that the datumfeature is sufficiently exact for its purpose.

Toleranced Datum Area Line Area PointFeature Feature --------- Datum Target Indications -------




Symbol Indication on Drawing Tolerance Zone Interpretation

for single features only:-

Straight -ness

The axis of the cylinder, to whichthe tolerance frame isconnected, is to be containedwithin a cylindrical zone of diameter 0.08.

Flatness

The surface is to be contained

betweentwo parallel planes 0.08 apart.

Circularity

The circumference of eachcross-sectionis to be contained between twoco-planar concentric circles 0.1 apart.

Cylindricity

The considered surface is to becontained between two coaxial

cylinders 0.1 apart.

for single or related features:-

Profileof any

line

(bilateral)

In each section parallel to theplane of projection, theconsidered profile is to becontained between linesenveloping circles of diameter 0.04, the centres of which are

situated on a line of theoreticallyexact geometrical profile.

Profileof any

surface

(bilateral)

The considered surface is to becontained between two surfacesenveloping spheres of diameter 0.02, the centres of which aresituated on a surface of theoretically exact geometricalform.

for related features only:-

Parallelismof a line (axis)

with reference

to a datum line

The toleranced axis is to becontained within a cylindricalzone of diameter 0.03, parallel tothe datum axis A (datumline).

Perpendic -ularityof a line (axis)

with reference

to a datumsurface

The axis of the cylinder is to becontained between two parallelplanes 0.1 apart, perpendicular to the datum surface.




Angularityof a line (axis)

with reference

to a datum

surface

The axis of the hole is to becontained between two parallelplanes 0.08 apartwhich are inclined at 60 todatum surface A.

Positionof a line

The axis of the hole is to becontained within a cylindrical

zone of diameter 0.08, the axisof which is in the theoreticallyexact position of the consideredline, with reference to thedatum surfaces A and B.

Coaxialityof an axis

The axis of the cylinder is to becontained within a cylindricalzone of diameter 0.08 coaxialwith the datum axis A-B.

Symmetryof a median

plane

The median plane of the slot isto be contained between twoparallel planes, which are 0.08apart and symmetricallydisposed about the medianplane with respect to the datumfeature A.

Circular run-out

radial

The radial run-out is to be notgreater than 0.1 in any plane of measurement during onerevolution about the datum axis

A-B.

Total run-outradial

The total radial run-out is to benot greater than 0.1 at any pointon the specified surface whilstrevolving about the datum axis A-B, and with relative axialmovement between workpieceand measuringinstrument. With relativemovement the measuringinstrument or the workpiece is tobe guided along a line having thetheoretically exact form of thecontour and being in its correctposition relative to the datumaxis.




TheoreticallyExact Dimension32mm

Used to specify theoretical (ideal)positions of features, etc.

Theoretically Exact Dimensionshave zero tolerance.

ProjectedTolerance Zone

Means of specifyingwhere adjoiningcomponents needparticular dimensions.

Important whenassembling:consequences areoften very great.

Maximum MaterialConditionqualification

Least MaterialConditionqualification

Means of specifyingmaximum sizeof a feature,combiningdimensionalandgeometricaltolerances.

EnvelopeRequirement

Means of specifyingoverall sizeof afeature.

R I Davidson [email protected]

mailto:[email protected]

mailto:[email protected]

Rd Reflex Tolerances

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