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Concepts IConcepts ITolerances, Limits, Fits and Tolerances, Limits, Fits and

Surface FinishesSurface Finishes

Tuesday 6Tuesday 6thth WeekWeek

Kogan Kogan #2 Redo Feedback#2 Redo Feedback

50% of those eligible redid assignment50% of those eligible redid assignmentnn Those who did not continue to make poor Those who did not continue to make poor

decisionsdecisions

All but one person of those attempting All but one person of those attempting assignment again raised their grade to a C assignment again raised their grade to a C –– excellent.excellent.

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Tolerances Tolerances (In week 2 we spoke briefly about Tolerances)(In week 2 we spoke briefly about Tolerances)

No feature of a component can be perfect (i.e., no No feature of a component can be perfect (i.e., no surface flat, no hole round etc), because of the surface flat, no hole round etc), because of the manufacturing processmanufacturing processThus, when dimensioning any featureThus, when dimensioning any featurenn Basic nominal dimensionBasic nominal dimensionnn Permitted variability (tolerance)Permitted variability (tolerance)nn Dimensions should be given in as large a tolerances as possible Dimensions should be given in as large a tolerances as possible

without interfering with the function of the part to reduce without interfering with the function of the part to reduce production costs.production costs.

20,1519,99

20 ± 0,01

Limits Unilateral

20 + 0,15- 0,01

Bilateral

LimitsLimitsExampleExample§§ Journal bearings are designed to operate at high rotational speeJournal bearings are designed to operate at high rotational speedd§§ If the clearance between the inner and outer diameter is to smalIf the clearance between the inner and outer diameter is to small the l the

bearing will seizebearing will seize§§ If the clearance is to big the shaft will vibrateIf the clearance is to big the shaft will vibrate§§ Limits on the size of the shaft and hole provide correct operatiLimits on the size of the shaft and hole provide correct operationon

ClearanceClearance

Nominal diameter = 20mmNominal diameter = 20mmClose running fit = H8/f7Close running fit = H8/f7f7: Shaft = 19,980 to 19,959f7: Shaft = 19,980 to 19,959H8: Hole = 20,000 to 20,033H8: Hole = 20,000 to 20,033Clearance = 20 to 74 micronClearance = 20 to 74 micronφφ20,00 H8/f720,00 H8/f7

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Limits (continued)Limits (continued)ExampleExample§§ Spool valve has a shaft that translatesSpool valve has a shaft that translates§§ This time the clearance should be a sliding fitThis time the clearance should be a sliding fit

ClearanceClearance

Nominal diameter = 20mmNominal diameter = 20mmSliding fit = H7/g6Sliding fit = H7/g6g6: Shaft = 19,993 to 19,980g6: Shaft = 19,993 to 19,980H7: Hole = 20,000 to 20,021H7: Hole = 20,000 to 20,021Clearance = 7 to 28 micronClearance = 7 to 28 micronφφ20,00 H7/f620,00 H7/f6

Ranges in TolerancesRanges in TolerancesRange of tolerances are a function of:Range of tolerances are a function of:§§ Range of measuring tool accuracyRange of measuring tool accuracy§§ Range of manufacturing processesRange of manufacturing processes§§ Range of Limits and fitsRange of Limits and fits§§ Range of materialRange of material

As IT # increases so does the rangeAs IT # increases so does the rangeAs nominal size increase so does the rangeAs nominal size increase so does the range

Nominal size range ISO Tolerances in micronsOver Up to IT1 IT2 IT3 IT4-- 3 0,8 1,2 2,0 3,03 6 1,0 1,5 2,5 4,06 10 1.0 1,5 2,5 4,0

Process Tolerance rangeLapping IT4 – IT 5Reaming IT6 – IT10Drilling IT10 – IT11Casting IT9-IT11

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Clearance fit Clearance fit [provide similar running performance[provide similar running performance with suitable with suitable lubrication]lubrication]nn Loose running fitLoose running fitnn Free running fitFree running fitnn Close running fitClose running fitnn Sliding fitSliding fit

Interference fit Interference fit [create an internal stress that is constant through [create an internal stress that is constant through size ranges]size ranges]nn Locational Locational interferenceinterferencenn Medium drive fitMedium drive fit

Transitional fit Transitional fit [use for location purposes][use for location purposes]

Fits and TolerancesFits and Tolerances

[See Figure 5.11 in text book][See Figure 5.11 in text book]

Geometry and TolerancesGeometry and TolerancesOccasionally geometry itself needs to have tolerances Occasionally geometry itself needs to have tolerances setsetnn FlatnessFlatness = a surface is flat when all elements are in one plane= a surface is flat when all elements are in one planenn StraightnessStraightness = a surface is straight if all it elements are in a = a surface is straight if all it elements are in a

straight linestraight linenn ParallelismParallelism = a surface or a line is parallel when all of its points = a surface or a line is parallel when all of its points

are equal distance from a are equal distance from a reference planereference plane

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Geometry and TolerancesGeometry and Tolerances(continued)(continued)

LabelingLabelingnn Tolerance box Tolerance box = contains datum label, or information about the = contains datum label, or information about the

tolerancetolerancenn Leader lineLeader line = touches feature or datum and tolerance box= touches feature or datum and tolerance boxnn DatumDatum = some geometries require a reference surface (e.g., = some geometries require a reference surface (e.g.,

parallelism) we label these reference surfaces as a datumparallelism) we label these reference surfaces as a datum10

20

0,3 A

A

Surface FinishSurface FinishSurface texture is the variation in the surface including Surface texture is the variation in the surface including roughness, waviness, lay and flaysroughness, waviness, lay and flaysRoughness describes the finest irregularities in the Roughness describes the finest irregularities in the surfacesurface

Zoom inAverage

Roughness

∑=

=n

iiy

nRa

1

1Measures:Ra = average peak heightRp = largest peak heightRv = lowest peak valleyRt = peak to valley distance

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Surface Finish (continued)Surface Finish (continued)LabelingLabeling

10

20

0,3 A

A

d

abcx

f e

a = 2D parameter 1b = 2D parameter 2c = 2D parameter 3d = manufacturing methode = lay patternf = allowancex = not allowed

MilledU Ra max 3,1L Ra 0,9