Limit, Fits, Tolerance and Gauges

By Prof N D Sadaphal

Assistant Professor

Sanjivani College of Engineering, Kopargaon (Maharashtra State) 423601

Mechanical Engineering

7/21/2016

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Limits, Fits & Tolerances

Limit -These are the extreme permissible sizes within which the operator is expected to make the component. Fit –It is the general term used to signify the range of Looseness and Tightness of mating partsTolerances –Allowable variations from the specified part dimensions that are permitted in manufacturing.

Spring 2005

Limit Fits & Tolerances

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Tolerance Dimensioning• Tolerance is the total amount that a specific dimension is permitted to vary;• It is the difference between the maximum and the minimum limits for the dimension.• For Example a dimension given as 1.625 ± .002 means that the manufactured part may be 1.627”or 1.623”, or anywhere between these limit dimensions.

Why is tolerancing necessary?It is impossible to manufacture a part to an exact sizeor geometryAssemblies: Parts will often not fit together if theirdimensions do not fall within a certain range of values

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Types of TolerancesA Geometric tolerance is the maximum or minimumvariation from true geometric form or position that maybe permitted in manufacture.

Geometrical Tolerances (Form)

Spring 2005

Types of Tolerances

The tolerance for a single dimension may be specified with the dimension based on one of the following methods

• Unilateral tolerance• Bilateral tolerance

Specific Tolerances

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Spring 2005

Specific TolerancesUnilateral ToleranceVariation from the specified dimension is permitted in only one direction,either positive or negative, but not both.

Spring 2005

Specific TolerancesBilateral ToleranceVariation is permitted in both positive and negative directions fromthe nominal dimension.

It is possible for a bilateral tolerance to be unbalanced; for example, 2.500 +0.010, -0.005

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Spring 2005

Fits between Mating PartsFit is the general term used to signify the range of

Looseness and Tightnessof mating parts

Spring 2005

Fits TypesClearance Fit occurs when two tolerance mating parts will always leave a space or clearance when assembled.

Interference Fit occurs when two tolerance mating parts will always interfere when assembled.

Transition Fit occurs when two tolerance mating parts are sometimes an interference fit and sometimes a clearance fit when assembled.

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Spring 2005

Important TermsNominal size: a dimension used to describe the general size.Basic size: the theoretical size used as a starting point for the application of tolerances.

Actual size: the measured size of the finished part after machining.

A specified system of fits for holes and shafts

Spring 2005

Tolerancing Holes and Shafts

- Hole basis •Hole is kept constant and Shaft sizes are vary to give various types of fits.• Uses the symbol “H” in the tolerance specification

- Shaft basis•Shaft is kept constant and Hole sizes are vary to give various types of fits.•Uses the symbol “h” in the tolerance specification

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Spring 2005

Clearance and Interference fits between two Shafts and a Hole

Spring 2005

Transition fit between a Shaft and a Hole

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Spring 2005

Metric Symbols of Fits

Terms related to Metric Limits & Fits

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Some Definitions• Deviation: is the difference between the basic sizeand the hole or shaft size.• Upper Deviation: is the difference between thebasic size and the permitted maximum size of thepart.• Lower Deviation: is the difference between thebasic size and the minimum permitted size of thepart.• Fundamental Deviation: is the deviation closest to the basic size.

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Gauges

•Gauges are inspection tool of rigid design, without a scale,which serves to check the dimensions of manufacturingparts.• Gauge do not indicate the actual value of the inspected part of the component.• They are used to determine whether the part is made within the specified limit.

Prof N D Sadaphal

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THE TAYLOR PRINCIPLE• Go Gauge is designed to check the Maximum Metal Condition/limit

(MMC) whileNo GO gauge should be designed to check Minimum metalcondition/limit.

• GO-gauge should check all related dimension (roundness, size etc)NO-GO gauge should check one element of dimension at a time.

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Classification of Plain GaugeGauges are classified:• According to their type :(a) Standard gauges- made as an exact copy of mating part(b) Limit gauges- made to the limits of the dimensions• According to their purposes :(a) Workshop Gauge: To Check Dimension after manufacture.used by operator during manufacturing.(b) Inspection Gauge: To check part before final acceptance.used by inspector after manufacturing.(d) Purchase Inspection Gauge: To check part of other factory(c) Reference or master gauges: To check the dimension of Gauge

Prof N D Sadaphal

7/21/2016

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Classification of Plain Gauge

According to the form of the tested surface :(a) Plug gauges for checking holes.

(b) Snap and ring gaugesfor checking shafts.

Prof N D Sadaphal

TAPER GAUGE• The most satisfactory method of testing a taper is to use taper Gauges. • They are also used to gauge the diameter of the taper at some point. • Taper gauges are made in both the plug and ring styles and in general, follow the same standard construction as plug and ring gauges.

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Screw Thread Gauge

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To Check Circular Shaft

Ring GaugeShould be usedas Go Gauge

Length should beequal to thelength of theshaft

Prof N D Sadaphal

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SCREW PITCH GAUGE

RADIUS AND FILLET GAUGE

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Feeler Gauge/Thickness Gauge

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KEY WORDS Standard Gauges : These are made to the nominal size of the partsto be tested.Limit Gauges : These are “Go” and “No Go” gauges.Plug Gauges : These are used for checking holes of many differentshapes and sizes.Ring Gauges : External diameter measuring gauges.Taper Gauges : Taper testing gauges.Snap Gauges : These are used for checking shafts.Thread Gauges : These are used for pitch diameter of the thread.Form Gauges : These are used to check the contour of a profile.Feller Gauges : For checking the clearance between the matingsurfaces.Indicating Gauges : To measure the position of the surfaces.Prof N D Sadaphal