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GEOMETRIC DIMESIONING & TOLERANCING REF : 1) ASME Y14.5M-1994. (American Society of Mechanical Engineers)

2) MNM STD A 00 0020 - REV A

Basic GD&T

PRESENTATION BY :- KOTWAL P. H.

PROJECT INGENIO, NASIK

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GD&T -

1) GEOMETRIC DIMENSIONING AND TOLERANCING IS A METHOD OF STATING AND INTERPRETING DESIGN REQUIREMENTS.

2) IT IS A TOOL AVAILABLE TO MAKE COMMUNICATION BETTER BETWEEN DESIGNER AND MANUFACTURER, INSPECTION.

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14 SYMBOLS USED IN GD&T

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10 MODIFIERS USED IN GD&T

OLD PRACTICE

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GD&T IS COMBINATION OF SYMBOLS,MODIFIERS, TOL. VALUES & DATUMS ---- IS CALLED FEATURE CONTROL FRAME (FCF)

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RULES IN GD&T:-RULE #1 - WHEN ONLY A TOLERENCE OF SIZE IS SPECIFIED, THAT CONTROLS BOTH SIZE AND FORM.

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RULE #2 - RFS IS IMPLIED FOR ALLGEOMETRIC TOLERANCE.

RULE #2a - RFS MAY BE SPECIFIED ON THEDRAWING W.R.T. INDIVIDUAL TOL., DATUMREF., OR BOTH AS APPLICABLE.

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RULE #3 - FOR SCREW THREADS, SPLINESAND GEARS THE TOLERANCE AND DATUMREFERENCE ORIGINATE FROM PITCHCYLINDER AXIS.

RULE #4 - A VIRTUAL CONDITION EXISTSFOR A DATUM FOS WHERE ITS AXIS ORCENTRE LINE IS CONTROLLED BY AGEOMETRIC TOLERANCE.

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General Terms used in GD&t

1) FOS - Feature Of Size2) MMC - Maximum Material Condition3) LMC - Least Material Condition4) RFS - Regardless Of feature Size.5) VC - Virtual Condition6) TOP - Tolerance Of Position7) BONUS TOLERANCE8) DATUM

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1) FOS - FEATURE IS A PHYSICAL PORTION OF

A PART AND DIMENSION SHOWN TO THEPORTION IS CALLED FEATURE OF SIZE.

E.g. FEATURES E.g. FEATURES OF SIZE

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2) MMC :-

MAXIMUM MATERIAL CONDITION IS A

CONDITION IN WHICH FEATURE OF SIZE

CONTAINS MAX AMOUNT OF MATERIAL

EVERYWHERE WITHIN THE STATED LIMITS

OF SIZE.

E.G. - SHAFT DIA 20.0 +/-0.2

I.e. MMC = 20.2

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3) LMC :-

LEAST MATERIAL CONDITION IN WHICH AFEATURE OF SIZE CONTAINS LEASTAMOUNT OF MATERIAL WITHIN THE STATEDLIMITS OF SIZE.

e.g. SHAFT DIA 20.2 +/-0.2 I.e. LMC OF SHAFT IS 20.0

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4) RFS -REGARDLESS OF FEATURE SIZE IS A TEARM THAT INDICATES A GEOMETRICTOLERANCE APPLIES AT ANY INCREMENT OF FEATURE WITHIN ITS SIZE TOLERANCE. OR RFS IS THAT THE GEOMETRIC TOLERANCE APPLIES AT WHATEVER SIZE OF PART IS PRODUCED• RFS applies, with respect to the individual tolerance, datum reference, or both, where no modifying symbol is specified.

• RFS does not permit bonus tolerance.

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e.g. RFS Tolerance Zone

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When & Where apply Modifiers like MMC, LMC& RFS.

1) MMC - When function of FOS is Assembly

2) LMC - When function of FOS is to ensure MIN distance on a part

3) RFS - To ensure symmetrical relationship.

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5) VC - Virtual Condition- VC is a collective actionof FOS at MMC or at LMC and geometrictolerance for that material condition.INNER BOUNDRY (IB) = LMC + GEOMETRIC TOL

OUTER BOUNDRY (OB) = MMC + GEOMETRIC TOL

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WORST CASE BOUNDRY (WCB) = TERM REFERS TO THE EXTREME BOUNDRY OF A FOS THAT IS THE WORST FOR ASSEMBLY. e.g.

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MMC Virtual Condition :- When a MMC modifier is applied in FCF of FOS, the VC of FOS gets affected.A) In case of External FeatureVC = MMC + Geometric Tolerance

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B) - In case if Internal FeatureVC = MMC - Geometric Tolerance

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LMC Virtual Condition :- When a LMC modifieris applied in a FCF of FOS, the VC of FOS isaffected.

A) In case of External FeatureVC = LMC - GEOMETRIC TOLERANCE

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A) In case of Internal FeatureVC = LMC + GEOMETRIC TOLERANCE

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MULTIPLEVIRTUALCONDITION :-The FOS mayhave several VC

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6) Tolerance Of Position :- TOP is a geometrictolerance that defines the location tolerance ofa FOS from its true position.

a) TOP can aplly with RFS,MMC & LMC.

Theoretically exact locationdefined of FOS is defined bybasic dimensions is calledTRUE POSITIONof the FOS

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b)comparison of CO-ORD. tolerancing with TOP tol zone

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c) Guide to use TOP

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d) TOP using Projected Tol. Zone - Projectedtolerance zone is tolerance zone that is projectedabove the part surface.

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e.g. Projectedtolerance Zone

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e) PLTZF & FRTZF used to define pattern type features. PLTZF - PATTERN LOCATING TOLERANCE ZONE FRAME WORK. It restricts the zone in which the entire pattern of features can be located FRTZF - FRAME RELATING TOLERANCE ZONE FRAME WORK. This tolerance is tighter & restricts the relationship of individual feature location with each other within the pattern.

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e.g. PLTZF & FRTZF

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7) BONUS TOLERANCE

- Bonus tolerance is an additional tolerance for a geometric control. - Bonus tolerance is only permissible when at MMC (or LMC) modifier is shown in the tolerance portion of a FCF- Bonus tolerance comes from FOS tolerance.

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8) DATUM -a) Implied Datum- Implied Datums are imaginaryplane, axis or centre point from measurement ismade

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Consequence of Implied plane 1) Good parts may reject 2) Rejected parts may accept

b) Planer Datum - A datum is a theoreticallyexact plane from which dimensions is made. A datum feature is a part feature that contactsa datum. A Planer Datum is the true geometricalcounterpart of a planer datum feature.

Next figure shows the presentationof Planer Datums

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How to decide datum sequence- When FCFrefers datum, the sequence is determined byreading the FCF from left. The first compartment that contains a datum reference denotes the datum feature that is to contact the inspection equipment first, second datum feature with second inspection equipment & third with third.

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ComparisonCo-ord Tol & GDT tol.

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