ME 546Geometric Dimensioning and Tolerancing

ASME Y14.5M-1994 Standard

History of Geometric Dimensioning and Tolerancing

• Geometric Dimensioning and Tolerancing symbols have been in use since at least the turn of the century. GDT was especially important during the Second World War in relation to extremely high volume production of Liberty Ships, aircraft, and ground vehicles. The automotive industry, with its high volumes, has also benefited from GDT. The American National Standards Institute publication in 1982 of ANSI Y14.5M-1982 was a turning point in the rigorous, unambiguous standardization of the methodology.

Geometric Dimensioning and Tolerancing: Why ?

• Geometric Dimensioning and Tolerancing (GDT) is a method for precisely defining the geometry of mechanical parts.

• It conveys the product function and design relationship.

• Variability— Mass Production.

Dimensioning

• All dimensions either locate or indicate a size of a feature

• Basic dimension: a theoretical dimension

• Limits of Size: acceptable amount of variation allowed on a dimension

– Limit Dimensions =

– (+/-) Tolerances = .455

.499

.495

+.003-.002

Tolerance Accumulation

Emphasis is on proper Dimensioning.

Chain Dimensioning: point-to-point dimensioning-allfeature are dimensioned from one another. This dimension type has the greatest tolerance accumulation effect.

Base Line: all the dimensions extend from a baseline. This dimension schema lowers tolerance accumulationseen by chain dimensioning.

Direct: the utilization of both direct and baselineDimensioning is captured here.

GTOL Symbology

• The 5 types are:– Form– Orientation– Profile– Runout– Location

• Other symbols:– Dimensional values that

require classification via symbols

– Example: DIA. Symbol

Modifying Symbols

• Symbols to remember:– MMC– LMC– RFS

• Used often in Industry– MMC

Feature Control Frame

A B C.011 M

Position Tolerance Zone Symbol

Tolerance Zone Shape

Tolerance Zone Size

Material Condition Modifier

Primary Datum

M

Secondary DatumDatum Material Condition Modifier

Tertiary Datum

All Geometric Tolerances are specified in a FCF.

Datum

• What is a datum?

• A datum is the origin from which the location or geometric shape of features of a part are established.

• It is a theoretically perfect point, line or plane.

• Tolerance specifications will reference these datum's.

Datum Reference Frame

• Positioning the part with relation to three mutually perpendicular planes: the datum reference frame.

• This reference frame exist in theory only• In some cases a single datum may be

sufficient

Datum Reference Frame

Datum Simulation

Theoretical

Datum simulation is required since fabrication and inspection operationsmust be made in relationship to thespecified datum

Features of Size

• Features of size are often classified as datum features.

• Examples are: holes, slots, tabs and shafts

• Placement of a datum feature symbol with a size dimension indicates that the feature of size is a datum feature

External Feature (Datum Call-Out)

Internal Feature (Datum Call-Out)

Material Condition Modifiers

• Material condition modifiers influence tolerance zone sizes

• Regardless of Feature Size (RFS)– The specified tolerance value is to remain constant

regardless of the size of the toleranced part.

– The tolerance zone does not change

– RFS is typically used for applications where holes have zero clearance or press fits.

Maximum Material Condition (MMC)

• Definition: MMC is the condition in which a feature of size contains the maximum amount of material within the stated terms of the size.– For external features we add the tolerance– For internal feature we subtract the tolerance

• MMC is used for clearance hole applications.

– For example: As my hole size increases my positional tolerance increases.

– Bonus Tolerance

Least Material Condition (LMC)

– Definition: LMC is the condition in which a feature of size contains the least amount of material within the stated terms of the size.

– For external features we subtract the tolerance– For internal feature we add the tolerance

Bonus Tolerance

• When departing from the specified material condition modifier, the positional tolerance zone size will increase.

.014.000.520 (LMC)

.015.001.519

.016.002.518

.017.003.517

.018.004.516

.019.005.515

.020.006.514 (MMC)

Position Tol.Bonus Tol.Hole Size

Positional Tolerance at LMC

LMC Applied to Boss and Hole

Axes Relation to Tolerance Zone

Virtual Condition

• The position resulting from the collective effects of size and geometric tolerancing applied to the feature.

• Virtual condition for external feature: Tab– MMC + geometric tolerance.

• Virtual Condition for Internal feature: Hole– MMC – geometric tolerance.

Virtual Condition cont’d

• A virtual condition can be thought of as the required size for a mating part, assuming he mating part is perfect.

• For a pin with perpendicularity assigned: VC is the size of a perfect hole that fits over the pin when the pin is at MMC and is in the worst permitted orientation.

Virtual Condition Example

• If a hole is at .510 dia. (+/- .005) with a tol. zone dia. of .002 and a pin is at .498 dia. (+.003/-.006) with a tol zone of .003, will the pin fit into the hole under virtual conditions?

Answer_______.

Answer!!!

• NO!

• Pin: .498 + .003 + .003 = .504 Dia.

• Hole: .510 - .005 - .002 = .503 Dia.

Interference fit by .001

Confused?

• Any Questions?

• There will be a Quiz.

• Date: TBD