geometric dimensioning and tolerancing course number instructor’s name planchard copyright 2012

Geometric Dimensioning and Tolerancing

Course Number

Instructor’s name

Planchard Copyright 2012

What is Geometric Dimensioning and Tolerancing “GDT”• It is a system of symbols, rules and definitions

used to define the shape (Roundness, Flatness, Cylindricity, etc.) and design Intent (parallel, perpedicular, concentric, et.) of the model.


Objectives of GDT

• Define parts based on function

• Include geometric as well as size tolerances

• Facilitate inspection and quality control

• Reduce need for notes

• Capture design intent

• Clarify tolerances for international use

• Eliminate tolerance stack-upPlanchard Copyright 2012

Objectives of GDTOrthogonal Datum Planes “3-2-1” Rule

3 points of contact

1 point of contact

A rule that defines the minimum number of contact points necessary to properly locate a part within the datum reference frame. The primary datum requires three points, the secondary datum two points, and the tertiary datum one point.

2 points of contact


Simple Part with GDT

Dimension tolerance does not say anything about the shape of the part!GDT controls the shape and the initial design intent.

Tolerance of the feature

Design IntentRoundness symbol of the feature

Primary datumSecondary datum

Tolerance of the dimension

Design IntentPerpendicularity symbol of feature



Tertiary datum

Secondary datum

Primary datum

Flatness symbolof the feature

Perpendicularity symbolof the feature

Position symbolof the feature: Hole

Dimension tolerance does not say anything about the shape of the part!GDT controls the shape and the initial design intent.


Tolerance of the datumfeature



Roundness: (ASME Y14.5M 1994 Section 2.7.1.3, paragraph (a)).

Circular runout

Secondary Datum plane

Primary Datum planeCylindrical face

Tolerance of the datum featured




Yes No




Primary Datum plane



Angularity symbolof the feature

View the selected Datumplanes, the angularity symbolwas added afterwards!




Note: The different Datum planes selectedto display the Angularity symbolof the feature from the previous slide!

Angularity symbolof the feature





Dimension tolerance does not say anything about the shape of the part!GDT controls the shape and the initial design intent

Datum C is the face of the feature: Simple Hole

Position symbolof the feature


Offset, arrows will not fit.



Dimension tolerance does not say anything about the shape of the part!GDT controls the shape and the initial design intent

Position symbolof the feature




Parts Do Not Have Center Lines—Features Have Axes Center lines are a drafting convenience. Associate the datum feature symbol with datum features!

Total runout symbolof the feature

No “For Construction” Centerline!



Coaxial Diameters - Two (or more) diameters that are shown on the drawing as being on the same centerline (axis).

Revolved Base Feature : Axis of revolutionsVs. Extruded Base Feature.

Circular runout symbolof the feature

Circular runout symbolof the feature



Profile of a Surface symbolof the feature

Tertiary Datum

Primary Datum

Secondary Datum


GDT Symbols

http://www.wisc-online.com/objects/index_tj.asp?objID=ENG18804Planchard Copyright 2012

Default Document Datum Tolerances (DimXpert)


Material Condition Symbols

• Maximum Material Condition (MMC)– largest shaft, smallest hole

• Least Material Condition (LMC)– smallest shaft, largest hole

• Regardless of Feature Size (RFS)

M

L

S


Tolerance Zones without GDTMeans this!

The tolerance area or boundary with that forms the Feature may vary when no tolerance of the feature is provided


GDT Rule 1

• Individual Feature of Size– Where only a tolerance of size is specified,

the limits of an individual feature prescribe the extent to which variations in its geometric form, as well as size, are allowed.


Location Controls

• Position

• Concentricity

• Runout


GD&T True Positioning

Note circular tolerance zones


Straightness


Flatness

The surface must be within the tolerance of size and must lie between two parallel planes: 0.25 apart (featuretolerance)

SymbolTolerance of the feature


Roundness

Each circular element of the Surface in any plane perpendicular to A common axis must be within the specified tolerance of size. It also must lie between two concentric circles – one having a radius of 0.25 larger than the other!

SymbolTolerance of the feature


Cylindricity

The cylindrical surface must be within thespecified tolerance of size, in this example 0.25and must lie between two Concentric cylinders – one having a radius of 0.25 larger than the other.

Symbol

Tolerance of the feature


Orientation Controls

• Perpendicularity

• Parallelism

• Angularity

• Line Profile

• Surface Profile


Parallelism for Planes

The surface must be within the specifiedTolerance (0.12) of size and must lie between two planes (0.12) apart which areparallel to Datum Plane A.

Symbol

Tolerance of feature

Datum Plane


Parallelism for Axis and Plane

The feature axis must be within the specifiedtolerance (0.12) of location and must lie betweentwo planes (0.12) apart which are parallel to thedatum plane (A) regardless of the feature size!


Parallelism for Two Axes

The feature axis must be within the specified tolerance (0.05) of location. Where the feature is at Maximum Material Condition (10.00). The MaximumParallelism tolerance is (0.05).


Perpendicularity of Axis vs. Plane

The feature axis must be within the specifiedtolerance(0.05) of location. Where the feature is at MMC (15.984) the Max perpendicularity tolerance is 0.05. Where the feature departs from it’s MMC size, and increase in the Perpendicularity tolerance is ALLOWED which is equal to the amount of the departure.


All parts are not created equal!

Profile of a line is a funny control. It works well for parts like extrusions where individual slices will be checked. However, when you start adding datum references, it can turn into a profile of a surface control

CNC issues!


geometric dimensioning and tolerancing course number instructor’s name planchard copyright 2012

Documents