basic gd&t datums

Data Analysis Lab

ME 288

L7 abc

GD & T

• Is a method (systems of symbols) for defining a part’s geometry– it goes beyond the form description

– based simply on tolerance dimensions

– as directed under rule #1

• A basic dimension is a theoretical

perfect size, location

or orientation dimension.12

Varies

Basic Dimension

2h

These symbols are used when dimensioning.

1.5h

12

Varies

Basic Dimension

ØDiameter

1.5h

0.3h

0.3 h

2h

( )(

ST

R SR SØ CR

X

2h

h h

90°

h

h

0.6h60°

h 2h

0.5h0.5h

30°

h

h

h

15°

2.5h

1.5h0.8h

60°

Counterbore Countersink Depth (or deep)

Dimension Origin Conical Taper Square

Arc Length Reference Slope

Radius

Places or By

Statistical Tolerance(dimensional)

SphericalRadius

SphericalDiameter

ControlledRadius

These symbols are used when dimensioning.

2h

h h

90°

h

h

0.6h 60°

h 2h

0.5h0.5h

30°

h

h

Counterbore Countersink Depth (or deep)

Dimension Origin Conical Taper Square

h = height of lettering

These symbols are used when dimensioning

1.5h

12

Varies

Basic Dimension

ØDiameter

1.5h

.3h

1.6

2h

( )(

ST

R SR SØ CR

X

h

15°

2.5h

1.5h0.8h

60°

Arc Length Reference Slope

Radius

Places or By

Statistical Tolerance

(dimensional)

Spherical

Radius

Spherical

Diameter

Controlled

Radius

A feature control frame (the backbone of GD & T) modifies a part’s geometry.

0.2 A B C

Modifier

Numeric tolerance(feature tolerance)

Geometric tolerance(characteristic)symbol (position)

Tertiary datum

Secondary datum with modifier

Primary datum

2h

Diameter symbol

Two additional examples of feature control frames modifying a part’s geometry.

0.08 0.05 A

Geometric tolerance(flatness)

Numeric tolerance

Geometric tolerance(perpendicularity)

Diameter symbol

Numeric tolerance

ModifierDatum

(A) (B)

Geometric characteristic(tolerance) symbols are categorized in two ways.

**

Perpendicularity

Angularity

Runout Circular

Runout Total

Profile surface

Profile line

Symmetry

Cylindricity

Position

Concentricity

Circularity

Straightness

Parallelism

Flatness

Either filled or unfilled

Symbol Description

Form

Pro

file

Ori

enta

tio

nLo

cati

on

Ru

no

ut

Type oftolerance

Individualfeatures

Individual orrelated features

Relatedfeatures

Geometric Characteristic Symbols

*

No datum or datums needed

A datum or datumsare required

Depending on the situation

GD&T geometric characteristic symbols illustrated.

1.5h M1.5h h 0.8h

2h1.5h

0.6h

h

1.5h

Concentricity Circularity Modifier

Straightness Parallelism Flatness

1.5h

1.5hh

2h

h h

60°

1.5h

h

h2h

2h1.2h

0.5hCylindricity Position

All round Profilesurface

Profile

line

Symmetry



h0.8h

0.8h3h

0.6h

1.5h

2h 1.5h

30°

45°

0.6h

1.5h

1.1h

**

Perpendicularity Angularity

Runout Circular Runout Total

Between

*

Filled or unfilled*

Table lists GD&T geometric characteristic symbol modifiers.

M

L

P

F

T

Maximum Material Condition

Least Material Condition

Projected Tolerance Zone

Free State Variation

Tangent Plane

Modifiers

ST Statistical Tolerance (geometric)

Between





Tangent Plane

Modifiers

Between





Tangent Plane

Modifiers

*Between

*Filled or unfilled

Modifiers

Maximum material condition, MMC(M)

• It should be taken literally.

• The geometric feature or size is as large as it can be.

• In the case of a hole, as small as it can be.

Least material condition, LMC(L)

• It should be taken literally.

• The geometric feature or size is as small as it can be.

• In the case of a hole, as large as it can be.

Rule #1:“Where only a tolerance of size is

specified, the limits of size of an

individual feature prescribe the extent of

which variations in its geometric form, as

well as size, are allowed.”ANSI Y14.5 – 1994

When only size tolerance is specified the object’s form can vary within the stated size limits.

Ø11.8

(B)

(C)

Ø12.2

Ø11.8

Ø12.2

Ø11.8

Ø12.2

Ø11.8

Ø12.2

Ø11.8 for entire length Ø11.8 for entire length

External dowel plug Internal hole

(A)

at Ø12.2 MMC must be perfect form

at Ø11.8 MMC must be perfect form

Ø12.2

11.8Ø

12.211.8

A cylinder can have a variety of shapes yet stay within the limits of size.

(A) (B) (C)

Min Min Min

Max Max Max

The rectangular prism can vary in shape as long as it stays inside the volume of the limits of size.

1614

119

97

2220

8

7(A) (B)

1816

16.015.8

Ø8.0 MMC

Ø7.8 LMC alongentire lengthof dowel

Ø6.8MMC

Ø7.0LMC

Ø 7.06.8

Ø8.07.8

Ring gage

Plug gage

(A) Checking geometric form with ring gage

(B) Checking geometric form with plug gage

Checking the size limits envelope:

A ring gage and plug gage are used to check the geometric form of a pin and hole.

Standard Stock Item

• Items whose geometry are already controlled by established industrial or government standards

– bars, sheet stock, tubing or structural shapes

Datums

• A datum is an exact surface, line, point ,axis or cylinder from which measurements are taken.

• Ex. A surface plate or a polished slab of granite (simulated datum).

• 3 points define a plane.

• Datum feature – is the surface of the part in contact with the simulated datum.

A height gage measures the height of an object from the simulated datum surface of a surface plate.

Datumfeature

Simulateddatum(surface plate)

Measuredheight

Dial face

ProbeUp

Down

Note: the datum feature rests on the simulated datum.the height is measured from the simulated datum and not from the datum feature.

Degrees of freedom (12) allow movement in two directions along each axis and rotation about each axis (clockwise and counterclockwise).

+Y

-X

+Z

-Y

+X

-ZIn order to measure geometric features

• part motion must be restricted.

A datum reference frame consists of three intersecting planes at 90° to each other.

The part can still move in the positive X, Y and Z directions“part motion must be restricted”

What if we temporarily clamped the part? → Measurements can be taken from the simulated datums.

Datum surfaces must be indicated on the drawing

2H for single lettering4H for double lettering

A2H H

H

60°

16

G

H

E

F

Filled orunfilled

(A) H = height of lettering

(B) Applications

Varies

The datum symbol is applied to solid cylinders.

KJ

J

Ø14Ø8

Ø14

J

-or-

F G HØ0.1

Ø12

N

P

RM

Ø6

Ø12

Ø6

M

The datum symbol is applied to holes.

Note: letters “I”, “O” and “Q” are not used to indicate datums because they may be confused with numbers one(1) and zero(0).

Double letters can be used e.g. AA, BB etc.

Order of Datums:Primary datum S has 3 pts of contact, secondary datum T has 2 pts and tertiary datum U 1pt of contact.

(A)This drawing

symbology

(B) Means this

SU

T

3 pts of contact1 pt of contact

2 pts of contact

S T UØ0.1

TS12

2X Ø6 ± 0.2

6

M

8

U

10

The order of the datums is critical!

(1 pt)

(2 pts)

(3 pts)

V W XØ0.1

2X Ø6 ± 0.08

M

X

W

V

(1 pt) (3pts)

(2 pts)

Trueheight

VX

W

Correct inspection procedure

True width(1 pt)

(3 pts)(2pts)

VX

W

Falseheight

Incorrect inspection procedure

Falsewidth

The datum axis is formed by two intersecting planes.

GH

(A) This drawing

8

8

8 8

Ø24

0.5 G H4X Ø4±0.5 Intersecting planes

perpendicular to G

Primarydatum G

Datum axis

(B) Means this

The plug gage establishes the datum axis.

(A) This drawing

(B) Means this

C

Ø10.8±0.1

Simulated datumcylinder C - largest Ø that fits into hole

Datum axis

Datum feature C

Plug gage

The smallest circumscribed cylinder establishes the simulated datum and the datum axis.

(A) This drawing

(B) Means this

D

Datum feature DSimulated datum D

Datum axis

19±0.5

The simulated central datum plane is established by the center plane of

the largest block that fits into the groove.

B) Means this

A) This drawing 24.223.8

12.211.8

0.4 Y Z

Z

Y

Central datum plane

Simulateddatums atmaximumseparation

Datum features

The simulated central datum plane is established by the center plane located by the two blocks at minimum separation.

B) Means this

A) This drawing

18.2

Simulated datumplanes at minimumseparation

Central datum plane

0.6 A B

B

A

17.8

basic gd&t datums

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