Introduction to GD&T

Session 2: Rules and Concepts of GD&T

An exploration of the

language known as

Geometric Dimensioning

and Tolerancing

Instructor:

John-Paul Belanger

Review

• Benefits of GD&T

• The GD&T standard: ASME Y14.5-2009

• Features: Surfaces vs. features of size

• Actual local size vs. mating envelope

• Basic dimensions

Today’s Agenda

The “feature control frame”

Maximum Material Condition

Least Material Condition

Regardless of Feature Size

Rule #2

Datums

Datum precedence

Datum targets

Wrap-up

J 0.25

The Feature Control Frame

A Previous Example

Other Examples

Other Examples

Maximum Material Condition (MMC)

–Ext. feature: largest size

–Int. feature: smallest size

15.3

Least Material Condition (LMC)

–Ext. feature: smallest size

–Int. feature: largest size

14.7

Using the “M” Modifier Only a hole made at “MMC” must be within the 0.4…

Still Two Things to Measure

Actual

Size

Stated

Geo Tol

Bonus Tol MMC – act. size

Total Geo Tol (stated geo tol +

bonus tol)

Inner

Boundary (size – total geo

tol)

17.9 0.4 0 0.4 17.5

18.0 0.4 0.1 0.5 17.5

18.1 0.4 0.2 0.6 17.5

18.2 0.4 0.3 0.7 17.5

18.3 0.4 0.4 0.8 17.5

18.4 0.4 0.5 0.9 17.5

18.5 0.4 0.6 1.0 17.5

MMC Modifier Often Helps!

Allows a fixed-size functional gage

Reduces scrap rate

Ensures assembly

Saves $

With the “L” Modifier… Only a hole made at “LMC” must be within the 0.4…

L

If the “L” Modifier Is Used

Actual

Size

Stated

Geo Tol

Bonus Tol LMC – act. size

Total Geo Tol (stated geo tol +

bonus tol)

Outer

Boundary (size + total geo

tol)

18.5 0.4 0 0.4 18.9

18.4 0.4 0.1 0.5 18.9

18.3 0.4 0.2 0.6 18.9

18.2 0.4 0.3 0.7 18.9

18.1 0.4 0.4 0.8 18.9

18.0 0.4 0.5 0.9 18.9

17.9 0.4 0.6 1.0 18.9

Rule #2

• First, a definition: RFS stands for “regardless of feature size”

• Now, the rule: Unless the MMC or LMC modifier is given, assume that a geometric tolerance (and any datum references) are RFS.

Constant tolerance

of 0.1 -- RFS

Datums

• Origin for a measurement From where does a location, orientation, runout, or

profile dimension originate?

85 ± 0.4

Datums

• Definition: A theoretically exact point, axis, or plane that serves as the origin from which a measurement is made.

• Also: A “datum feature” is the physical surface used to create the datum.

Datums

• Datums are the key to consistency:

Product Design

Process Design

Manufacturing

Inspection

Symbol

(Triangle may be filled or unfilled)

(Old datum feature symbol)

Datum vs. Datum Feature

datum plane A datum feature A

Selecting Datums

The most important consideration is…

… part / product ___________

• What features on the detail orient and

locate it in the assembly?

Selecting Datums

• Other concerns: Accessibility for mfg. and inspection

Repeatability

Stability

Cost

Time

Easy: A Single Datum

More Common: Two or Three

• Three mutually perpendicular planes:

1. Primary

2. Secondary

3. Tertiary

P S

T

First Plane

• Primary datum plane:

Requires ____ points of contact (min)

Second Plane

• Secondary datum plane:

Requires ____ points of contact (min)

Third Plane

• Tertiary datum plane:

Requires ____ point of contact (min)

Restricting Degrees of Freedom

• One plane, one hole, one slot:

1. Primary

2. Secondary

3. Tertiary

Restricting Degrees of Freedom

• One plane, one hole, one slot:

1. Primary

2. Secondary

3. Tertiary

Restricting Degrees of Freedom

• One plane, one hole pattern:

1. Primary - 3 degrees of freedom

2. Secondary - 3 degrees of freedom

Polling Question #3

What is the maximum number of datums allowed on a print?

a. 1

b. 2

c. 3

d. 23

e. There is no maximum

Surface vs. FOS Datum

Surface vs. FOS Datum

Surface and FOS

“M” on a Datum Reference

The geometric tolerance

starts at 0.2, but can be

“fudged” by 2 modifiers:

Ø 0.2 stated tolerance

+ Ø 0.8 potential bonus tolerance

+ Ø 0.3 potential shift tolerance

Ø 1.3 max. effective position variation

Shift vs. Bonus

Multiple Datum Features

Multiple Datum Features

Datum Targets

• A “datum target” is a way to identify specific

points

lines

areas

which are then used to establish the true datum.

Good for unusual-shaped parts, or if the function dictates

that only a portion of the surface will be contacted.

Datum Targets

A

A1 Ø15

A2 Ø15

A3 Ø15

Datum Target Simulator

Datum Targets

Datum Targets

Datums -- Summary

Q&A

Please type any questions you have

into the Q&A panel on your screen.

Thank you!

Questions?

Tooling U-SME 866.706.8665 www.toolingu.com/ilt John-Paul Belanger 248.652.1397 belanger@gdtseminars.com

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