Introduction to GD&T Session 2: Rules and Concepts of GD&T

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1 Introduction to GD&T Session 2: Rules and Concepts of GD&T An exploration of the language known as Geometric Dimensioning and Tolerancing

2 Instructor: John-Paul Belanger

3 Review Benefits of GD&T The GD&T standard: ASME Y Features: Surfaces vs. features of size Actual local size vs. mating envelope Basic dimensions

4 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

5 The Feature Control Frame 0.25 J

6 A Previous Example

7 Other Examples

8 Other Examples

9 Maximum Material Condition (MMC) Ext. feature: largest size Int. feature: smallest size 15.3

10 Least Material Condition (LMC) Ext. feature: smallest size Int. feature: largest size 14.7

11 Using the M Modifier Only a hole made at MMC must be within the 0.4

12 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)

13 MMC Modifier Often Helps! Allows a fixed-size functional gage Reduces scrap rate Ensures assembly Saves $

14 With the L Modifier Only a hole made at LMC must be within the 0.4 L

15 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)

16 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 RFS

17 Datums Origin for a measurement From where does a location, orientation, runout, or profile dimension originate? 85 ± 0.4

18 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.

19 Datums Datums are the key to consistency: Product Design Process Design Manufacturing Inspection

20 Symbol (Triangle may be filled or unfilled) (Old datum feature symbol)

21 Datum vs. Datum Feature atum plane A datum feature A

22 Selecting Datums The most important consideration is part / product What features on the detail orient and locate it in the assembly?

23 Selecting Datums Other concerns: Accessibility for mfg. and inspection Repeatability Stability Cost Time

24 Easy: A Single Datum

25 More Common: Two or Three Three mutually perpendicular planes: 1. Primary 2. Secondary 3. Tertiary T S P

26 First Plane Primary datum plane: Requires points of contact (min)

27 Second Plane Secondary datum plane: Requires points of contact (min)

28 Third Plane Tertiary datum plane: Requires point of contact (min)

29 Restricting Degrees of Freedom One plane, one hole, one slot: 1. Primary 2. Secondary 3. Tertiary

30 Restricting Degrees of Freedom One plane, one hole, one slot: 1. Primary 2. Secondary 3. Tertiary

31 Restricting Degrees of Freedom One plane, one hole pattern: 1. Primary - 3 degrees of freedom 2. Secondary - 3 degrees of freedom

32 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

33 Surface vs. FOS Datum

34 Surface vs. FOS Datum

35 Surface and FOS

36 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

37 Shift vs. Bonus

38 Multiple Datum Features

39 Multiple Datum Features

40 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.

41 Datum Targets A Ø15 A1 Ø15 A2 Ø15 A3

42 Datum Target Simulator

43 Datum Targets

44 Datum Targets

45 Datums -- Summary

46 Q&A Please type any questions you have into the Q&A panel on your screen. Thank you!

47 Questions? If you would like to learn more, please contact: Tooling U-SME John-Paul Belanger

Terms The definitions of 16 critical terms defined by the 2009 standard 1

856 SALT LAKE COURT SAN JOSE, CA 95133 (408) 251 5329 Terms The definitions of 16 critical terms defined by the 2009 standard 1 The names and definitions of many GD&T terms have very specific meanings.