Geometric Dimensioning and Tolerancing Unit 6 Datums GDT

GDT 1. Establish a reference coordinate system. 2. Tell me where ‘perfect’ is. 3.

Introduction • Datums are nothing more and nothing less than the physical features of

Datum Feature Symbol • Datum Feature Symbol EML 2023 Department of Mechanical and Aerospace

Three-Plane Concept • define primary, secondary, and tertiary datum planes • the primary datum

Three-Plane Concept • the secondary datum plane is to the primary datum • secondary

Three-Plane Concept • the tertiary datum plane is to the primary & sec. datums

Three-Plane Concept - Circular • circular parts also require a three-plane concept for repeatable

Establishing Datums from Primary Plane Surfaces 15. 9 ± 0. 2 5 H 7(5/5.

Datum Features without Size P L 5 H 7(5/5. 012) 15. 9 ± 0.

Datum Features of Size C 15. 9 ± 0. 2 R 5 H 7(5/5.

Datum Feature Symbol Placement is Important 6. 21

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Geometric Dimensioning and Tolerancing Unit 6 – Datums

GDT 1. Establish a reference coordinate system. 2. Tell me where ‘perfect’ is. 3. Tell me how far off from ‘perfect’ is allowed. EML 2023 Department of Mechanical and Aerospace Engineering 2

Introduction • Datums are nothing more and nothing less than the physical features of parts from which to make repeatable measurements. • These features are considered theoretically exact. • Datums define a coordinate system from which to measure everything. • Datums are usually a plane, a line, or a point. • We will need to label the datums on our drawing so that the part inspector will know from where to measure. EML 2023 Department of Mechanical and Aerospace Engineering 3

Datum Feature Symbol • Datum Feature Symbol EML 2023 Department of Mechanical and Aerospace Engineering 4

Three-Plane Concept • define primary, secondary, and tertiary datum planes • the primary datum is the one that functionally is usually the most critical feature or surface of the part • the primary datum feature of the part will make contact with theoretical datum plane in a minimum of three points EML 2023 Department of Mechanical and Aerospace Engineering 5

Three-Plane Concept • the secondary datum plane is to the primary datum • secondary datum feature is the one that functionally is the 2 nd most critical feature or surface of the part • 2 point minimum contact EML 2023 Department of Mechanical and Aerospace Engineering 6

Three-Plane Concept • the tertiary datum plane is to the primary & sec. datums • 1 point minimum contact EML 2023 Department of Mechanical and Aerospace Engineering 7

Three-Plane Concept - Circular • circular parts also require a three-plane concept for repeatable orientation • the primary datum is usually one flat surface of the part, • defines a datum axis along the center of cylinder should be a construction line (or omitted) should have a center line EML 2023 Department of Mechanical and Aerospace Engineering 8

Features with and without Size 1. 9

Establishing Datums from Primary Plane Surfaces 15. 9 ± 0. 2 5 H 7(5/5. 012) 0. 2 25± 0. 3 LL M-10 x 1. 25 12. 7± 0. 1 25 ± 0. 3 H 2 X 6± 0. 25 Theory Datum features H and L are mated with a theoretical perfect plane contacting the high points of the datum feature. Physical Datum features H and L are mated on the high points of the manufacturing or inspection equipment 6. 15

Datum Features without Size P L 5 H 7(5/5. 012) 15. 9 ± 0. 2 25± 0. 3 M-10 x 1. 25 N M K 2 SURFACES 12. 7± 0. 1 H 2 X 6± 0. 25 25 ± 0. 3 S 17± 0. 02 Datum planes established from planar datum features Datum feature N establishes a plane on the high points of the left surface. Datum feature L establishes a plane on the high points of the rear surface. Datum feature K establishes a plane on the high points of two surfaces. 6. 16 Datum feature H establishes a plane on the high points of the bottom surface. Datum feature M establishes a plane on the high points of the left side of the slot.

Datum Features of Size C 15. 9 ± 0. 2 R 5 H 7(5/5. 012) 25± 0. 1 M 10 x 1. 25 E F B 12. 7 ± 0. 1 A 25± 0. 1 2 X 6± 0. 25 0. 2 M SR 8. 5 0. 4 D Theory Datum axis E is the axis of theoretical datum feature simulator 6. 18 G Physical Simulated datum axis E is the axis of a collet, chuck, CMM etc.

Datum Features of Size C 15. 9 ± 0. 2 R 5 H 7(5/5. 012) 25± 0. 1 M 10 x 1. 25 E F B 12. 7 ± 0. 1 A 25± 0. 1 2 X 6± 0. 25 0. 2 M SR 8. 5 0. 4 D 6. 19 G

Datum Features of Size C 15. 9 ± 0. 2 R 5 H 7(5/5. 012) 25± 0. 1 M 10 x 1. 25 E F B 12. 7 ± 0. 1 A 25± 0. 1 2 X 6± 0. 25 0. 2 M SR 8. 5 0. 4 D Theory 6. 19 G Physical Datum center plane B is the center plane of the datum feature simulator. Sim ulated datum center plane B is the center plane of the gage block.

Datum Features of Size C 15. 9 ± 0. 2 R 5 H 7(5/5. 012) 25± 0. 1 M 10 x 1. 25 E F B 12. 7 ± 0. 1 A 25± 0. 1 2 X 6± 0. 25 0. 2 M SR 8. 5 0. 4 D Theory 6. 20 Datum axis C is the axis of the datum feature simulator G Physical Simulated datum axis C is the axis of the gage pin

Datum Features of Size C 15. 9 ± 0. 2 R 5 H 7(5/5. 012) 25± 0. 1 M 10 x 1. 25 E F B 12. 7 ± 0. 1 A 25± 0. 1 2 X 6± 0. 25 0. 2 M SR 8. 5 0. 4 D Theory Physical Datum D axis and two planes are established from a pattern of two holes 6. 20 G Simulated datum feature D axis and two intersecting planes established from gage

Datum Feature Symbol Placement is Important 6. 21