Geometric Dimensioning and Tolerancing

Geometric Dimensioning and Tolerancing Chapter 3, Datums

GDT • Establish a reference coordinate system. • Tell me where ‘perfect’ is. • Tell me how far off from ‘perfect’ is allowed.

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. • Datumsdefine 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.

Datum Feature Symbol • Datum Feature Symbol

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 the theoretical datum plane in a minimum of three points

Three-Plane Concept • the secondary datum plane is  to the primary datum • secondary datum feature is the one that functionally is the 2nd most critical feature or surface of the part • 2 point minimum contact

Three-Plane Concept • the tertiary datum plane is  to the primary & sec. datums • 1 point minimum contact

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

Three-Plane Concept - Circular • datum A defines an axis along the center of the cylinder

Features with and without Size 1.9

Establishing Datums from Primary Plane Surfaces 5 H 7 ( 5 / 5 . 0 1 2 ) 1 5 . 9 ± 0 . 2 0 . 2 2 5 ± 0 . 3 L L M - 1 0 x 1 . 2 5 1 2 . 7 ± 0 . 1 2 5 ± 0 . 3 H 2 X 6 ± 0 . 2 5 T h e o r y D a t u m f e a t u r e s H a n d L a r e m a t e d w i t h a t h e o r e t i c a l p e r f e c t p l a n e c o n t a c t i n g t h e h i g h p o i n t s o f t h e d a t u m f e a t u r e . P h y s i c a l Datum features H and L are mated on the high points of the manufacturing or inspection equipment 6.15 D a t u m D a t u m W o r k p i e c e f e a t u r e f e a t u r e W O R K P I E C E S U R F A C E D a t u m T E f e a t u r e s i m u l a t o r T o o l & g a g e P h y s i c a l d a t u m f e a t u r e s i m u l a t o r v a r i a t i o n S i m u l a t e d ( s u r f a c e p l a t e o r T h e o r e t i c a l d a t u m p l a n e p r o c e s s i n g e q u i p m e n t ) d a t u m P l a n e M a g n i f i e d V i e w

D a t u m f e a t u r e H e s t a b - D a t u m f e a t u r e N e s t a b - l i s h e s a p l a n e o n t h e h i g h l i s h e s a p l a n e o n t h e h i g h p o i n t s o f t h e b o t t o m p o i n t s o f t h e l e f t s u r f a c e . s u r f a c e . D a t u m f e a t u r e L e s t a b - l i s h e s a p l a n e o n t h e h i g h p o i n t s o f t h e r e a r s u r f a c e . D a t u m f e a t u r e M e s t a b - l i s h e s a p l a n e o n t h e h i g h p o i n t s o f t h e l e f t s i d e o f t h e s l o t . D a t u m f e a t u r e K e s t a b l i s h e s a p l a n e o n t h e h i g h p o i n t s o f t w o s u r f a c e s . Datum Features without Size P L 5 H 7 ( 5 / 5 . 0 1 2 ) 1 5 . 9 ± 0 . 2 0 . 2 2 5 ± 0 . 3 M - 1 0 x 1 . 2 5 N M 2 S U R F A C E S K 1 2 . 7 ± 0 . 1 H 2 5 ± 0 . 3 2 X 6 ± 0 . 2 5 S 1 7 ± 0 . 0 2 D a t u m p l a n e s e s t a b l i s h e d f r o m p l a n a r d a t u m f e a t u r e s 6.16

C R 1 5 . 9 ± 0 . 2 5 H 7 ( 5 / 5 . 0 1 2 ) 2 5 ± 0 . 1 M 1 0 x 1 . 2 5 E F A 1 2 . 7 ± 0 . 1 B 2 5 ± 0 . 1 2 X 6 ± 0 . 2 5 S R 8 . 5 0 . 2 M 0 . 4 D G Theory Physical Simulated datum axis E is the axis of a collet, chuck, CMM etc. Datum axis E is the axis of the theoretical datum feature simulator Datum Features of Size 6.18

C R 1 5 . 9 ± 0 . 2 5 H 7 ( 5 / 5 . 0 1 2 ) 2 5 ± 0 . 1 M 1 0 x 1 . 2 5 E F A 1 2 . 7 ± 0 . 1 B 2 5 ± 0 . 1 2 X 6 ± 0 . 2 5 S R 8 . 5 0 . 2 M 0 . 4 D G Datum Features of Size 6.19

C R 1 5 . 9 ± 0 . 2 5 H 7 ( 5 / 5 . 0 1 2 ) 2 5 ± 0 . 1 M 1 0 x 1 . 2 5 E F A 1 2 . 7 ± 0 . 1 B 2 5 ± 0 . 1 2 X 6 ± 0 . 2 5 S R 8 . 5 0 . 2 M 0 . 4 D G Theory Physical D a t u m c e n t e r p l a n e B i s t h e c e n t e r p l a n e o f t h e S i m u l a t e d d a t u m c e n t e r p l a n e B i s d a t u m f e a t u r e s i m u l a t o r . t h e c e n t e r p l a n e o f t h e g a g e b l o c k . Datum Features of Size 6.19

C R 1 5 . 9 ± 0 . 2 5 H 7 ( 5 / 5 . 0 1 2 ) 2 5 ± 0 . 1 M 1 0 x 1 . 2 5 E F A 1 2 . 7 ± 0 . 1 B 2 5 ± 0 . 1 2 X 6 ± 0 . 2 5 S R 8 . 5 0 . 2 M 0 . 4 D G Theory Physical Datum axis C is the axis of the datum feature simulator Simulated datum axis C is the axis of the gage pin Datum Features of Size 6.20

C R 1 5 . 9 ± 0 . 2 5 H 7 ( 5 / 5 . 0 1 2 ) 2 5 ± 0 . 1 M 1 0 x 1 . 2 5 E F A 1 2 . 7 ± 0 . 1 B 2 5 ± 0 . 1 2 X 6 ± 0 . 2 5 S R 8 . 5 0 . 2 M 0 . 4 D G Theory Simulated datum feature D axis and two intersecting planes established from gage Physical Datum D axis and two planes are established from a pattern of two holes Datum Features of Size 6.20

Datum Feature Symbol Placement is Important 6.21

Evaluation specified dimension , orientation planes, axis, center lines, edges, points, areas no three 90 circle X size X X tolerance X X convey rocking X

Geometric Dimensioning and Tolerancing