1. Geometric Dimensioning and Tolerancing
Unit 6 – Datums

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

3. 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.

4. Datum Feature Symbol
Datum Feature Symbol

5. 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

6. 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

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

8. 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

9. 1.9 Features with and without Size
If you remember earlier in unit 1 we talked about features of size and features without size. Any feature can be a datum feature, so datum features can be of size or without size.
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10. Establishing Datums from Primary Plane Surfaces 6.15 D a t u m f e r s. 2 M - 1 x 5 H 7 ( / ) 9 3 6 X L D a t u m f e r s H n d L w i h o c l p g . T y
Since datums are mandatory lets look at how datums are called out and how datums are established from datum features. Remember datum features are the actual part features and datums are the theoretical perfect. The following slides show how to get from the datum feature to the datum. (point, line or plane)
The next few slides will first deal with datum features without size (Basically plane surface datums, also can not have datum modifiers applied). Afterwards, we will cover datum features with size (These features may have datum modifiers applied).
Notice the datum feature symbol H applied to the surface it will establish a plane. Notice the theory and physical are both shown. 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 D a t S u m U R F A C E T o o l
& g a g e f e a t T u E r e s i m u l a t o r 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 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 d a t u m P l a n e M a g n i f i e d V i e w

11. Datum Features without SizeP L 1 5 . 9 . 2 5 H 7 ( 5 / 5 . 1 2 ) . 2 2 5 . 3 M - 1 x 1 . 2 5 N M K 2 S U R F A C E S 1 2 . 7 . 1 H 2 5 . 3 2 X 6 . 2 5 S 1 7 . 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 D a t u m f e r H s b - l i h p n o g c . D a t u m f e r N s b - l i h p n o g c . D a t u m f e r L s b - l i h p n o g c .
Click thru the next few slides to show the other plane surface datums identified on the drawing. Pay particular attention to how the datum feature symbols are applied, sideways, up, down, on a feature control frame etc. The students will have to properly apply datum feature symbols in a later exercise.
Notice datum K, to be clear it is identified as 2 surfaces.
Notice datum L it has a flatness specified, it is not mandatory but very common because this is usually where the part mounts and we want a stable mount. D a t u m f e r M s b - l i h p n o g d . D a t u m f e r K s b l i h p n o g w c .
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12. Datum Features without SizeP L 1 5 . 9 . 2 5 H 7 ( 5 / 5 . 1 2 ) . 2 2 5 . 3 M - 1 x 1 . 2 5 N M K 2 S U R F A C E S 1 2 . 7 . 1 H 2 5 . 3 2 X 6 . 2 5 S 1 7 . 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 D a t u m f e r H s b - l i h p n o g c . D a t u m f e r N s b - l i h p n o g c . D a t u m f e r L s b - l i h p n o g c . D a t u m f e r M s b - l i h p n o g d . D a t u m f e r K s b l i h p n o g w c .
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13. Datum Features without SizeP L 1 5 . 9 . 2 5 H 7 ( 5 / 5 . 1 2 ) . 2 2 5 . 3 M - 1 x 1 . 2 5 N M K 2 S U R F A C E S 1 2 . 7 . 1 H 2 5 . 3 2 X 6 . 2 5 S 1 7 . 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 D a t u m f e r H s b - l i h p n o g c . D a t u m f e r N s b - l i h p n o g c . D a t u m f e r L s b - l i h p n o g c . D a t u m f e r M s b - l i h p n o g d . D a t u m f e r K s b l i h p n o g w c .
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14. Datum Features without SizeP L 1 5 . 9 . 2 5 H 7 ( 5 / 5 . 1 2 ) . 2 2 5 . 3 M - 1 x 1 . 2 5 N M K 2 S U R F A C E S 1 2 . 7 . 1 H 2 5 . 3 2 X 6 . 2 5 S 1 7 . 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 D a t u m f e r H s b - l i h p n o g c . D a t u m f e r N s b - l i h p n o g c . D a t u m f e r L s b - l i h p n o g c . D a t u m f e r M s b - l i h p n o g d . D a t u m f e r K s b l i h p n o g w c .
6.16

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

16. Datum Features of Size Theory Physical 6.18. 2 D M B 6 5 X 1 7 S R 8 C F A E x H ( / ) 9 4 G
Datum axis E is the axis of the theoretical datum feature simulator
Theory
Physical
Simulated datum axis E is the axis of a collet, chuck, CMM etc.
This slide shows in theory how an axis is established from an imperfect feature.
The left part of this slide shows in practical how an axis is established from an imperfect feature. Notice now how important is was to learn all the family of datum terms.
The designer works with the datum (perfect axis) and the best the manufacturing or inspection can do is with the practical simulated datum. There is small mis-match there and error has to do with the accuracy of the inspection equipment and the fixturing and tooling. Once we know the accuracy of the equipment and tooling we can quantify the differences and factor that into our measurements.
6.18

17. Datum Features of Size 6.19 . 2 D B 6 ± 5 X 1 7 C F A E x H ( / ) 9. 2 D M B 6 5 X 1 7 S R 8 C F A E x H ( / ) 9 4 G
This is an example of a slab type feature or a slot identified as a datum feature. The slot is the datum feature. What is the datum derived from the is feature, point line or plane? How is it derived. The next few slides will illustrate the principles.
6.19

18. Datum Features of Size Physical Theory 6.19 S i m u l a t e d c n r p. 2 D M B 6 5 X 1 7 S R 8 C F A E x H ( / ) 9 4 G S i m u l a t e d c n r p B s h o f g b k .
Physical D a t u m c e n r p l B i s h o f d .
Theory
Example from the book showing the datum plane and the theoretical datum feature simulator of a slot, 2 parallel planes at maximum separation.
Example from the book showing the physical datum plane and the simulated datum, gage block. The largest gage block is the simulator and the centerplane of the gage block is the simulated datum.
6.19

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

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

21. Datum Feature Symbol Placement is Important
This is a good time to remind the students of the importance of the placement of the datum feature symbols. Click thru the following examples and explain the differences in the application of datum feature symbols to plane surfaces and features of size.
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22. Datum Feature Symbol Placement is Important
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23. Datum Feature Symbol Placement is Important
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24. Datum Feature Symbol Placement is Important
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25. Datum Feature Symbol Placement is Important
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26. Datum Feature Symbol Placement is Important
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27. Datum Feature Symbol Placement is Important
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28. Datum Feature Symbol Placement is Important
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29. Datum Feature Symbol Placement is Important
This slide shows the application of partial datums. These are used by the designer when only a portion of the feature is to be used rather than the entire feature. In some case it may be necessary to hatch the partial datum as in the top example and in other cases it may not be necessary.
6.21