TECH 104 – Technical Graphics Communication Week 7: Dimensioning.

TECH 104 – Technical Graphics Communication

Week 7:

Dimensioning


Week 7: Dimensioning

Here’s what we talked about last time…..



“Half” sections show the interior of an object where the Cutting Plane has penetrated only half way through the object.

NOTE:

The cutting plane line may only have one arrow on it.



“Revolved” section views.

“Removed” section views.



“Offset” section views.

“Assembled” section views.



Drawing styles may sometimes be combined with one another. Here we see an auxiliary and section view combined.



Examples of sectioning conventions...

…more examples…



…More examples of sectioning conventions.



Let’s now take a look at how dimensions are used to help describe the object we want to manufacture…..


Measurement is used to tell us:

* How tall?

* How heavy?

* How many?

* How much?



Standards of Measurement:

1. Inch System2. Metric (ISO)3. Military (diverse)4. Associations (also diverse)



Dimensions are applied to objects in a variety of styles:

1. Chaini. Alignedii. Unidirectional

2. Tabular

3. Coordinate/Baseline

4. Ordinate



Dimension elements….


Dimensions are used to show an object’s:

1. Overall: Width

Depth

Height

2. The actual size of features (rounds,

fillets, holes, arcs, etc.)




3. And where features are located such as centers, angles, etc.



The proper placement of dimensions is critical to ensure that the part can be read and manufactured to specifications.

Here are some practices that need to be followed when applying dimensions to an object:



Dimensions should be stacked in a “broken chain” format to aid in the readability of the plate.

“Breaking the Chain” refers to leaving out one dimension as shown above so that manufacturing tolerances are maintained.

1.



Stacked dimensions that show position and size.



Do not place dimensions directly on the object unless it is unavoidable.

2.



As a general rule…Stay off the object as much as possible.



Extension lines can be shared and even broken to clarify crowded dimensions.

3.



Some features are dimensioned from their center lines.

The center line may also be used as an extension line.

4.


Dimensions may be laid out in different configurations.

Unidirectional dimensioning is the current standard in most industrial applications today.


5.



Leaders with dimensions are used to show negative cylinders (holes).

The leader should always be placed to penetrate the center of all round features.

6.


Features such as counterbores, countersinks and spot faces are all dimensioned using a leader.

Note: Each of these features has a special dimensioning symbol that can be used to show:

a. Diameterb. Shapec. Depth




Here we see several examples where notes are used instead of symbols to dimension features.

Notes that describe a specific feature are known as “local notes”.

“General notes” are used to describe a characteristic that effects the entire part, i. e. materials, production instructions, etc.



Arcs are always dimensioned as a radius. Full circles are dimensioned showing their diameter value.

7.



When dimensioning a part, always start with the inner-most dimensions and work to the outer-most values.

Remember:

Dimensions are used to show both the size and location of features.

8.



Always dimension features and not lines…..and remember….

NEVER, NEVER, NEVER dimension to hidden lines!

9.



Here are some rules (See Table. 15.1) on how to dimension properly:

1. The overall Width, Height, and Depthmust be shown on the object.

2. Each feature should be dimensioned in the view where it appears true shape and size. Never dimension a feature where it appears only as a line.

3. NEVER dimension to hidden lines!!!



4. The measurement standards must be maintained and clearly noted on the print.

5. Dimension lines should never cross.

6. Leaders must point at an angle that allows them to penetrate the centre of the object which they are describing.

7. Chains MUST be broken to allow for tolerance variation in the part.



8. Dimensions should be placed to allow order to the print and promote ease of reading.

9. Negative cylinders (holes) must always be measured by their diameter.

10. Arcs are always shown by specifying a radius value.

11. When laying out centres, specify one common reference point for the X and Y axes.



12. Dual dimensions may be used, but they must be consistent and clearly noted.

13. Angles may be dimensioned by showing their degree(s), or their 3 point location.

14. Only dimension each feature once!!!



Week 8:

Mid-Term Exam

Date post:	13-Dec-2015
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TECH 104 – Technical Graphics Communication Week 7: Dimensioning.

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