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Dimensioning & Projections. Dimensions. Dimensions are used to describe the sizes and relationships between features in your drawing. Dimensions are used to manufacture parts and to inspect the resulting parts to determine if they are acceptable.

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Dimensions l.jpg
Dimensions

  • Dimensions are used to describe the sizes and relationships between features in your drawing.

  • Dimensions are used to manufacture parts and to inspect the resulting parts to determine if they are acceptable.

  • Drawings with dimensions and notes often serve as construction documents and legal contracts.

  • ANSI Y14.5M-1994 is the current standard. Other standards may apply.


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Standards for Your Career Field

  • Standards are different in different career areas.

  • Most of the examples in this lecture will be of mechanical parts.

  • Civil, Electrical, Construction, and other areas follow similar practices, but sometimes with less need for precision in measurements.

  • Dimensioned drawings are a part of a contractual document.


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Vocabulary

Dimension line, Extension line, Leader, Dimension offset or gap, Centerline,

Finish mark, Dimension value

Baseline dimensioning, Chained dimensioning


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Tolerance

Definition: The total allowable variation an acceptable part can have from the specified dimension.

The less variation allowed, the more the part will cost to make.


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3 Things for Good Dimensioning

  • Good technique of dimensioning

  • Good choice of dimensions

  • Good placement of dimensions


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Dimensioning Technique

  • describes how the dimensions in your drawing should look.

  • defined by various standards like ANSI Y14.5-1994.

  • help you create dimensions that are plainly visible and can be easily interpreted.

  • specifies sizes for creating dimensions relative to the paper size of your final plot.


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arrowheads

extension lines

dimension lines

sizes

use thin, dark, lines (HB lead)

make object stand out clearly!

Dimensioning Technique





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Orientation forDimension Values

  • Unidirectional:

    • read horizontally from bottom of sheet

  • Aligned:

    • align with dimension line and read from bottom or right side of sheet







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Choice of Dimensions

  • the dimensions you specify determine the way the the part is manufactured and the way the tolerance is applied

  • consider the purpose of the part and its function in the assembly

  • consider how easy it will be to check the measurement on the actual part

  • fully dimension each part

  • do not overdimension, each dimensions should appear only once


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Don’t over dimension

Give the diameter of circular shapes, the radius of arcs.

No redundant or superfluous dimensions

Give size dimensions for features.

Give location dimensions to show how features relate to one another.

Choosing Which Dimensions to Show



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Baseline

each dimension is specified from a common baseline

tolerances do not stack

Chained

each dimension continues from the previous one

tolerances stack

Effect of Tolerance on Dimensioning


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Units

  • Fractional Inch

  • Decimal inch dimensions are typically specified to 2 decimal places.

  • Metric values are typically given in whole millimeters or to one decimal place.


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Placement of Dimensions

  • Rules-of-thumb for dimension placement help ensure that others will be able to interpret your drawing

  • Where placement practices conflict, remember that your goal is to clearly communicate the purpose of the drawing. Use the practice you feel will make the drawing easy to understand.


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Avoid dimensioning on object (face of part).

Avoid dimensioning to hidden lines.

Place dimensions between views when possible.

Don’t “float” dimensions.

Place dimensions where feature shows shape.

Give overall dimensions where possible.

Group dimensions around a central view.

Placement Practices








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Summary

  • Good dimensioning is a combination of choosing dimensions which reflect your design intent, proper technique in creating the details of the dimension line, extension line, arrowheads and dimension values, and placing the dimensions on the drawing so that they can be read clearly.

  • Dimensioning drawings correctly can be as important or more important than drawing the shapes correctly.

  • Good dimensioning requires practice and thought!



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Orthographic Projection

a system of drawing views of an object using perpendicular projectors from the object to a plane of projection




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The Glass Box

  • Imagine that the object you are going to draw is positioned inside a glass box, so that the large flat surfaces of the object are parallel to the walls of the box.

  • From each pointon the object, imagine a ray, or projector perpendicular to the wall of the box forming the view of the object on that wall or projection plane.




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Unfolding the Glass box

  • For Third Angle Projection (the method in the U.S.)

  • Imagine that the walls of the box are hinged and unfold the views outward around the front view.

  • This will give you the standard arrangement of views for 3rd Angle Projection which is used in the US, Canada, and some other countries.



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The Standard Arrangement of Views

TOP

LEFT FRONT RIGHT

BOTTOM

REAR

Why must views be arranged so that they align?

To make it possible for someone to interpret the drawing.



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Using a Miter Line to Transfer Depth

1.

Draw miter line at 45 degrees

at a convenient distance to produce the desired view.


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Sketch light lines projecting depth locations for points to miter line and then down into side view as shown.

2.


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Project additional points, surface by surface. miter line and then down into side view as shown.

3.


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4. miter line and then down into side view as shown.

Draw the view locating each vertex of the surface on the projection and miter line.


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Necessary Views miter line and then down into side view as shown.

A sketch or drawing should only contain the views needed to clearly and completely describe the object. Choose the views that show the shape most clearly, have the fewest hidden lines, and show the object in a usual, stable, or operating position.

One view drawing

of a shim

One view drawing

of a connecting rod


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Position of Side Views miter line and then down into side view as shown.

An alternative postion for the side view is

rotated and aligned with the top view.


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First Angle Projection miter line and then down into side view as shown.


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Symbols for 1st & 3rd Angle Projection miter line and then down into side view as shown.

Third angle projection is used

in the U.S., and Canada


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Summary miter line and then down into side view as shown.

  • The six standard views are often thought of as produced from an unfolded glass box.

  • Distances can be transferred or projected from one view to another.

  • Only the views necessary to fully describe the object should be drawn.


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