4

Dimensioning

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Objectives

Review and understand the standard dimensioning practices

for mechanical drawings.

Apply the standard dimensioning practices.

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1 Dimensioning

Geometrics:

It is the science of specifying and tolerancing the shapes and

locations of features on objects.

Once the shape of a part is defined with a set of orthographic

drawings, the size information is added in the form of dimen-

sions. Dimensioning a drawing also identifies the tolerance (or

accuracy) required for each dimension.

2 Size and Location Dimensions

A well-dimensioned part or structure communicates the size

and location requirements for each feature

Designs are dimensioned based on two criteria:

1. Basic sizes and locations of features.

2. Details for construction and for manufacturing.

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2.1 Units of measure

Most countries outside of the United States use the metric sys-

tem of measure, or the international system of units (SI), which

is based on the meter.

The common metric unit of measure on engineering drawings is

the millimetre, abbreviated as .

Angular dimensions are shown either in decimal degrees or in

degrees, minutes, and seconds. The symbol used for degrees

is °, for minutes ', and for seconds ". Where only minutes and

seconds are specified, the number of minutes or seconds is

preceded by the 0°.

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2.2 Dimensioning terminology

There are a number of terms relevant and important to dimen-

sioning practices (refer to drawing).

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Dimension - Numerical value that defines the size, shape, location, surface texture, or geometric charac-teristic of a feature. Normally, dimension text is

or high, and the space between

lines of text is or

Basic Di-mension

- A numerical value defining the theoretically ex-act size, location, profile, or orientation of a feature relative to a coordinate system.

Basic dimensions have no tolerance.

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Reference Dimension

- A numerical value enclosed in parentheses, provided for information only and not directly used in the fabrication of the part.

It is a calculated size without a tolerance used to show the intended design size of a part.

Arrows - Symbols placed at the ends of:

dimension lines to show the limits of the di-mension

leader lines (leaders)

cutting plane lines Uniform in size and style, regardless of the size of the drawing.

Arrow heads are usually about long and should be one-third as wide as they are long.

Visible gap

- There should be a visible gap of between the feature’s corners and the end of the exten-sion lines.

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Leader line

- Thin, solid line used to indicate the feature with which a dimension, note, or symbol is associ-ated.

A leader line is generally a straight line drawn at an angle that is neither horizontal nor verti-cal.

Leader lines are terminated with an arrow touching the part or detail. On the opposite end of the arrow, the leader line will have a short, horizontal shoulder ( long). Text is ex-tended from this shoulder such that the text height is centered with the shoulder line.

Adjacent leaders on a drawing should be drawn parallel to each other.

Tolerance - The amount that a particular dimension is al-lowed to vary. All dimensions (except reference dimensions) have an associated tolerance.

The tolerance is the difference between the maximum and minimum limits.

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2.3 Size and location dimensions

A size dimension might be:

(a) the overall width of a part or structure

(b) the diameter of a drilled hole

A location dimension might be the length from the edge of an

object to the centre of a feature. The basic criterion is,

“What information is necessary to manufacture or construct the object?”

The drawings below give examples of horizontal, vertical, di-

ameter and radius dimensions.

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2.4 Standard practices

The guiding principle for dimensioning a drawing is clarity.

2.4.1 Placement

2.4.2 Spacing

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2.4.3 Grouping & Staggering

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2.4.4 Extension Lines

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2.4.5 Reading Direction

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2.4.6 View Dimensioning

2.4.7 Repetitive Features

Use the symbol to dimension repetitive features.

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3 Detail Dimensioning

The drawing that follows gives examples of how to correctly

and incorrectly dimension a hole.

If it is not clear whether a hole extends completely through a

part, the word THRU can follow the numerical value.

Symbols are used for spotface, counterbored, and countersunk

holes. These symbols must always precede the diameter sym-

bol.

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3.1 Symbols for Drilling Operations

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3.2 Dimensioning a Blind Hole

The depth symbol is used to indicate the depth of a hole. The

depth symbol precedes the numerical value. When the depth of

a blind hole is specified, the depth is to the full diameter of the

hole and not to the point.

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3.3 Diameter v/s Radius

If a full circle or an arc of more than half a circle is being di-

mensioned, the diameter is specified, preceded by the diameter

symbol. If the arc is less than half a circle, the radius is speci-

fied, preceded by an R.

Concentric circles are dimensioned in the longitudinal view

whenever practical.