ENGINEERING GRAPHICS

2110013

DIMENSIONING, SCALES, LINES AND MULTIPLE PROJECTIONS

Dimensions A dimension is for size and position (of the designed/modeled

shape).

A DIMENSION is a numerical value expressed in appropriate units of measurement and used to define the size, location, orientation, form or other geometric characteristics of a part.

A method of communication to machinists in the Production facility.

Different kinds:

– Linear

– Aligned

– Angular

– Radius/Diameter

– Reference

Dimensions The purpose of dimensioning is to provide a clear and complete

description of an object. 

A complete set of dimensions will permit only one interpretation needed to construct the part.

Dimensioning should follow these guidelines. 

Accuracy:   correct values must be given. 

Clearness:    dimensions must be placed in appropriate positions. 

Completeness:  nothing must be left out, and nothing duplicated. 

Readability:   the appropriate line quality must be used for legibility.

The first dimension line should be approximately 12 mm (0.6 in) from the object. Extension lines begin 1.5 mm from the object and extend  3 mm from the last dimension line.  

A leader is a thin line, used to connect a dimension with a particular area (see the hole  diameter arrow shown here.)    A leader may also be used to indicate a note or comment about a specific area.

When there is limited space,   a heavy black dot may be substituted for the arrows,  again clarity is the rule -  it should be a clear point into which the dimension lines ends.

Also in this drawing, shown here, two holes on the main surface of the flat panel are identical, allowing the "2x" notation to be used and the dimension to point to only one of the circles.  This is more preferable than cluttering the space with duplicate information.

Types of Dimensioning To show dimensions of all parts of drawing

is mandatory for engineer.

All lines in drawing would have length, width or radius to indicate.

The use of proper dimensioning method will tends to full marks for correct drawing.

Now let us discuss methods of dimensioning in brief.

There are two Methods of Dimensions. (1) Uni-directional System  (2) Aligned System.

1) Uni-directional System 

In Unidirectional Method of  Dimensioning the dimension line should be cut at center and dimensions should be placed in the middle of dimension lines as shown into the figure.

2) Aligned SystemIn Aligned Method of Dimensioning the dimension line should be continuous and dimensions should be placed in the middle of dimension lines as shown into the figure.

Line styles and types A variety of line styles graphically represent

physical objects. Types of lines include the following:

visible – are continuous lines used to depict edges directly visible from a particular angle.

hidden – are short-dashed lines that may be used to represent edges that are not directly visible.

Line styles and typescenter – are alternately long- and short-dashed lines that may be used to represent the axes of circular features.

cutting plane – are thin, medium-dashed lines, or thick alternately long- and double short-dashed that may be used to define sections for section views.

section – are thin lines in a pattern (pattern determined by the material being "cut" or "sectioned") used to indicate surfaces in section views resulting from "cutting." Section lines are commonly referred to as "cross-hatching."

Line styles and types

Scales A scale is defined as the ratio of the linear dimensions

of the object as  represented in a drawing to the actual dimensions of the same.

There is a wide variation in sizes for engineering objects. Some are very large (eg. Aero planes, rockets, etc) Some are vey small ( wrist watch, MEMs components) There is a need to reduce or enlarge while drawing the objects on paper. Some objects can be drawn to their actual size. The proportion by which the drawing of aan object is enlarged or reduced is called the scale of the drawing.

Scales

Scales Types of Scale :-

Engineers Scale :  The relation between the dimension on the drawing and the actual dimension of the object is mentioned numerically  (like 10 mm = 15 m).

Graphical Scale: Scale is drawn on the drawing itself. This takes care of the shrinkage of the engineer’s scale when the drawing becomes old.

Scales

Types of Graphical Scale :-Plain Scale

Diagonal Scale

Vernier Scale

Comparative scale

Scale of chords

Scales

Plain Scale Diagonal Scale

Multiple views and projections The orthographic projection shows the

object as it looks from the front, right, left, top, bottom, or back, and are typically positioned relative to each other according to the rules of either first-angle or third-angle projection. The origin and vector direction of the projectors (also called projection lines) differs, as explained below.

Multiple views and projections In first-angle projection, the projectors originate as if

radiated from a viewer's eyeballs and shoot through the 3D object to project a 2D image onto the plane behind it. The 3D object is projected into 2D "paper" space as if you were looking at a radiograph of the object: the top view is under the front view, the right view is at the left of the front view.

In third-angle projection, the projectors originate as if radiated from the 3D object itself and shoot away from the 3D object to project a 2D image onto the plane in front of it. The views of the 3D object are like the panels of a box that envelopes the object, and the panels pivot as they open up flat into the plane of the drawing. Thus the left view is placed on the left and the top view on the top; and the features closest to the front of the 3D object will appear closest to the front view in the drawing. 

Prepared by …. Class FX students Akash Ambaliya_140030119003 Vivek Aghara_140030119001

Thank You…