Technological Design: February 2010

[edit] Engineering drawings: common features
Drawings convey the following critical information:
Geometry – the shape of the object; represented as views; how the object will look when it is viewed from various standard directions, such as front, top, side, etc.
Dimensions – the size of the object is captured in accepted units.
tolerances – the allowable variations for each dimension.
Material – represents what the item is made of.
Finish – specifies the surface quality of the item, functional or cosmetic. For example, a mass-marketed product usually requires a much higher surface quality than, say, a component that goes inside industrial machinery.

Line styles and types

Standard engineering drawing line 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.
center – 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."
Lines can also be classified by a letter classification in which each line is given a letter.
Type A lines show the outline of the feature of an object. They are the thickest lines on a drawing and done with a pencil softer than HB.
Type B lines are dimension lines and are used for dimensioning, projecting, extending, or leaders. A harder pencil should be used, such as a 2H.
Type C lines are used for breaks when the whole object is not shown. They are freehand drawn and only for short breaks. 2H pencil
Type D lines are similar to Type C, except they are zigzagged and only for longer breaks. 2H pencil
Type E lines indicate hidden outlines of internal features of an object. They are dotted lines. 2H pencil
Type F lines are Type F[typo] lines, except they are used for drawings in electrotechnology. 2H pencil
Type G lines are used for centre lines. They are dotted lines, but a long line of 10–20 mm, then a gap, then a small line of 2 mm. 2H pencil
Type H lines are the same as Type G, except that every second long line is thicker. They indicate the cutting plane of an object. 2H pencil
Type K lines indicate the alternate positions of an object and the line taken by that object. They are drawn with a long line of 10–20 mm, then a small gap, then a small line of 2 mm, then a gap, then another small line. 2H pencil.

[edit] Orthographic projection
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.
First angle projection is the ISO standard and is primarily used in Europe. The 3D object is projected into 2D "paper" space as if you were looking at an X-ray of the object: the top view is under the front view, the right view is at the left of the front view.
Third angle projection is primarily used in the United States and Canada, where it is the default projection system according to BS 8888:2006, the left view is placed on the left and the top view on the top.
Not all views are necessarily used, and determination of what surface constitutes the front, back, top and bottom varies depending on the projection used.

[edit] Isometric projection
The isometric projection show the object from angles in which the scales along each axis of the object are equal. Isometric projection corresponds to rotation of the object by ± 45° about the vertical axis, followed by rotation of approximately ± 35.264° [= arcsin(tan(30°))] about the horizontal axis starting from an orthographic projection view. "Isometric" comes from the Greek for "same measure." One of the things that makes isometric drawings so attractive is the ease with which 60 degree angles can be constructed with only a compass and straightedge.
Isometric projection is a type of axonometric projection. The other two types of axonometric projection are:
Dimetric projection
Trimetric projection
[edit] Oblique projection
An oblique projection is a simple type of graphical projection used for producing pictorial, two-dimensional images of three-dimensional objects:
it projects an image by intersecting parallel rays (projectors)
from the three-dimensional source object with the drawing surface (projection plan).
In both oblique projection and orthographic projection, parallel lines of the source object produce parallel lines in the projected image.
[edit] Perspective
Perspective is an approximate representation on a flat surface, of an image as it is perceived by the eye. The two most characteristic features of perspective are that objects are drawn:
Smaller as their distance from the observer increases
Foreshortened: the size of an object's dimensions along the line of sight are relatively shorter than dimensions across the line of sight.