This page contains information to
help new users understand
the basics of computer aided drafting
(CAD).
Note: In these answers we will follow a few shorthand conventions for describing user-interface
procedures.
Key combinations will be presented like this: Ctrl+Alt+Delete, which means that you
should press and hold down the
Control key, the Alt key, and the Delete key at the same time.
Menu selections will be presented like this: File->Open,
which means that you should open the File
menu, and then make the Open selection.
- Fundamentals
- Vector versus Raster
- Draw each object once
- Geometric Elements
- Using_a_Grid
- The Coordinate System
- Layers
- Drawing in 2D
- Modeling_in_3D
-
A "paint"
program lets you manipulate each pixel in an array of pixels that make up
an image. This is a Raster based approach. If the overall picture shows a
car and a building, there is no logical distinction between them - its
just an array of colored dots you can manipulate with painting tools.
-
A "draw"
program goes a step further - it is composed of separate entities or
objects, such as circles, lines, etc. This is a Vector based program. It
may provide facilities to group these into "car" and "building", but the
final result is still an image described in terms of its appearance on
paper.
-
A CAD program
introduces the concept of real-world measurement. A car or building can be
drawn as if it were life-size, and later arranged into sheets and printed
on paper at any desired scale. Valid measurements may be taken from the
drawing and they should properly correspond to the real world object, if
the drawing was created correctly.
- CAD systems draw or model from a set of elementary geometric
elements or primitives.
- In a two-dimensional program, these primitives are the point,
line and curve, and also other entities like text and
hatching.
- You can draw polygons with any number of vertexes, or figures
like circles arcs, or ellipses.
- Connected lines, or polylines can be open or closed.
- In three-dimensional programs, the primitives can be surfaces
(planes and grids) or solids. In surface modeling, a
three-dimensional object, like a cube for example, is composed
of 6 faces. In solid modeling, a cube is a single primitive that
can be combined with other primitives by means of Boolean
operations (e.g. add, slice, intersection, subtraction) to
create more complex solids.
- Three-dimensional solid objects can be created from 2d
entities, by means of SWEEPING and EXTRUDING operations.
- For example, a rectangle can be extruded along a
linear path to generate a rectangular box.
- Extrusions can also follow a line or curve and vary in
scale.
- Other objects can be created by rotating a surface around
an axis. For example, rotating a rectangle around one of its
edges produces a cylinder. This operation is known as rotational
sweep.
- A 6 copy sweep will produce a hex, such as a bolt head
- Sweep can be used to create spirals and screw
threads by adding offset values.
- A surface object can be created as an extrusion of an open
shape (not a plane), for example, some connected lines. A plane,
however, will always produce a solid object.
- As you work, you can turn the Display Grid and Snap Grid
on and off, and you can change the grid and snap spacing.
- The Display Grid is a pattern of lines or dots that
extends over the area you specify as the drawing limits. Using the grid is
similar to placing a sheet of grid paper under a drawing. The grid helps you
align objects and visualize the distances between them. The grid is not
plotted. If you zoom in or out of your drawing, you may need to adjust grid
spacing to be more appropriate for the new magnification.
- The Snap Grid restricts the movement of the crosshairs to
intervals that you define. When Snap Grid is on, the cursor seems to adhere,
or "snap," to an invisible grid. Snap is useful for specifying precise points
with the arrow keys or the pointing device. You control snap precision by
setting the X and Y spacing.
- Snap spacing does not have to match grid spacing. For
example, you might set a wide grid spacing to be used as a reference but
maintain a closer snap spacing for accuracy in specifying points.
-
The
coordinate system is a method of locating points in the drawing area.
It
enables you to locate points by specifying distances from a fixed
reference
point. You can locate a point by giving its distance in the
horizontal direction,
vertical direction, measuring along an angle, etc.
-
The
coordinate system is available when a function requires data input in the
form of point locations. You may use it while drawing, editing or any time
you
need to locate a point. The most common coordinate systems are as
follows:
-
Cartesian
Coordinates
-
Polar
coordinates
Cartesian
Coordinates
-
Cartesian
coordinates is a rectangular system of measurement that enables
you to
locate points with the help of horizontal and vertical coordinates.
-
The
horizontal values, called X-coordinates,
are measured along the X-axis.
-
The
vertical values, called Y-coordinates,
are measured along the Y-axis.
-
The intersection of the X- and Y-axes is called the origin point,
which
represents the 0,0 location of the coordinate system.
-
The
positive X values are measured to the right and the positive Y values
are
measured above the origin point.
-
The negative X and Y values are measured
to the left and below.
-
To enter a coordinate, you need to enter both the X
and Y values
separated by a comma (X, Y).
-
Example:
To locate a point two units to the right and one unit
above the origin
point, enter X = 2, Y = 1 or (2,1). Similarly,
to locate a point two
units to the left and one unit below,
you need to enter X = -2, Y = -1
or (-2, -1).
-
You can use fractions as needed to enter the
coordinate values.
Polar
Coordinates
The
Format to Enter Coordinates
-
Absolute
format is a way of measuring distances from a fixed reference
location
(origin point), which is the 0,0 location of the coordinate system.
Consider this point to be stationary at all times. In some CAD programs
this point remains visible at the left bottom corner of the drawing area,
while in others it is invisible. You can use this point as a reference to
measure any distance in the drawing. Absolute coordinates are primarily
used to adjust the alignment of diagrams in a drawing, to align one
drawing with another or to make plotting adjustments.
-
Relative
format is a way of measuring distances from the last point
entered. All
measurements are taken the same way as the absolute
coordinates, with the
only difference being that the relative coordinates
are measured from the last point entered instead of the origin point.
When a point is entered,
it becomes the reference for entering the next
point and so on. This mode
of measurement is frequently used for drawing
because it is always
convenient to place the drawing components relative
to each other rather
than a fixed reference point.
-
Polar format
is a way of measuring Distance and Angle from from the last point
entered.
When a point is entered,
it becomes the reference for entering the next
point and so on. This mode
of measurement is frequently used for drawing site maps
because it allows using not only geometric angles, but Bearing
entries as well. IE: N 69°54'05" E 35
-
In
CAD programs are the equivalent of aligned transparent overlays. Each
layer can be displayed and printed individually or in combinations. A CAD
file of a complete project may contain 50 or more layers, with each layer
containing specific details for things such as existing grades, proposed
grades, building details, etc. Because of this, CAD files can be quite
large.
-
General naming
guidelines:
There
are some standards for naming CAD layers, known as 'Layer Naming
Conventions'
Some are as follows:
- Designing in 3D is far easier than 2D because 3D design
methods tie in with peoples' natural visualization. This improves the quality
of the design immensely, and thus allows projects to be concluded more quickly
and cheaply. It is one of many powerful benefits that 3D brings.
- Views do not have to be re-drawn to achieve the traditional
2D projections. 2D views are derived from looking at the 3D model from its
top, sides or ends. Once a model exists, the draftsman can create as many
views as he would like and each projection is "free."
- Solid Shaded views can co-exist with Wireframe views.
- Views can be created at any time during, or at the end
of, the design phase.
- 3D does not automatically mean working with solid modeling.
3D means working in 3 dimensions.
- This includes
- Wireframe methods.
- Surfaces methods.
- Solid Modeling.
- As awareness of 3D grows, a widespread mis-belief is that
the CAD system will need changing, and that any new system must have all the
latest 3D capabilities such as parametrics. This view is unfortunate for two
reasons. Firstly, the most significant single benefit comes from simply
working in 3D - the functionality of latest releases merely adds convenience.
Secondly, changing CAD systems is hugely inconvenient.
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