| This chapter describes the general commands used in AutoCAD for drawing
most graphic entities. As has been explained previously, entities are the
basic building blocks of all AutoCAD drawings. Once drawn, entities can
be manipulated and changed in various ways to suit the needs of the draftsman
or designer. Subsequent chapters will explain these processes.
These notes concentrate on describing the function of these commands
as well as the options available. There is little attempt to explain how
to use the commands. Generally, once AutoCAD's prompting methods (as described
in the last chapter) and the function of each command is understood, the
operation of each of these commands is fairly self-evident. The tutorial
notes and exercises are intended to supplement these notes and help with
their application. In addition, there are extensive help facilities in
AutoCAD (accessed by the command HELP) that can be explored in order to
discover more of its features. During the operation of any command, you
are able to get help on that specific command by typing 'HELP (the apostrophe
is part of the syntax - it means the command is "transparent" and can be
executed within another command sequence).
Before introducing the basic entity drawing commands, it is necessary
to discuss a number of commands that are used to establish certain aspects
of the drawing environment. |
The commands in this section can used at any time, but are generally
used at the start of the drawing process to set up certain aspects of the
drawing environment. This list is not exhaustive. A number of similar commands
will be introduced later in their appropriate contexts. Still others will
not be covered in these notes at all.
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Units
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Used to specify the drawing units. This is usually set by the prototype
drawing, but may be set explicitly with this command for any drawing. The
effect of this command is only to control the display format and precision
(that is, whether it is a decimal number or a fraction, and the number
of decimal places, etc.) rather than the meaning of the number (feet, metres,
etc.). AutoCAD does not really "care" what the number means, but only how
to represent it.
This command also prompts for the way in which angles are to be
represented (decimal degrees, radians, etc.) and how angles are to be measured
(normally anti-clockwise beginning from "east" taken as 0).
Limits
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Used to set limits to the size of the drawing. The lower left corner and
the top right corner must be specified. Limits checking may also be turned
ON or OFF. If limits checking is on, then points cannot be specified outside
the current limits.
Qtext
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Sets quick text ON or OFF. If quick text is ON, then all text is represented
on the drawing by simple boxes showing its extent only.
Viewres
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This command performs two functions. The first is to allow the user to
disable fast zoom. (There is little point in disabling fast zoom, except
to conform to earlier versions of AutoCAD that had no fast zoom capability.)
The second function is to permit control of the smoothness of circles and
arcs as drawn on the display. There is a trade-off here between regeneration
time (when zooming and panning) and quality of the displayed image. A circle
zoom percent value of about 20 seems to be a good compromise. This command
does not affect circles on plotted drawings.
Blipmode
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Controls the use of blips (small crosses) by AutoCAD to mark any screen
positions pointed to by the user. These blips can be useful reference points,
but can also become irritating as they begin to clutter up the drawing.
Note that blips are not part of the drawing, and are removed whenever a
redraw is done.
Fill
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Used to determine whether solids, doughnuts and wide polylines are solid
filled (with colour) or simply outlined. Again, this represents a trade-off
between regeneration time and quality of image. Note that this command
does affect the plotted output of the drawing.
Status
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Lists the environment settings for the current drawing.
|
This section lists all the major commands that are used to produce
entities on the drawing. The list is not quite exhaustive, but it certainly
covers enough to meet all architectural needs.
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Line
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This command initiates the drawing of a sequence of one or more straight
lines. The user is prompted "From point:" and should either specify a point
or press RETURN to start at end of the previous line or arc that was drawn.
The prompt "To point:" is then displayed repeatedly, permitting the user
to either specify a sequence of points or type C to close the polygon,
U to undo last line segment or RETURN to complete the command.
Point
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This command is used to draw a single marker or point. By default, the
marker is a single "dot". Its major use in architectural drafting is probably
as a means of setting up node points in blocks to be used by the NODE object
snap operation (described in Chapter 6). The AutoCAD screen menus provide
a range of marker styles that can be selected instead of the "dot".
Circle
-
AutoCAD provides a number of alternative ways to construct a circle. The
default way is to specify the centre point followed by the size of either
the radius or the diameter. Alternatively, a user can specify three points
on the circumference, two end-points of the circle's diameter, or the radius
of the circle plus two other lines and/or circles (which already exist
on the drawing) to which the new circle is to be tangential (option TTR
in the prompt).
Arc
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Arcs, like circles, can be constructed in a number of ways. The default
method is to specify three points. The other methods are:
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start point, centre and end point;
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start point, centre and included angle;
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start point, centre and length of chord;
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start point, end point and radius;
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start point, end point and included angle;
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start point, end point and starting direction;
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start point and direction of previous line or arc plus end point.
The various methods are selected by following the prompts. The last method
is selected by pressing space bar or RETURN in response to the initial
prompt.
Ellipse
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This command is used to construct ellipses. The simplest method requires
that the two end points of one of the axes (major or minor) be specified
followed by a distance value that defines half the length of the other
axis. Alternatively, the same method is used only the first axis (major
or minor) is specified by the centre of the ellipse and one of its end
points. There are other options explained in the AutoCAD reference manual.
Pline
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This command is used to draw what are known as polylines. These are continuous
sequences of straight line and/or arc segments that can have varying line
width. Individual segments can even have tapering thickness. Thick segments
are bevelled at intersection points. Polylines can be closed to form a
polygon. To construct a polyline, simply follow the prompts. Most options
are quite clear.
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Polylines are very important in AutoCAD because they are treated as
single entities for many operations. For example, a single command can
be used to fillet each corner of a polyline or to form a smooth curve from
a polyline. Further, the area and perimeter of a polyline can be also calculated.
Finally, polylines can be exploded into separate line and arc entities
when necessary.
Polygon
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This command is used to draw a regular polygon. After entering the number
of edges, either specify its centre and "radius" (being the radius of either
an inscribed or circumscribed circle about the polygon) or locate the end
points of any one of its edges. Polygons are in fact closed polylines and
can be treated as such, once constructed.
Hatch
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This command is used to draw a cross-hatch pattern within a closed boundary
or between closed boundaries. The boundaries must be defined by entities
which are accurately connected at their endpoints (it does not cope well
with "T" junctions). If the boundaries are not well defined, then the hatching
can leak out in quite unexpected ways. AutoCAD comes with an extensive
set of pre-defined patterns that can be selected. Alternatively, you can
create your own patterns and add them to the standard set or even create
a hatch pattern "on the fly" using the "User" option and following the
prompts. If you select "Hatch..." from the Draw pull- down menu, then AutoCAD
displays an icon menu to make it easier to select a pattern. Note that
some caution has to be exercised when using standard hatch patterns as
the whole pattern (as illustrated in the icon menu) is scaled to fit within
a 1x1 unit square. Thus, when you are prompted for a scale factor, you
need to visualise the size of a 1x1 square relative to the area you wish
to fill in order to estimate the required scale to create the effect you
want. If you do choose a scale factor which is too small, then you need
to abort the cross-hatching process by pressing Ctrl-C (hold down the Control
key and press c) and then erasing the abortive hatching by typing erase
last followed by RETURN.
Bhatch
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This is a new command with Release 12 and overcomes some of the boundary
limitations of the normal HATCH command (above). It supports what is called
boundary hatching, where you are able to pick a point adjacent to the boundary
that you want and AutoCAD will search for the nearest entity (presumably
on the boundary that you want) and then construct a closed boundary by
tracing anti- clockwise looking for intersection points and connecting
lines or arcs. This command is driven by a dialogue box and provides a
great number of options, including the ability to preview the hatching
before committing yourself. It is beyond the scope of these notes to detail
all those options: there is on- line help available, and you really need
to just experiment.
Dtext
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Text can be entered anywhere on a drawing with any height (measured in
drawing units) and with any rotation. It can be left justified (default),
right justified, centred, stretched to fit neatly between two points (option
F - with fixed height) or sized in order to align perfectly between two
points (option A - where character size is adjusted to fill the available
space). The text itself can be underscored or overscored and special characters
like the degree symbol ("o") can be included (details in AutoCAD reference
manual).
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AutoCAD supports a variety of standard character fonts which can be
combined with character attributes (height, width, obliquity angle, back-to-
front, upside-down and orientation) to form user- defined styles. These
are set up using the STYLE command. The "Fonts..." item in the Options
pull- down menu causes AutoCAD to display the available fonts on the screen
in the form of an icon menu.
In addition to these entity commands, there are commands to set up the
following 3D objects: 3D polylines (3DPOLY); 3D faces (3DFACE); 3D meshes
(3DMESH for any mesh, RULESURF for ruled surfaces, TABSURF for a tabulated
surface mesh, REVSURF for surfaces of revolution and EDGESURF for surface
meshes defined by 4 adjoining edges). |
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