NORTHERN VIRGINIA COMMUNITY COLLEGE
ANNANDALE CAMPUS
AutoCAD Reference for Engineering Graphics
by
Rodolfo R. Napisa
Assistant Professor
Mechanical Engineering Technology
Computer Aided Design and Manufacturing
Table of Contents:
It is the intent of this paper to assist students of engineering graphics in the use of the AutoCAD software. However, the content of this paper is limited and by no means a substi-tute for an AutoCAD reference book or textbook.
AutoCAD software, a product of AutoDESK Inc., current DOS version requires a minimum of "386" architecture with a complementary math co-processor. The 80386 family of processors with an 80387 math co-processor and at least 8 megabytes of RAM (random access memory) are the base requirements for the central processing unit. The 486 Intel chip family also belongs to the "386" architecture family and is widely used in this application.
Besides the central processing unit which is the heart of the system, a video graphics card and an appropriate graphics monitor are also required to run the system. The most popular type of video cards are the VGA (video graphics adapter) and SVGA (super video graphics adapter). Display resolution is the factor in describing the difference between the two graphics cards. The display is measured in the number of pixels or dots per inch. The pixel size is also a factor in selecting a good monitor.
For rapid screen movement it is recommended to use a mouse or a digitizer. The mouse that most systems come with is usually is sufficient for this application. However, should the user have a need to trace or digitize an existing drawing to CAD, a digitizer is highly recommended. The digitizer is also capable of being programmed to achieve more effi-cient operation. Cost is always a consideration in deciding whether to use a digitizer or a mouse.
The AutoCAD drawings can be plotted or printed depending on the requirements. Laser Jet printers are the most popular output device because of their ease of use. The plot size is restricted to 8 1/2 x 11 sheet paper or "A" size. Unfortunately, engineering practices re-quire drawing on a bigger paper size. This leads to the use of a plotter. The most popu-lar plotters in the market currently are the "ink jet" type. Their worry-free ease of use is the reason for their existence in most small to large engineering firms.
Computer Aided Drafting and Traditional Board Drafting:
The basic difference between CADD and traditional board drafting is the layout of de-sign. In the traditional board arena you are automatically faced with the paper limitation, there-by requiring you to predetermine the scale of the drawing on the sheet of paper. If some-how there was an error on the selection of the scale of the drawing to the paper size, the drafter has to re-create the drawing on a larger sheet or scale the drawing differently further down or up.
In CADD, the drawings are created in full size and the titleblock, including
the paper size, is scaled up. This preserves the integrity of the drawing
database and further provides a vehicle to adjust the drawing scale accordingly.
It is important to know that the titleblock and selection of paper size is
essential for presentation and preserves the legibility of textual contents.
The final step in obtaining a drawing from a CADD system is plotting. The
plot routine allows the user to scale his/her drawings to a desired size.
The determina-tion of the scale is completed prior to plotting. This
differentiates the traditional board ap-proach from the CADD approach.
Introduction to AutoCAD's Graphics Screen:
The graphics screen contains all the necessary tools for the creation of engineering draw-ings. The screen menu located at the far right side of the screen provides a complete array of commands necessary to generate a drawing. The pull-down menus are co-located with the coordinates display and the modes area display at the top of the screen. The pull-down menu contains features that are commonly used and are preprogrammed to suit the needs of the user. Icon menus are embedded in the pull-down menu. They provide a graph-ics display selec-tion as opposed to the trivial textual displays in both pull-down and screen menus.
The software has provided the user with various options from screen, pull-down, icon and tablet menus to facilitate its use. With all the different menu options, the user should avoid complacently neglecting the command prompt area. This prompt area is located at the bottom of the screen and directly interfaces with the user. Failure to read and understand the prompts displayed in this area causes unnecessary frustration.
The AutoCAD software has preprogrammed the function keys for ease of use of the designer, initially. However, these keys can be changed by reprogramming the keyboard func-tion through AutoLISP. The following describes the function keys for DOS versions of the soft-ware.
F1 - flip screen (switches from graphics to text and vice versa)
F2 - F5 - null function (unassigned for DOS version)
F6 - coordinates on/off (defines also the different types of coordinate display)
F7 - screen grid on/off
F8 - ortho on/off
F9 - snap on/off (grid snap)
F10 - tablet on/off (available only during digitizing operation)
In addition to the function keys described, the up/down and left/right arrow
keys are used in lieu of a digitizer or mouse. The movement on the screen
of the cursor can be increased through the use of the page up key and decreased
by pressing the page down key. The insert key allows the user to access the
screen menu commands. The use of these keys is not recommended because it
slows down the operations of the
system.
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Engineering graphics rely primarily on the use of the finite numerical values for deter-mining distance or length of a given line or radius of an arc or circle. The user should be able to translate the different engineering graphics figures in computer generated graphics with a similar approach. The software requires the user to define points that will be used to eventu-ally de-scribe a graphics entity.
Absolute Coordinate System:
This coordinate system always uses the absolute "0,0,0" of the sys-tem to determine the position of the point entry. It takes the form:
for 2D drawings: x,y (for example 3,4)
for 3D drawings, a third entry is introduced to define the depth: x,y,z (for example 3,4,4)
Relative Coordinate System:
The relative coordinate system uses the last point entry as the refer-ence. Unlike the absolute coordinate system it has a moving refer-ence. After an entry has been used to generate the next point, the latest point entry supercedes the previ-ous and now becomes the reference for the next point entry. It takes the form:
for 2D drawings: dx,dy (for example @3,4)
for 3D drawings a third entry is introduced to define the depth: @dx,dy,dz (for example @3,4,4)
Polar Coordinate System:
The existence of the polar coordinate system facilitates the point entry using a given length and the angle where it lies. The polar coordinate system also uses the last point entry as the reference as op-posed to the true "0,0,0" absolute coordinate. It takes the form:
for 2D drawings: @dist<angle (for example @3<45)
for 3D drawings a third entry is introduced to define the second angle: @dist<angle1<angle2 (for example @3<45<30)
Setting Up for Engineering Drawing:
It is not unusual that at the beginning of the project the drawing scale is undefined. The paper size normally is defined ahead of time since it becomes part of the deliverables. This should not hold back the design activity. But effort should be made to determine the plot scale at an early stage so that the legibility of the text and dimensions can be preserved. As previously men-tioned, the CADD system is capable of producing drawings in full-size scale and reduced or increased during the plotting.
In selecting the drawing units, it is important to understand the system=s capabilities and limitations. Each unit measured by the computer is then associated to either "one inch" or "one millimeter" during plotting. This means that you have to select the proper unit assign-ment ahead of time to facilitate the use of the system.
Currently, under the main menu "SETTING" or the pull-down menu "SETTING", the system allows the user to select different "UNITS" settings. The purpose of which is to give the user their engineering discipline format for coordinate entry. For example, if the system is set to accept architectural units, the user can then enter feet, inches and fractional inch combinations. On the other hand, when using the angular measurement option, one can select surveyors units, which define angles in a different form (N23D45'30"E).
The system allows switching of units in the middle of drawing operations. This option permits the user to adapt to any information that he/she is currently working on. For in-stance, if an electrical engineer has to layout a fiber optics installation in a secured building, he/she should anticipate that the base drawings will be provided in an architectural format. Howev-er, the units can be changed to facilitate the completion of the project by simply setting it to engineering units.
The following are the options provided for selection of units:
Linear:
Scientific - displays decimal distances with exponents [ 0.0000E+01]
Decimal - displays decimal distances with no associated measurement [ 0.0000
]
Engineering - displays feet and decimal inches [ 0'- 0.0000" ]
Architectural - displays feet, inches and fractions of an inch [ 0'- 0 0/0"
]
Fractional - displays fractional distance with no associated measurement
[ 0 0/0]
Angular:
Decimal Degrees - simple decimal number [ 0.0000 ]
Deg/Min/Sec - provides angular measurement in deg/min/sec combination [ 0d0'-0"
]
Grads - provides angular measurement in gradients [ 0.0000g]
Radians - provides angular measurement in radians [ 0.0000r ]
Surveyor - provides angular measurement in surveyors units [ N0d0'0"E ] --
N/S provides the vertical direction and E/W provides the horizontal direction
The system allows the user to save their work on magnetic media. The filename is re-stricted to 8 characters maximum and an extension of ".DWG" is added by the system. When beginning a new drawing, it is essential to give the drawing name when prompted, which will facilitate the "SAVE" function in saving your drawing to the magnetic media. The first pull-down menu provides the "SAVE" command and also the "SAVEAS" command. The "SAVES" command will save your drawing to another drawing filename and use that filename as the current file. Be reminded that it is necessary to save your drawing periodically should system failure occur.
The main menu and the pull-down menu allow the user to begin drawing different entities, i.e. points, lines, arcs, circles, etc.
POINT - allows the user to draw a point on the screen using the different coordinates systems specified above. Caution: a regular point is defined only as a pixel and sometimes is not obvious. However, a point can be as-signed different modes and sizes using the PDMODE and PDSIZE commands.
COMMAND: Point (return) (enter point)
LINE - use in generating straight lines when given at least two points. The LINE command begins by asking for the "first point" and subsequent points use the "to point" follow - up prompt. To terminate the process use the "CRTL -C" keys or use a simple "RETURN", e.g.:
COMMAND: Line(return)
FROM POINT: (enter a point)
TO POINT: (enter another point)
TO POINT: (CTRL-C or return)
ARC - generates an arc given different sets of parameters. The arc is normally generated counterclockwise unless otherwise overridden by the inputs. The following abbreviations are used to define the different functionality of the arc command:
S - start point
C - center point
E - endpoint
L - chord length
A - included angle
R - radius
D - direction
CIRCLE - generates a circle given different sets of parameters. A circle can be defined given the "center point and radius"; "center point and diam-eter"; "2P - describing the diameter of the circle"; "3P - given three points on the perimeter of the circle"; and "TTR - tangent, tangent and the radius of the circle".
POLYGON - draws a regular polygon with different number of sides. The system allows for the different input such as the "center point" and "radius of circle" over which a polygon can be inscribed or circumscribed. The other option is to allow the user to specify the length of an edge that even-tually defines the polygon.
POLYLINES - are used when there is a requirement of connectivity of adjoining lines. This also helps the user to do curve fitting and generate wider lines. The use of polylines should be restricted because it uses more memory than a line.
DTEXT - allows the user to place multiple lines of text in a drawing, while the TEXT command is limited to a single line ot text.
HATCH - allows the user to define the texture of the material when a section is being depicted in the drawing. Using BHATCH, a deriva-tive of HATCH, facilitates the use of the routine. BHATCH known as boundary hatch has the ability to define the area of hatch by selecting a given point inside the boundary of entities. The HATCH command, on the other hand, requires the user to define the boundary by selecting the entities. Users can also assign different hatch patterns by selecting from the options pro-vid-ed by the soft-ware. The user can also define his or her own pattern by writing codes that will be interpreted by the system.
OBJECT SNAP FUNCTIONS:
OBJECT SNAP FUNCTIONS - provide the ability to capture a defined point on an entity. These functions are transparent com-mands and when used the prompt requires the user to provide a point on the screen or a coordi-nate.
CENTER - captures the center point of an arc and circle
ENDPOINT - captures the endpoint of an entity
INSERT - captures the insert of a "block" or "text"
INTERSECTION - captures the intersection of any two or more
entities
MIDPOINT - captures the midpoint of an entity
NEAREST - captures any point on the entity that is near-est to the cursor
selection
NODE - captures a point
PERPENDICULAR - given an initial point it will draw a line perpendicular
to the second entity selected
QUADRANT - captures the quadrants of the circle, i.e., 0, 90, 180, 270 &
360 degrees
TANGENT - given an initial point, it will draw an entity tangent to the reference
entity
OBJECT SELECTION PROCESS - in any editing function it is mandatory to identify a collection of entities that you desire to affect. The basic premise of the selection processes is the ability to select and deselect different collections of entities. While it is important to discuss the editing routine, it is equally important to understand the function of the selection process to reduce the frustration in editing complex groups of entities.
Implied selection process is when you begin the selection of entities by passing the cursor on the entity itself, or when a blank spot of the screen is selected which becomes an initial point for a window or crossing selection. After that initial point is selected, the next point selected towards the right causes the system to use the window option and the next point selected towards the left causes the crossing function to be selected.
The window option is defined as a selection over which only the entities entirely inside the window are affected. Crossing selection means entities inside and partly inside the imaginary rectangular box are affected. Since both window and crossing are rectangular-based selection processes, the system allows irregularly shaped polygons to function similarly to window and crossing. They are "cpolygon" and "wpolygon". Meanwhile, the "fence" selection option uses only entities that cross the boundary and not the entities inside the boundary. "Last" selects the last entity added to the draw-ing while "previous" addresses entities previously selected. Use caution in using the "all" option, for it selects all of the entities that are currently active on the screen.
Despite all the selection options available, the user can also deselect the selected entities through the "remove" function, and thereafter continue selection by using the "add" option of the selection pro-cess. The user should look at each of the edit functions and associate the selection routine as part of the command. The selection routine is a sub-routine that needs to be terminated by pressing "return" to indicate completion of the selection process, and the primary edit command is ready to commence its opera-tion.
ERASE deletes entities activity displayed on the screen
COPY -- allows the user to re-pli-c-ate entities or a collec-tion of enti-ties by indicating the first point or base point and the second point in reference to the base point.
MOVE -- unlike Acopy@ the move command does not replicate the entities selected but it relocates these entities from the base point to the second point.
OFFSET -- this command creates the same entity at a given distance from the origi-nal. In the case of lines, a new line is generated parallel to the original entity given the side on which to be created. For circles or polylines, it tends to shrink or enlarge the entities maintaining the same profile.
FILLET -- creates rounded edges when given two entities and a radi-us. How-ever, the user can set the radius to zero thereby using this com-mand to find the intersec-tion of two lines.
CHAMFER -- similar to "fillet" command except that it creates a mitered corner when given two distances. These distances dictate the generation of the mitered figure.
ROTATE -- allows the rotation of a group of entities from a selected base point and a given angle along the z-axis.
MIRROR -- generates a mirror image of entities given two points that de-scribe the mirror line. The user also has the option to delete the selected object and retain just the mirror image.
CHANGE -- provides various options that allow the user to modify the enti-ties select-ed. The proper-ty of the entities can be changed using this com-mand. However, "change" can also modify the actual dimen-sional char-acteristics or values assigned to an entity.
CHPROP -- an abbreviated form of the CHANGE command that only affects entity property.
DDMODIFY -- is the interactive equivalent of CHANGE command.
TRIM -- allows the user to remove portions of entities that cross another entity. The system expects the user to define the side of the boundary to trim.
EXTEND -- allows the user to extend some entities to a given boundary.
One of the benefits that the CADD system offers is consistency of the drawing. To attain such benefit, users are taught "to generate entities once and use them plenty". The block func-tion in AutoCAD enables the user to maintain consistency and also reduces the disk space required by the drawing. This is similar to the templates that are being used in board draft-ing.
There are two types of blocks -- internal and external. The internal block allows the use of that particular group of entities only on that particular drawing. It cannot be shared by several drawings unless that internal block is changed to an external one. The external block can be generated using the WBLOCK function or by simply creating a draw-ing and saving it to a filename. XREF or external reference blocks are different from the simple block in that they do not get inserted into the drawing but are merely being refer-enced. The user should be cautioned that this external reference should be delivered along with its depen-dent drawings.
COMMAND:BLOCK (return)
BLOCK NAME: titleblk
INSERTION POINT: (enter a point)
SELECT OBJECT: (select entities)
COMMAND:WBLOCK (return)
BLOCK NAME: titleblk (if given the option is completed; if return key is
pressed, fol-low the sequence for BLOCK command)
Under the BLOCK menu and the DRAW menu, the INSERT command is accessi-ble. It allows the insertion of the block in a given drawing. The order of precedence is to search for the internal block, if available; if not then a search for a match on the disk drive in the current subdirectory or in a library location set by the software configuration is done.
EXPLODE, an edit function, is used to ungroup this collection of entities. The original defini-tion of each entity is recalled and allows the user to modify the exploded block of information.
The display functions allow the user to complete his/her drawings given the screen limitation set forth by the monitor. The ability to have a close-up view of the drawing is virtually unlimited, as is the ability to zoom out of the drawing. The ability to scroll through the drawing using the same magnification height is provided by the "PAN" command.
Display functions:
ZOOM
ALL/CENTER/DYNAMIC/EXTENTS/LEFT/PREVIOUS/-VMAX/-WINDOWS/<SCALE (X/XP)>:
ALL - displays the drawing limits or extents, whichever is greater
CENTER - allows the user to define the center point and the magnification height of the next display
DYNAMIC - allows the user to graphic size the display through the use of the pick button and the "RETURN" key
EXTENTS - displays the extents of the drawing. The ex-tents is defined as the smallest rectangle that can be built around your drawing.
LEFT - similar to the center option except that this time the left-hand corner of the drawing is defined prior to specifying the magnification height.
PREVIOUS - allows the user to recall the previous display.
VMAX - allows the user to display the maximum size of the drawing without causing the system to regenerate.
WINDOW - allows the user to zoom in by specifying the "first point" and "second point" of a window
SCALE - allows the user to input a desired scale; if the nu-meric value is by "X" then the next display is scaled relative to the current display
REDRAW - refreshes the screen and removes the blip marks or residue left during the drawing creation.
REGEN - refreshes the screen and recalculates the database; at times this takes longer time than "REDRAW". Should the drawing require regeneration the function is invoked by seve-ral zoom functions and need not be used to refresh the screen.
The layer command allows the user to group entities according to their functionality. This function permits the selection of groups of entities to be active or inactive. Furthermore, the user can display selected groups of entities using the layer command. The following are the different options associated with layering.
ON/OFF - allows the entities to be removed from the display or the plot, howev-er, the enti-ties are still active in the drawing data-base.
FREEZE/THAW - similar to "on/off", the entities are not displayed or plotted, and they are not active in the drawing database.
LOCK/UNLOCK - allows the user to refer to the selected group of entities but they cannot be selected for editing.
COLOR - used to assign a different color to a given layer name.
LTYPE or LINETYPE - use to associate different line type for a given layer.
NEW - allows the user to create a new layer.
SET - allows the user to set or reset the current layer.
MAKE - a combination of NEW and SET, it allows the user to create a new layer and then reset the current layer to the newly defined layer.
AutoCAD has no limitation in the number of layers that can be created at a given time. However, it is prudent to create only layers that will be used to reduce the amount of disk space used by the drawing. Layer names are not limited in number of characters but it is important to note that the name should be meaningful and associated to actual function of the entities generated in that layer.
The software uses a semi-automatic dimensioning command. Though it has made dimensioning simple, the user should be cautioned that each dimension line is a sepa-rate entity and at times the system does not recognized its existence (overlapping of dimen-sion lines). The system can be tailored to different types of dimensioning, i.e. architectural, engi-neering, ordinate, etc. The numerical value displayed during this processes is linked with the current unit assignment. There are two different ways to create a dimension line: 1) by capturing points using the object snap functions; 2) by selecting the entity to be dimensioned. The following options are the most commonly used subroutines in dimensioning.
HORIZONTAL - ignores the difference in the Y-axis and only processes the X-axis distance
VERTICAL - ignores the X-axis values and utilizes only the Y-axis values for the calcula-tions of the distance
ALIGN - uses both X and Y axes= values and generates a dimension line that is parallel to either the entity or points selected.
ROTATED - facilitates dimensioning entities that have a simi-lar angle of rotation
ANGULAR - allows the user to dimension angles; note there must be two lines in order to create an angular dimension
EDIT FUNCTION - contains a list of different functions that enables the user to edit existing dimensions. For example, dimension text location, dimension scale, units and others.
DIM VARIABLES - dictates the different types of dimen-sion lines. It is essential to set your variables ahead of time in order to avoid improper dimension lines.
The inquiry routines allow the user to make active the on-line help function. Each com-mand is defined in enough detail for the user to continue their assigned task. Besides the on-line help, the inquiry menu contains commands that can query the system about a selected entity. It also provides options to calculate area, perimeter, distances and point coordinates.
The outcome of working on a CADD system is still a hard copy. Though govern-ment and industry have tried to promote a paperless environment, all contract require-ments still require paper output. The plotting routine allows the user to determine the differ-ent images that he/she wishes to obtain as a hard copy. The user can plot the Display, Extents, Limits, Window or a View (by-product of VIEW command). At the same plotting prompt the user can adjust the scale of the drawing to fit into a given papersize and control the plotter operation through the software itself.