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MECH 200 Engineering Drawing

Lab 1: Basic AutoCAD 2-D Drawings

Objective

Introduction & Concepts

AutoCAD is one of the more popular drafting packages around and is often considered as the baseline skill for CAD Technicians and engineers. Students of Engineering sometimes assume that CAD is a low-level task they do not require any abilities in. This is a false assumption, since Engineers are often required to produce schematics and illustrations to document their designs. Reports and presentations are essential in the proposal of new ideas, and often the Engineer does not have the luxury of a personal CAD Technician at this stage of the project. As a result, those Engineers with CAD skills are the ones whose ideas get approved for production.

As a student, you will find mastery of AutoCAD invaluable. You will get more job interviews in the co-op process, produce superior project and work-term reports, and be able to communicate your ideas more effectively with a minimum of extra effort.

The History of AutoCAD

AutoCAD has been around almost as long as the Intel 8086 family of processors (almost 20 years now). The first versions of AutoCAD were written to run on Personal Computers based on the ancient 286 processor. On these first PC's memory was incredibly scarce by today's standards (under 640 kB) and disk drives were amazingly tiny (a 40 MegaByte hard drive was considered massive, and floppy disks were actually floppy). These machines sported the very first VGA display adapters, giving them a 640 x 400 graphics screen mode in addition to their "blocks of characters" text mode.

In order to run on these primitive systems, these first versions of AutoCAD were amazingly stingy with memory and drive space, and had none of the graphical interface features such as the push-button icons, pull-down menus and dialog boxes that users now take for granted. AutoCAD expected the user to type drawing commands and answer questions on a command line while simultaneously entering coordinates with the mouse. Everything was done this way, and users of AutoCAD had to be quick typists with good memories to be effective.

Although times have changed since the primitive days of the eighties, AutoCAD's way of doing things has not. As computers have gotten faster and more sophisticated, AutoCAD has become fancier and more "Windows 95-like" but it still demands typed commands and answers from its users according to the same syntax it expected in 1986. In this way, people who learned how to use Version 9 or earlier can get good results from current versions.

The AutoCAD Text Screen

Just as it did on DOS based 286 machines, AutoCAD uses both a text screen and a graphics screen to interact with its user: The text screen behaves exactly like a command line in DOS or a terminal window in Unix, and is used by AutoCAD to record past commands and their responses. Certain commands depend on this screen to list large amounts of data, so don't think it's unimportant: There will be times in the future when AutoCAD will trick you into thinking it has crashed when in fact it is merely waiting for you to enter something in this screen. Don't minimize this window into an icon... that's just asking for trouble!

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Figure 1: The AutoCAD Text Screen

The AutoCAD Graphics Screen

The graphics screen is where 98% of AutoCAD's business gets done. In ancient times, the early versions of AutoCAD would toggle between this screen and the text screen whenever the user pressed the F1 key on their trusty 286. In the windowed environments we use today, this screen lives in its own window, sometimes referred to as the drawing editor.

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Figure 2: The AutoCAD Graphics Screen

The AutoCAD graphics screen is divided into four parts. To the right is the Side Menu , at the bottom is the Command Line, along the top is the Status bar, and the rest of the screen is the Drawing Area.  There are also pull-down program menus above the status bar, but these are standard equipment on all windowed operating system programs, so we won't get into them here.

The side menu displays your current command options. Long before pull-down menus even existed, this was the only type of menu that AutoCAD offered. The side menu is very effective at speeding up interaction between you and AutoCAD. Mouse-clicking on any of the options currently listed has the same effect as typing them on the command line. The command line itself is where you enter commands for AutoCAD and answer its various questions. The status bar displays current information about your drawing, and the drawing area is where you create and edit your drawings using the mouse.

Interacting with AutoCAD

Most people with computer experience are familiar with pull-down menus. After all, every Microsoft Windows, Macintosh, and X-Windows programs uses them. As a result, you might be tempted to use AutoCAD's pull-downs in the same manner and ignore the side menu and command line to save yourself some hassle...  Don't do that: It will cause you more trouble than it saves.
 

AutoCAD's entire way of thinking revolves around the command line concept. You give AutoCAD a command, and it asks you a series of questions regarding the command. When all the questions are answered, AutoCAD completes the command.  This approach doesn't mesh well with pull-down menus for anything other than simple operations, so it is recommended you bite the bullet and learn the command line way before you start drawing with AutoCAD's pull-down menus.

AutoCAD informs you it is ready for instructions with the following command line prompt:

    command:

After you have typed a command and hit the <Enter> key, you will see the command line scroll one line up and display a new prompt asking you for any new information required by the current command. The side menu will also change at this point to show the subcommands and options available to you: Clicking on any of these side menu items will automatically enter them at the new command line prompt. Once you have answered the question, AutoCAD will either prompt you for more information or complete the command if no more interaction is required.

A Command Example

Now we'll look at a simple example of command line interaction: Let's draw a line:

AutoCAD says...
    command:

You type in the line command...
    command: line

After you hit the <Enter> key, AutoCAD asks for the first point of the line...
    command: line
    From point:

You answer by typing in a 2-dimensional coordinate (2,3). We could pick the point by clicking the mouse in the drawing area, but let's keep things on the command line for simplicity's sake.
    command: line
    From point: 2,3

(Do you notice how brackets around the coordinates aren't required on the command line?)

AutoCAD now asks for the second point of the line. You type in coordinates (6,7)...
    command: line
    From point: 2,3
    To point: 6,7

AutoCAD now asks for the third point of the line. You could type in as many coordinates as you want in the line, repeating the last interaction as many times as necessary. For the sake of argument though, let's say two points is all you require. Simply hit the <Enter> key at the "To point:" prompt.

    command: line
    From point: 2,3
    To point: 6,7
    To point: (Hit <Enter> here, rather than type in any more coordinates...)

To AutoCAD, you are refusing to answer the question by hitting <Enter> at this point, so it assumes the command is done. AutoCAD now prompts you for the next command as follows...
    command:

Canceling a Command

There is bound to come a time when you start the wrong command by mistake. To cancel a command in progress, simply hit the <Ctrl > and  < c > keys together: AutoCAD will drop whatever it's doing and return to the  "Command:" prompt.

Command Aliases

We all know that Engineers hate to type (proper spelling is often required). Command aliases are abbreviations that AutoCAD understands when they are typed at the command line. Some of the favorite ones are:
    u        undo
    e        erase
    r        redraw
    z        zoom
    m        move
There are many more than those listed above. Experiment by typing the first letter of any command you wish to execute. Over the course of a drafting session, you will be amazed at how much time and hassle these aliases can save you!

Command Line Options and Defaults

As you will see over the next few months, some of AutoCAD's commands have many steps from start to completion. AutoCAD uses a command line syntax remind you of the subcommands currently availiable, as well as the default subcommand that will be selected if you hit <Enter>  without making a choice.

AutoCAD's Inner Universe

Right now, you probably think of AutoCAD's drawing area as a flat surface of fixed dimensions where you draw pictures - It's actually a window into a virtual 3D universe of incredible size and resolution where you can create not only drawings, but three dimensional models of great scope and detail.

AutoCAD uses single precision floating point numbers to define the coordinates of its universe: The numerical value of these coordinates can be from -1038 to  1038 (approximately), and the resolution between two consecutive numbers in this range is approximately 10-38. The resulting scope and resolution of the universe accessible through the drawing area is truly mind-boggling if you think about it.

The Coordinate System

The lab manual will tell you that AutoCAD uses a rectangular cartesian coordinate system with the X axis running horizontally across the screen and the Y axis increasing vertically toward the top of the screen. This is a simplification made to help you get started quickly without too much deep thought or worries.

In actual fact, AutoCAD's coordinates system is fully 3D, with a Z axis that is related to the X and Y axes by the right-hand rule: When you use your right hand to rotate the X axis into the Y axis (The heel of your hand stays fixed at coordinates (0,0,0)), your thumb will point in the positive Z axis direction.

Imagine that you are in a virtual spacecraft, and the viewscreen is none other than the AutoCAD drawing area. You are free to pilot anywhere in AutoCAD's universe, rotating your ship to look in any direction. As you might guess, your ship would get lost pretty fast if it didn't have some sort of guidance system built in. This 3D virtual compass is called the UCS Icon.

http://www.ece.uvic.ca/%7Eengr150/Graphics/Lab1/ucs_icon.gif

Figure 3: The UCS Icon

UCS stands for User Coordinate System, and this Icon is always shown at the bottom left-hand corner of the drawing area. It gives you a clear indication how your view lines up with the AutoCAD's coordinate system. For simple two dimensional drawing purposes, it makes sense to have your view oriented as described in the lab manual, with the X axis running horizontally across the screen and the Y axis increasing vertically toward the top of the screen.

 

 

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Figure 4: Four views of your Virtual Spacecraft in AutoCAD's Universe

The figures above demonstrate the best way to visualize the relationship between the AutoCAD drawing area and the virtual universe inside each drawing you create. You are aboard the spacecraft and the drawing editor is your viewscreen. You can adjust the magnification of this viewscreen to include as much or as little of this universe as you see fit, and (in later labs) you can move and rotate your ship around this universe to create three dimensional views of your drawings.

Now that you know AutoCAD's universe is fully 3D, you can safely ignore the Z Axis. As long as our virtual spacecraft stays oriented as shown above, everything will appear and behave two dimensional. If no Z coordinate is supplied when you enter points at the command line, AutoCAD will assume Z = 0, and mouse-picked points will always have a Z coordinate of zero from this virtual viewpoint.
 

Redefining the Limits of AutoCAD's Universe

People use AutoCAD to draw from circuits to skyscrapers and beyond. It follows that different drawing files require different sized regions of AutoCAD's virtual universe to accomodate the objects they depict. As the user, you are expected to set these regions yourself for each drawing file you create.  This process is called setting the drawing's limits and it should be done before you draw anything in a new AutoCAD file. Set your drawing's limits with the limits command. Choose limits large enough to fit everything you want to draw. AutoCAD will then prompt you for the lower left hand corner limit (keep this at 0, 0) followed by the upper right hand corner limit.

You'll need to use the zoom all command after you redefine the limits in order to change your screen magnification to match your new limits. To AutoCAD, "All" means to adjust the zoom factor to include everything within the drawing file within the drawing area, or to zoom to fit the current limits if nothing goes beyond their rectangular boundary. 

 

Creating a Grid

Once your limits are set, you will need to define a grid within them. This is accomplished with the grid command. This command asks you for a numerical value which it uses to define the grid spacing. The grid is a matrix of points that covers the defined limits. The grid will not extend beyond the limits, so take care not to draw anything outside of its boundaries (any drawing entities placed outside of the limits will not print to hard copy). Many drawing programs that use grids have a "snap to grid" option that only allows the user to only mouse-pick points that are also grid points. Be careful! AutoCAD's grid has no effect on the selection of points like this. It is only a visual tool.
 

Once defined, your grid can be toggled on and off with the F3 key. Note that if you zoom too far out or define your grid too densely for the current magnification, it won't be displayed: Use a different grid spacing if this happens. 

 

How to Mouse-Pick with Precision

Since the resolution of AutoCAD's coordinate system is unbelievably high (10-38 is a decimal place with 38 zeros and a one after it), the probability of correctly mouse-picking a desired point is practically zero. This high resolution can therefore cause inaccuracies and errors in your drawings if you mouse-pick your points while staring at the coordinate display in the graphics window's status bar: This practice is called eyeballing, and it is to be avoided at all times!

If you employ the techniques described in this section, you will learn to use this resolution to your advantage. Remember that technical drawings must be 100% accurate to be of any use, and that a sloppy drawing's innacuracies will always cause trouble and eventual loss of employment in the real world...

Defining a Snap

The most obvious tool AutoCAD offers for precision mouse-picking is called the Snap Mode. Think of the snap as an invisible user defined grid that can be toggled on and off as required. When the snap is on, you may only mouse-pick points that lie on this invisible grid. When the snap is off, the full resolution of AutoCAD is applied to mouse-picked points.  The snap is defined with the snap command. AutoCAD will prompt you for a numerical value which it uses to space the points of the invisible snap. Once defined, your snap can be toggled on and off with the F3 key. A good rule of thumb is to define your snap and grid so that the grid spacing is 5 or 10 times that of the snap spacing. This will give you a visible grid with five or ten invisible snap points in the space between its visible points.

Using The @ Symbol and Polar Point Notation

Consider the following situation: You are drawing a rectangular plate that is 23.4537 units tall and 17.2 units wide. You have started drawing the line that depicts the left side of the plate. The lower left-hand corner of the plate (the first point of your line, and the last point you have entered) is at coordinate (13.25, 19.131).

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Figure 5: Drawing the Side of the Rectangular Plate

 

In order to complete the side of this plate, you must work out the coordinates of the top left hand corner by adding 25.4537 to the Y coordinate of the last point you entered (19.131). Should you do this calculation in your head, work it out on paper, or do you use a calculator? 

Thanks to the success of the word-wide web, everyone knows about the @ symbol. In AutoCAD, this symbol is a quick way to reference the last point you have entered. In the situation above, the last point you entered was the lower left hand corner of the plate (13.25, 19.131). By simply typing "@" on the command line, you reference this point and its coordinates as components for the next point's calculation. We know the next point required is 25.4537 units directly up the Y Axis from our last point. AutoCAD can calculate this for us if we use polar point notation as well as the @ symbol as follows:

     @25.4537<90

If you are having problems understanding this last paragraph, think of it this way. The @ symbol reminds AutoCAD of the last point entered. Next we give a distance (25.4537) and a direction (90 degrees), separated by the lesser-than symbol "<" which is AutoCAD's substitute for an angle symbol.

After entering @25.4537<90, we find that @ now refers to the upper left hand corner...we are therefore free to continue using polar point notation to draw the top of the plate with a line to the upper right hand corner:

    @17.2<0

Using The OSnap Tools

As your draw new objects, the object database within your drawing file will expand to store them. This internal database is how AutoCAD keeps track of everything that exists in its virtual space, and is available when you mouse-pick points if you know how to use the OSnap modes.

OSnap is an abbreviation of Object Snap: It's a way to mouse-pick points that are part of objects you have already drawn. If you need to pick the center of a circle, the end of a line, or the tangent point of an arc, all you need to do is use the appropriate osnap just before mouse-picking a point in the general area of the object's feature of interest.

 

Click the right mouse button when the cross-hairs are within the drawing area to call up the OSnap pop-up menu. Of the available osnap tools, the following are relevant to the beginner:

Center        cen
This osnap returns the center-point of any circle selected. Remember you pick a circle by mouse-picking a point along its circumference, not within its interior!

Endpoint        end
This osnap returns the end-point of any line or arc selected. The end-point selected will be the closest to the point you actually mouse-pick.

Intersection      int
This osnap returns the intersection of any two objects. Mouse-pick the intersection as close as possible to give the osnap a good starting point to search from. Don't use this one if osnap endpoint will do, it's more accurate.

Midpoint        mid
This osnap returns the mid-point of any line selected.

Nearest        near
This osnap returns ANY point on the object selected. You won't use it often.

Perpendicular    perp
This osnap finds the point along any selected line entity required to make a 90 degree angle with the last point entered. Experiment to get a better idea on how this works...

Quadrant        quad
This osnap returns the closest quadrant point of any circle selected. The quadrant points of a circle are located on the circumference at bearings 0, 90, 180, and 270 degrees.

Tangent        tan
This osnap finds the point along any selected arc or circle required to draw a tangent line from the last point entered. Experiment to get a better idea on how this works...

None         none
This osnap turns off any currently set osnap mode. More on these in the next section...

Setting OSnap Modes

The Osnap tools shown above work as one-shot commands that apply only to the next mouse-picked point. To make any of these tools the default mouse-picking mode, you need to run the osnap command and select any one of the osnap tools. This osnap tool will be used EVERY TIME you mouse-pick unless you override it with another osnap (using the pop-up menu described above), or run the osnap command again to change the mode back to none.

Using Osnaps and Polar Notation to create Construction Geometry

There will be many times when you will draw temporary lines and circles whose only purpose is to serve as the basis for final objects. These objects are called Construction Geometry, and they serve a similar purpose to the scaffolding used by real-life construction crews raising a building. To get an idea of how osnap, polar notation and construction geometry can work together in your drawings, consider the following situation:

This lab asks you to draw a 30 x 20 rectangular tab, centered around a circle of radius 5 at point (55,50). Drawing the circle is easy enough, you enter the centerpoint and radius and it's done.

Likewise, drawing the tab is also easy if you use polar notation: run the line command and pick any nearby point, followed by @30<180, @20<270, @30<0, and then close to complete the tab.

Here's the tricky part - Centering the tab around the circle. First, you draw a horizontal line (which is construction geometry, by the way) 30 units long and move it so that its midpoint is positioned on the center of the circle: Start the move command and then

  1. Pick the line and hit <Enter> to select the line for moving,
  2. type mid, and pick the line again to specify the base point of the move, and then
  3. type cen, and pick the circle to specify the second point of displacement.

Now you can move the whole tab so that the midpoint of one of its sides coincides with the endpoint of the construction line: Start the move command and then

  1. Select all of the lines that make up the tab and hit <Enter> to select them for moving,
  2. type mid, and pick either of the vertical sides of the tab to specify the base point of the move, and then
  3. type end, and pick the corresponding end of the construction line to specify the second point of displacement.

You can now erase the construction line, because it has served its purpose!

Chamfers & Fillets

Examine your computer desk. Chances are the desktop has rounded or blunted corners to stop anyone from bruising themselves on a sharp edge. The two techniques used in manufacturing to achieve this effect are Chamfering and Filleting.

Chamfering

Chamfering is an operation whereby the sharp corner is effectively flattened by trimming it off with a linear cut:

http://www.ece.uvic.ca/%7Eengr150/Graphics/chamfer.jpg

 

Figure 6: A Chamfered Corner

You can chamfer any corner in your AutoCAD drawing with the chamfer command. This command prompts you to choose the lines that  make up the corner, and then for the x and y distances from the corner to place the ends of the chamfer cut.

Filleting

http://www.ece.uvic.ca/%7Eengr150/Graphics/fillet.jpg

Figure 7: A Filleted Corner

 

Filleting is an operation that rounds a sharp corner into an arc of arbitrary radius: This is achieved in AutoCAD with the fillet command. This command has two modes of operation: The first mode allows you to set the fillet radius, while the second prompts you to choose the lines that make up the corner to be filleted. This second mode will continue to prompt you for corners to fillet until you press Esc or Enter.

Arrays

AutoCAD's array command is designed to create armies of clones from a single selection of objects. How these copies are placed relative to the original depends entirely on whether you choose the rectangular or polar options.

A rectangular array is defined by a horizontal and vertical cell spacing between members, while a polar array is defined by a radial and angular cell spacing about a common centerpoint. To create a polar array of eight objects around a common centerpoint, you must

  1. Select the original object to copy,
  2. Specify a polar array,
  3. Pick the centerpoint of the array,
  4. Enter the number of objects in the final array (eight in this case),
  5. Specify the angular sweep covered by the array (full circle equals 360 degrees), and
  6. Indicate if you want the objects rotated as they are copied.

Linetypes

AutoCAD's way of handling different linetypes is quite different to that of any other graphics program you might have experienced before. Linetype definitions must be loaded into any drawing before they may be used: This saves a little bit of memory for AutoCAD and reduces the size of your drawing file (not too essential today, but it meant a lot in the days of the 286 PC). Linetypes are loaded with the linetype command.

 

Once loaded, you will want to adjust the scale factor of your linetypes. This is a global variable called ltscale that can be any positive number. Redefine this variable to change the spacing of your lineteypes, but be careful, since it governs every linetype in your drawing.

 

One way to change your linetype spacing while avoiding the ltscale variable is to use the 2 and X2 variations of your desired linetype. For example, the linetype dashed has a variation called dashed2 that is twice as dense, and a variation called dashedX2 that is twice as sparse. This naming convention holds true for all the linetypes you might wish to use.

 

 

Figure 8: The 2 and X2 Linetypes

 

Blipmode

As you mouse-pick points in AutoCAD you will notice tiny cross-hair markers being left behind. Eventually, your screen will be covered with the remains of these "blips", and the appearance will not be unlike specks of dirt left all over your drawing. Most people hate this effect and would turn it off if they could. Fortunately, this is easily achieved by changing the blipmode variable to OFF. After this has been done, no more blips will be drawn, although the existing specks will remain until you command AutoCAD to redraw the screen.

Redraw and Regenerate

On occasion you will find your AutoCAD screen getting "dirty" with half deleted lines and negative images from moved and erased entities. The quickest cure for this is the redraw command, which refreshed the display from AutoCAD's video buffer. Occasionally, even a redraw is not sufficient to clear up some display problems, and so you will find that the regenerate command is required. The regenerate command actually rebuilds the video buffer from AutoCAD's object database, and is guaranteed to fix any corruption of the display.

Lock Files and Drawing Filename Extensions

The version of AutoCAD running on the University lab workstations is a shared program. There can be up to 50 people using it at any one time, and therefore certain problems can arise that aren't possible on a desktop computer.

Imagine you and a partner are working on a drawing project. Each of you are responsible for drawing different parts of the same drawing, and you usually take turns working on it. Late one night, you decide to log in and get some work done... Unfortunately, unknown to you, your partner has also logged in before you and is making all kinds of progress of her own. From the moment you open the drawing file, a bizarre game of tennis begins. She saves her work, you save your work, she saves her work, and so on... Whoever saves their work last wins the game. The loser will have to do everything all over again!

The horrifying scenario above is not possible because AutoCAD implements a feature known as file locking. When you open a drawing file (i.e. lab1.dwg), AutoCAD creates a lock file to safeguard the drawing file (in this example it would be called  lab1.dwk). As long as the lock file exists, AutoCAD will refuse to load the drawing for editing by anyone else, thereby avoiding the nightmarish situation above. Whenever you finish editing a drawing and exit AutoCAD, the lock file is deleted, and anyone with access to the drawing file can once again open it for editing.

Although file locking prevents the above problem (sometimes referred to as version-itis), a problem does arise whenever AutoCAD crashes in the middle of editing a file. The lock file is not deleted in the crash, and you consequently get locked out of your own drawing! The next time you try to edit your drawing, AutoCAD will see the lock file and think someone else is currently using the drawing file: You'll be denied access to your drawing unless you get rid of the lock file yourself.

To remove a lock file, you must go to a terminal window such as the pulley shell window you will find on your workstation's desktop. Simply change to the directory where your drawing file is located and delete the file with the .dwk extension and the same name as your drawing. Do not delete the .dwg file by accident: It is your drawing file!!!

In UNIX, you change directories with the cd command, list their contents with the ls command, copy files with the cp command, and delete files with the rm command

To recap the drawing extensions that AutoCAD uses, we consider a drawing called monster.

AutoCAD will create three files to support this drawing:

  1. monster.dwg    This is the actual drawing file: It contains all your work to the last save.
  2. monster.bak    This is the backup of the drawing file: It contains all your work to the 2nd last save.
  3. monster.dwk    This is the lock file: It exists only when you are editing the drawing.

Procedures

Creating the Plate

  1. Save the drawing as plate. AutoCAD will automatically add the .dwg extension to your filename.
     
  2. Examine the Plate Specification Drawing and set the drawing limits to comfortably fit what you are about to draw with  the limits command)
     
  3. Set the grid to a spacing of 5 with the grid command.
     
  4. The default limits for AutoCAD is 12 x 9, which you reset in step 2. Zoom the magnification of the drawing area to encompass your new limits and grid with the zoom all command.
     
  5. Set the snap mode to a spacing of 1 (integer values only): Use the snap command.
     
  6. Refer to the Plate Specification and draw the rectangular outer boundary connecting points A to B, B to C, C to D and D to A. Leave the corners of the plate sharp and unchamfered for now.
     
  7. Use the circle command to create the circles concentric around point E. repeat this command to create the lower left-hand circle centered on point F.  Note that the http://www.ece.uvic.ca/%7Eengr150/Graphics/Lab1/diam_symbol.gif symbol means "diameter" in the Plate Specification.
     
  8. Draw the lower left-hand rectangular tab around point F. You can avoid mental coordinate calculations by using the osnap modes, polar notation and construction geometry.
     
  9. Fillet the rectangular tab with a radius of 3. Use the fillet command.
     
  10. Copy the tab and circle from base point F to multiple second points G, H and I. Use the copy command with the multiple option.
     
  11. Draw a construction line joining the 0 degree quadrants of the concentric circles about point E. Use osnap quad to achieve this in one command. Next, draw a circle of radius 7 whose centerpoint is also the midpoint of this construction line (hint: use osnap mid). Finally, erase the construction line.
  12. Use the array command to create a polar array of eight circles from the one you just drew.
     
  13. Run the chamfer command and set the first and second chamfer distances to 10. Repeat the command and pick the corners of the plate to chamfer each of them automatically.
     
  14. Use the offset command to set the offset distance to 10. Now repeat the command and offset the outer edge of the plate (including the chamfers) to create the inner boundary line (consult the Plate Specification Drawing for details if this sounds confusing). These offset generated lines will overlap, but don't worry. You'll fix them in the next step.
     
  15. Trim off the unwanted ends of the offset lines with the trim command: This command asks you to select objects that will act as cutting edges, and then to select the ends of the objects to trim. Try it out - it's easy, although you will have to run the command twice to get the desired effect: Once to trim the chamfer offsets to the right length, and a second time to trim off the unwanted vertical and horizontal line ends.

Creating the Disk

  1. Open a New drawing and save it as disk. AutoCAD will automatically add the .dwg extension to your filename.
     
  2. Examine the Disk Specification Drawing and set the drawing limits to comfortably fit what you are about to draw with  the limits command. Make the limits as tall as they are wide, and choose even numbers for the upper-right coordinate: (200,200) for example.
     
  3. Set the grid to a spacing of 5 with the grid command.
     
  4. The default limits for AutoCAD is 12 x 9, which you reset in step 2. Zoom the magnification of the drawing area to encompass your new limits and grid with the zoom all command.
     
  5. Set the snap mode to a spacing of 1 (integer values only): Use the snap command.
     
  6. Refer to the Plate Specification and draw the three concentric circles that define the disk's inner diameter http://www.ece.uvic.ca/%7Eengr150/Graphics/Lab1/diam_symbol.gif92 and outer radii R75 and R90. Choose the centerpoint to be one half of the upper-right coordinate: (100,100) for example. Hint - After you have entered this value the first time, you can refer to it by typing the @ symbol on the command line when prompted during the next two runs of the circle command.
     
  7. Draw a construction line from the 0 degree quadrant point (see osnap quad for details) to the centerpoint of the circle, and then use the rotate command to rotate this line by +15 degrees about the centerpoint.
     
  8. Create a horizontal mirror image of the construction line with the mirror command: The mirror line should go from the centerpoint of the circle to the 0 degree quadrant: Do not agree to delete the original line when asked by AutoCAD, because you still need it.
     
  9. Run the array command and create a polar array from your two construction lines. You want 4 items, swept over 360 degrees, rotated as they are copied.
     
  10. Draw a circle with diameter 14, centered on the 0 degree quadrant of the R75 circle; Rotate it +30 degrees around the disk's centerpoint.
     
  11. Repeat the last step but rotate the circle +60 degrees about the disk's centerpoint this time.
     
  12. Run the array command and create a polar array from the two new rotated circles. You want 4 items, swept over 360 degrees, rotated as they are copied.
     
  13. Draw a line from the 0 degree quadrant of the R90 circle to its 180 degree quadrant . This will be the horizontal centerline notation when we are done. Now draw a line from the 90 degree quadrant of the R90 circle to the 270 degree quadrant: This will become the vertical centerline notation in time.
     
  14. Run the linetype command: AutoCAD will ask you which linetype to load from the acad.lin file. load the center linetype.
     
  15. Change the linetype property of the vertical and horizontal centerline by running the chprop command: You must select both lines, and then change their ltype from BYLAYER to center.
     
  16. You will likely not see any difference in the centerlines after the previous step: This is because the drawing's ltscale variable is set too low: Type ltscale at the command line to set this variable to a larger value (15, for example).
     
  17. Now you can trim away all the unwanted lines from your disk drawing. Refer to the Disk Specification Drawing for details, and be careful! Remember you can always use the undo command if you make a mistake.

Deliverables

·         In this lab you will be producing two drawings in AutoCAD which are shown in the Specification Viewer.

1.      Plate 2-D drawing and

2.    Disk 2-D drawing

·         A single top view drawing of each object is sufficient to specify the details.