When you have completed this module, you will be able to:
1. Describe a circle and an arc and their characteristics.2. Describe and use the CIRCLE and ARC commands to draw circles and arcs.
Geometry LessonCircles
A circle is defined as a closed curve in which all points are the same distance from its centerpoint. The center point is a single XY coordinate. Study the drawings in Figure 7-1 for adescription of the characteristics of a circle.
AutoCAD Command: CIRCLEThe CIRCLE command is used to draw circles.
Shortcut: C
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Geometry LessonArcs
An arc is defined as an open curve in which all points are the same distance from its centerpoint. Study the drawings in Figure 7-2 for a description of the geometry of an arc.
AutoCAD Command: ARCThe ARC command is used to draw arcs.
Shortcut: A
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7 - 3
Circles and Arcs
When you are drawing arcs, you should know that AutoCAD wants to drawarcs counterclockwise. To AutoCAD, this is the positive arc angle. Whenyou use the Center-Start-End option to draw arcs, there is no way to drawthe arc clockwise or in the negative arc angle. If you use the Angle option,AutoCAD does allow you to enter a negative angle and it will draw the arcfor you.
Step 1 Start a new drawing using the template Module Template A.
Step 2 Save and name the drawing AutoCAD 2DWorkalong 07-1.
Step 3 Create layer Object, color red, as shown inFigure Step 3.
Step 4 Set layer Object as the currentlayer and enter the LINE and ARCcommands shown below to draw theobject shown in Figure Step 4.
Command: LINESpecify first point: 2.5,5Specify next point or [Undo]: @4,0Specify next point or [Undo]:Command: ARCSpecify start point of arc or [Center]: C
(If possible, always draw arcs starting withthe center point first.)
Specify center point of arc: 2.5,5(Enter the center point.)
Specify start point of arc: @4,0
Figure Step 3
The Object to be Drawn
(Enter the start point. Here the trick is to give AutoCAD the start pointand the radius of the arc at the same time.)
Specify end point of arc or [Angle/chord Length]: ASpecify included angle: 90
(Enter the angle of the arc. In this case, it is 90 degrees.)Command: LINESpecify first point: 2.5,5Specify next point or [Undo]: @0,4Specify next point or [Undo]:Command:
Step 5 Enter the CIRCLE commands shown below to complete the object. See FigureStep 5.Command: CIRCLESpecify center point for circle or [3P/2P/Ttr (tan tan radius)]: 3.5,6
(Since this is the first circle inserted, enter an absolute coordinate for the center point.)Specify radius of circle or [Diameter]: D
(Circles are usually dimensioned with diameters rather then a radius,enter D)
Specify diameter of circle: .65Command: CIRCLE
(Press enter or space as that will repeat the last command.Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: @1,0
(Notice here, since AutoCAD always remembers the lastpoint, use an@. The lastpoint was the center of the first circle placed.)
Specify radius of circle or [Diameter] <0.3250>: DSpecify diameter of circle <0.6500>: .75Command: CIRCLE
(Press enter or space as that will repeat the last command. Figure Step 5Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: @0,1.5
(Again use an @ which is from the center of the last circle placed.)Specify radius of circle or [Diameter] <0.3750>: DSpecify diameter of circle <0.7500>: 1Command: CIRCLE
(Press enter or space as that will repeat the last command and save youentering it again.)
Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: @-1,0Specify radius of circle or [Diameter] <0.5000>: DSpecify diameter of circle <1.0000>: .5Command: CompletedStep 6 Save and close the drawing. Drawing
When you are drawing arcs, it isbest to specify the center pointfirst then the start point. Whenyou give the location of the startpoint, you can specify the radius
and the direction at the same time, then enterthe angle or the end point whichever is easier.Command: ARCSpecify start point of arc or [Center]: CSpecify center point of arc: 3,4Specify start point of arc: @0,-2
(Specifying the radius and the direction for thestart point.)
Specify end point of arc or [Angle/chord Length]: ASpecify included angle: 180Command:
Using the CIRCLE Command - TTR OptionStep 1 Start a new drawing using the template Module Template A.
Step 2 Save and name the drawing AutoCAD 2D Workalong 07-2.
Step 3 Create layer Object, color red, as shown inFigure Step 3.
Step 4 Set layer Object as the current layer andenter the LINE and CIRCLE commands shownbelow to draw the object shown in Figure Step4A, 4B, 4C and 4D.Command: LINESpecify first point: 1.25,4.5Specify next point or [Undo]: @6,0Specify next point or [Undo]: @0,4Specify next point or [Close/Undo]: @-6,0Specify next point or [Close/Undo]: CCommand: CIRCLESpecify center point for circle or [3P/2P/Ttr (tan tanradius)]: TTRSpecify point on object for first tangent of circle: P1
(Using the graphic cursor, pick one of the lines the
Figure Step 3
Object to be Drawn
circle is tangent to. Note how the symbol that automatically appears. SeeFigure Step 4A.)
Specify point on object for second tangent of circle: P2(Pick the other line.)
Specify radius of circle <0.7000>: 1(Enter the radius of the circle. Be careful, since circles are usuallydimensioned in diameters you will have to divide it in half.)
Command: CIRCLESpecify center point for circle or [3P/2P/Ttr (tan tan radius)]: TTRSpecify point on object for first tangent of circle: P3Specify point on object for second tangent of circle: P4Specify radius of circle <1.0000>: 1.25
Command: CIRCLESpecify center point for circle or [3P/2P/Ttr (tan tanradius)]: TTRSpecify point on object for first tangent of circle: P5Specify point on object for second tangent of circle: P6Specify radius of circle <1.2500>: 1
(See Figure Step 4C and 4D.)Command:
Figure Step 4C
Step 5 Save and close the drawing.
Figure Step 4DCompleted
Drawing
Drafting LessonReading Dimensions for Circles and Arcs
When you are reading thedimensions for circles and arcs,consider the following:
Circles are dimensioned asdiameters. For example: 2.0 DIA.
Arcs are dimensioned as radiuses.For example: 1.5 RWhen there is more than one circle ofthe same diameter, they are onlydimensioned once. For example:0.5 DIA., 4 PLACES
Sometimes multiple arcs are dimensioned as typical (TYP.). For example: 2.0 R TYP.Typical simply means that there is at least one more arc of the same size.
A useful feature in AutoCAD is the Undo/Redofeature. You can undo all the commands in thecurrent drawing session. You can also reverse anundo with a redo. To undo the last command, usethe U command and to redo an undo, use the
REDO command. There are icons on the Standard toolbar to helpyou undo and redo.
Command: ULineCommand: REDOLine
For 2000-2002 UsersYou can use the U command to undo as many commands as youwish but the REDO command must be executed before any othercommands are entered or it will not work. Redo will only undothe last undo.
The Key Principles in Module 71. AutoCAD always draws arcs counterclockwise.
2. When you are drawing arcs, it is best to specify the center point first then the start point,which sets the radius, and then an angle or the endpoint.
3. An useful feature in AutoCAD is the Undo/Redo feature. You can undo all thecommands in the current drawing session. You can also reverse an undo with a redo.
AutoCAD 2D Lab 07-1 Module Template A Inches N/A N/A
Note: Color, Linetype, and Lineweight are all ' ByLayer ' unless otherwise instructed.
Layering Scheme
Objects on Layer Name Color Linetype LineweightAll Objects
Instructions:Object Red Continuous Default
1. Start a new drawing using the template shown above.2. Save and name the drawing with the name shown above.3. Setup the layers using the Layering Scheme shown above.4. Draw the object shown below using the layering scheme above. In this
drawing, all objects should be on layer Object and display red.5. Check your drawing with the key. The key name is the same as the drawing
name shown above. If you have trouble doing this, redo Module 3.6. If you have any errors, correct your drawing.
AUTOCAD SELF-P ACED LE ARNI NG MODULES
7. Turn layer Key off.8. Save and close the drawing.
Construction TechniqueThe following steps are the construction technique suggested by the author to help you learn how toconstruct objects using AutoCAD. It is only the suggested method and if you can complete the drawingaccurately using a different construct technique, that is what is important. You may want to compare yourconstruction technique with the authors.
Construction HintsDo your best to complete the lab exercise drawing without using the following hint(s). If you get stuck andcan't complete it on your own, use the following hint(s) to help you.
Note: Color, Linetype, and Lineweight are all < ByLayer < unless otherwise instructed.
Layering Scheme
Objects on Layer Name Color Linetype LineweightAll Objects
Instructions:Object Red Continuous Default
1. Start a new drawing using the template shown above.2. Save and name the drawing with the name shown above.3. Setup the layers using the Layering Scheme shown above.4. Draw the object shown below using the layering scheme above. In this
drawing, all objects should be on layer Object and display red.5. Check your drawing with the key. The key name is the same as the drawing
name shown above. If you have trouble doing this, redo Module 3.6. If you have any errors, correct your drawing.7. Turn layer Key off.8. Save and close the drawing.
Construction TechniquesThe following steps are the construction technique suggested by the author to help you learn how toconstruct objects using AutoCAD. It is only the suggested method and if you can complete the drawingaccurately using a different construct technique, that is what is important. You may want to compare yourconstruction technique with the authors.
Construction HintsDo your best to complete the lab exercise drawing withoutusing the following hint(s). If you get stuck and can't completeit on your own, use the following hint(s) to help you.
Hint 1Command: CCIRCLE Specify center point for circle or [3P/2P/Ttr (tan tanradius)]: 85,170Specify radius of circle or [Diameter]: DSpecify diameter of circle: 10Command: CCIRCLE Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: @40,0Specify radius of circle or [Diameter] <5.0000>:Command: CCIRCLE Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: @0,20Specify radius of circle or [Diameter] <5.0000>:Command: CCIRCLE Specify center point for circle or [3P/2P/Ttr (tan tanradius)]: @-40,0Specify radius of circle or [Diameter] <5.0000>:Command:
