chapter 6 Creating Objects - lnpu.edu.cnjixiexy.lnpu.edu.cn/zlgc/jpkc/jsjht/doc/No7-AutoCAD.doc · Web viewChapter 6 Creating 2D-Curved Objects (2) 6.3 Drawing Curved Objects You

辽宁石油化工大学机械工程学院计算机绘图双语教学讲义 No7 Tang Xiaochu Li Jing

Chapter 6 Creating 2D-Curved Objects (2)

6.3 Drawing Curved Objects

You can create a variety of curved objects with AutoCAD, including circles, arcs, ellipses, spline curves, and donuts.

6.3.1 Drawing Circles You can create circles in several ways. The default method is to specify the center

and radius. You can also specify the center and diameter or define the diameter alone with two points. You can define the circle's circumference with three points. You can also create the circle tangent to three existing objects or create it tangent to two objects and specify a radius. In the following illustrations, the darker circles are the ones being drawn. as shown in the illustration

Draw a Circle Tangent to Other Objects

The tangent point is a point where an object touches another object without intersecting it. To create a circle that is tangent to other objects, select the objects and then specify the radius of the circle. In the illustrations below, the bold circle is the one being drawn, and points 1 and 2 select the objects to which it is tangent. To create a circle tangent at three points, set running object snaps ( OSNAP) to Tangent and use the three-point method to create the circle.

6.3.2 Drawing Arcs


You can create arcs in many ways. The default method is to specify three points start point, a second point on the arc, and an endpoint. You can also specify the included angle, radius, direction, and chord length of arcs. The chord of an arc is a straight line between the endpoints. By default, AutoCAD draws arcs ounterclockwise.

6.3.2.1 Draw Arcs by Specifying Three Points

You can draw an arc by specifying three points. In the following example, the start point of the arc snaps to the endpoint of a line. The second point of the arc snaps to the middle circle in the illustration.

6.3.2.2 Draw Arcs by Specifying Start, Center, End

When you know the start point, center point, and endpoint, you can draw an arc by specifying either the start point or the center point first. The center point is the center of a circle that the arc is part of.

6.3.2.3 Draw Arcs by Specifying Start, Center, Angle

When you have a start point and a center point you can snap to and you know the included angle, use the Start, Center, Angle or the Center, Start, Angle option. The included angle determines the endpoint of the arc. Use the Start, End, Angle method when you know both endpoints but cannot snap to a center point.

6.3.2.4 Draw Arcs by Specifying Start, Center, Length


When you have a start point and a center point you can snap to and you know the chord length, use the Start, Center, Length or the Center, Start, Length option. The length of the chord of the arc determines the included angle.

6.3.2.5 Draw Arcs by Specifying Start, End, Direction/Radius

When you have a start point and an endpoint, use the Start, End, Direction or the Start, End, Radius option. The illustration on the left shows an arc drawn by specifying a start point, endpoint, and radius. You can specify the radius by entering a length or by moving the pointing device clockwise or counterclockwise and clicking to specify a distance. The illustration on the right shows an arc drawn with the pointing device by specifying a start point and an endpoint and a direction. Moving the cursor up from the start point and endpoint draws the arc concave to the object, as shown here. Moving the cursor down draws the arc convex to the object.

6.3.2.6 Draw Contiguous Arcs and Lines

Immediately after you complete an arc, you can start a line tangent to the arc at an endpoint by starting the LINE command and pressing ENTER at the Specify First Point prompt. You need to specify only the line length.

Conversely, after you complete a line, you can start an arc tangent to the line at an endpoint by starting the ARC command and pressing ENTER at the Specify Start Point prompt. You need to specify only the endpoint of the arc.

You can connect sequentially drawn arcs in the same way. To create connected arcs using a menu, choose Arc from the Draw menu, and then choose Continue. In both cases, the resulting object is tangent to the previous one. You can use the shortcut menu to repeat the Continue option.


6.3.3 Drawing Spline Curves A spline is a smooth curve passing through a given set of points. AutoCAD uses a

particular type of spline known as a nonuniform rational B-spline (NURBS) curve. A NURBS curve produces a smooth curve between control points. Splines are useful for creating irregular-shaped curves, for example, drawing contour lines for geographic information system (GIS) applications or automobile design.

AutoCAD creates "true" splines-which are NURBS curves-with the SPLINE command. You can also create linear approximations of splines by smoothing polylines with PEDIT. You can convert 2D and 3D smoothed polylines to splines with SPLINE.

Creating true spline curves rather than editing polylines to approximate splines has three advantages:

Spline curves can be created by interpolating the spline through a set of points that lie on the desired path of the curve. This method creates curved boundaries far more accurately than polylines for both 2D drafting and 3D modeling.

Splines can be edited easily either with the SPLINEDIT command or with grips, and the spline definition is maintained. This definition is lost with PEDIT-smoothed polylines.

A drawing containing splines uses less disk space and memory than a drawing with smoothed polylines.

You create splines by specifying points. You can close the spline so the start point and endpoint are coincident and tangent. You can also change the spline-fitting tolerance while drawing the spline to see how closely the spline fits the set of specified fit points. The lower the tolerance, the more closely the spline fits the points. At zero tolerance, the spline passes through the points.

6.3.4 Drawing Ellipses


You can create full ellipses and elliptical arcs, both of which are exact mathematical representations of ellipses. The default method of drawing an ellipse is to specify the endpoints of the first axis and the distance, which is half the length of the second axis.

The longer axis of an ellipse is called the major axis, and the shorter one is the minor axis. The order in which you define the axes does not matter.

In the following procedure, you draw an ellipse using the default method and the pointing device. Here, the first axis is the major axis, and the second is the minor. The distance increases as you drag the pointing device away from the midpoint.

To draw a true ellipse using endpoints and distance1 From the Draw menu, choose Ellipse Axis, End.2 Specify the first endpoint of the first axis (1).3 Specify the second endpoint of the first axis (2).4 Drag the pointing device away from the midpoint (3) of the first axis and click

to specify the distance.

Ellipses created by specifying axis endpoints and distanceIn the illustrations above, points 1 and 2 are the endpoints of the first axis, and point

3 defines the distance (half the length) of the second axis. The ellipse at the left is drawn by specifying the center (1) and two axes. The endpoint of the first axis is at point 2, and point 3 defines half the length of the second axis.

The illustrations below show two different ellipses created by specifying axis and distance. The third point specifies only a distance and does not necessarily designate the axis endpoint.

Command line ELLIPSEYou can provide a rotation angle instead of a distance or draw the ellipse based on a

center point, an endpoint of one axis, and half the length of the other axis. If you are drawing on isometric planes, use an ellipse to represent a circle viewed

from an oblique angle. The easiest way to draw an ellipse with the correct shape is to use the Isocircle option of ELLIPSE.


To draw an isometric circle

1. From the Tools menu, choose Drafting Settings. 2. In the Drafting Settings dialog box, Snap and Grid tab, select Isometric Snap.Choose OK.

3. Command: isoplane4. Current isoplane: Left5. Enter isometric plane setting [Left/Top/Right] <Top>: t6. Current isoplane: Top7. Command: _ellipse8. Specify axis endpoint of ellipse or [Arc/Center/Isocircle]: i9. Specify center of isocircle:10. Specify radius of isocircle or [Diameter]:

6.3.5 Drawing Elliptical Arcs

The default method of drawing elliptical arcs uses the first axis endpoints and the second axis distance, as for full ellipses. You then specify start and end angles. The apex of the angle is the center of the ellipse, and the major axis is defined as 0 degrees. The start angle defines the start point of the elliptical arc. The end angle defines the endpoint, and the arc is drawn between these points in the direction set by the ANGDIR system variable. If ANGDIR is 0, the angles are measured in a counterclockwise direction. If ANGDIR is 1, they are measured in a clockwise direction.

If the start and end angles are the same, you create a full ellipse. You can also specify a start angle and an included angle. The included angle is measured relative to the start point instead of from 0 degrees.

In the following procedure, start and end angles are measured from point 1, the first endpoint of the first axis, ANGDIR is set to 0, so the angles are measured counterclockwise from point 1. The start angle (4) is 230-degrees and the end angle (5) is 50-degrees.

To draw an elliptical arc using start and end angles1 From the Draw menu, choose Ellipse Arc.


2 Specify endpoints for the first axis (1 and 2).3 Specify the distance of the second axis (3).4 Specify a start angle (4).5 Specify an end angle (5).Related ARC creates arcs. PLINE creates arc segments within polylines.

6.3.6 Drawing DonutsDrawing donuts is a quick way to create filled rings or solid-filled circles. Donuts

are actually closed polylines that have width. To create a donut, specify its inside and outside diameters and its center. You can continue creating multiple copies with the

same diameter by specifying different centers until you press ENTER to end the command. To create solid-filled circles, specify an inside diameter of 0.

Donuts created as filled rings and solid-filled circlesTo draw a donut1 From the Draw menu, choose Donut.2 Specify the inside diameter (1).3 Specify the outside diameter (2).4 Specify the center of the donut (3).5 Specify the center point for another donut or press ENTER to end the

command.Command line DONUTSystem variables DONUTID stores the inside diameter value of a donut;

DONUTOD stores the outside diameter value. FILLMODE controls the display of donuts and other wide polylines.

Related PEDIT edits donuts. EXPLODE converts a donut to two arcs. If you explode a donut, its line width reverts to 0.

6.4 Creating Point Objects

Point objects can be useful, for example, as node or reference points that you can snap to and offset objects from. You can set the style of the point and its size relative to the screen or in absolute units.

To set point style and size1 From the Format menu, choose Point Style.2 In the Point Style dialog box, select a point style.


3 Under Point Size, specify a size.4 Choose OK.Command line DDPTYPETo create a point marker1 From the Draw menu, choose Point Single Point.2 Specify the point location.Command line POINT

System variables PDMODE sets different point styles. PDSIZE controls point size.

6.5 Creating Text

The text you add to your drawings conveys a variety of information. It may be a complex specification, title block information, a label, or even part of the drawing. Use the TEXT command to create line text .

Creating Line TextUsing TEXT you can create one or more lines of text and end each line when you

press ENTER. Each text line is a separate object that you can relocate, reformat, or otherwise modify.

Formatting Line TextYou can format text as you create it using the options on the command line. Justify

determines how the characters in the text line align with the insertion point. Style sets the default format characteristics

Assigning a Style to Line TextAll text in a drawing has a style associated with it that sets the font, size, angle,

orientation, and other text characteristics. When you enter text, the TEXT command uses the current text style. You can assign a different, existing style by entering its name at the Style prompt.

6.6 Creating Solid-Filled Areas


You can create triangular and quadrilateral areas filled with a color. For quicker results, create these areas with the FILLMODE system variable off, and then turn on FILLMODE to fill the finished area. You don't see the area outline until it is complete.

Solid-filled areas used to depict buildingsTo create a triangular solid-filled area1 From the Draw menu, choose Surfaces 2D Solid.2 Specify the first point (1).3 Specify the second point (2).4 Specify the third point (3). Then press ENTER.5 Press ENTER again to exit the command.

When you create a quadrilateral solid-filled area, the sequence of the third and fourth points determines its shape. Compare the following illustrations:

Command line SOLID

6.7 Creating Regions

Regions are 2D enclosed areas you create from closed shapes called loops. A loop is a curve or a sequence of connected curves that defines an area on a plane with a boundary that does not intersect itself. Loops can be combinations of lines, polylines, circles, arcs, ellipses, elliptical arcs, splines, 3D faces, traces, and solids. The objects that make up the loops must either be closed or form closed areas by sharing endpoints with other objects. The objects must also be coplanar (on the same plane).

You can create regions out of multiple loops and out of open curves whose endpoints are connected and form loops. If the open curves intersect in their interior, they cannot form a region. Objects such as 3D polylines and face meshes can be converted to regions by being exploded. You cannot form regions from open objects that intersect to form a closed area: for example, intersecting arcs or self-intersecting curves.

You can apply hatching and shading to regions, and you can analyze properties such as their area and moments of inertia.

6.8 Hatching Areas

Hatching fills a specified area in a drawing with a pattern. You can hatch an enclosed area or a specified boundary using the BHATCH and HATCH commands.

BHATCH creates associative or nonassociative hatches. Associative hatches are linked to their boundaries and are updated when the boundaries are modified. Nonassociative hatches are independent of their boundaries. BHATCH defines boundaries automatically when you specify a point within the area to be hatched. Any whole or partial objects that are not part of the boundary are ignored and do not affect the hatch. The boundary can have overhanging edges and islands (enclosed areas


within the hatch area) that you choose to hatch or leave unhatched. You can also define a boundary by selecting objects.

HATCH creates nonassociative hatches only. It is useful for hatching areas that do not have closed boundaries. See "Creating a Hatch Using Point Acquisition." HATCH is available only on the command line.

After you've created a hatch, you can edit it with HATCHEDIT or explode it into composite lines using the EXPLODE command.

6.9 Precision Drawing with Osnap

AutoCAD provides object snap tools to help you draw quickly and accurately.By using these tools, you can draw precisely without having to enter coordinates or perform tedious calculations.

Snapping to Points on ObjectsDuring drawing commands, you can snap the cursor to points on objects such as

endpoints, midpoints, centers, and intersections. For example, you can turn on object snaps and quickly draw a line to the center of a circle, the midpoint of a polyline

segment, or the apparent intersection of two lines.

When the command prompts you to specify a point, choose an object snap using one of the following methods:

·Click a toolbar button on the Standard toolbar Object Snap flyout or on the bject Snap toolbar.·Press SHIFT and right-click in the drawing area, and choose an object snap from the shortcut menu.·Enter an object snap abbreviation on the command line.

Then Move your cursor over the snap location and click.After you click to the snap point, the object snap clears.

The following table shows the AutoCAD object snaps, the toolbar buttons, and command line abbreviations you can use to start them.

Object snap Command line Snaps toEndpoint END Object endpointsMidpoint MID Object midpointsIntersection INT Object intersectionsApparent Intersection APP Apparent intersection of objectsExtension EXT Object extension pathsCenter CEN Center points of circles, arcs, or ellipsesNode NOD Point objects drawn with the POINT command


Quadrant QUA Closest quadrant of arcs, circles, or ellipsesInsertion INS Insertion point of a block, shape, text, attribute, or attribute definitionPerpendicular PER Object points forming a perpendicular (normal) alignment

Parallel PAR Point on an alignment path that is parallel to the selected objectTangent TAN Point on a circle or arc that, when connected to the last point, forms a line tangent to the objectNearest NEA Object snap point closest to the selection pointNone NON Turns off object snaps for the next point selection

6.10 Using AutoTrack Drawing

AutoTrack helps you draw objects at specific angles or in specific relationships to other objects. When you turn on AutoTrack, temporary alignment paths help you create objects at precise positions and angles. AutoTrack includes two tracking options: polar tracking and object snap tracking. You can toggle AutoTrack on and off with the Polar and Otrack buttons on the status bar. Object snap tracking works in conjunction with object snaps. You must set an object snap before you can track from an object's snap point; the AutoSnap aperture settings control how close you must be to the alignment path before the path is displayed.

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chapter 6 Creating Objects - lnpu.edu.cnjixiexy.lnpu.edu.cn/zlgc/jpkc/jsjht/doc/No7-AutoCAD.doc · Web viewChapter 6 Creating 2D-Curved Objects (2) 6.3 Drawing Curved Objects You