Relative Coordinates

Transcription

1 AutoCAD Essentials Most drawings are created using relative coordinates. This means that the next point is set from the last point drawn. The last point drawn is stored as temporary 0,0". AutoCAD uses (shift 2) as a prefix to the X,Y coordinates to denote relative input mode. Any measurements after are referenced from the last point drawn rather than from the world space 0,0. 33

2 AutoCAD Essentials Relative linear input - 2D drawings Relative input is the most common way to draw. The next point in the sequence is referenced from the last point drawn. Two forms for relative input are available. character must precede the numerical input. Linear Polar {angular} This sequence of commands has drawn a closed polygon. +X moves to the right -X moves to the left +Y moves up -Y moves down letter c closes the polygon. Polar (angular) input 2D indicates relative mode. The < is a shift-comma which indicates an angle is to follow. Angle 0 (zero) is usually to the right (east). Angles may be + or - from zero. In the last frame, OBJECT SNAP ENDPoint was used to close the polygon. (See next page) 34

3 AutoCAD Essentials Object Snaps - assuring accurate connections Object snap options may be accessed by use of pop-down menus, the toolbox, or by typing the option during a command sequence. Object snaps are used during a regular command. They modify the current command sequence to assure accuracy. View... Toolbars. Turn on Object Snap Drag the Toolbar any place on the screen or dock it along an edge of the screen. Object Snaps may be invoked by: Typing the option Picking from Toolbar Setting a Running Osnap And on some systems with a 3 button mouse, Click the middle mouse button. Object snap commands are extremely important for accuracy. Drawing data may be exported for analysis, machine control (CAD/CAM) or other uses. Lines must connect. Tangents must be perfect. Etc... Eyeball is never good enough! In fig a. the start of the second line was picked with what appeared to be good accuracy. Zooming in, however, shows the lines do not connect. (fig b). Use object snap ENDPoint to assure a mathematically perfect connection. Invoking ENDpoint object snap during a command produces a pickbox at each endpoint. Place the cursor over the end of the first line (anywhere inside the box) and AutoCAD will search for a connection. fig a fig b 35

4 AutoCAD Essentials Object Snap: endpoint This will link to the end of a line, arc or other entity. Place the pickbox near the end of the line and click. Command: l (line) Type or select endp and click near the desired endpoint. Object Snap: intersection This will link to the intersection of lines or other entities. Place the pickbox near the intersection and click. Type or select int and click near the desired intersection. Note: Endpoint is the quickest selection and is preferred whenever possible. Even if an intersection or other geometry is present. When typing, be sure to type endp not end otherwise, you may end the drawing by mistake! Note: Intersections take more time to identify since two entities must be located in the drawing database. On a very large drawing the time may be considerable. Use endpoint when appropriate. (Endpoint will not work in this example.) Object Snap: center This will link to the center of a circle or arc. Place the cursor near the circle not the center of the circle. AutoCAD will start a line from the center of the entity. Object Snap: perpendicular This will snap perpendicular to an entity. Note: You cannot object snap perpendicular from an entity. 36

5 AutoCAD Essentials Object Snap midpoint will snap to the center of an arc, line or other entity. Object snap quadrant will pick the top (12:00 o clock) right (3:00 o clock) bottom (6:00 o clock) or left (9:00 o clock) points on a circle or arc. Object snap tangent does not always show a rubberband line at the start. Start the line command. Select osnap tangent and point near the top of the first circle. Select osnap tangent again and point near the top of the second circle. Tangent is a very powerful command. You must pick in the area where the connection is to be made. Since there are many possible combinations, the location picked is used to determine the actual connection. Object snap nearest will connect to an entity at the nearest point to the crosshair. Note that this is somewhat inaccurate for location but it is very accurate for connection. 37

6 AutoCAD Essentials Object snap node will snap to a point previously drawn. A point is the smallest entity on a drawing. Points are very useful for marking locations - especially in 3D space. Object snap insert will snap to the base point of a symbol or block previously inserted into the drawing. Using Object Snaps Object Snaps are invoked during a command. First start the command, then invoke the object snap. Object snaps may be used to end a drawing sequence as well as to start a drawing sequence. Object snaps guarantee mathematical accuracy of connections. Running object snaps - pick Tools... Drafting Settings At times it is convenient to have the same object snap (or combination of object snaps) stay current. In this way it is not necessary to select the snap mode at each step. Some Osnaps have a marker shape which appears on the screen as the cursor nears that entity option. Set the color and other options on the AutoSnap Tab. 38

7 AutoCAD Essentials Picking a Reference point... (resetting the temporary zero) Often a construction requires measurement relative to an existing location on the drawing. This presents a problem to the drafter: How to set up the construction? Circle b is essentially located from point a on the drawing. Relative measurements are in reference to the last point set. In order to measure from point a in the example above, a must be set temporarily as the last point. Using Inquiry... ID to set a temporary reference point. One command which is very useful for this purpose is the Inquiry... ID. (ID is short for identify.) The command will display the location of a particular point on the drawing. Usually, the actual numbers are not of much interest. Invoking the command does set the temporary last point to the point ID d. The next command can use this last point as a means of measuring to the next point. Command: ID Be sure to use an object snap to select the intersection or endpoints at a. Note: This is a method of setting up this construction which works with early versions of AutoCAD. Object Snap From. This new option simplifies the steps shown above. From must be typed. Below is the command window used to draw the 0.50 diameter circle 0.50 to the right and 0.75 below corner point A. 39

8 AutoCAD Essentials Tutorial - Relative and polar mode drawing. The drawing shown below combines a number of relative (linear) and polar (angular) drawing inputs. To start the drawing, start the line command and pick any point for A. Command: line From point: pick any point To point :@1.50,0 (a to b) To point:@1.80<34 (angle from 0 btoc) To point:@1.50,0 (c to d) To point:@0,1.25 (d to e) To point:@-1,1 (e to f) To point:@1,1 (f to g) To point:@0,.75 (g to h) To point:@-2.75,0 (h to i) To point:@-1.5,-1.25 (i to j) To point:@0,-1.5 (j to k) To point:@1.46<239 (angle from zero ktol) c to close. ( Or, osnap endp at a ) angle efg =90 Note: Key in this example. Verify the steps, math and reasoning. {ID point c } {Draw circle} {ID point f } { continue the drawing. These commands will be seen at the bottom of the screen. } Some numbers will vary. {Draw polygon circumscribed} 40

9 AutoCAD Essentials Print/Plot Drawing Save the drawing. Plotting may result in a crash if the network/plotter/prin ter is not working right. From the File menu pick Print. In later versions of AutoCAD, a large menu will appear as seen at the right. Select the printer or plotter. Click Arrow to expand options. Click on the word Window< Window the drawing as shown to the right These steps insure that the drawing is correctly selected for plotting. Each drawing, especially if different units or sizes are used, must be Picked. This sequence is typical of later AutoCAD versions. Other versions may vary. 41

10 AutoCAD Essentials Click Plot Preview. This allows a check of the actual plot before sending it out. Press ESC key to exit preview. If the plot is turned the wrong way, pick Rotation and Origin box to change. Preview again to make sure the plot looks O.K. Click O.K. To print. This sequence seems rather complex at first try. It is a very good way to adjust the plot and verify everything visually before going to the actual plot. Dimensioning. Next, try placing dimensions on the drawing. Nearly all drawings require dimensions. Pop down the Dimension menu and note the commands available. AutoCAD must meet the needs of many types of drawings and systems of measurement. There are different standards for Architecture, Civil Engineering, Mechanical, Metric, etc. Many settings are possible to meet these needs. 42

11 AutoCAD Essentials Dimension Settings Pop down the Dimension menu. Command: DDIM (Dialog Display Dimensions) Pick Style. Custom styles may be created and saved. There are many differences in the way an architectural drawing is dimensioned and the way a machine part is dimensioned. The Dimension Styles dialog box provides sub menus for settings. Styles may be saved by name. Saved styles may be recalled and applied to similar types of drawings. Pick Modify Pick Text. Set the Text Height to.125 Number of Decimal Places Pick Primary Units Set two decimal place Precision ( 0.00). Four decimal places is too many for most dimensions. More work will be done with dimension settings and creating style sheets in later chapters. For now, these two temporary settings will be O.K. Some classroom computers may not allow saving custom styles. 43

12 AutoCAD Essentials Other Dimension Settings Pick Lines and Arrows. Set Arrow Size =.125 Notice the other settings which affect the appearance and placement of dimensions. Alternate Units will dual-dimension using inches and metric units for example. Use the Help menu for examples of dimension settings. Linear Dimensions Horizontal, Vertical and Aligned dimensions are shown. AutoCAD automatically draws the extension lines, dimension lines, arrowheads and calculates the correct numerical value for each dimension. Use Object Snaps to pick the endpoints of the lines. Ouick method: press Enter and pick the line. Drag the dimension. Press Enter. Use Radius for arcs. Use Diameter for circles Angular Dimensions Angles are dimensioned by picking two sides of the angle. The dimension may be placed as an acute angle or as an obtuse angle by pointing with the cursor. Practice dimensioning the previous tutorial. 44

13 AutoCAD Essentials Exercises Problem 1A. Layout as shown. Print or plot. Problem 1B. Dimension approximately as shown. Print or plot. Problem 2A. Layout as shown, Print or plot. Problem 2B. Dimension as shown. Print or Plot. 45

14 AutoCAD Essentials Notes: 46