Introduction to AutoCAD


Chapter 4 - Drawing Editing Commands


The commands covered in this chapter are all concerned with editing and in some way manipulating existing graphics in a drawing. They fall into four major groupings: deletion operations; transformations; editing and alteration; and administrative activities.
Deletion Operations
There are two commands in this category. Erase This command is used to remove a selected group of entities. The selection set (as described in Chapter 5) can be built before or after the command is entered. After entry of this command, if there is not already a selection set, the user is required to construct one. Upon pressing the final RETURN, AutoCAD removes all the selected objects from the drawing. Since one of the selection options is "Last", it is possible to erase the most recent object added to the drawing. It is further possible to repeat that command any number of times in order to delete backwards through the whole drawing process. Oops This command restores the most recent group of objects that were deleted using the ERASE command. It will only restore one group of erased objects, so it cannot be repeated.
Transformation Operations
There are a number of commands which permit the selection of a group of objects so that they can be transformed in some way. The following five commands work in this way. Move After selecting the objects to be moved, a displacement can be specified by picking two points which between them define a vector indicating the direction and length of the move. Copy This is similar to the MOVE command in operation, except that the original group of objects are not effected in any way. The copied objects are not scaled or rotated and are completely independent of the original (as if they had been drawn "manually"). A special "Multiple" option permits multiple copies to be made of the one group of objects. Rotate Used to rotate the selected group of objects about a specified base point through a given angle. Positive angles yield anti-clockwise rotation, while negative angles are taken as clockwise. The rotation angle can also be specified by "Reference". That is, the current angle in degrees is first specified (probably by pointing) and then the new angle is given (relative to the direction of the positive X-axis). This can be useful, as it allows AutoCAD to calculate the angle of rotation instead of the user. Scale The selected group of objects can be enlarged or shrunk by setting a base point for the scaling and applying an appropriate factor. As with rotation, the scaling factor can also be determined by "Reference". In this case, a reference length is set from the drawing and the required new length is specified. This is useful when scaling something to the same length as an existing entity on the drawing. Mirror With this command a mirror copy of a selected group of objects is produced by specifying the two ends of the mirror line. The original objects may be deleted or retained. The user can also control whether or not text is mirror inverted or kept readable during the mirroring process. There are three other commands for moving and duplicating entities that are included in this group. Stretch This command permits a group of entities (generally selected with the "Crossing" option) to move through a given displacement (like the MOVE command), but with the difference that all other entities that connect to the "moved" ones are stretched or shrunk in order to maintain their connections. This would, for example, allow a door or window that is set into a wall to be moved while still maintaining its links to the wall. Array This command produces multiple copies of selected objects arranged in a rectangular or circular (polar) array or pattern. Offset This command is used to construct a new entity that is parallel to an existing entity. The entity can be any single line, polyline, arc, circle or curve. The user must either specify the side to offset from and the distance, or a point through which the offset entity is to pass.
Editing And Changing Existing Entities
This group of commands permit various editing changes to be made to existing entities in the drawing. Change This is a general command that permits a wide variety of changes to existing entities. The options are too complex and varied to fully explain in these notes. However, in general there are two types of change that can be made: changes to properties that are possessed by all entities such as layer name, colour and line type; and changes to the geometry and attributes of specific types of entities (select "Change point" option). For example, if the entity is text, then this command allows the user to change its position, style, height, rotation angle and the actual text string itself. Pedit This command is used to make all kinds of editing changes to polylines. Options include: changing the uniform width of a whole polyline; changing width and taper of individual segments; closing an open polyline, or opening a closed one; breaking a polyline into two; assembling existing (connected) entities to form a new polyline; moving vertices of a polyline; adding and deleting vertices; and finally, fitting a curve to a polyline. Break This command is used to split an existing line, arc, circle or polyline into two parts, generally (though not necessarily) with an erased portion between. Trim This command may be used to trim parts of some objects in the drawing so that they finish precisely at some cutting edge (or edges) that are established by one or more other objects. One or more lines, arcs, circles or polylines must first be identified to serve as cutting edges (these may be selected by any of the available methods). Pointing is then used to select the parts of objects to be trimmed away. Any number of objects may be selected in this way for trimming, including those that had been specified as cutting edges. Extend This command is the complement of TRIM. It operates in exactly the same way, except that selected lines are extended to end precisely at the boundary edges that are specified. Fillet This command connects two existing lines, circles or arcs by means of a smoothly fitted arc of a specified radius (referred to as a fillet). The command allows the user to change the current default radius before filleting. This command can also be used to fillet all the corners in a selected polyline. If a fillet of zero radius is used, then two lines can be made to join at a single point (useful for closing corners in a plan drawing). Chamfer This command is similar to and operates in the same way as FILLET except that it chamfers the corners with a straight edge which intersects the two objects at a specified distance from their point of intersection. The two chamfer distances are specified separately and can be different values. The next two commands permit the placement of points (as in the POINT command - using the current point style) at regular distances along a single line, arc, circle or polyline. These points can then be used as node points for object snap operations. Alternatively, these commands can insert instances of a block at each of the point positions, optionally rotated about their insertion points so that they are aligned with the object. Divide This divides the selected object into a specified number of equal divisions (number in range 2 to 32767). Measure This command starts at one end of the object and measures out segments of a specified length between each point or block.
Error Recovery
There are three commands in AutoCAD that can be used to recover from errors made during the editing process. U The U command can be used to undo the effect of the most recent editing or drawing operation. It can be used repeatedly to undo backwards through a whole drawing session. Redo This command is used immediately after any undo operation in order to redo what was undone. Undo This command is a more versatile version of the simplified U command. It supports the following operations: undo several commands at once; set Mark points and later undo Back to those points; Group operations together (terminated by End) so that they can be later undone as a group; and set various Controls and limitations on the undo process.
Enquiry Operations
The final section of this chapter lists a few commands that may be used in order to obtain information about the position and nature of the objects in a drawing. List This command will list a whole host of stored and derived information about any entities that are selected (using the normal method of selection) on the current drawing. Dist This command is used to calculate and display (in the command region) the distance and angle between two designated points on the drawing. Id This displays (in drawing units) the X Y and Z (height above the X-Y plane - used in 3D work) coordinates of a designated point on the drawing. Area This will calculate and display the area and perimeter of any region that is defined by a sequence of specified points on the drawing. The points are assumed to form a closed polygon. Each of these commands has the added benefit that they store their results in such a way that the advanced user can retrieve them for use in subsequent operations. This is explained in the AutoCAD reference manual.