Drawing and Editing Commands for 2D Geometry

Drawing and Editing Commands for 2D Geometry

AutoCAD's power comes from combining a relatively small set of draw commands with a larger set of modify commands. Once you master the command ladder in this lesson, you can construct virtually any 2D manufacturing or marine part outline with speed and precision.

Workspace Safety and File Discipline

Before touching the keyboard, apply the CAD equivalent of shop safety: save your file every 10–15 minutes using QSAVE (Ctrl+S), and keep AUTOSAVE enabled (set SAVETIME to 10 minutes in Options). Corrupt or lost files cost real project time. When working near plotters or large-format printers, keep fingers clear of moving carriage arms — these are pinch hazards. Electrical cords to workstations must not create trip hazards across aisles.

Draw Commands: Building the Geometry

LINE creates straight segments between picked or typed endpoints. Type `L`, press Enter, pick or type your start point using absolute coordinates (`X,Y`), relative coordinates (`@ΔX,ΔY`), or polar relative (`@distance<angle`). Press Enter or Esc to end the command. Use Close (`C`) to close a polygon back to the start point.

CIRCLE offers several methods: center-radius, center-diameter, 2P (two points on diameter), 3P (three points on circumference), and Tan-Tan-Radius. For a ⌀1.250" hole centered at a known point, type `C`, pick the center, then type `0.625` for the radius.

ARC defaults to three-point entry but also accepts Start-Center-End, Start-Center-Angle, and other combinations found in the Draw menu. Arcs always draw counterclockwise from start to end by default.

RECTANGLE (RECTANG) draws a closed four-sided polyline. Specify two opposite corners, or use the Length and Width options for exact sizes. The result is a single polyline object — useful for gasket outlines and plate boundaries.

POLYGON creates regular polygons from 3 to 1024 sides, inscribed in or circumscribed about a circle. Specify the number of sides, center, and radius.

ELLIPSE requires an axis endpoint, the other axis endpoint, and a half-axis distance for the minor axis, or use the Center option.

After drawing each object, practice ERASE (select, then press Delete or type `E`), UNDO (Ctrl+Z), and REDO (Ctrl+Y) immediately. This command-ladder drill builds muscle memory for error recovery.

Modify Commands: Shaping the Part

OFFSET creates a parallel copy at a specified distance. Used constantly for wall thickness, gasket cross-sections, and deck plate borders. Type `O`, enter the distance, pick the source object, then click the side to offset toward.

TRIM removes portions of objects that cross a cutting edge. Type `TR`, select cutting edges (or press Enter to select all), then click the portions to remove. In AutoCAD 2021 and later, pressing Enter immediately at the cutting-edges prompt selects all objects as potential edges.

EXTEND stretches objects to meet a boundary edge — the logical counterpart to TRIM. Same workflow: select boundary, then click the end of the object to extend.

FILLET rounds corners with an arc of a specified radius. Set the radius first with the Radius option (`R`), then pick two lines. For manufacturing drawings, fillets represent actual machined radii — always confirm the radius value matches the callout on the sketch.

CHAMFER cuts a beveled corner. Use the Distance option (`D`) to set two distances, or the Angle option to set one distance and one angle. Chamfers appear frequently on marine plate edges and machined fastener bores.

ARRAY copies objects in a pattern. Rectangular Array places copies in rows and columns; Polar Array distributes copies around a center point at equal angular spacing. Both are critical for bolt-circle patterns on flanges and deck fittings.

MIRROR reflects objects about a mirror line. You specify two points defining the line. Decide whether to erase the source objects. Used for symmetric outlines — half the gasket can be drawn, then mirrored.

ROTATE pivots objects about a base point by a typed angle (positive = counterclockwise) or by reference from one known angle to another.

Verifying Your Work: DIST, MEASURE, and AREA

After construction, always verify geometry before plotting or submitting.

• DIST (or MEASUREGEOM > Distance) reports the true 3D distance between two picked points. Use it to check edge lengths and hole-center spacing.
• AREA (or MEASUREGEOM > Area) calculates enclosed area. Select the boundary with the Object option if it is a closed polyline, or pick corner points in sequence. Compare the reported area to the sketch notation. A mismatch signals an open gap in the boundary or an incorrect dimension.
• MEASUREGEOM is the modern combined command; it contains Distance, Radius, Angle, Area, and Volume sub-options in one command.

For the deck fitting plate exercise, use AREA with the Object option on a closed polyline. If the boundary is not a single closed polyline, use PEDIT > Join to merge connected lines and arcs before running AREA.

Precision Entry Best Practices

Always type coordinates rather than picking freehand when a dimension is given. Use Object Snaps (OSNAP) — Endpoint, Midpoint, Center, Intersection, Quadrant — to snap to exact geometry rather than near-misses. Confirm SNAP and ORTHO settings match your task: ORTHO constrains lines to horizontal and vertical; turn it off when drawing angled geometry.

Save a version of your file before each major modify operation. Undo history is lost when you close the file.