AutoCAD

AutoCAD is the classic 2D drafting (and basic 3D) tool. Learn the interface, coordinate input, draw and modify commands, layers, blocks, dimensions, model vs paper space, scales and plotting, and automating with scripts and AutoLISP.

For decades, most engineering drawings — layouts, schematics, plant plans, sheet-metal flat patterns — have been drawn in AutoCAD or DWG-compatible tools. Even in a 3D world, 2D drafting skills remain essential for layouts, P&IDs and manufacturing drawings.

Mastering precise input, layers and layouts makes drawings accurate, consistent and easy for others to use.

An analogy

AutoCAD is a digital drawing board with an infinitely precise ruler: you draw at full size in model space, then frame and scale views on sheets in paper space.

Precise drawing

Draw with coordinates (absolute x,y; relative @dx,dy; polar @L<θ), object snaps, ortho/polar tracking and dynamic input. Core commands: LINE, PLINE, CIRCLE, ARC, RECTANG; modify with MOVE, COPY, ROTATE, MIRROR, TRIM, EXTEND, OFFSET, FILLET, CHAMFER, ARRAY.

Layers and standards

Put different kinds of objects on layers (outlines, centrelines, hidden, dimensions, text) with colours, linetypes and lineweights. Company standards (layer names, text styles, dimension styles, title blocks) make drawings consistent — often enforced with templates (.dwt).

Model space, paper space and scales

Draw everything full-size (1 unit = 1 mm) in model space. In a layout (paper space), create viewports at standard scales (1:1, 1:2, 1:5, 1:50…) and add title block, annotations and notes. Annotative text and dimensions adjust automatically to viewport scale.

Blocks, xrefs and dimensions

Blocks are reusable symbols (bolts, valves) with attributes for data extraction. External references (xrefs) link other drawings, such as a building grid, so teams work in parallel. Dimensions follow ISO/ASME conventions with tolerances and GD&T symbols.

Automation and interoperability

Scripts (.scr), AutoLISP, .NET and Dynamic Blocks automate repetitive tasks. Sheet sets manage large drawing packages. DWG/DXF exchange with CAM (laser/plasma nesting) and 3D tools; verticals (Mechanical, Electrical, Plant 3D) add standard parts and intelligence.

Formulas

Worked example: Choosing a scale

Given: Plant layout 30 m long; A1 sheet usable width ≈ 780 mm

Find: Suitable scale.

Answer: 1:50

Worked example: Polar input

Given: Line 100 mm at 30°

Find: Endpoint offset.

Answer: (+86.6, +50.0)

Command-line input (typed sequence)

LINE
0,0            ; absolute start point
@100,0         ; relative: 100 right
@100<90        ; polar: 100 at 90° (up)
@-100,0
C              ; close the rectangle

CIRCLE 50,50 20            ; centre 50,50, radius 20
OFFSET 5 (select circle) (click outside)
LAYER M Hidden C 1 Hidden L HIDDEN Hidden   ; make layer, red, hidden linetype
DIMLINEAR (pick two endpoints) (place)

Typing coordinates gives exact geometry; object snaps (END, MID, CEN, INT) pick exact points on existing objects.

AutoLISP: draw a bolt-hole circle

(defun c:BOLTCIRCLE (/ cen pcd n d i ang pt)
  (setq cen (getpoint "\nCentre: ")
        pcd (getreal "\nPitch circle diameter: ")
        n   (getint  "\nNumber of holes: ")
        d   (getreal "\nHole diameter: ")
        i   0)
  (while (< i n)
    (setq ang (* i (/ (* 2 pi) n))
          pt  (polar cen ang (/ pcd 2)))
    (command "_.CIRCLE" pt (/ d 2))
    (setq i (1+ i)))
  (princ))

AutoLISP automates repetitive drafting; this command places n holes evenly on a pitch circle.

Common mistakes

Applications

Key takeaways

Going further