Engineering drawings

An engineering drawing is a legal and technical instruction. A fabricator in another city, who has never spoken to you, must be able to make exactly the part you intend from the drawing alone, and if something goes wrong, the drawing is what lawyers and inspectors will read. Words describe ideas; drawings describe geometry precisely enough to build.

Most drawings today are produced in CAD (computer-aided design) software, and buildings and infrastructure are increasingly modeled in BIM (building information modeling). But the conventions underneath have not changed: projection, line types, dimensions, tolerances and title blocks. This chapter teaches you to read and produce drawings that follow them, using a handrail bracket and the plan of our accessible entrance as examples.

Kinds of drawing

Different jobs need different drawings.

• Sketches: freehand, quick, for thinking and discussing. Every engineer should be able to sketch a clear isometric view in a minute.
• Detail (part) drawings: one part, fully dimensioned and toleranced, with material and finish. The fabricator works from these.
• Assembly drawings: how parts fit together, with a parts list (bill of materials) and balloons identifying each part.
• Plans, elevations and sections: the language of civil and building work. A plan looks down; an elevation looks horizontally at a face; a section shows a cut.
• Schematics and diagrams: electrical circuits, piping and instrumentation diagrams (P&IDs) and flow sheets, where connections matter more than physical shape.

Orthographic projection

A three-dimensional object is shown on flat paper by projecting it onto planes at right angles to each other. Each projection, called a view, shows the object as seen looking straight at one face, with no perspective. Usually three views (front, top and right side) are enough; simple parts may need only two, and complex ones more.

The front view is chosen to show the most characteristic shape of the part, with the fewest hidden lines. The other views are then placed in fixed positions relative to it, and features line up between views: the width in the front view matches the width in the top view, and the height in the front view matches the height in the side view.

First-angle and third-angle

There are two conventions for arranging the views, and confusing them produces mirror-image parts.

• Third-angle projection, standard in North America: each view is placed on the same side as the face it shows. The top view goes above the front view, the right side view to the right.
• First-angle projection, standard in Europe and much of Asia: each view is placed on the opposite side. The top view goes below the front view, the right side view to the left.

Every drawing states which it uses, with a small symbol in the title block showing a truncated cone. Check it whenever you receive a drawing from overseas.

Line types

Lines carry meaning through their weight and pattern, standardized in ISO 128 and ASME Y14.2.

• Visible (object) lines: thick, continuous; edges you can see.
• Hidden lines: medium, short dashes; edges behind a surface.
• Center lines: thin, long-short chain; axes of holes and symmetry.
• Dimension and extension lines: thin, continuous, with arrowheads on dimension lines.
• Section (cutting-plane) lines: thick chain with arrows showing the viewing direction; the cut surface is hatched with thin parallel lines.

Section views

When the inside of a part matters, a section view shows it as though the part were cut along a plane and the front piece removed. The cut faces are cross-hatched. Sections replace confusing hidden lines with clear solid ones. A full section cuts all the way through; a half section cuts a symmetric part to its center line, showing inside and outside in one view. In building drawings, a wall section through the entrance would show the slab, insulation, drainage and handrail post connection in a way no plan can.

Pictorial views: isometric

Pictorial drawings show three faces at once and are easy for non-specialists to understand. The most common is isometric: the three axes are drawn 120° apart, usually with one vertical and the other two at 30° to the horizontal. All three axes are foreshortened equally.

In a true isometric projection, lengths along each axis appear at \(\sqrt{2/3} = 0.816\) of their real size. In practice, engineers usually make an isometric drawing at full length along the axes, which is simply 22% larger and much easier to measure. Lines that are not parallel to an axis (such as diagonals) are not true length in either version and must be located by plotting their end points.

Circles become ellipses in isometric. A quick freehand method: draw the isometric square (a rhombus) that would enclose the circle, then sketch the ellipse tangent to the midpoint of each side.

Scale

Most objects do not fit on paper at full size. A drawing scale states the ratio of drawn size to real size. Metric drawings use preferred scales such as 1:1, 1:2, 1:5, 1:10, 1:20, 1:50 and 1:100. American building drawings usually use architectural scales, such as 1/4 in = 1 ft-0 in (1:48) and 1/8 in = 1 ft-0 in (1:96), and site and civil drawings use engineering scales such as 1 in = 20 ft (1:240). Machine parts are often drawn full size (1:1) or larger.

Never scale dimensions off a printed drawing to build from. Printers and PDF viewers resize sheets, and drawings carry the note "Do not scale" for that reason. Dimensions are written on the drawing; if one is missing, ask for it.

Dimensioning

Dimensions define size and location. A well-dimensioned drawing gives every dimension needed to make and inspect the part, once, in the view where the feature is clearest.

• Dimension each feature once. Duplicate dimensions can conflict, and the conflict will be discovered by the fabricator.
• Dimension to features that matter for function: hole centers from the surface that locates the part, not from an arbitrary edge.
• Place dimensions outside the views where possible, with extension lines leaving a small gap from the object.
• Show diameters with Ø and radii with R. On metric drawings, dimensions are in millimeters and units are omitted. Many American drawings use decimal inches instead (ASME Y14.5 covers both); the title block states which, and on inch drawings the number of decimal places often implies the tolerance.
• Avoid dimensioning to hidden lines.

Chain versus baseline dimensioning

How dimensions are arranged affects accuracy. Chain dimensioning gives each feature relative to the previous one; baseline dimensioning gives every feature relative to one datum surface. Because each dimension carries a tolerance, chains accumulate error.

Tolerances

No part is made exactly to size. A tolerance states how much variation is acceptable. Tighter tolerances cost more, often a lot more, so the designer's job is to make tolerances as loose as function allows.

• Limit dimensions: 25.00 / 24.95 (the largest and smallest allowed).
• Bilateral tolerance: 50 ± 0.1 (variation both ways).
• Unilateral tolerance: 50 +0.2 / 0 (variation one way only).
• General tolerances: a title block note such as "unless otherwise stated, ±0.5" covers all dimensions without their own tolerance.

Fits

When a shaft goes into a hole, the tolerances on both decide whether the fit is a clearance fit (always a gap; the parts slide or rotate), an interference fit (always overlap; the parts are pressed together and stay put), or a transition fit (could be either). The ISO system (ISO 286) names tolerance zones with a letter for position and a number for grade: capital letters for holes, small letters for shafts. Most designs use the hole-basis system, where the hole is H (its lower limit is the nominal size) and the shaft tolerance is chosen to give the desired fit.

Tolerance stack-up

When several parts add up to a gap, their tolerances add up too. A worst-case analysis assumes every part is at its worst limit simultaneously. A statistical analysis (root sum square, RSS) recognizes that this is very unlikely when parts vary randomly, and combines tolerances as the square root of the sum of squares.

Geometric dimensioning and tolerancing

Size tolerances alone cannot control shape. A hole can be the right diameter but in the wrong place, or a surface the right height but warped. Geometric dimensioning and tolerancing (GD&T), defined in ASME Y14.5 (2018 edition) and the ISO GPS standards, adds controls for form (flatness, straightness), orientation (perpendicularity, parallelism), location (position) and runout, all measured from datums: reference features that mimic how the part is held and located in use. GD&T is a course of its own; for now, recognize the boxed feature control frames on drawings and know that they define exactly how each feature will be inspected.

The title block and revisions

Every sheet carries a title block, normally in the bottom right corner, containing: the drawing title and number, the project, the scale, the projection symbol, units, general tolerances, material and finish (for parts), the sheet number, the names or initials of who drew, checked and approved it, and the engineer's seal where required.

Drawings change, and everyone must know which version is current. The revision block records each change with a letter or number, a description and a date. A drawing marked "Issued for construction" is authorized for building; earlier issues ("for review", "for bid") are not. In the United States, drawings submitted for permits and public works are sealed by the PE in responsible charge, as covered in ENGG-100. Working from an outdated revision is a classic and expensive site error.

CAD, BIM and the drawing of the future

CAD software generates views from a 3D model automatically, so the views always agree with each other, a big improvement on hand drafting. BIM goes further for buildings: the model carries information about every element (material, fire rating, supplier, maintenance schedule) and is shared among architects, engineers, contractors and owners. Some industries are moving toward model-based definition, where the annotated 3D model, not a 2D drawing, is the legal definition of a part. Whatever the medium, the same conventions of projection, dimensioning and tolerancing make the information unambiguous.