Engineering Drawing Symbols: What Each One Means, With Examples

What do the dimension symbols on a drawing mean?

Dimension symbols sit in front of a number and say what kind of size it is. Ø is a diameter, R a radius, ⌴ a counterbore, ⌵ a countersink, ↧ a depth and □ a square. A number with no symbol is a straight distance. These are the symbols you meet on almost every machined part.

Older and some ISO drawings write DIA instead of Ø, and the word DEEP instead of ↧. They mean the same thing. Units are not written next to each number: the title block says whether the whole drawing is in millimetres or inches.

How do you read a counterbore or countersink callout?

Read a hole callout left to right, one line per operation. The first line is the drilled hole, the next line is the counterbore or countersink on top of it. For example, 4X Ø6.6 THRU, then ⌴ Ø11 ↧ 6.8, means four through holes with a flat-bottomed pocket for an M6 socket head cap screw.

• 4X Ø6.6 THRU / ⌴ Ø11 ↧ 6.8. Four holes 6.6 across, all the way through. Each has a counterbore 11 across and 6.8 deep, so the screw head sits below the surface.
• Ø5.5 THRU / ⌵ Ø11 × 90°. A through hole with a 90° countersink 11 across at the surface, for a flat-head screw that sits flush.
• Ø8.5 ↧ 20 / M10×1.5 - 6H ↧ 16. A blind tap-drill hole 20 deep, threaded M10 for 16 of it.
• Ø20 ⌴ SF Ø30. A spotface: a shallow counterbore that only cleans up a rough or curved surface for a washer to sit on. If no depth is given, it is just deep enough to clean up.

A counterbore takes a cap screw with a flat underside; a countersink takes a cone-headed screw. For sizes by screw, see countersink vs counterbore and the metric clearance hole chart.

What do the GD&T symbols mean?

GD&T symbols control shape and position rather than size. They sit in a feature control frame, a row of boxes: the symbol, the tolerance zone, then the datums it is measured from. ⌖ Ø0.2 A B C means the feature’s centre must lie within a 0.2 circle around its exact location, measured from datums A, B and C.

• Datum feature symbol. A capital letter in a box attached to a filled triangle on a surface or feature. It names the reference the frames measure from.
• Ⓜ and Ⓛ. Maximum and least material condition. Ⓜ after a position tolerance gives bonus tolerance as the hole grows from its smallest size.
• Ⓟ, Ⓕ, Ⓣ. Projected tolerance zone, free state and tangent plane.

Form tolerances such as flatness need no datum. Orientation, location and runout always do. Basic dimensions, the boxed numbers, give the exact location the frame’s tolerance is applied to.

How do you read a surface finish symbol?

The surface finish symbol is a check mark, √, touching the surface it controls, with the roughness written on it. On a metric drawing Ra 1.6 means an average roughness of 1.6 micrometres; inch drawings write 63 for 63 microinches, about the same finish. A smaller number is smoother.

• Plain √. Any process may be used.
• √ with a bar across the top. Material must be removed, by machining or grinding.
• √ with a circle in the V. No material may be removed: leave the surface as cast, forged or rolled.
• A symbol in the title block or next to it. The default finish for every surface not marked otherwise.

Typical values: Ra 3.2 (125) is a normal milled face, Ra 1.6 (63) a good machined finish, Ra 0.8 (32) a fine one for seals and bearing seats.

How do you read a thread callout?

A thread callout gives the size, pitch and fit class in one line. M8×1.25 - 6H is a metric internal thread, 8 mm across with a 1.25 mm pitch. 1/4-20 UNC-2B is an inch internal thread, a quarter inch across with 20 threads per inch. The letter says internal or external.

• Metric. Capital letter (6H) for an internal thread, small letter (6g) for an external one. If the pitch is left off, as in M8 - 6H, it is the coarse pitch.
• Inch. UNC is coarse, UNF fine. Class 2B is internal, 2A external; 3B and 3A are tighter fits.
• Depth. ↧ after the thread gives its full-thread depth. THRU means it runs through the part.
• Pipe threads. NPT and similar callouts are tapered threads for sealing, not a standard bolt thread.

Drawings usually show threads as a simplified dashed circle or line, not the helix. See how to add threads to a part for what that means in a 3D model.

What do the different line types mean?

Line weight and dash pattern tell you what an edge is. Thick continuous lines are edges you can see, thin dashed lines are hidden edges, and long-short dashed lines are centre lines. Reading these correctly is how you tell a hole from a boss.

In a section view, hatching shows where the cutting plane passes through solid material. A hole cut in half shows as a gap in the hatching, which is often the clearest way to read a counterbore’s depth.

What does the projection symbol mean?

The projection symbol, a cone seen from the side and the end, says how the views are laid out. In third-angle projection, standard in the US, the right side view sits to the right of the front view. In first-angle, common in Europe and Asia, it sits to the left.

Check it before you read anything else. Read with the wrong projection, the part comes out mirrored, and every dimension still looks correct. The symbol is in or next to the title block; older drawings may write THIRD ANGLE PROJECTION in words. See 2D drawing to 3D model for how the views fit together.

What do the common abbreviations on a drawing mean?

Drawings use short capitalised abbreviations to save space in callouts and notes. THRU, TYP, REF and UNO are on nearly every print. ASME Y14.38 is the full list, but the ones below cover most machined parts.

What does the title block tell you?

The title block, usually bottom right, holds the rules for everything else on the sheet: units, the default tolerances, the projection symbol, material, finish, scale, drawing number and revision. Read it first, because any dimension without its own tolerance takes the default from here.

• Default tolerances. Often by decimal places: .XX ±.01 and .XXX ±.005 on inch drawings, or a general tolerance class such as ISO 2768-m on metric ones.
• Units. “Dimensions in mm” or “inches”. Some drawings show both, with the second unit in brackets.
• Revision. A letter or number, with a revision block listing what changed. Make sure you are working from the latest one.
• Notes. Numbered notes cover what symbols can’t: break all sharp edges, heat treatment, plating, deburring.

Tolerances add up across a chain of dimensions; see tolerance stack-up basics for how to check that they still fit.

Frequently asked questions

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