Dimensioning Conventions

Dimensioning conventions are the agreed rules by which an engineering drawing states a part’s size and the position of its features. A drawing communicates in two languages: shape, shown by the views this wiki treats under reading engineering drawings, and size and position, stated by dimensions — the numbers, lines and symbols that say how big a feature is and where it sits. Geometric tolerancing, the GD&T system of symbols and feature control frames, controls form and orientation; conventional dimensioning is the everyday language of size, and it follows rules of its own — how a dimension is drawn, which dimensioning system carries which meaning, and how holes, threads and other features are called out. The machinist reads those rules constantly, because a dimensioning scheme that looks similar can mean very different things on the machine. This entry sets out the conventions that matter most to the person making the part.

The anatomy of a dimension

Every dimension is drawn from a standard set of parts, and reading them begins with recognising those parts. The dimension line carries the number and runs between the two points being dimensioned; extension lines reach from its ends out to the part, showing exactly which points the dimension measures between; and arrows point at the extension lines. A leader line does a different job: it runs from a note or value to a particular feature — a hole, a chamfer, a surface finish — calling out something that cannot be dimensioned between two lines. The value itself is written plainly, and the drawing’s conventions carry the rest: in metric drawings no unit symbol is used — a dimension of 25 means 25 millimetres by agreement — while imperial drawings state inches, and the title block declares which system governs.

Symbols of size

Size dimensions are written with symbols that tell the machinist what kind of feature they refer to. A diameter is marked with the diameter symbol before the value, dimensioning a hole or a cylinder by its diameter, the natural way to give a round feature its size. A radius is marked with the letter R, dimensioning an arc or a rounded corner through a leader to the curve. A square feature may carry the square symbol, a depth may be written after the size, and an angular dimension is measured in degrees between the two lines that form the angle. The symbols matter because they select the feature: a bore called Ø20 and an arc called R20 are different geometry, and the machinist who reads them correctly has already understood which tools will make the feature.

Chain, baseline and ordinate dimensioning

The positions of features can be dimensioned in different systems, and the choice is not cosmetic — it decides how errors add up on the part and how coordinates read on the machine. Chain dimensioning places each dimension from the feature before it, feature to feature along the part. It is compact and easy to draw, but its meaning is insidious: the tolerance on each dimension adds to the tolerance on the next, so the last feature carries the accumulated error of all before it. Baseline dimensioning measures every feature from a single reference surface — a common edge or centre — so each feature’s position is independent and carries only its own tolerance, and the reference surface is exactly the surface this wiki calls a datum in setup. For CNC work a drawing is often dimensioned, or re-dimensioned, as ordinate dimensioning: all positions given from one origin, with the coordinates tabulated or written at the features rather than drawn with dimension lines. Baseline and ordinate systems are the ones a CNC machinist wants, because they read straight into the absolute coordinates of the work offset: the part’s dimensions become the program’s X and Y values without arithmetic.

Calling out holes and threads

Some of the most important conventions govern the callout of holes, because a hole’s note tells the machinist the whole sequence of making it. A plain hole is called by its diameter and, where it does not go through, its depth: Ø10 and Ø10 × 20 deep are different jobs, the first a through hole, the second drilled to a depth and no further. A threaded hole is called by its thread — the size and pitch, such as M6 or M8 × 1.25 — with the depth stated and sometimes the tap drill size given; the machining of such holes is the work this wiki treats under drilling and holemaking. Where the top of a hole is opened out for a screw head to sit flush, the drawing calls a countersink or counterbore with its own size, angle or depth, often on a leader to the feature. Reading a hole’s callout is reading the process: the machinist sees the diameter, the depth, the thread and the headroom, and from the note the tools and the operations follow.

Tolerances beside the dimensions

Size and position stated, the drawing must still say how close is close enough, and the tolerance is written beside the dimension in one of a few conventional forms. The commonest is the plus-and-minus tolerance: a nominal size with a limit above and below it, such as 25 ± 0.1, or with different plus and minus values where the designer wants a band not symmetric about the nominal. Alternatively the drawing may use limit dimensions, stating the largest and smallest acceptable sizes directly — 25.1 over 24.9 — so the machinist reads the band without arithmetic. Where no tolerance is written beside a dimension, the title block’s general tolerance governs, as this wiki’s entry on reading drawings describes; and where a size must fit another part, the dimensioning may defer to the system of fits this wiki treats under fits and limits. Every form says the same underlying thing: the size is not a single number but a band the part must fall inside, and the form tells the machinist how the feature will be measured and whether an ordinary cut will hold it.

Reading conventions into a program

Dimensioning conventions matter to the machinist because the drawing’s scheme becomes the machine’s plan. A drawing dimensioned from datums tells the programmer where to set the work coordinate system; a baseline or ordinate scheme feeds the program’s absolute coordinates directly, while a chain-dimensioned drawing forces the programmer to watch the build-up and decide which end of the chain truly matters. The symbols and callouts tell which tools and operations each feature needs, and the tolerances tell which features will be measured closely and which will be cut and forgotten. The conventions of the drawing are the interface between the designer’s intent and the machinist’s work, and the machinist who reads them — who sees in a chain dimension the error stacking, in a baseline the datum, in a callout the process — is reading, in the drawing’s own language, the job that the machine is about to do.