A drawing is not read top to bottom like a piece of text. It is read in layers, and each layer answers a different question: what shape it is, what sizes it has, how much those can vary, and how the surface has to come out.
First: the title block
The bottom right corner is the first thing to look at, and it is what most people skip. That is where you find the material, the scale, the projection system, the general tolerance that applies to every dimension without one of its own, the drawing revision and the default surface finish.
Two specific warnings. If the drawing is in inches and you do not notice, everything that follows is wrong. And if the revision is not the latest, you are machining a part that has already changed.
The views and where they come from
The views are projections of the part. Europe uses first-angle projection and America third-angle, and the symbol in the title block tells you which. The difference is where each view is placed: in first angle the right-hand view is drawn on the left; in third angle, on the right. Mixing them up means machining the part mirrored.
Sections show the inside, with hatching across the cut material. Dashed lines are hidden edges: they exist even though you cannot see them from outside.
Dimensions: what each one measures, and from where
The important thing about a dimension is not the number, it is where it is measured from. A part dimensioned in a chain accumulates error; the same part dimensioned from a common datum does not. And that datum is usually the face you will use as the work offset, so pay attention: dimensioning and setting from the same face stops you dragging error through the job.
Symbols that always turn up: Ø diameter, R radius, □ square, × a number of identical features ("4× Ø6" is four 6 mm holes), SR spherical radius, and C or the chamfer symbol.
Dimensional tolerances
They come in three forms: with explicit deviations (20 +0.02 / 0), with an ISO code (20 H7), or with nothing at all, in which case the general tolerance from the title block applies. That last one is the most ignored, and it is what gets a part that "was fine" rejected.
If the ISO system rings a bell but you do not have it down, it is worked through in the guide to tolerances and fits.
Geometric tolerances: what is not a measurement
A part can have every dimension right and still be useless. A shaft can measure 20.00 at every point and be bent; a face can be in the right place and not be square. That is what geometric tolerances are for, in boxes with a symbol, a value and sometimes a datum letter.
The ones that come up most:
- Flatness: how much a flat surface may bow.
- Straightness: the same for a line or an axis.
- Perpendicularity and parallelism: relative to a datum, marked with a letter (A, B, C) inside a triangle.
- Concentricity and coaxiality: that two diameters share an axis.
- Position: where the centre of a hole may sit. The most common one on hole patterns.
- Runout: total deviation as the part is turned about its axis. Widely used on turned parts.
The datum letter matters as much as the value: perpendicularity to A is not the same as perpendicularity to B, and it defines how the part has to be clamped and measured.
Surface finish
The little triangle symbol with a number gives the maximum allowable roughness, normally as Ra. If there is a general one in the title block and a specific one on a face, the specific one wins. This drives the process: some values simply cannot be reached by milling, as explained in the guide to surface finish.
The order worth reading it in
A sequence that works:
- Title block: material, scale, projection angle, general tolerance, revision.
- Views: understand the shape in three dimensions before looking at a single number.
- Overall dimensions: how big the part is and whether your stock will cover it.
- Critical dimensions: the ones with tight tolerances. Those drive the order of operations.
- Geometric tolerances and their datums: they define the workholding.
- Finishes: they decide whether another process is needed.
And one last check that saves parts: if a dimension looks impossible or contradicts another, do not interpret it. Ask. A badly dimensioned drawing is more common than you would think.
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