Blueprint reading for technicians: how to read a mechanical drawing

Blueprint reading means interpreting an engineering drawing so that you can make, check, order or fit a part without asking the person who drew it. The name is historical. The original blue-background copies came from a photographic printing process that is long obsolete, and modern prints are usually black lines on white paper or a PDF, but the trade still calls them blueprints.

Technicians use mechanical drawings every week even when they never machine anything. You read one to order the right replacement shaft, to check whether a worn bore is still inside its limit, to find which bracket holds a sensor on a conveyor, or to work out what a fabricator should make from a sketch. You do not need to be a draftsperson. You need to find the same six things on every drawing, in the same order, until it becomes automatic.

This page is about mechanical part and assembly drawings. Process drawings (P&IDs) and electrical drawings use their own symbols and are covered on separate pages and in the Drawings and Documentation track linked below.

A reliable routine stops you jumping to a dimension before you know what the drawing is. Follow these steps each time.

1. Title block: find the part name and number, the drawing number, the material, the units, the scale and the revision letter or number.
2. Revision block: confirm you have the latest revision. Making a part from an old revision is a classic and expensive mistake.
3. General notes: read them before the views, because they can change how everything is made, such as a finish, a heat treatment or a note on sharp edges.
4. Views: work out what each view shows, and which one shows the feature you care about.
5. Dimensions: read the sizes, the positions of features and the tolerances.
6. Section views and details: check these for internal features that the outside views cannot show.

The title block is a boxed area, usually in the bottom right corner, that identifies the drawing. Exact layouts vary between companies, but you can expect the part name, a drawing or part number, the material, who drew and approved it, the date, the units of measure, the scale, and often a default tolerance that applies to any dimension without its own tolerance.

Check the units first. A drawing in millimetres and one in inches look identical until you read the block, and a bore of 25 is a very different part in each. The scale tells you how the drawn size relates to the real size, for example 1:2 means the drawing is half size. Never measure a drawing with a ruler to find a size, because prints are copied, shrunk and printed to fit a page. The dimension written on the drawing is the authority.

The revision block, normally near the title block, lists each change with a letter or number, a short description and a date. If the number on the part in your hand does not match the latest revision on the drawing, find out what changed before you order or fit anything.

A part is shown by several flat views, each looking at it from a different side. The usual set is a front view, a top view and a right side view, and enough of them are drawn to show every feature. This system is called orthographic projection. Each view is lined up with the others, so a feature you can see in the front view sits directly above or below, or beside, the same feature in the next view.

There are two layouts for placing the views, and they matter when you read an overseas drawing. In third-angle projection, common in the United States, the top view sits above the front view. In first-angle projection, common in Europe and much of Asia, the top view sits below the front view. The drawing carries a small projection symbol, usually in the title block, to say which one it uses. If you read the layout the wrong way round you will picture the part back to front.

A section view shows the part as if it were cut open along a marked line, so you can see inside. The surfaces the cut passes through are filled with thin parallel diagonal lines, called hatching. A cutting plane line with arrows, labelled with letters such as A-A, tells you where the cut was made and which way you are looking.

Different lines carry different meanings, and mixing them up is the fastest way to misread a drawing.

A dimension gives a size or a position. A tolerance says how far a real part may differ from that size and still be acceptable, because no process makes a part exactly to the number. Tolerances are written in a few common ways: a bilateral tolerance such as 50.00 plus or minus 0.10, a unilateral tolerance that allows variation in one direction only, or limit dimensions that simply state the largest and smallest allowed size.

The tolerance is what turns a drawing into something you can inspect. A shaft that has to slide into a bearing needs a tight tolerance. A bracket that only holds a sensor may allow a loose one. When a drawing gives no tolerance beside a dimension, the general tolerance in the title block or notes applies.

Geometric dimensioning and tolerancing, known as GD&T, uses a set of symbols in boxes to control form, orientation and position, not just size. It is a separate subject with its own standards, and it is not taught on this page.

A part drawing shows one item. An assembly drawing shows how several parts fit together, with numbered circles called balloons pointing to each part. The balloon numbers match a bill of materials, a table that lists the part number, name, quantity and sometimes material for every item. For a technician, the assembly drawing and bill of materials are how you find the right part number to reorder.

Notes can appear anywhere on a drawing but are usually grouped. Read them. They carry instructions that no view can show, such as a required surface finish, a coating, a break-sharp-edges instruction or a requirement to mark the part. A surface finish symbol on a face tells the maker how smooth that face has to be.

There are three kinds of drawing a controls or maintenance technician reads: mechanical drawings of parts and machines, process drawings (P&IDs) and electrical drawings. This page is the first. Our Drawings and Documentation track teaches the other two in depth: P&ID symbols and instrument tags, tracing a control loop, reading a schematic and wiring diagram, loop diagrams, I/O lists, cable schedules and commissioning documents. It does not teach machine-shop mechanical blueprints or GD&T, and we would rather say so than imply it does.

If you want to practise the process-drawing side right now, the interactive P&ID viewer needs no account. For the mechanical side of machines, such as gears, belts, couplings and fasteners, the Mechanical Foundations track covers how those parts work, which makes the drawings that show them far easier to read.