How To Read Fabrication Blueprints

Here Be Dragons

Drawing a diagram to explain a relative position or a movement has been around since the dawn of time. Taking a stick and sketching an attack plan in the dirt is as old as human warfare (yet as fresh as last week’s neighborhood flag football game).

While written instructions in cuneiform can be found on Mesopotamian clay tablets dating to the bronze age (around 3000 BCE), it is likely that the first illustrated set of instructions was the Egyptian Book of the Dead, a set of papyrus scrolls that were buried with the dead beginning around 1500 BCE to give them spells to use in the afterlife. Ancient Greeks later created illustrated medical texts on individual sheets of papyrus in the 3rd or 4th century BCE.

Medieval mapmakers would ink out their best understanding of trade routes and open seas to help guide travelers, occasionally inscribing an illustration of mythological creatures like sea monsters or dragons in uncharted regions thought best to be avoided. Meanwhile, inventors like Leonardo Da Vinci were sketching out their concepts in detail, allowing them to be easily followed by others.

Why So Blue?

Following the Middle Ages, hand-drawn sketches were the only practical way of reproducing multiple copies of architectural and engineering documentation, though they could vary from copy to copy and often lacked precision and clarity. When the process known as blueprinting was invented in the 19th century, a more efficient and accurate way to reproduce technical drawings became available.

In 1842, photographer and chemist Sir John Herschel invented a process called cyanotype (meaning “dark blue impression”), which produced a white image on a blue background. A technical drafter could make an ink or pencil drawing on semitransparent paper which could then be overlayed on light sensitive material and then exposed to light. The light would turn the background blue, but leave the areas covered by the lines white, basically creating a negative of the original that came to be known as a blueprint.

A century later, blueprints were replaced by diazo prints with blue lines on a white background, which were easier to read and faster to create. Known as whiteprints or bluelines, diazo prints were—and still are—often just called blueprints. Even in today’s digital era, the usually black and white architectural or engineering documentation created via computer-aided design (CAD) will often still be referred to as blueprints just because of tradition.

Understanding Fabrication Blueprints

The key to understanding a manufacturing blueprint, such as a set of metal fabrication directions, is to know what pieces of information such schematics contain and where they are located.

The components of a fabrication blueprint will include most of the following:

  • Diagram of the Part. The bulk of the fabrication blueprint will be an illustration of the part itself, usually shown from more than one angle. The part will be shown to scale, meaning that an inch in the illustration will equal a specific amount—larger or smaller—on the physical item. The drawing of the part will consist of different types of lines, such as object lines (thicker lines showing the edges of the part), hidden lines (dashed lines showing edges that are behind the current view of the part), dimension lines (showing a distance between two points), center lines (dashed lines that indicate the center axis of an object), phantom lines (indicating movement, like a hinged part), and break lines (designating that only the two ends of a longer section are being shown).
  • Title Block. Blueprints will contain one or more information blocks, which are boxes that contain details about the part. The most important of these is the title block. Located in the lower right corner of a blueprint, the title block usually contains the name and address of the organization that commissioned the blueprint, the name of the engineers who created the drawing and the date of their work, the name or description of the part, a part or drawing number used to identify the component, and the dimensions of the image. The material used in the project should be identified here, and it may list other requirements like heat treating or a surface finish. The title block is often where the scale of the image will be listed as a ratio, with the first number indicating the part’s size and the second being the size in the image. So, a ratio of 1:2 would mean that the drawing is twice the size of the actual part, while 2:1 denotes that the drawing is only half the scale of the part.
  • Revision Block. Usually right above or to the left of the title block, a revision block indicates changes that have been made to the drawing, including who modified it and when.
  • Tolerance Block. A tolerance block lists the acceptable range of variation—upper and lower limits—for a particular dimension and will usually be located near that dimension. A general tolerance block, typically located near the title block, will identify dimensions that aren’t explicitly mentioned on the drawing.
  • Bill of Materials Block. A bill of materials (BOM), also called a parts list, will be included if additional materials need to be added to complete the part. It will usually be found near the title block or in the upper left corner of the print.
  • Legend. A key to what each symbol on the blueprint means will be found in a legend area or block located adjacent to the drawing.
  • Notes. Any details or specifications about the part that can’t readily be seen in the drawing will be included in a notes section, usually located in the upper or lower left corner of the blueprint.
  • Grid. Many blueprints will include a grid or a set of numbers and letters that run along two edges at equal intervals to designate zones or locations on the diagram so that a specific area in the image can be referred to easily (such as “B3”).

It is important for the fabricator to keep their measuring equipment calibrated so that all measurements and tolerances in the blueprint can be precisely matched.

A quality set of blueprints will give any metal fabricator everything they need to know to create the specified part and will allow for very accurate quality control as the finished parts are inspected and compared to the printed information.

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