How to Read a Mechanical Drawing

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Mechanical drawings are a type of technical illustration that describe the dimensions and features of 2-dimensional and 3-dimensional objects. The level of detail in a mechanical drawings varies depending on the intended use of the sketch and the complexity of the item. For example, a mechanical drawing detailing the construction of a table is unlikely to include requirements for the flatness of the wood surfaces because it is assumed that the surface of each plank is relatively level. This type of simplified mechanical drawing is the kind that most non-engineers encounter, however, laymen can navigate more complex mechanical drawings with the assistance of a drafting symbol dictionary.

Look for a non-mechanical sketch or title line that identifies the object. If a title or sketch is not included, look for a drawing that shows the object in profile.

Note the purpose of each structural line. Lines that are used to construct the exterior walls of the item may be a different color than lines that divide sections made from different materials. For example, a tube that has a copper wire core and PVC shell may be drawn in two colors or indicate each material with a label.

Group lines together to form the boundaries of different components, such as the sides of a single piece of lumber.

Look for the dimensions of each line or component. Length and width are typically written within brackets alongside the item they are describing. Multiple measurements can be written stacked vertically radiating from a piece. For example, if two pilot holes are indicated, measurements could indicate the distance between the edge of the wood, the first hole and the second hole, as well as the width of each hole and each screw head.

Isolate a sequence of symbols. Symbols are set apart from the drawing contained within a rectangle near the line that they are describing.

Read the symbols from left to right; the first symbol stands for the dimension being discussed and the second stands for the amount of variance that is tolerable in the stated dimension. Additional symbols explain what the tolerance is relative to. Use a symbol dictionary to decode the sequence. For example, a straight followed by ?.2? and "A" means that the straightness of the indicated line can vary by .2 relative to the line labeled A.

Review your impressions before beginning construction to ensure that you correctly interpreted all lines and measurements. Some lines can be deceptively simple. For example, a straight line can represent the dimension straightness, a joint, a structural wall or it can be a typo.

Things You'll Need

  • Magnifying glass
  • Symbol dictionary