A Beginner's Guide on how to read Mechanical Drawings

Jordan Elkurdi

Jordan Elkurdi

Founder of Mercury Nero and Engineering Innovation Advocate

Published Oct 19, 2015

In my years of mechanical design I have encountered a vast array of drawings that have both conveyed an elaborate understanding of the assembly or led me on a journey into the fires of Mount Doom. The latter is obviously a drawing that details little information and leaves much to assumption. Nevertheless all the drawings have conveyed enough information to be able to carry out manufacturing or assembly of the finished product. The difference between the two dwells in the assumed technical knowledge of the audience and the amount of time required to decipher the "code". Personally I don't like making assumptions and will tend to over define the drawing for the sake of clarity and leave little to chance.

This article aims to empower the inexperienced readers with some tools to be able to read most General Assembly drawings whilst diverting the assumptions to those with the technical background in the field.

The key to reading the drawings, elaborate or vague, is to follow a simple process that relies on the many similarities of most drawing. Before progressing, my suggestion is to have a drawing handy whilst going through below process.

  1. Always start with reading the details in the title block. This information is regarded as the context in which the drawing should be perceived. The details should give you the following information;
    • General tolerances
    • Projection details
    • Scale used in the drawing
    • Status of the drawing (Preliminary, Approved, etc.)
    • Name of the component or assembly
    • Contact details of the drawing owner
    • Mass
    • Units used in the drawing
    • Sheet number and number of sheets.
  2. The next step is to read the notes in the drawing (outside the title block). Any information in the notes that conflict with the title block should be considered as the correct information hence the notes will in all instances, supersede the title block information.
  3. Read the Bill of Material (BOM). The bill of materials is a list of the components and the quantity that makes up the general assembly. The BOM is usually tabulated on the first page.
  4. Make sure you have at hand all the relevant pages for the assembly drawings. Drawings could be part of series and hence the entire series should be present in order to be able to translate the information.
  5. Understand the difference between the visible lines, hidden lines and phantom lines.
    • Visible lines indicate an edge is visible in relevant view.
    • Hidden lines indicate the edge is behind a face
    • Phantom lines indicate edges of structure that are relevant but not included in the drawing or phantom lines could also mean a tangent line (where a curve starts or ends)
    • Centre lines indicate the geometric centre of the assembly.
  6. Understand the projections, sections and details. View the drawing itself disregarding the dimensions and try to visualise how and what the assembly looks like in 3D. Most new drawings will have an isometric view to assist. Use the sections and details as a start point and you will soon realise that the details and sections have been created to highlight important components or features.
  7. Using the BOM try to find the components in the drawing in order to understand the role the component plays.
  8. Look at the notes that have a leader (arrow) to the assembly, this information is usually vital to the assembly and its functionality.
  9. If in doubt, ask. Drawings are essentially a transmittal of instructions and if you don't understand the instructions then it is likely that they may have been vague in order to prompt an enquiry with the owner of the drawing. This is common practise in highly elaborate assemblies that are not a mass produced.
  10. Use the dimensions in the drawing to be able to grasp the size of the component/Assembly and even use a tape measure or ruler to gage the size.

Remember this process may take up to an hour or even more depending on the complexity of the assembly and on the experience of the reader. If the drawing is too much to absorb, then it is better to put the drawing down and resume after a few hours.  This will allow a better perception of the lines and also allow the subconscious to mull over the details whilst the conscious is distracted.

I hope this has been helpful and by all means if you have more suggestions please add them in the comments below. I only attempt to dispense the very basics as a foundation for growth.

 Happy reading

Jordan Elkurdi

Others also viewed