How to read technical drawings
Understanding how to read part drawings is essential to designing a product. Technical drawings, also called mechanical drawings, mechanical working drawings or manufacturing drawings, provide the details needed to manufacture the product being depicted.
They can also be used after the fact. Let’s say you have the technical drawing of a fastener that’s already been manufactured. You can use the drawing to see the following information:
- Geometry
- Dimensions
- Tolerances
- Materials
- Construction
Manufacturing technical drawings are not replacements for 3D CADs, which are excellent representations of products. But they do not provide the information that technical drawings give you. The technical-drawing definition states that it is a precise and detailed drawing. Below is the drawing for a tapered cap, which protects profiles from damage and debris, moisture and liquid ingress during manufacturing, storage and transportation.
Technical-drawing standards
Let’s start with the technical drawing information box on the right-hand side. This is a technical drawing title block example, and tells you the product designer, which in this case is Essentra Components.
On the bottom of the title block, the mechanical engineering drawing provides other information that tells you more about the cap.
1. All dimensions measured in mm
The figures in the table at the left of the box are in millimetres.
2. Do not scale
This is standard in engineering diagrams and means that you shouldn’t take measurements from the drawing itself. The stated dimensions give you the information you need.
3. Linear Tolerance
Another standard piece of information. Once manufactured, all parts vary slightly in their actual measurements. Linear tolerance is the amount of error acceptable in the dimensions of the product.
Understand how to read tolerances. In the example above, there are three columns. Each column indicates the tolerance of a dimension, depending on the number of decimal places that the dimension has. So the column headed 0 means a dimension with no decimal places, while 0.0 is a dimension with one decimal place and 0.00, a dimension with two decimal places.
As an example, let’s take the dimensions for d1 in the drawing, which is 11.3 mm. The size puts it in the 10-30 mm range. To discover the tolerance, we go to the column headed 0.0, because 11.3 has just one decimal place. The tolerance is therefore more or less 0.20 mm.
4. Third angle projection
The engineering title block also includes the symbol for third-angle projection, meaning that’s the projection in which the cap is shown. Only the U.S., UK, Japan, Australia and Canada use third-angle projection. All other countries use first-angle projection. As you can see, they are mirror images of each other.
Third-angle projection tells an engineer that the top view of the product comes above the front view, and the right-side view is drawn to the right side of the elevation. As a comparison, first-angle projection the top view comes below the front view and the right-side view is drawn to left side of the elevation.
While the technical-drawing example of the cap shows the symbol and states that this is a third-party projection, some designers just show the symbol.
5. Angular tolerance
The angular tolerance of a dimension. Like linear tolerances, this tells you how much error is allowed.
6. Unspecified Radii
When radii of bends or fillets (rounding of a part’s corner) are the same, or if any radius is predominant, it’s standard not to draw the radii dimensions on the image. Instead, it’s noted as unspecified radii in the engineering drawing title box.
7. Draft-Angle
A requirement for injection-molding designs. Calculated in degrees, the draft angle is measured from the vertical axis of the mold to account for shrinkage, which is common with thermoplastics. Note, the material shown in the box is LDPE, a popular thermoplastic.
The other pieces of information are self-explanatory, such as item number and the name of the product.
Mechanical-drawing symbols
Technical-drawing symbols provide more information. So do types of lines in engineering drawings. Below are ISO drawing standards:
Plus or minus: communicates the precision of an approximation in the information box.
Continuous, thick line: indicates outlines or edges.
Continuous, thin line – used for several purposes: imaginary lines of intersection, projection lines, hatching, bending lines, outlines of revolved sections and short center lines.
Dashed, thin lines – can also be thick, but must be consistent within drawing. Indicates hidden outlines and hidden edges.
Dashed, thick lines with dots – does not communicate anything about geometry. Indicates surface requirements.
Dashed, thin lines with dots – axis lines in front of section planes.
Thin chain line with thick ends – identifies the plane in which the part is cut. If the cut line is on more than one plane, the directional change is designated with thicker ends.
Continuous, thin zigzag line – signals a break when a part needs to be shortened for ease of visibility.
Continuous free hand – indicates breaks or cuts.
Long, thin dash and double short-dash lines – show adjacent components and also alternative or extreme positions. Where bends are indicated, these lines represent the initial outlines before bending. Can also show parts in front of the cutting plane.
Section line – drawn at 45˚ use to show interior view of solid areas of cutting plane line.
Third-angle projection, which shows the right view.
First-angle projection, which shows the left view.
Reading dimensions
Below is a section of the drawing along with the key. We already know that the measurements are in millimeters, so simply match d1 (dimension 1) to the table, and we see that it measures 11.3 mm.
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