Technical drawings are everywhere in engineering. They can be used to define how a part should be manufactured and inspected or to explain how the different parts of a system fit together. Ultimately, they're tools that engineers use to communicate, so being able to understand them is an important skill but it can sometimes feel like they're quite difficult to decipher.
In this video, we'll cover everything you need to know to make sense of them, so let's get started.
Types of Engineering Drawings
Engineering drawings come in all shapes and sizes. Assembly drawings show how all of the different components of an assembly fit together and the functional relationship between them.
Detail drawings, on the other hand, fully define the geometry of a single component. Although they don't normally define the manufacturing methods that should be used, they provide all the information needed to fabricate and inspect a specific part.
Other types of drawings include layout drawings that are used to illustrate a design approach and so don't include much detail, and interface control drawings that identify any interfaces with other components.
Standards and Structure of Drawings
Drawings often follow conventions that are defined in technical standards with the most common being ASME Y14 and the various ISO standards. Standards go into a lot of detail and companies usually have their own requirements anyway, so in this video we'll just cover the fundamentals and some best practices without focusing on any one particular standard.
Regardless of the standard being used, drawings all have the same general structure.
| Drawing Section | Description |
|---|---|
| Title Block | Located usually at the bottom right corner, contains company logo, drawing title, drawing number, scale, author, checker, approver, and sometimes material and finish information. |
| Revision History | Usually at the top right corner, lists changes to the drawing. |
| Drawing Space | Where views of the component or assembly are shown. |
Understanding Views and Projection Methods
The primary views of a drawing include the front, side, top, and bottom views that are key to any detailed drawing. These are created using orthographic projection, where visible edges of the part are projected onto an imaginary plane aligned with the face of the object.
One view is chosen as the front view, usually the one that provides the most information about the object. Additional views show other sides to fully define the object in three dimensions.
Views are arranged using either third angle projection, common in North America, or first angle projection, more common in Europe. The difference lies in the placement of the views relative to the front view.
Additional Views and Details
Other views such as isometric or exploded views may be added to improve clarity or show how parts fit together. Hidden lines indicate internal geometry, often shown as dashed lines, or sectional views show a sliced object with hatched surfaces representing cut areas.
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Drawings may include tables like a bill of materials listing parts and quantities, and notes specifying important information such as torque ranges or material specifications.
Dimensions and Tolerancing
Dimensions provide all necessary sizes to manufacture a part, including lengths, diameters, and radii. Callouts identify features such as fillets or holes, often indicating the number of occurrences or typical application.
Proper dimensioning avoids placing dimensions inside parts and uses sectional views rather than hidden lines for details. Center lines reinforce circular features for clarity.
Holes and Threads
Hole callouts include diameter and depth, with counterbored or countersunk holes showing appropriate symbols and dimensions. Threaded holes are drawn with two concentric circles and specified by standardized thread types.
Metric threads use the ISO standard starting with M followed by diameter and pitch, while Unified threads use nominal size and threads per inch with class of fit.
Tolerances and Their Importance
No part can be manufactured exactly to nominal dimensions, so tolerances define acceptable deviations. These can be specified as limits or plus/minus values. General tolerances apply where none are explicitly stated.
Tolerances impact manufacturing cost and inspection difficulty, so specifying only what's necessary is advised.
| Dimensioning Method | Description | Effect on Tolerances |
|---|---|---|
| Chain Dimensioning | Dimensions applied consecutively between features. | Can accumulate tolerance stacking. |
| Datum Dimensioning | Dimensions referenced from a common datum feature. | Avoids tolerance stacking and simplifies inspection. |
Geometric Dimensioning and Tolerancing GDNT
Traditional tolerancing only considers size, but GDNT controls form, orientation, and position, allowing more precise control over features like flatness or roundness.
Drawings using GDNT include feature control frames defining these additional requirements, overriding normal tolerances.
GDNT is a complex topic deserving its own detailed coverage.