This website and blog naturally focus on GD&T, but it’s a good time to discuss the importance of simple print-reading skills as a prerequisite to learning GD&T. As I travel around teaching classes on GD&T, you’d be surprised how many people don’t fully understand some of the simple rules of drafting, view layouts, and notation on drawings.
First, note that there can be different terms for this skill; the title of this blog entry mentions “blueprint” reading, but nobody uses actual “blue” prints anymore. (This name was given because at one time they really were blue, due to the chemical process used in producing these drawings; see here for more on the history of this.) I suppose a more proper term today would be an “engineering drawing” but if you want to call them blueprints still, hey, go ahead.
If GD&T is to make sense, then the object being toleranced must certainly be understood first. Most drawings use “orthographic” projection, which is simply a fancy name referring to the straight-on, flat view of a part from a particular angle. Think of a cube: each of the six sides can be flattened out to display six orthographic views. Depending on the part, there may be fewer or more than six orthographic views (in addition to other views such as section views).
Here is a simple example showing three orthographic views of the same part:
The trick is to look at a series of flat pictures and be able to visualize a three-dimensional object from those views. Some people are born with that gift of spatial viewing, others can get it with practice. Plus, on many modern drawings a 3-D “isometric” view is given, which certainly helps to visualize the part.
(Note: the traditional orthographic views are usually laid out in a specific arrangement. Here in North America, the standard arrangement is “third-angle projection,” which places the top view above the front view, and the right side view off to the right side of the page. In Europe and many other countries, the predominant arrangement is “first-angle projection,” where the top view is placed below the front view and the right side view is placed to the left. They are both acceptable, but simply different customs. To be sure, always read the title block or notes to determine which system is being used!)
Here’s one reason why all this is all so important to GD&T: Suppose a surface is labeled as datum A in a certain view. When we look at another view we may see a feature control frame tolerancing another surface back to datum A. But if we make a mistake in the visual interpretation, then we may end up applying the tolerance to the wrong face of the part!
Even if you may feel embarrassed about not being proficient at print reading, don’t hesitate to ask for help or seek out some self-study training materials. Happy new year to everyone…
