Engineering drawing interpretations and GD & T
Engineering Drawing Interpretation
Understand elements of an Engineering Drawing.
First Angle and Third Angle Orthographic Projection
In the first angle projection, Front view is at the top and top is below from view aligned with it. Left hand view is placed on the right side of front view and vice versa.
In the third angle projection, Front view is at the bottom and top is above from view aligned with it. Left hand view is placed on the left side and right side view is placed on the right side of front view.
Isometric Views and Isometric Projections
Isometric views are 3-D pictorial views. The isometric x and y axes are 30\(^\circ\) to the horizontal line. z axis is still vertical. In isometric projection, an isometric scale is constructed to account of reduction in apparent length of objects. Circles in represented by isocircles which are ellipses with an angle of 35\(^\circ\)16’.
To avoid isocircles in isometric view, sometimes simplified version of pictorial representation is made where front surface is still perpendicular to the observer. So now, circles are represented as they are instead of isocircles, which increase the speed of drawing. General, Cabinet and Cavalier Oblique view share same concepts, however variations in z axis angle and length are there.
Perspective views have vanishing points to give illusion of depth. They are useful in architectural drawings or drawings where distortion due to large depth is significant. One point, two point and three point perspective views can be drawn based on how much realistic drawing is required.
Title block is the information box generally located at the bottom right corner of a drawing. It contains information regarding company name, Title, scale, date, scale, part number and drawing number. It may contain additional information such as company logo, copyright information, first/third angle symbol etc.
Working drawings are comprehensive drawings ready to be released for manufacturing personnel. They generally contain required orthographic views (Front, top and side) and may have pictorial representation in terms of isometric view. Depending upon the complexity of manufactured part, section view or detailed views may be added. It can also have special instructions regarding tolerances and manufacturing processes.
Bill of Materials: (BOM)
it is the parts list which lists how many parts are there in an assembly. Generally the tabular information contains part name, quantity and material. The parts in the drawing views are referred then with balloons having same serial numbers as in part list.
A table which shows information regarding chronological revisions made in the parts. Typically includes revision number, what revision was made, who authorized it and the date on which it was revised.
If a part has multiple holes, then instead of cluttering the drawing with hole related dimensions, it is preferred to have a separate table showing size and location of holes in the part.
Development of Surfaces and Flat patterns
In air conditioning, where ducts meet at an oblique angle, development of surfaces is required. Special parts such as boxes need to be created out of a cut-out sheet metal surface which is shown in flat pattern.
GD and T
Translate geometric callouts into plain English with meaning.
Explain the major rules found in ASME Y14.5-2009.
Explain the tolerance zones for the 14 geometric characteristics.
Recognize proper application of GD & T.