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The Principles of Orthographic Drawing & Dimensioning

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Orthographic engineering drawings have the purpose of making something. The drawing provides enough views and information for someone to interpret it and make the part. Making these drawings requires visualizing the part and deciding how to show features, such as holes, for clarity and dimensioning purposes. Reading one requires understanding orthographic drawing and dimensioning practices.

Orthographic Drawings

Orthographic drawings are the projection of views on a drawing. It is necessary to show only enough views to show all the features. There are six basic views: top, front, bottom, back, right side and left side. The most commonly used are top, front and right side views. The drafter can include other views such as an isometric view which shows it at an angle. A section is showing the piece as if it had part of it cut off. Drafters use section views to illustrate a specific feature.

Top View

Decide how to present the piece to best show all the features. This will identify the views. The top view is looking straight down on the object. All views remain aligned with each other throughout the drawing. Draw the outside of the object with a thick heavy line called an object line. Draw features such as holes in their correct location with object lines. Put this view at the top of the drawing, and position other views accordingly.

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Front, Back, Side and Bottom Views

To identify the front view, rotate the top view 90 degrees clockwise. Align the front view exactly under the top view. The bottom view shows the front view as if rotated 90 degrees clockwise further. Exactly above the top view, and rotated 90 degrees counterclockwise, is the back view. The right side view is rotated 90 degrees counterclockwise from the front view. Show the right side view on the right of the front view. The left side view is rotated 90 degrees counterclockwise from the front view, and shown to the left of the front view. The front view is rotated 90 degrees clockwise to show the bottom view.

Dimensions

The purpose of dimensions is to provide a complete and clear description. The center, overall length, width and all features need dimensioning. This includes thread specifications for boltholes, slots and angles. The American Society of Mechanical Engineers (ASME) publishes “Geometric Dimensioning and Tolerancing Y14.5.” It is the accepted industry authority for dimensioning practice. Its purpose is to provide a consistent guide for those using engineering drawings. It gives accepted callouts and practices to dimension features on a drawing. These standards include the gap between the object and extension line, placement of the dimension line and arrowhead size and placement of dimension text.

About the Author

Now living in Arizona, Les Moore has written reports of motorcycle races for "Cycle News" and "Midwest Motorcycling" since 1969. He has provided technical and procedural data for the Intra and Internet. Moore received a Certificate of Drafting from San Jose Community College in 1982.

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