Quick Reference For Using Technical Drawings

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Quick Reference For Using Technical Drawings

How to read plans, lines, views, and more

by John A. Nelson


Technical drawing is a simple “graphic language” that has its own special rules and standard practices. Drafters and designers around the world work from this very tight set of rules or drafting standards. The rules are updated and modified every ten or twelve years.

Understanding this language will make your woodworking projects easier and allow you to read and understand most of the drawings used today. It will also save you time and, in some instances, save you material.

Like any other language, some of the technical points can get very complex, but learning some of the basics can start you on your way to reading technical drawings with ease.

The following basic rules of reading technical drawings are written using the latest drafting standards. Note, however, that the following information is the way it should be done. Now and then you will find drawings that are very poor and do not follow these rules.

Figure 1: Line Thickness. Note the difference between a thick line and a thin line.

In this article, we will look at two basic ideas: recognizing types of lines and their functions and understanding orthographic projection. If you understand these basics, technical drawings will become easy to read.

Identifying Lines

The first step to reading technical drawings is understanding the language of lines. Each line on a drawing represents something very specific, such as a visible surface, a center of a feature, a cut away surface and alternate positions or parts. The chart on page 60 and the following information will help you recognize the different types of lines and what they represent.

Figure 2: Object, Center and Hidden Lines.

Line Appearance is your first clue to determining what type of line you are seeing and what purpose it serves. There are two major kinds of lines—thick and thin. (See Figure 1.) Thick lines typically represent visible object lines—lines that you could see if you were looking at the actual object—cutting plane lines and break lines. Thin lines are hidden lines, center lines, section lining, phantom lines, extension lines and dimension lines. (See Figures 2, 3, 4 and 6.) A general rule of thumb is that thick lines are the most important lines on the drawing. All lines on a drawing should be black regardless of the thickness.

Another characteristic for distinguishing between lines is noting whether they are solid or dashed. In addition to determining whether a line is solid or dashed, also note if the line has a repeating dash or a repeating dashed pattern. For example, center lines have a repeating long line-short dash-long line pattern (See Figure 5.) whereas hidden lines have equal-length repeating dashes. Making these observations will help you to recognize the function of the line.

Figure 3: Cutting Plane Line and Section Lining.

The addition of arrowheads and dimensions to lines are other keys to determining line type. If, for instance, a line has a dimension in the middle of it, it is almost certainly a dimension line. (See Figure 7.) Arrowheads are commonly associated with dimension lines and cutting plane lines.

Line Use itself will help you to determine what type of line you are seeing. Ask yourself questions about what the line is showing. Is it extending from the object? Then it is probably an extension line. Is it marking the center of a circle? It’s probably a center line.

Using these tools to identify the lines used in technical drawing is a good start to proficiently reading many woodworking plans. Let’s move on to orthographic projection.

Understanding Orthographic Projection

Figure 4: Thick Break Line. This figure shows a 1⁄4″ square bar that is 48″ long. The end of the square bar is drawn full-size but the length is drawn 2″ long. Two free hand break lines in the center of the bar indicate that the dial is broken. The dimension notes the full size, or 48″.

Orthographic projection is a multiview system used to illustrate a three-dimensional object on a flat piece of paper. Using this system, any three-dimensional object can be fully illustrated on a flat piece of paper, regardless of how complicated it is.

All objects have a length (L), a width (W) and a depth (D). To capture a three-dimensional object on a two-dimensional piece of paper, we show the length, width and depth of an object through drawings of each of its planes—front, back, right side, left side, top and bottom. Projections of complex projects will often use all of the views of the object; however, simpler projects may not require all of the views.

Figure 5: Center Line. This figure shows a 1⁄4″-diameter rod 2″ long. The center line indicates that the surface is round. Note how the center line is used in the end view.

Always remember that the front view is the most important view. In most simple projects, the front view is the only view needed. Take a look at any one of your scroll saw patterns. The pattern you chose probably shows only the front view. Most scroll saw patterns do not need a top view or a side view because these views only show the thickness of the material. However, a more complex scroll saw project may require an assembly drawing with more than one view. A very complicated project may need a section view to show the relationship of the interior parts. The more complex the object, the more views are needed.

Figure 6: Phantom Line. To the left, the pendulum is drawn with a thick object line, as far left as it can go. The phantom line is drawn to the right to indicate the maximum range of motion to right. To the right, the phantom line shows the relationship and location of the mating parts.

As an example, Figure 8 shows a pictorial view of a house. Each arrow around the house points in a different direction to view the house. Standing directly in front of the house, you would see the front view illustrated in Figure 9. Imagine walking around to the right side of the house for the next view. The house now looks like the right side view in Figure 9. Continue walking around the house to the rear. This view is the rear view in Figure 9. Completing the walk around the house, the left side would look like the left side view in Figure 9. Now imagine being lifted in a helicopter to a point directly above the house. You would see the top view in Figure 9. Finally imagine burrowing beneath the house and looking up to see it from the bottom as in the bottom view in Figure 9.

Figure 7: Dimension and Extension Lines. Here are various methods used to add dimension lines and dimensions.

Study each view by itself. Notice that looking at the front view alone will not give anyone enough information to build the house. The front view shows the length and height but not the depth of the house. Two windows and a door are located on the front view, but the rest of the windows and doors in the house are not shown. Also note that none of the five remaining views alone describe the house well enough to build it. One view alone is helpful, but not complete.

Now consider all the views together. Using all of the views together, the shape of the house and the location of the doors and windows are easily understood. By using orthographic projection, we are able to present all of the important information for constructing the house.

Figure 8: Pictorial View of a House.

Remember, especially when you are creating your own multi-view drawings, that the front view is the most important view and the starting point of every orthographic projection, no matter how many views are used. All other views are projected from the front view. In Figure 9, the largest number of views available to describe an object are shown; however, most objects can be described using three views— the front, top and right-side views.

To help you better understand orthographic projection, refer to the drawing in the pull-out pattern section. Glue it to a piece of heavy paper, cut it out carefully and fold it along the dotted lines. It should result in a cube. Tape the cube together with transparent tape. The cube will illustrate the relationship between all of the views and why they are placed on the paper the way they are.

Figure 9: Orthographic Projection of a House.

By using the basic ideas and tools presented here, you can tackle the plans for woodworking-based scrolling projects with confidence and ease.

Line Appearance Use
Object line Thick, solid Most important kind of line. Shows outline of object.
Hidden line Thick, dashed Represents any surface that can not be seen.
Center line Thin, dashed (long-short-long pattern) Notes center of object, hole, or radius. Also illustrates the center of object with curved or round surfaces.
Cutting plane line Thick, dashed, arrows at each end Represents where imaginary saw cut through object, splitting it in half. Shows what object would look like if it were cut in half. Arrows point to direction you are looking after object is cut in half.
Section lining Thin, solid Added to surfaces where imaginary saw cut through. (Think of thin section lines as a painted surface) Drawn at 45-degree angle and spaced about 1/8″.
Break line Thick or thin, solid, drawn to match object line thickness Indicates that object is broken, or not the full size in a certain direction.  
Phantom line Thin, dashed (long line-two dashes-long line pattern) Shows alternate position of part, such as clock pendulum. Shows where a mating part is located.
Extension line Thin, solid. Sometimes dimension is added between extension lines. Other times, dimension placed to side of extension lines, due to space considerations. Extend from object to be dimensioned.
Dimension line Thin, solid, arrowhead at each end, usually dimension is positioned in center of line. Shows actual dimension between the arrowhead tips. In dimensioning a radius (R), actual dimension is distance between swing point and tip of the arrow regardless of where the arrowhead is placed.

About the Author

John A. Nelson is the author of Fox Chapel’s popular Scroll Saw Workbook, available at www.foxchapelpublishing.com.






This article was originally published in Scroll Saw Woodworking & Crafts Fall 2004 (Issue 16).





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