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24 Chapter 3 Freehand Sketching
In this chapter you will learn useful techniques for freehand sketching to create both two-dimensional orthographic sketches and three-dimensional pictorial sketches. You will learn how to quickly make rough sketches to convey a concept and how to make more refined sketches of objects that are more complex.
An integral part of the creative design process is ideation , the generation of concepts or ideas to solve a design problem. Often freehand sketching can be used to explore and communicate mental concepts that come about in the mindās eye. The process of sketching can solidify and fill out rough concepts. Furthermore, sketching captures the ideas in a permanent form that can be used to communicate the concept to others. In this way, sketches often act as stepping stones to refine and detail the original concept or generate new ideas. Many great design ideas are first sketched on the back of an enve- lope or in a lab notebook, such as the freehand sketch of one of helicopter inventor Igor Sikorskyās designs, (Figure 3.1). While computers are the workhorses for engineering graphics, initially generating ideas on a computer screen is very rare. A more common scenario is sketching an idea on paper and subsequently refining the concept on paper using more rough sketches. This often occurs simply because all that is needed for a freehand sketch is a pencil and a paper. Freehand sketching quickly translates the image of the concept in the mindās eye to paper. Engineers often communicate via rough freehand sketches to refine and improve the design. Sketches are much more useful than detailed CAD drawings early in the design process, because they are informal, quickly and easily changed, and less restrictive. It is only after clarifying the design concept by iterating through several free- hand sketches that it is possible to draw the object using computer graphics. In fact, often an engineer will sit down to create a CAD drawing of an object using a freehand sketch as a guide. This chapter focuses on the rudimentary elements of freehand technical sketch- ing, because in many ways freehand sketching is the first step in CAD.
Freehand sketching requires few tools: just a pencil and paper. It may be tempting to use straight-edged triangles or rulers for drawing straight lines and a compass to draw circles. But these instruments often slow down the process and distract from the pur- pose of sketching, which is to create a quick, rough graphical representation of the image in the mindās eye. Generally sketching has three steps, although the steps are usually subconscious. First, the sketch is planned by visualizing it in the mind including the size of the sketch on the paper, the orientation of the object, and the amount of detail to be included in the sketch. Second, the sketch is outlined using very light lines to establish the orientation, proportion, and major features of the sketch. Finally, sharp- ening and darkening object lines and adding details develops the sketch. All sketches are made up of a series of arcs and lines, so the ability to draw circles and straight lines is necessary. A straight line is sketched in the following way. First, sketch the endpoints of the line as dots or small crosses. Then place your pencil on the
26 Chapter 3 Freehand Sketching
design engineers are the ones who can sketch an idea clearly in a minute or so. The goal of the sketch in this case is not to show the details of the part, but to provide another person with a clear concept of the idea. For example, a design engineer may need to show a sketch to a manufacturing engineer to get input on the manufacturability of a part. If the concept is at an early phase, CAD drawings would not have been created yet. So the design engineer needs to use a freehand sketch of the part. The sketch of Sikorskyās helicopter in Figure 3.1 exemplifies the power of free- hand sketching. A brief glance at this sketch provides immediate insight to the concept that is being shown. One does not need to study the sketch to know what is being sketched, even if the viewer has never seen the concept before. These quick ideation sketches are not difficult to draw and require no artistic talent, just some practice. Two types of pictorial sketches are used frequently in freehand sketching: oblique and isometric. The oblique projection places the principal face of the object parallel to the plane of the paper. The isometric projection tilts the part so that no surface of the part is in the plane of the paper. The advantage of the oblique projection is that details in the front face of the object retain their true shape. This often makes oblique freehand sketching easier than isometric sketching, where no plane is parallel to the paper. The disadvantage of the oblique projection is that it does not appear as āphotorealisticā as an isometric projection. In other words, an isometric projection is similar to what a photo- graph of the object would look like.
Often freehand sketching begins with light thin lines called construction lines that define enclosing boxes for the shape that is being sketched. Construction lines are used in several ways. First, the construction lines become the path for the final straight lines of the sketch. Second, the intersection of construction lines specify the length of the final lines. Third, points marked by the intersection of construction lines guide the sketching of circles and arcs. And finally, construction lines guide the proportions of the sketch. This last item is of crucial importance if the sketch is to clearly represent the object. For example, if an object is twice as wide as it is high, the proportions in the sketch must reflect this. Proper proportions of the boxes defined by the construction lines will result in proper proportions of the sketch. An oblique freehand sketch is easy, since it begins with a two-dimensional repre- sentation of the face of the object. Figure 3.4 shows the steps in quickly sketching a part with a circular hole.
Figure 3.
Section 3.3 Basic Freehand Sketching 27
Step 1: Horizontal and vertical construction lines are lightly drawn to outline the basic shape of the main face of the part. This is known as blocking-in the sketch. If you are using a pencil or felt-tip marker, press lightly when drawing the construc- tion lines to produce a thin or light line. If you are using a ball-point pen, draw a single, light line. Step 2: Sketch in the face of the part using the construction lines as a guide. How you sketch the outline of the part depends on the type of pen or pencil that you are using. The idea is to thicken the lines of the part compared to the construction lines. If you are using a pencil or a felt-tip marker, pressing hard for the outline of the part will result in heavy or dark lines. If you are using a ball-point pen, the line width does not depend much on how hard you press. In this case, the outline of the part is sketched with a back and forth motion of the pen to thicken the lines of the part compared to the construction lines as shown in Figure 3.4. The straight lines are usually sketched first, followed by the arcs. The circle for the hole in the part is added last to complete the face of the part. Step 3: Sketch receding construction lines (lines into the plane of the paper labeled a ) at a convenient angle. All of the receding lines must be parallel to each other and are usually at an angle of 30Ā° to 45Ā°. The receding lines end at the appropriate depth for the object. Then vertical and horizontal lines at the back plane of the part are added (lines labeled b ). This blocks in the three-dimensional box enclosing the object. Step 4: Sketch in and darken the lines outlining the part. Again it is usually easiest to sketch in the straight lines first, then the arcs, and finally any details. Because the construction lines are light compared to the outline of the part, they are not erased.
The final sketch, while rough and lacking detail, clearly shows the design intent for the part.
Figure 3.
Section 3.4 Advanced Freehand Sketching 29
The sketching methods described in the previous section were focused on sketches in which the face of the object is in a single plane. Freehand sketching is somewhat more difficult when the face of the object is not in a single plane. The difficulty here is accu- rately depicting the depth of the object. Oblique and isometric projections are still use- ful, though somewhat more complicated than those in the previous section. In addition, orthographic projections are also valuable.
An example of the steps leading to an oblique freehand sketch of a complicated object are shown in Figure 3.6. Because the face of the base of the object and the face of the upper portion of the object are in different planes, it is necessary to begin with a box that encloses the entire object before sketching either face. Some of the construction lines are removed after they are used in this example. This was done here to make the sketch more clear. However, this is not necessary in practice, if the construction lines are drawn as light lines.
Step 1: To begin, construction lines to form a box that encloses the object are drawn to block-in the sketch. Notice that the front and back faces of the box are rectangular with horizontal and vertical sides. The receding construction lines are parallel and at an angle of 30Ā° to 45Ā° to horizontal. The easiest way to draw this box is to first draw the front rectangle ( a ). Then draw an identical second rectan- gle above and to the right of the first rectangle ( b ). Finally connect the corners with receding construction lines ( c ). Step 2: Now the front face of the base of the object can be sketched in the front rectangle. The lines are appropriately darkened. Step 3: Certain features of the front face of the base extend backward along or parallel to the receding construction lines. For example, the lines ( d ) forming the chamfer (angled cut on the right side of the base) can be sketched parallel to receding lines. Likewise the receding line for the upper left corner of the base can be sketched ( e ). Then the base can be finished with a horizontal line on the back face ( f ). Now it is possible to block in the upper rounded portion of the object to create a box ( g ) that encloses the upper protrusion within the larger box that encloses the entire object. Step 4: The front face of the upper portion of the object can be sketched in this box. Then receding lines corresponding to the chamfer and the left edge of the base can be darkened. In addition, the lines forming the back face can be sketched. Note that the line forming the back edge of the chamfer is parallel to the line forming the front edge of the chamfer. Construction lines ( h ) on the front face of the upper portion are drawn to center of the circle for the hole. Step 5: A receding construction line ( i ) extending from the peak of the front face to the plane of the back face is sketched to aid in aligning the curved outline of the back of the upper portion. The back face is identical to the front face except that it is shifted upward and to the right. This results in the left side of the back face being hidden. A darkened receding line ( k ) finishes the left side of the upper por- tion of the object. Finally, four construction lines ( m ) are sketched to block in the circle for the hole.
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Step 6: Now the hole can be sketched in and darkened. The back edge of the hole is also added to complete the sketch. The construction lines may be erased, but usually the construction lines are retained if they are made properly as light lines.
Oblique sketching is often aided by the use of graph paper with a light, square grid. The process is identical to that shown in Figure 3.6, but it is easier to keep the pro- portions correct by counting the number of boxes in the grid to correspond to the approximate dimensions of the part. Graph paper further improves the sketch by help- ing keep lines straight as well as more accurately horizontal or vertical.
Isometric freehand sketches of more complex objects start with an isometric box to block in the sketch. Then faces are sketched and additional features are blocked in.
Figure 3.
32 Chapter 3 Freehand Sketching
b, c, and d in Figure 3.7). The procedure for using isometric grid paper is the same as that described above, but using the isometric grid paper keeps proportions of the part consistent. One simply counts grid boxes to approximate the dimensions of the object. The grid paper also aids in sketching straight lines parallel to the isometric axes.
In some cases it is necessary to sketch orthographic projection views rather than oblique or isometric pictorial views. Because orthographic views are two-dimensional represen- tations, they are not as difficult to sketch as pictorial views. But there are several tech- niques that make freehand sketching of orthographic views easier and more efficient.
Figure 3.
Section 3.4 Advanced Freehand Sketching 33
The process for sketching three orthographic views of the object in the previous two fig- ures is shown in Figure 3.8.
Step 1: Begin by blocking in the front, top, and side views of the object using the overall width, height, and depth. The construction lines extend between views to properly align the views and maintain the same dimension in different views. For instance, line ( a ) represents the bottom edge and line ( b ) represents the top edge in both the front view and the right-side view. The distance between lines ( a ) and ( b ) is the height dimension in both views. The space between the views should be large enough so that the drawing does not look crowded and should be the same between all views. Step 2: In the second step the upper protrusion is blocked in. Note that line ( c ) extends across the top and front views, to assure that the width of the protrusion is consistent in both views. Likewise, line ( d ) extends across the front and right-side views. Step 3: The outline of the object is darkened to clearly show the shape of the object in all three views. Care must be taken in darkening lines. For instance, the right corner of the front view should not be darkened, because the detail of the chamfer has not yet been added. Step 4: Construction lines for the holes and other details are added next. The cen- ter of the hole is positioned with construction lines ( e ). Then construction lines that block in the hole ( f ) are drawn. These construction lines extend between views to project the hole to the top view and to the right-side view. Construction lines extending between views ( g ) are also added for the chamfer. Step 5: Now the hole and chamfer are sketched and darkened to show the com- pleted object. Step 6: Finally, centerlines (long-dash, short-dash) that indicate the center of the hole are added. Hidden lines (dashed lines) that indicate lines hidden behind a surface are also added. Construction lines may be erased as was done in this fig- ure, but this is not usually necessary.
Figure 3.
Section 3.4 Advanced Freehand Sketching 35
Figure 3.
Figure 3.
36 Chapter 3 Freehand Sketching
Figure 3.
