Explanation Patterns: Understanding Mechanically and Creatively


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However, if the object in figure 2 had a hole on the back side, it would not be visible using a single isometric drawing.

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In order to get a more complete view of the object, an orthographic projection may be used. Imagine that you have an object suspended by transparent threads inside a glass box, as in figure 3. Then draw the object on each of three faces as seen from that direction. Unfold the box figure 4 and you have the three views. We call this an "orthographic" or "multiview" drawing. Which views should one choose for a multiview drawing? The views that reveal every detail about the object. Three views are not always necessary; we need only as many views as are required to describe the object fully.

For example, some objects need only two views, while others need four.

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The circular object in figure 6 requires only two views. We have "dimensioned" the object in the isometric drawing in figure 7. As a general guideline to dimensioning, try to think that you would make an object and dimension it in the most useful way. Put in exactly as many dimensions as are necessary for the craftsperson to make it -no more, no less. Do not put in redundant dimensions. Not only will these clutter the drawing, but if "tolerances" or accuracy levels have been included, the redundant dimensions often lead to conflicts when the tolerance allowances can be added in different ways.

Repeatedly measuring from one point to another will lead to inaccuracies. It is often better to measure from one end to various points. This gives the dimensions a reference standard. It is helpful to choose the placement of the dimension in the order in which a machinist would create the part. This convention may take some experience.

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There are many times when the interior details of an object cannot be seen from the outside figure 8. We can get around this by pretending to cut the object on a plane and showing the "sectional view". The sectional view is applicable to objects like engine blocks, where the interior details are intricate and would be very difficult to understand through the use of "hidden" lines hidden lines are, by convention, dotted on an orthographic or isometric drawing. To prepare a drawing, one can use manual drafting instruments figure 12 or computer-aided drafting or design, or CAD.

The basic drawing standards and conventions are the same regardless of what design tool you use to make the drawings. In learning drafting, we will approach it from the perspective of manual drafting. If the drawing is made without either instruments or CAD, it is called a freehand sketch. An isometric view of an "assembled" pillow-block bearing system is shown in figure It corresponds closely to what you actually see when viewing the object from a particular angle.

We cannot tell what the inside of the part looks like from this view. We can also show isometric views of the pillow-block being taken apart or "disassembled" figure This allows you to see the inner components of the bearing system. Isometric drawings can show overall arrangement clearly, but not the details and the dimensions. A cross-sectional view portrays a cut-away portion of the object and is another way to show hidden components in a device.

Imagine a plane that cuts vertically through the center of the pillow block as shown in figure Then imagine removing the material from the front of this plane, as shown in figure This is how the remaining rear section would look. Diagonal lines cross-hatches show regions where materials have been cut by the cutting plane. This cross-sectional view section A-A, figure 17 , one that is orthogonal to the viewing direction, shows the relationships of lengths and diameters better.

These drawings are easier to make than isometric drawings.

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Seasoned engineers can interpret orthogonal drawings without needing an isometric drawing, but this takes a bit of practice. The top "outside" view of the bearing is shown in figure It is an orthogonal perpendicular projection. Notice the direction of the arrows for the "A-A" cutting plane. A half-section is a view of an object showing one-half of the view in section, as in figure 19 and The diagonal lines on the section drawing are used to indicate the area that has been theoretically cut.

These lines are called section lining or cross-hatching. The lines are thin and are usually drawn at a degree angle to the major outline of the object.

The spacing between lines should be uniform. A second, rarer, use of cross-hatching is to indicate the material of the object. One form of cross-hatching may be used for cast iron, another for bronze, and so forth. More usually, the type of material is indicated elsewhere on the drawing, making the use of different types of cross-hatching unnecessary. Usually hidden dotted lines are not used on the cross-section unless they are needed for dimensioning purposes.

Also, some hidden lines on the non-sectioned part of the drawings are not needed figure 12 since they become redundant information and may clutter the drawing. The cross-section on the right of figure 22 is technically correct. However, the convention in a drawing is to show the view on the left as the preferred method for sectioning this type of object.

The purpose of dimensioning is to provide a clear and complete description of an object. A complete set of dimensions will permit only one interpretation needed to construct the part. Dimensioning should follow these guidelines. The dimension line is a thin line, broken in the middle to allow the placement of the dimension value, with arrowheads at each end figure An arrowhead is approximately 3 mm long and 1 mm wide. That is, the length is roughly three times the width. An extension line extends a line on the object to the dimension line. The first dimension line should be approximately 12 mm 0.

Extension lines begin 1. A leader may also be used to indicate a note or comment about a specific area. When there is limited space, a heavy black dot may be substituted for the arrows, as in figure Also in this drawing, two holes are identical, allowing the "2x" notation to be used and the dimension to point to only one of the circles. The dimensions should be placed on the face that describes the feature most clearly. Examples of appropriate and inappropriate placing of dimensions are shown in figure In order to get the feel of what dimensioning is all about, we can start with a simple rectangular block.


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With this simple object, only three dimensions are needed to describe it completely figure There is little choice on where to put its dimensions. We have to make some choices when we dimension a block with a notch or cutout figure It is usually best to dimension from a common line or surface. This can be called the datum line of surface. This eliminates the addition of measurement or machining inaccuracies that would come from "chain" or "series" dimensioning.

Notice how the dimensions originate on the datum surfaces. We chose one datum surface in figure 27, and another in figure As long as we are consistent, it makes no difference. We are just showing the top view. In figure 29 we have shown a hole that we have chosen to dimension on the left side of the object.

Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively
Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively
Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively
Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively
Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively
Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively
Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively
Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively
Explanation Patterns: Understanding Mechanically and Creatively Explanation Patterns: Understanding Mechanically and Creatively

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