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Technical Note 09
Designing a "Trapezoid" Box

A common box shape used in automotive designs is a "trapezoid" box shape. The reason we have placed the word trapezoid inside quotation marks is because in Geometry the word trapezoid is used only to describe a two-dimensional shape. When talking about a three-dimensional shape, a box whose cross-section (or side view) looks trapezoidal is properly referred to as a truncated pyramid (a pyramid with its top cut off). This is illustrated below:

The difference between a trapezoid and truncated pyramid.

The truncated pyramid shape is quite flexible as shown below. The top can be smaller than the base or the base can be smaller than the top or any combination.

The truncated pyramid can produce four general shapes.

Eminence Designer always assumes that a trapezoidal side is the front of the box (this is depicted as the shaded side in the examples below). However, you don't have to use the box this way. If you specify the same wall thickness for all sides, then it doesn't matter which side is used for the front when you build the box. In the samples below, the box is rotated in different directions to show how the "top" and "bottom" can also be substituted as the "front".

Although the truncated pyramid cannot be rotated in Eminence Designer, there is nothing to stop you from picking a different side for the front of your box.

Note however that Eminence Designer doesn't allow you to rotate the truncated pyramid drawings. If you use a different side for the front, you'll need to add a note to the printouts so others will know which side is being used as the front.


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