Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (25 page)

4.15 CROSSHATCHING

Referring again to Figure 4.7, one can see that the Section View has areas of the
object geometry that has a pattern of diagonal lines. This indicates that this part
of the object is actually internal and solid. The crosshatching shows the imagi-
nary geometry that would be apparent if the object was actually cut open.

The sample in Figure 4.7 shows a pattern of 45 degree lines evenly spaced
for the crosshatching. This is the typical pattern. However, there are various types
of crosshatching patterns that could be used. The different patterns can be used to
indicate different types of materials (such as iron, aluminum, nonmetallics, etc.)
Refer to standard ANSI Y14.2M. Standard ISO 128-1982 (E) paragraph 4.2 indi-
cates that the meaning of the hatching shall be clearly defined.

4.16 DESIGN METHODOLOGIES USING

DRAWINGS

The use of drawings for design activity lends itself to a certain methodology. This
methodology involves a sort of hierarchy of tasks, and each level of this hierarchy
usually works with a certain type of drawing. For some companies or design ac-
tivities, all these levels may not actually be present, but for a large company or a
system integration design activity these levels are most likely present. Some com-

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mon examples of the methodology would be in the automotive, aerospace, or en-
gineered equipment sectors. Virtually all these companies are using 3-D
processes now, though, so this drawing methodology may no longer be relevant.
However the same principles can be applied to 3-D design methodologies (this is
discussed in later chapters).

4.16.1 Specifications

The top level of the 2-D design methodology is the General Arrangement (GA)
drawing. As mentioned earlier in this chapter, this drawing basically shows the
end product being designed. This is the drawing that would be used to form mar-
keting assessments and the engineering specification. The engineering specifica-
tion or “spec” is a document (often referring to the General Arrangement
drawing) that is supposed to specify the requirements that the design is supposed
to meet. This specification may also be vital to the ISO 9001 quality system for
the engineering department.

Obviously, then, the General Arrangement drawing is often created first
(before any other drawings for design). This stage of the design process is often
called Conceptual Design. At this stage, the basic concepts of the design are be-
ing assessed and adjusted to meet the various market requirements. It is very im-
portant to be as accurate as possible in this stage of the process. If a major
problem with design is discovered later in the project (after 100s or 1000s of
drawings have been made), then it will be very costly to correct. If problems can
be anticipated at this stage, there is only the General Arrangement drawing to be
corrected.

The General Arrangement drawing would be typically controlled by a
project engineer (although this person may or may not create the drawing). The
project engineer is usually assumed to work on the project over its entire life cy-
cle. As the design proceeds to a level or greater and greater detail, problems with
the original assumptions of the design will arise, and the General Arrangement
drawing will then be revised to reflect the evolution of the design. Because the
General Arrangement drawing is at such a high level, it is rarely used in the ac-
tual manufacturing of the product. However, it is often used in consultations with
prospective and actual customers. This drawing is also likely to be used in post-
sales situations such as retrofits, product support and maintenance, technical
manuals, and product liability.

4.16.2 Layouts and Assembly

Assuming the Conceptual Design phase is successfully completed (the General
Arrangement drawing being finalized as much as possible), the next phase is usu-
ally Preliminary Design. The type of drawings used at this stage are called layouts.
These drawings are used to actually start sizing and fitting together components,

Drawings and 2-D Design 103

parts, and systems. As with General Arrangement drawings, layouts are not really
used in the manufacture of the product, and they would generally not be “re-
leased” (meaning they are not used outside of the engineering department).

Layouts are generally going to start from the graphical information avail-
able in the General Arrangement. That is, the views of the General Arrangement
would be copied and probably scaled to larger size. The layouts will often also
concentrate on specific areas of the product being designed. For an automotive
example, the group responsible for the engine would work on an engine layout,
and one designer would be given the main control over the engine layout draw-
ing. The layout drawing is also probably going to have more views that most
other drawings; it may show the Front View, Right View, Left View, Top View,
Bottom View, etc.

The person doing the layout would then try to figure out how individual
components and parts would fit into the overall design. This usually means that
a specific component is being selected by the designers (and whether it is going
to be purchased or manufactured). Based on that decision, the component would
be drawn in all the views necessary on the layout. This often involves creating
an image of the component in one or two of the views, and then using a method
known as projection to figure out what it would look like in the other views.
This was done with specific mechanical drawing methods prior to CAD sys-
tems, but now the CAD system should be able to do this for the designer. It is
very important that a proper understanding of scale be maintained in the layout
drawing. Components will not fit properly in the eventual physical product if the
components are not properly scaled to the view scale being used in the layout
drawing.

Once the detail design process described below has made sufficient pro-
cess, the layout drawing would often be the basis for an assembly drawing. The
assembly drawing would show how to assemble components. The assembly
drawings would be the drawings that have the “balloons” or “bubbles” mentioned
earlier.

4.16.3 Detailed Design and Drafting

Eventually, the layouts progress to the point that the geometry or shape of spe-
cific components have been determined, and the layout designer feels that the
components have been positioned and sized properly to fit and work properly.
Based on the views of the component in the layout drawing (say Front View,
Right View, and Top View), a “detailer” or detail designer can create a drawing
that can be used by manufacturing for that single component. This type of draw-
ing is called a detail drawing or just a detail, and the individual component in the
design is often just called a detail. In most cases, a 3-D CAD system would refer
to the detail as a Part or a Part Model.

104 Chapter 4

Since detail drawings are used to dictate or guide the manufacturing pro-
cess, these drawings are the ones that would show most of the dimensions, toler-
ances, GD&T, etc. Since these drawings are used in processes outside of the
engineering department (such as manufacturing or purchasing), these drawings
are usually the type that need to be released. They will need the appropriate Title
Block with approvals and revision levels. The detailer needs to know the proper
way to document the individual components; it is best that they also understand
the manufacturing processes that are likely to be used to create the component.
However, a detailer may not need to know anything about the overall system de-
sign and performance.

4.16.4 Drawing Package

With the detail drawings completed, a full package could be created that included
all the information assumed to be needed to build the entire product. The full
package would include the General Arrangement drawing, the assembly or instal-
lation drawings, the detail drawings, and all the Bills of Material or parts lists.
Prior to the adoption of CAD systems, all this information would typically be
captured on “miniature” media such as microfilm (photographic film type of me-
dia with different frames on a roll showing drawings or lists), microfiche (a pho-
tographic film card that may contain dozens of images of drawings), or aperture
cards (mainframe-style punch cards with a small photographic film image em-
bedded in the card). This miniature media could be then be filed and archived for
maintenance and future reference. Although these types of systems may still be
used for legacy information, most drawings and packages are now controlled via
electronic imaging systems.

4.17 CONCLUSION

This chapter has presented basic information on mechanical drawings. There are
many more features of these drawings, but other resources can be consulted for
more detailed information. A few of these additional features would be surface
finish symbols (generally indicating how rough or smooth surfaces need to be),
weld symbols (indicating how parts are to be welded together), screw and thread
representations and notes, and “broken lines” (where objects are too long to be
shown completely). However, what has been presented are the most essential ele-
ments to allow someone to become basically familiar with drawings.

4.18 CHAPTER EXERCISES

1. Try to locate a board drawing (drawing created by hand) from your
campus or company. Record the date it was created, the project is was made for,

Drawings and 2-D Design 105

the drawing size, the revision level, and how it was approved. In a campus situa-
tion, check with the on-campus machine shop, plant engineering, or plant ser-
vices department if needed.

2. Create a hand sketch drawing for a simple object using a Front, Right,
and Top View. If you can find them, devices such as a drawing board, T-square,
triangle, compass, and scale could be used.

3. Study a copy of an ASME Y14 or ISO 128 standard. Record whether
the drawing in Exercise 1 (or any other drawing from your campus or company)
conforms to the standard.

4. Study copies of a corporate drawing or drafting standard. Record
whether the sample drawing or drawings conforms to the corporate standard.

4.19 CHAPTER REVIEW

1. In the First Angle projection system, the Right View for an object is

placed on the right or the left of the Front View?

2. Which projection system is standardized in the United States?
3. If a view scale is set to 0.5, and a line on a drawing is drawn to a length

of 20 mm, what is the value that needs to be shown in the dimension

for the length of that line?

4. If a face of a part is oblique and not directly viewed by the Front, Top,

or Right Views in a drawing, what type of view should be created to

document and dimension features in that face—an Auxiliary View or a

Detail View?

5. In an ISO standard dimension, is the value shown above the dimension

line or in the middle of the dimension line?

6. What is a datum?

5

Two-Dimensional CAD

5.1 INTRODUCTION

This chapter provides information on CAD systems relevant to 2-D design and
drawings. The previous chapter explained what drawings really are and what re-
quirements they are expected to meet. That chapter also discussed how drawings
(CAD or manual) are used in implementing a 2-D design methodology. Hope-
fully the information in this chapter will help the reader (a user, manager, or ad-
ministrator) now understand how the CAD system can implement these
methods.

5.2 BACKGROUND

CAD systems have developed over a number of decades, and they are continuing
to become more powerful. Throughout the 1970s, these systems became more
and more capable, but they were quite expensive and generally ran on mainframe
computers. As the minicomputer (such as the former Digital Equipment Corpora-
tion’s VAX) became available in the early 1980s, the CAD systems started to be-