Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (22 page)

Engineering sketches No A simple drawing to document new concepts or

to prepare for laboratory analysis.
Layout No A drawing for overall design system configura-

tion. Detail drawings are made to fit into the

context of the layout.

Detail Yes A drawing of a single component or part.
Assembly or
Yes A drawing that shows how various parts (or de-

Installation
tails) are put together. The Bill of Material is

usually used in conjunction with the assem-

bly drawing to specify which specific items

are put together.

General Arrange-
Yes A drawing that shows the most general design

ment or Final
information for the end product of the design.

Assembly
It tries to show the whole design as it appears

in the field. Also called a G.A.

Schematic Yes A drawing that documents designed circuits

used within a product; it may not be geomet-

rically correct, but it is functionally correct.
Word Yes A drawing that contains no geometry of a de-

sign; it only contains text information (notes)

with the drawing format. It is generally em-

ployed to officially record some text-based

design or manufacturing information.

The kind of drawings that are used for “official” production and design
control are often referred to as “released” drawings. This implies that it is avail-
able throughout the company and/or its suppliers. Table 4.3 indicates whether the
type of drawing is of this type.

4.7.1 Engineering Sketches

An engineering sketch is assumed to be less than a normal drawing. Instead of
documenting a design for production and sale, this drawing would be used to
document a prototype or basic parameters for a potential product. It may not have
a format at all. In many cases, the sketch is made by an engineer (often primarily
responsible for calculations); while official drawings are made by designers (of-
ten primarily responsible for commercialization of a new product).

Drawings and 2-D Design 85

4.7.2 Layouts

A layout drawing is often also not a normal drawing. It is a special drawing that
often is a master of the released drawings. This is where components are shown
in the context of the entire design, to basically figure out how things fit. For a car,
the layout drawing would actually look like many views of the car, and the posi-
tion of the engine, doors, etc. would be determined from this drawing. This can
still be done with 2-D CAD software, but much more typically this type of activ-
ity is done with 3-D models instead.

Often the layout drawing would be controlled by a head designer for a
product. This drawing would then be used by other designers in the more detailed
design of “lesser” assemblies. Some CAD software uses data management meth-
ods to continue this paradigm with 2-D CAD data shown overlaid or as layers.
The master designer can change the layout, but the lesser assembly designers are
not allowed by the software to alter the overall layout. Again, 3-D CAD generally
does an even better job with this project control function.

4.7.3 Details

A “detail” drawing is really a drawing of a single component within a larger de-
sign. It is a drawing of a single part. It usually has an assigned number called the
part number. The detail drawing should be completely sufficient to have someone
manufacture the part. Using the car analogy, the button one would press to open a
door on a car would be a single part. This button would have a drawing of its
own, and the button would have a part number and be manufactured as a discreet
item. A drawing showing just this button could then be called a detail (or a detail
drawing).

4.7.4 Assembly or Installation Drawings

An assembly drawing shows how various parts (details) are put together. For the
car door example, the button would need to be shown connected to the handle
and latches, etc. that form a door handle; the assembly drawing shows this rela-
tionship and how to put the parts together. Keep in mind, that the Bill of Material
is usually used in conjunction with the assembly drawing to show or call out
which specific items are to be put together.

Also, an assembly like a door handle may then need to be shown being in-
stalled on the car door. This type of drawing (which simply shows a larger con-
text of assembly function) can be referred to as an installation drawing.

As with the detail drawing, the assembly drawing should be sufficient to
assemble the parts together to meet the requirements of the product being
designed.

86 Chapter 4

4.7.5 General Arrangements

Eventually the assembly and installation drawings lead to a drawing that shows
the complete final product. This is the general arrangement drawing. It shows the
most general configuration for the product. It may not be necessary for actually
manufacturing the product. However, if this a top level drawing for an assembled
product (such as the car in previous examples) this drawing shows how the last
steps are taken for the assembly. Therefore, this type of drawing may be referred
to as the final assembly drawing or just the final drawing.

4.7.6 Schematics

A schematic is a graphical tool for showing circuits of various types. Usually
they would be for electrical, hydraulic, or pneumatic (air pressure) systems.
Schematics are essential elements for documenting the design of products that
use these types of systems. Therefore, they need to be carefully documented and
controlled in conjunction with the overall product design state.

In order to apply revision control and information release control, the sche-
matics can be shown on the typical mechanical drawing format (with Title Block,
Revision Block, etc.).

4.7.7 Word drawings

A word drawing does not contain any geometric information that graphically
shows the component documented by the drawing. Instead, it is specified or de-
scribed in written text (notes) shown on the drawing.

Using the car door analogy, there may be certain lubricants that need to be
carefully specified. These lubricants may have a special formulation or chemical
characteristics. The lubricant characteristics drawing document could be indi-
cated as an item that is shown or called out on the door assembly drawing. Then
using the part number for the call out, one could then find the word drawing for
that part number. Then when one looks at the word drawing, the lubricant charac-
teristics can be determined.

4.8 DRAWING STANDARDS

There are a number of industry standards that can be referred to for drawing
standards. Often these form the basis of a company’s drawing procedures, al-
though companies also usually need to have some customization applied to these
standards. Table 4.4 lists some of the more important standards from groups such
as ASME International (mechanical engineering professional society) and ISO
(an international standards body).

Drawings and 2-D Design 87

TABLE
4.4

Some Important Standards for Drawings

Standard (recent edition) Description

ANSI Z210.1 (1973) Rules for numerical rounding

ASME Y14.1 (1995) Standard for the drawing formats and sizes (U.S. or inch

drawings)

ASME Y14.1M (1995) Standard for the drawing formats and sizes (Metric or mm

drawings)

ASME Y14.2M (1992) Standard for line conventions and lettering

ASME Y14.3M (1994) Standard for how to show views and projections
ASME Y14.5M (1994) Standard for dimensions and tolerancing

ASME Y14.36M (1996) Standard for notation that indicates surface texture (relates

to how closely manufactured parts approximate totally

smooth or the planar geometry that is shown in the

drawing)

ASME Y14.8M (1996) Standard relating to information for castings and forgings
ASME Y14.100M (1998) Standard for drawing practices

DIN 6 Part 1 (1986) Projections in first angle and views

DIN 6771 Part 1 Title Blocks

ISO 128 Technical drawings—general principles of presentation
ISO 1302 (1992) Technical drawings—method of indicating surface texture
ISO 2768 (1989) General tolerances

ISO 5457 (1980) Technical drawings—Sizes and layout of drawing sheets

4.9 VIEWS

At this point, the basic outline of mechanical drawings has been presented. This
has included the sizes and formats for standard drawings without much discus-
sion of the graphical data that represents the product design. Much of the rest of
this chapter will be concerned with the geometric part of drawings. The most im-
portant concept with respect to this part of the drawing is called views or multi-
view projections or perhaps just “projections.”

As the name implies, a view relates to what an object looks like from a
particular view or viewing angle (refer to Figure 4.4). For instance a Front View
is what the object looks like in the front (front being determined by the person
making the drawing); then the Right View is what the object looks like from the
right direction; the Top View is then from the top. The basic idea here is to fully
document in a 2-D medium the complete definition of an actual 3-D object by
looking at it from a set of more or less standard directions. The person reading
the drawing is then expected to be able to reconstruct or mentally visualize the
object’s true 3-D configuration.

88 Chapter 4

FIGURE
4.4

Viewing directions or angles for an object.

One needs to realize that when an object is shown in a drawing it is gener-
ally implied that is from a particular view. Referring to Figure 4.1, one can see
that there are 4 views shown. These views are all for the same object; it is not
showing 4 separate objects. The lower left view is assumed to be the Front View,
the lower right view is the Right View, the top left view is the Top View, and the
top right view is called the Isometric View (which is explained later).

4.9.1 Standardization of Views (Third Angle vs.

First Angle)

Unfortunately, there are two competing standardizations for the presentation of
views on a drawing. The sample drawing shown in Figure 4.1 uses what is known
as “third angle projection.” This is the standard in the United States. In this case,
the front view is selected first, and the view on the right is what the object looks
like from the right.

Many other countries (continental European countries, in particular) use
“first angle projection.” In this case, the view of the object from the right (the
Right View) is placed to the LEFT of the front view; the Top View is placed BE-
LOW the front view, etc. The idea here is that one imagines that the view from
the right projects onto a piece of paper held up on the left side of the part (like the
part’s view is projected onto the paper). This is in contrast to the idea of the “third
angle projection” where one imagines that the paper is “wrapped” onto the part
and then unfolded to reveal what is seen in the drawing. In either case, the graph-
ical information shown in the views is the same, the only difference is where the
views are placed on the drawing.

Drawings and 2-D Design 89

It is important to know which method was being used by the person making
the drawing. Therefore, there is a standardized note in the drawing format that
indicates first or third angle projection, and a small cone-like object is shown
with its Right View projection (the Right View being either to the left or right of
the Front View). Refer to Figure 4.5.

4.9.2 “True” Views

An important concept of the views on a drawing is that the angle at which the
object is viewed is “true.” This implies that the object is viewed head on or not at
a skewed angle. This is important since the drawing needs to show dimensions in
the views, and these dimensions should all be in a plane that coincides with the
face of the object being viewed.