Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (20 page)

4.3 TYPICAL MECHANICAL DRAWING

Figure 4.1 shows a typical mechanical drawing. The mechanical drawing is used
for a number of purposes, and there may be some variation among different com-
panies, but they all have some standardized elements. It is essential that anyone
working with drawings (3-D CAD users included) be aware of all these elements.
The basic elements of the drawing are explained in the following sections.

FIGURE
4.1

Typical mechanical drawing.

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4.3.1 Format

The format refers to the “border” or “frame” that bounds the drawing informa-
tion. It includes a number of important pieces including the title block, the
proprietary information statement, zone indicators, and the revision block. There
are industry standards covering formats such as ASME Y14.1, Y14.1M, and
ISO 5457.

4.3.2 Title Block

The title block is considered part of the format and supplies the basic identifica-
tion information for the drawing. It includes the name of the item shown in the
drawing, the drawing number, the revision level of the drawing, the name of the
company that owns the drawing, the size of the drawing, the sheet or page num-
ber, the person that created the drawing, etc. It is usually located at the lower
right part of the format. Since the title block is part of the format, it is covered in
the standards listed for the format.

4.3.3 Proprietary Information Statement

The proprietary information statement is a note that indicates the security of the
drawing. This note will often state that the drawing is the property of the com-
pany, that the drawing contains information that is to be considered a trade secret,
and that the receiver agrees to abide by this trade secret status once they accept a
copy of the drawing. The trade secret status provides very restrictive security for
a document. For instance, if someone steals the information in the drawing and
another company profits from the information, that other company can be liable
for paying back 3 times what they gained from the information (referred to as
triple indemnity). Therefore, anyone dealing with drawings in any way (including
CAD users) must be aware of this provision, and be certain to never violate it.
The proprietary information statement is often located as part of the format, al-
though it is a company standard (not an industry standard).

4.3.4 Zone Indicators

The zone indicators are letters or numbers shown in the margin or border of the
format with lines or tick marks delimiting them. The combination of the vertical
and horizontal zone indicators tells the reader to look at a particular region or
zone of the drawing (such as C-5 or D-3). This is most often used in conjunc-
tion with the revision block where the revisions are said to be made in a par-
ticular zone. The larger the drawing, the more important this issue becomes.
Since the zone indicators are part of the format, refer to the standards listed for
the format.

Drawings and 2-D Design 75

4.3.5 Revision Block

The revision block is a section of the drawing that indicates the revision or his-
tory of the design. As mentioned earlier, mechanical drawings are expected to
document a particular state of the design. This state may indicate the geometric
properties of the item, part attributes such as the material of which it is to be
made, the acceptable tolerances, and vendors approved by engineering to be able
to manufacture the item. This is all very important information, and knowing
when it was changed and by whom is vital to establishing control over the total
design. The drawing for one small part can have life or death consequences for an
entire machine such as an airplane or an automobile. Anyone working on draw-
ings (particularly CAD users) must keep this in mind. Therefore, revisions must
be carefully documented. Since the revision block is part of the format, refer to
the standards listed for the format.

4.3.6 Bill of Material or Parts List

The Bill of Material (BOM) or Parts List section of the drawing is a note that may
or may not be part of the format. It is associated with a type of drawing that
shows an assembly or group of individual parts (often called details). The bill of
material will identify each of the individual parts and their quantity in the assem-
bly. There is no widely adopted industry standard for paper-based Bills of Mate-
rial; it is generally a company standard. However, many companies now rely on
computer software to handle all Bills of Material.

4.3.7 Views

The part or assembly or machine that is actually documented by the drawing is
usually shown in the drawing (although not always). The part of the drawing
where the reader actually sees the design or the geometry is contained in group-
ings called views. They are called views because the design is shown from a
variety of view angles. For instance, the reader is shown what the design looks
like from the front, and the right side, and the top. This would correspond to the
Front View, the Right View, and the Top View respectively. Each of these views
is expected to be placed on the drawing in a particular location so that the
reader knows which view angle is being expressed. Unfortunately, this view
placement is not standardized globally. In the United States the views are
placed in one arrangement (called “first angle”); while in Europe the views are
placed differently (called “third angle”). Some relevant standards for views
and their projections (which are explained in Section 4.9) are ASME Y14.3,
DIN 6, and ISO 128.

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4.3.8 Notes

The notes on a drawing are simply instances of text or wording. They are used for
a wide variety of tasks. A note may indicate information that has no relationship
to the geometry of the design (i.e. the name of a company approved to build the
design). On the other hand, notes may have a relationship to specific features of
the design. In this case, a note may have an arrow with it that points to a hole, and
the note may indicate that the hole is supposed to be drilled and tapped (a hole
drilled and then screw threads cut into the sides of the hole). Of course, there are
countless other tasks for these notes. Also, an entire drawing may be just notes.
This is often referred to as a word drawing. Standards that can be referred to for
notes include ASME Y14.34M and Y14.35M.

4.3.9 Item Callouts

The callouts
on a drawing are a way of identifying different components or parts
on a drawing that actually shows more than one part. These separate parts are
often called details, and they are usually numbered. These numbers, then, are
usually recorded in the Bill of Material as items. The callouts then become a
means of connecting the information in the Bill of Material with the physical de-
sign. The callout numbers can be shown inside geometry such as a circle (called a
“balloon” for obvious reasons), or they are a larger size number with an under-
line. In any case, the number should have an arrow pointing to the part of the
drawing that clearly shows that particular part.

4.4 STANDARD DRAWING SIZES

Drawings are created in standard sizes. Table 4.2 shows a table of these sizes.
Note the difference between identification systems of inch and mm (metric)
drawings. Inch drawings use different capital letters; the mm drawings use a
number after the letter A.

Before CAD systems were developed that could electronically store and re-
trieve drawings, drawings were kept in flat files (extra wide file cabinets) that
were arranged by drawing size. For instance, there would be C-sized cabinets for
C-size drawings, D-sized drawers for D-size, etc. The drawings within the drawer
would then be identified by the drawing number shown in the title block. There-
fore, to locate a particular drawing amongst hundreds or thousands of drawings,
one would look at the letter and number combination (at least for inch drawings).
So a drawing referred to as D46372819 would be found by looking at the D-sized
cabinets and then using the number to find the appropriate drawer in the cabinet.
Within that drawer, one would sort through the drawings to find that one drawing.
This approach (with the leading letter designation) may still be found in the part

Drawings and 2-D Design 77

TABLE
4.2
Standard Drawing Sizes (refer to
ASME Y14.1, ASME Y14.1M, ISO 5457)

Inch or U.S. drawings Vertical Horizontal

E 34.0 in. 44.0 in.

D 22.0 in. 34.0 in.

C 17.0 in. 22.0 in.

B 11.0 in. 17.0 in.

A 8.5 in. 11.0 in.
mm or metric drawings

A0 841 mm 1189 mm

A1 594 mm 841 mm

A2 420 mm 594 mm

A3 297 mm 420 mm

A4 210 mm 297 mm

numbering system after a CAD system is installed in a company (although the
drawing size is no longer needed to find the drawing anymore).

Note that sizes A, C, E and B, D are meant to be multiples. That is, 2 C-size
drawings next to each other equal an E size drawing, 2 A-size drawings equal a C
size, etc. There are larger sizes of drawings beyond the E size, but they are not
very typical in mechanical design. There is also roll size drawings. In this case
the E size is kept in the vertical direction, but the drawing rolls out longer than
the E size. The horizontal dimension of the roll or R-size drawing could be 60 in.,
72 in., 84 in., etc. (each size another 12 in. longer). There are similar roll sizes for
the mm type of drawings.

4.5 TITLE BLOCK DETAILS

As mentioned earlier, the title block is a region of the drawing that contains vital
identification information such as drawing number, revision level, etc. This type
of information is central to proper document control and management. Document
control, in turn, is vital to meeting the requirements of quality systems (such as
ISO 9001) and is essential to having a viable system for managing the activities
of any design department. Although the title block is a feature of a drawing, this
sort of document management information forms the basis for CAD based engi-
neering data management systems (even 3-D CAD). Just because the data man-
agement system is automated does not mean that items such as revision level are
no longer important. There will still be revision control, and attributes such as
revision levels will still be used to maintain revision control.

A close up example of a standard Title Block is shown in Figure 4.2.

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FIGURE
4.2

Standard Title Block (reference ASME Y14.1).

4.5.1 Title

As shown in Figure 4.2, the title is the basic naming for the drawing. It should be
as succinct and descriptive as possible. Often these names are standardized to be
a noun followed by a progression of classifying adjectives. The following would
be an example:

PLATE ANGLE SUPPORT

This title indicates that the drawing is for a plate that has a bend, and that it
is meant to be a support of some kind.

Note that the title and all other notes on a drawing are generally in upper
case letters. One probable exception is for metric or SI units. Units such as kilo-
gram and millimeter are abbreviated as kg or mm.

Also note that the drawing title is often reflected in automated engineering
data management systems. A Bill of Material program will probably expect some
sort of name for items (in addition to the drawing or part number), and the draw-
ing title is often used for this purpose. However, this means that the drawing title

Drawings and 2-D Design 79

may have to conform to typical software data restrictions. This may include a
limit on the total number of letters or characters in the name. It may also include
restrictions on letters or characters that can be used. For example, double quotes
(“), dollar signs ($), and slashes (/ or \) are typical delimiters between character
data and may cause difficulties for the computer software that is integrated with
the CAD system. Therefore, these characters may not be permitted in the title.