number of the next larger assembly of which the
smaller unit or assembly is a part. The USED ON
column shows the model number or equivalent
designation of the assembled units part.
Finish marks (P) used on machine drawings show
surfaces to be finished by machining (fig. 1-6).
Machining provides a better surface appearance and a
better fit with closely mated parts. Machined finishes
are NOT the same as finishes of paint, enamel, grease,
chromium plating, and similar coatings.
NOTES AND SPECIFICATIONS
Blueprints show all of the information about an
object or part graphically. However, supervisors,
contractors, manufacturers, and craftsmen need more
information that is not adaptable to the graphic form
of presentation. Such information is shown on the
drawings as notes or as a set of specifications attached
to the drawings.
NOTES are placed on drawings to give additional
information to clarify the object on the blueprint (fig.
1-2). Leader lines show the precise part notated.
A SPECIFICATION is a statement or document
containing a description such as the terms of a contract
or details of an object or objects not shown on a blue
print or drawing (fig. 1-2). Specifications describe
items so they can be manufactured, assembled, and
maintained according to their performance require-
ments. They furnish enough information to show that
the item conforms to the description and that it can be
made without the need for research, development,
design engineering, or other help from the preparing
Federal specifications cover the characteristics of
material and supplies used jointly by the Navy and
other government departments.
LEGENDS AND SYMBOLS
A legend, if used, is placed in the upper right
corner of a blueprint below the revision block. The
legend explains or defines a symbol or special mark
placed on the blueprint. Figure 1-2 shows a legend for
an electrical plan.
THE MEANING OF LINES
To read blueprints, you must understand the use
of lines. The alphabet of lines is the common language
of the technician and the engineer. In drawing an
object, a draftsman arranges the different views in a
certain way, and then uses different types of lines to
convey information. Figure 1-6 shows the use of stan-
dard lines in a simple drawing. Line characteristics
Figure 1-6.Use of standard lines.