Machine Drawings
Introduction
In learning to draw and read machine drawings, you must first become
familiar with common terms, symbols, and conventions. The following
paragraphs cover common terms most used in all aspects of machine
drawings.
Standards
American industry and the Department of Defense (DoD) follow the standard
Geometrical Dimensioning and Tolerancing, ANSI Yl4.5M-1982. This is
the standard used in all blueprint production whether the master drawings are
drawn by human hand or by computer-aided drawing (CAD) equipment. It
standardizes the production of prints from the simplest hand-made job on site
to single or multiple-run items produced in a machine shop with computer-
aided manufacturing (CAM).
Refer to ANSI Y14.5M-1982 when creating
or altering machine drawings. Also refer to MIL-STD-9A for Screw Thread
Conventions and Methods of Specifying, ANSI 46.1 for Surface Textures, and
MIL-STD-12C for Abbreviations for Use On Drawings and In Technical-
Type Publications.
General terms
Tolerances, fillets and rounds, slots and slides, keys, keyseats, and keyways,
screw threads, gears, helical springs, and finish marks present common
problems to the draftsman. Standards offer uniform solutions to these
problems.
TOLERANCES: Tolerancing is a method of indicating acceptable variations
in size or surface and appears as a minus or plus a certain amount stated in
fractions or decimals. The minus or plus figures are the minimum and/or
maximum value prescribed for a specific dimension. The three ways of
showing tolerances are the unilateral method used when variation in design is
permissible in one direction only, the bilateral method, which shows the
acceptable minus or plus variations, and the limiting dimensioning method,
which states both the allowable minimum and maximum measurements.
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