Tc 9-524 fundamentals of machine tools

7-45

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TC 
9-524
The Brown and Sharpe 29° worm screw thread uses a 29°
angle, similar to the Acme thread. The depth is greater and the
widths of the crest and root are different (Table 7-9 in
Appendix A). This is a special thread used to mesh with worm
gears and to transmit motion between two shafts at right
angles to each other that are on separate planes. This thread
has a self-locking feature making it useful for winches and
steering mechanisms.
The square screw thread is a power transmitting thread
that is being replaced by the Acme thread. Some vises
and lead screws may still be equipped with square
threads. Contact areas between the threads are small,
causing screws to resist wedging, and friction between the
parts is minimal (Table 7-9 in Appendix A).
The spark plug thread (international metric thread type)
is a special thread used extensively in Europe, but seen
only on some spark plugs in the United States. It has an
included angle of 60° with a crest and root that are 0.125
times the depth.
Different types of pipe thread forms are in use that have
generally the same characteristics but different fits.
Consult the Machinery’s Handbook or a similar reference
for this type of thread.
THREAD FIT AND CLASSIFICATIONS
The Unified and American (National) thread forms
designate classifications for fit to ensure that mated threaded
parts fit to the tolerances specified. The unified screw thread
form specifies several classes of threads which are Classes 1A,
2A, and 3A for screws or external threaded parts, and 1B, 2B,
and 3B for nuts or internal threaded parts. Classes 1 A and 1 B
are for a loose fit where quick assembly and rapid production
are important and shake or play is not objectionable. Classes
2A and 2B provide a small amount of play to prevent galling
and seizure in assembly and use. and sufficient clearance for
some plating. Classes 2A and 2B are recommended for
standard practice in making commercial screws. bolts. and
nuts. Classes 3A and 3B have no allowance and 75 percent of
the tolerance of Classes 2A and 2B A screw and nut in this
class may vary from a fit having no play to one with a small
amount of play. Only high grade products are held to Class 3
specifications.
Four distinct classes of screw thread fits between mating
threads (as between bolt and nut) have been designated for the
American (National) screw thread form. Fit is defined as “the
relation between two mating parts with reference to ease of
assembly. ” These four fits are produced by the application of
tolerances which are listed in the standards.
The four fits are described as follows:
Class 1 fit is recommended only for screw thread work
where clearance between mating parts is essential for
rapid assembly and where shake or play is not
objectionable.
Class 2 fit represents a high quality of thread product and
is recommended for the great bulk of interchangeable
screw thread work.
Class 3 fit represents an exceptionally high quality of
commercially threaded product and is recommended only
in cases where the high cost of precision tools and
continual checking are warranted.
Class 4 fit is intended to meet very unusual requirements
more exacting than those for which Class 3 is intended. It
is a selective fit if initial assembly by hand is required. It
is not. as yet. adaptable to quantity production.