LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
(6) Square Screw Thread.
Because of their design and strength, square
screw threads are used for vise screws, jack screws, and other devices where
maximum transmission of power is needed. All surfaces of the square thread
are square with each other and the sides are perpendicular to the center
axis of the threaded part. Because the contact areas are relatively small
and do not wedge, friction between the matching threads is reduced to a
minimum under heavy pressure.
Thread Fit and Designations.
(1) Thread Fit.
(a) The Unified and American (National) Standard thread forms designate
classifications for fit to insure that mated threaded parts fit to the
(b) The Unified screw thread form specifies several classes of threads
which are classes 1A, 2A, 3A for screws or external threaded parts, and 1B,
2B, and 3B for nuts or internal threaded parts. Classes 1A and 1B 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 during 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
(c) Four distinct classes of screw thread fits between mating threads
(as between bolt and nut) have been designated for the American (National)
Standard 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 and by the tolerances which are listed in the
standards. The four fits are described as follows:
1 Class 1 Fit. This fit is recommended only for screw thread work
where clearance between the mating parts is essential for rapid assembly and
where shake or play is not objectional.