OTHER IMPORTANT TERMS
LEAD: The distance a screw thread advances
axially in one turn.
CUT THREAD: Threads are cut or chased; the
unthreaded portion of shank will be equal to major diameter
ROLLED THREAD: Threads are cold formed
by squeezing the blank between reciprocating serrated dies.
This acts to increase the major diameter of the thread over
and above the diameter of unthreaded shank (if any), unless
an extruded blank is used.
Classes of thread are distinguished from each other
by the amounts of tolerance and allowance specified. External
threads or bolts are designated with the suffix "A";
internal or nut threads with "B".
CLASSES 1A and 1B: For work of rough commercial
quality where loose fit for spin-on-assembly is desirable.
CLASSES 2A and 2B: The recognized standard
for normal production of the great bulk of commercial bolts,
nuts and screws.
CLASSES 3A and 3B: Used where a closed fit
between mating parts for high quality work is required.
CLASS 4: A theoretical rather than practical
class, now obsolete.
CLASS 5: For a wrench fit. Used principally
for studs and their mating tapped holes. A force fit requiring
the application of high torque for semi-permanent assembly.