Thread Callouts and Threaded Fasteners

Read diameter, spacing, series and class separately

In 1/4-20 UNC-2B, 1/4 is the nominal major diameter in inches, 20 is the number of threads per inch, UNC is the Unified coarse series, and 2B identifies the internal thread's fit class. The major diameter is the largest thread diameter; it is not the diameter of the hole drilled before tapping.

UNF identifies the Unified fine series. At a given nominal diameter, fine threads have more threads per inch and therefore a smaller pitch than coarse threads. In M6 × 1, M identifies a metric thread, 6 is the nominal major diameter in millimeters, and 1 is the pitch in millimeters. It does not mean one thread per inch.

Practice reading a callout

Thread class controls fit, not fastener strength

Unified external thread classes are 1A, 2A and 3A; internal classes are 1B, 2B and 3B. A means external, as on a screw; B means internal, as in a tapped hole. Class 1 permits a looser fit, class 2 serves general applications, and class 3 uses closer dimensional control.

The class number is not a material grade or a load rating. A class 3 thread does not by itself make a stronger bolt. Read the drawing for the required class and use the appropriate inspection limits. M6 × 1 alone does not specify a metric tolerance class; do not append an inch A/B class to it.

For the general distinction between a nominal dimension, its tolerance and a mating fit, see Tolerances and Fits. Exact thread acceptance limits require the applicable specification and gauges.

Pitch is spacing; lead is travel per revolution

Pitch is the axial spacing between adjacent thread crests. For an inch thread, pitch in inches = 1/TPI. A 20-TPI thread has a pitch of 0.050 in. Lead is the axial travel in one complete relative revolution.

On a single-start thread, lead equals pitch. On a multiple-start thread, lead equals pitch times the number of starts. Right- and left-hand threads move in opposite axial directions for the same rotation. Look for a left-hand designation rather than forcing a fastener that does not start normally.

The lathe topic owns the process of single-point thread cutting. The tapping topic explains preparing and making internal threads with taps.

Choose the fastener by how it locates and clamps the parts

Engagement, grade and locking are separate decisions

Engagement length is the axial length of mating threads carrying load. There is no single diameter multiplier that establishes adequate engagement for every combination of screw and parent material. Use the joint design or fastener-system requirements, and ensure the screw does not bottom before it clamps.

Strength identification must come from the specified fastener grade or property class and its traceable markings or documentation. Thread fit class and head style are not substitutes. Do not assign a strength grade to an unidentified fastener from its appearance.

A flat washer is not a locking device. A lock washer or thread-locking compound is not a guarantee against loosening in every joint. Use the specified locking method; for a compound, follow its compatibility, preparation, cure and removal instructions.