What do the numbers in a thread callout mean?
A thread is specified in this order:
- Nominal diameter. The largest diameter of the thread, called the major diameter. It can be written as a fraction, a decimal or a machine-screw number.
- Thread count or pitch. Inch threads use TPI, threads per inch. Metric threads use the pitch, the distance from one thread crest to the next in millimetres. A dash separates it from the diameter for inch threads (3/4-16), and an "x" for metric (M10 x 1.5).
- Class of fit. How tightly the internal and external threads fit together.
- Length. Whether you mean the overall length or the length of thread.
Add the hand, right or left, when it matters. A linkage that adjusts like a turnbuckle needs one of each, so the hand is part of the specification (Q1-01).
What is coarse and fine pitch?
Thread standards such as the Unified (inch), ISO/DIN (metric) and British Whitworth have coarse and fine versions of each size. A coarse thread has fewer threads per inch, or a bigger pitch. A fine thread has more, or a smaller pitch. For an adjuster, pitch sets how far one turn moves the link: a fine thread moves it less per turn, which allows finer adjustment (Q1-04).
The common series are Unified Coarse (UNC), Unified Fine (UNF), and metric coarse and metric fine. In an inch callout the series symbol follows the TPI, as in "¼"-20 UNC-1A". The symbol is part of the standard designation in ASME B1.1 (UNC, UNF, UNEF, UN, UNR), and some symbols, such as UNR, also say something about the thread root. Always write it.
What is a class of fit?
It says how much tolerance there is between mating threads. The two systems are these.
Inch (Unified). There are six classes: 1, 2 and 3, followed by A for an external thread (a bolt or stud) or B for an internal thread (a nut or a hole). Class 1A has the largest tolerances and eases assembly. Class 2A is tighter. Class 3A is the tightest, for precision work where a close fit is needed.
Metric. The class has a number and a letter. The number is the tolerance grade, and a smaller number is tighter. The letter is the tolerance position, and a letter nearer the start of the alphabet leaves more allowance. Lower-case letters are for external threads and capital letters for internal threads. The most common pair is 6g for the external thread and 6H for the internal thread. Both inch and metric tolerances are defined in standards, such as ASME B1.1 for Unified inch threads and ISO 965-2 for metric tolerances.
Male and female threads each carry their own class. A matched pair could read 3/4-16 UNF-3A-LH for the male thread and 3/4-16 UNF-2B-LH for the female thread, or M10 × 1.25 – 6g-LH and M10 × 1.25 – 6H-LH in metric.
Left-hand is written by adding LH at the end of the callout (…-2A-LH, M10 × 1.25 – 6g-LH). Right-hand is the default and is usually not written.
How do you choose the thread for your link?
Match what it screws into.
- Same size, same pitch. Match the thread size and hand to your rod ends. A rod end that has a 3/4-16 thread needs a 3/4-16 mating thread.
- Same hand where needed. On a right-hand and left-hand link, each end has its own hand, and each jam nut follows its end (Q1-05).
- Not mixed. Inch and metric are different families. Inch threads are listed by TPI and metric threads by pitch.
- Enough engagement. Choose the size and length so that enough thread stays engaged over the whole adjustment range (Q1-04; K04-Q1-23).
What should you write on the drawing or order?
- Nominal diameter and pitch or TPI of each end.
- The hand of each end.
- The class of fit of each thread.
- Which length you mean (Q1-04).
- Whether the part is male (external thread) or female (internal thread) at each end.