Thread Forms

Thread form refers to the profile of a screw thread. Thread standards that define thread form, series, class, allowance, tolerance and designation are usually issued by thread form.

Note: Methods of designating screw threads varies by standard. Refer to the individual standard for proper specification of screw thread requirements.

Examples

Sharp V

Sharp V threads were used in until about 1909. Sharp V threads had the disadvantages of being hard to produce and easily damaged. One advantage is that a single cutting tool can produce a variety of pitches because of the sharp root.

Sharp V threads were used until about 1909.

Unified

Unified threads are among the most common thread form used in the United States. There are several series of unified threads. The basic profile is shown here.

Basic form of a UN thread form.

ACME

ACME threads are commonly used in power transmission application such as in lead screws. ACME threads are similar to European DIN103 trapezoidal threads, but they are not interchangeable.

Basic form of an ACME thread form.

Buttress

Buttress threads are used when force will be applied in one direction more than the other. For example, in leadscrews for clamping operations. There are several versions of buttress threads in use today including ANSI B1.9, British standard buttress threads, and the Löwenhertz thread.

Basic form of a buttress thread.

Lamp Base

The “American Standard” lamp base thread is designed to be rolled into sheet metal components.

Electric lamp base threads.

Basic Profile

Most o the images shown above are of each thread form’s basic form. Actual thread dimensions are determined by the allowance (usually clearance between the external and internal threads) and the amount of tolerance as it deviates from the basic profile.

Illustration showing the space between the basic profile and the thread design profile.
Thread design limits are determined by the amount of allowance and tolerance applied to a basic form.

Examples of Various Thread Forms and Standards

SymbolStandardDescription
ACMEASME/ASME B1.5ACME Thread Form (ACME-G, General;
ACME-C, Self-Centering)
STUB ACME (B1.8)
AMOANSI/ASME B1.11 American Standard Microscope
Objective Thread
BUTTANSI/ASME B1.9Buttress Threads
(PUSH-BUTT, Push type)
MANSI/ASME B1.13Metric Screw Threads
MJ – (B1.21M)
NHANSI/ASME B1.20.7Hose Coupling Threads
NHR- for garden hose applications
NPANSI/ASME B1.20.1
ANSI/ASME B1.20.3
American Standard
Pipe threads – Various. Includes:
NPS for straight threads,
NPT for tapered threads,
PTF for dryseal.
UN ANSI/ASME B1.1Unified. Various. Includes:
UN – Constant-pitch series
UNC – Coarse-pitch series
UNF – Fine-pitch series and others.
UNJ – Rounded root (B1.15)
UNR – Rounded root
UNM – Miniature (B1.10)
UNS – Special
and more…
DIN 405Knuckle
DIN 780Worm
DIN 13Metric ISO V thread
Whitworth thread