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Threads are mostly coded with a cryptic combination of numbers and letters. The following article provides more detailed information about which dimensions are specified and which dimensions are taken where, in order to shed some light on the darkness of the thread markings.

The basics of measuring threads

  • Metric and imperial threads
  • Marking dimensions
  • Thread types and code letters (ISO)
  • Also read - pre-drilling a thread is precision work
  • Also read - Dimensioning threads in representation and implementation
  • Also read - Determine the thread with the caliper

Metric and imperial threads

Metric threads are recorded in the ISO standard and use metric units for all dimensions. That is centimeters and millimeters.

Inch threads, on the other hand, give the dimensions in inches and inches. They are not recorded in the ISO. The standards for these threads can be found in the so-called UTF (Unified Thread Standard).

Inch threads are standard in aviation, in house installation (Whitworth thread) and in the computer sector.

Marking dimensions

For the identification of threads - with ISO threads - certain, fixed dimensions are used. Of course, you have to differentiate between external threads (as with the screw) and internal threads (as with the nut).

There are also clockwise and counter-clockwise threads. With counterclockwise threads, a screw is screwed in exactly in the opposite direction, namely counterclockwise.

Nominal diameter (d or D)

In the case of external threads (screw), the nominal diameter is the external diameter of the thread, i.e. the largest measurable diameter. The nominal diameter of screws is denoted by d.

In the case of internal threads (nuts), the nominal diameter is the smallest measurable diameter on the inside. It is denoted by D.

core diameter

The root diameter is the smallest measurable diameter for screws and the largest measurable diameter for nuts. It is designated with d1 or D1, depending on whether it is an external or internal thread.

In the case of matching external and internal threads, the core diameter of the external thread is naturally always slightly smaller than the core diameter of the internal thread.

Pitch diameter

The pitch diameter is defined as the mean value between the external diameter of the external thread and the internal diameter of the associated internal thread, for example screw and nut. It is denoted by d2.

pitch

In the Iso system, the pitch is the distance between two thread peaks in mm. In the past, the term pitch was used for this.

In the non-metric system - i.e. with inch threads - the pitch is defined as the number of tips per inch.

Pitch angle

The slope angle can only be calculated. It is the arctangent from the division result of the slope divided by the pitch diameter x Pi.

division

The pitch only plays a role in multi-start threads. The so-called number of gears is also given there. The division results from the pitch divided by the number of threads and is specified as 60/20 for multi-part threads, for example.

Measure thread

External threads can be measured with the help of so-called ring gauges, flank micrometers and sometimes optical measuring instruments are used. Internal threads are measured with the help of plug gauges.

the most important thread types and their code letters (ISO)

  • Standard thread according to ISO M
  • Pipe thread for non-sealing connection G
  • Pipe thread for sealing connection R
  • Trapezoidal thread Tr or KT
  • Buttress threads S, GS, KS
  • Bicycle thread FG
Tips & Tricks Since all threads used today are standardized for most purposes, it is usually easy to determine which thread is present. Only in very rare cases does it actually need to be measured.

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