Main||Tolerances||Lead Angles||Over Wire

How to use the threaDoctor?

The ThreaDoctor is an advanced, online threading calculator. It uses an advanced algorithm for precise calculations of threading data. Please read the below manual to use it in the best possible way!

Units of Measurement

Data Input: The data input must be in the default units of the relevant threading standard.

Data Output: The results are displayed by default in the units of the relevant threading standard (mm or inches). You can change the display unit. However, you should beware that the formal results are in the default units of the thread standard, and other units of measurement are displayed for convenience only.


Thread Doctor - Color Explnations
  • White Background – Input fields.
  • Yellow Background – Calculated output fields.

Supported Threading Standards

United Inch Screw Threads Per ASME B1.1

Metric Threads Per ISO 68-1, ISO 261, and ISO 965-2

  • Coarse pitch series.
  • Fine pitch series.

Nominal thread paramters

These basic thread parameters depend only on the threading family, diameter, and pitch[12]. They remain the same in the different thread classes.

UN Thread External- Sktech

Allowance and Tolerances of the Thread parameters

These nominal thread parameters are a general guideline. To machine an actual thread, you need additional parameters: 

  • Is the thread Internal or External?
  • What is the tolerance class of the thread?
  • What is the Length of Engagement[9]? (Only for unified inch threads)

The threading calculator takes all the above into account to compute the allowed variations of the below parameters:

Unifeid Metric threads: Sketch of allowance parameters (External)

Important Note: The UN threads standard (ASME B1.1) lists all the data for the length of engagement of five times the pitch diameter[2] (5xP). The ThreaDoctor is more sophisticated and considers the length of engagement in the formulas. The results will equal the tabulated figures in ASME B1.1 when you set the engagement length[9] to 5xP. 

Lead Angles

The lead angle[11] of the thread depends on its pitch[12], number of starts, and diameter. Our threading calculator displays the lead angle based on these parameters.

Over Wire Measurements

Measuring the pitch diameter[2] becomes challenging when you machine a thread and don’t have the required Go/NoGo gauge. In such cases, the most common method is the “Over Wire[13]” measurement. The ThredDoctor includes a module that calculates the tolerance over the wire for your pitch diameter range and the wire diameter. Our threading calculator also shows you the ideal wire diameter and lets you select the wire from a list of standard market wires.

Page Glossary Terms
1. Tooth per inch (tpi) ( TPI ) TPI is an imperial unit of measurement for pitch, commonly used in the United States. As its name suggests, it is the number of times in which the feature appears over a linear distance of 1" (25.4 mm)
2. Pitch Diameter. The pitch diameter (also referred to as the effective diameter) is the diameter of an imaginary cylinder, which intersects it so that the distance between two intersection points at opposite flanks of a thread flanks equals half the nominal pitch of the thread.
3. Minor Diameter. The minor diameter is the diameter of an imaginary cylinder that touches the smallest diameter of a thread. It depends on the pitch and the thread's angle.
4. Major Diameter. The major diameter of a thread is the diameter of an imaginary cylinder that touches the largest diameter of a thread. The nominal value of the major diameter equals the nominal thread size. For example, in a UNC 1/4-20 thread, the nominal major diameter is 1/4" (6.35 mm)
5. Thread Height (Depth) ( Thread Depth ) The distance between the crest and root is called the thread height. It is designated with hs for external threads and hn for Internal threads. Because in all threads, either the crest or the root must be relieved, the height is a reference dimension and is usually not controlled.
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