ThreaDoctor: Advanced thread Calculator

Main||Tolerances||Lead Angles||Over Wire||Cutting Data

How to use the threaDoctor?

The ThreaDoctor is an advanced online thread calculator. It uses an advanced algorithm for precise calculations of thread 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 thread standard.

Data Output: The results are displayed by default in the units of the relevant thread 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 thread Standards

The ThreaDoctor includes 4 sub-apps: Metric Thread Calculator, UN thread Calculator, NPT thread Calculator, and BSP thread calculator.

United Inch Screw Threads Per ASME B1.1

  • UNC – Coarse pitch thread.
  • UNF – Fine pitch thread.
  • UNEF – Ultra fine pitch thread.
  • UN – Constant pitch (##-UN) thread.

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

National Pipe Threads Per ASME B1.20.1

British Pipe Threads

  • BSPTBrith Tapered Pipe Threads (Per ISO-7)
  • BSPPBrith Parallel Pipe Threads (Per ISO-228)

ACME trapezoidal Threads Per ASME B1.5

  • General Purpose (G)
  • Centralized (C)

Because of the huge amount of parameters, the results of the ThreaDoxctor are divided into 5 tabs. You can jump from one tab to the other via the top or bottom navigation bars.

ThreaDocotor Advanced Thread Calculator

Top Navigation Bar

ThreaDoctor Top Nav Bar

Bottom Navigation Bar

ThreaDoctor Bottom Nav Bar

Tab #1Nominal thread parameters

These parameters describe the theoretical thread profile (also called the basic profile). To manufacture and measure a thread, you need to obtain also the allowed tolerances, which are given in the next tab.

60° straight standard threads (M & UN)

  • The basic thread parameters depend only on the thread family, diameter, and pitch.
  • They remain the same for external and internal threads.
  • They do not depend on the thread tolerance class.
UN Thread External- Sktech

National Pipe Threads (NPT)

Pipe Thread (External)

NPT External Thread (Pipe) Basic Profile sketch

Fitting Thread (Internal)

NPT Internal Thread (Fitting) Basic Profile sketch

The Thread Calculator App provides the following basic parameters for NPT pipe threads:

  • Nominal Pipe Size.
  • Pipe Outside Diameter.
  • Pitch in TPI & Distance.
  • Thread Tapper Angle.
  • Thread V half Angle.
  • Height of sharp V & Thread height.
  • Pitch Diameter & Minor Diameter at the Pipe’s endpoint.
  • Length & Pitch diameter of Hand-Tight Engagement.
  • Last Complete Thread Pitch Diameter.
  • Overall Thread Length.
  • Length of Vanish threads
  • Pitch Diameter at the fitting’s end
  • Tap Drill Size for internal (Fitting) thread.

Examples of popular NPT pipe threads are 1/8-27 NPT and 1/4-18 NPT

Tab #2Allowance and Tolerances of the Thread Parameters

These nominal thread parameters represent the theoretical profile. To machine an actual thread, you need additional parameters.

60° straight standard threads (M & UN)

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

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

Unifeid Metric threads: Sketch of allowance parameters (External)
  • Thread Allowance.
  • Pitch Diameter.
  • Major Diameters.
  • Minor Diameter. 
  • Pitch.
  • Thread Angle.

Important Note: The UN threads standard (ASME B1.1) lists all the data for the length of engagement of five times the pitch diameter (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 to 5xP. 

National Pipe threads (NPT)

The NPT thread does not have tolerance classes. The tolerances depend only on the thread’s pitch and are provided by the Thread Calculator for these parameters:

Pipe Thread (External)

NPT External Thread (Pipe) Profile Details Sktech

Fitting Thread (Internal)

NPT Internal Thread (Fitting) Profile Details Sktech

Tab #3- Lead Angles (*)

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

Tab #4Over Wire Measurements (*)

Measuring the pitch diameter 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” 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.

Tab #5 Cutting Conditions (*)

The ThreaDoctor provides cutting conditions recommendations based on the selected thread and raw material for the 3 main thread machining techniques (Thread Turning, Thread Milling & Tapping):

Thread Turning Cutting Data

The calculated cutting parameters for Thread Turing are:

  • Cutting Speed
  • Feedrate
  • Total Depth
  • Number of passes
  • Depth of cut per pass to ensure constant chip thickness

Thread Milling Cutting Data

The calculated cutting parameters for Thread Milling are:

* The user can select the milling cutter diameter and number of flutes, and the results are recalculated accordingly.

Tapping Cutting Data

The calculated cutting parameters for Tapping are:

Tab #6 – CNC Threading programs (*)

Thread turning Gcode NC program

The ThreaDoctor writes for you a CNC Gcode program for the selected thread based on many configuration options:

Supported Controllers

  • Fanuc (1 and 2 lines format)
  • Okuma
  • Mazak
  • Haas
  • Mitsubishi

Supported In-Feed methods

  • Radial
  • Flank
  • Modified Flank
  • Alternating Flank

Supported Passes configurations

  • Constant Volume
  • Constant Depth

Additional configurations

  • Working directions (Away/Towards the chuck)
  • Handness of Tool and Thread.
  • Chamfer
  • Inch/metric CNC program

(*) Supported for Metric and unified inch threading standards only.

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