a) WHAT IS A THREAD OR THREADING

A thread or threading is a surface whose axis is contained in the plane and around it describes a helicoid (the name given to any curved line whose tangents form a constant angle (α), following a fixed direction in space) cylindrical path.


b) TYPES OF THREAD

Trapezoidal thread.

It is recommended when force is wanted to be directed in one direction. (For example, in jacks and bolts)

Square thread.

The square thread differs on the fact that almost all forces run almost parallel to the axis.

Acme thread.

Permits the use of a split or disengaging nut.

Acme thread with cut filet.


If space limitations exist, it is convenient to use this type of nut because it is more resistant to the power transmission applications.

Whitworth thread.

Whitworth thread screw is equivalent to British Standard Pipe (BSP) which is explained later.

Rounded thread

It is recommended when the threads have to be laminated or molded into metal sheet. (For example, bottle caps).

Sharp V-Thread

This thread is very rare and is usually applied in precision instruments where the most important matter is perfect fitting or grip by friction.



Below we present a table with most of the threads that are used in mechanical engineering, types of threads to be recognized by their code letter indicating the size of outer diameter and pitch.

The latter is directly indicated in millimeters for metric threads, while unified threads and Whitworth are indicated by the number of existing threads per inch.




Description

Other descriptions

UNF

Unified National Fine Thread.

NF, SAE

NPT

National Pipe Tapered Thread

ASTP

BA

British Association


ISO

International Standards Organisation


C

Bicycle Thread


BSP

Whitworth // The British Standard Pipe thread

R

Rd

Metric Knuckle Screw Threads (Rd)


Tr

Trapezoidal thread


NPSF

National Straight Pipe for Fuels thread


BSW

British Standard Whitworth normal pitch

W

BSF

British standard fine thread


BSPT

The British Standard Pipe thread

KR

NPTF

National Pipe Thread Tapered Thread


PG

Steel Conduit Thread

Pr

M

Metric screw thread

SIF

UNC

The Unified Thread Standard

NC, USS

M

Metric screw thread

SI

UNEF

Unified Extra Fine Thread

NEF

NPS

National Pipe Straight


E

Edison Thread


UNS

The Unified Thread Standard

NS

API

American Petroleum Institute





























As we have seen,  over  time many types of threads have been developed but today, there are 3 the most common internationally.

The Whitworth thread form was selected to connect pipe threads, since it is made ​​self sealing by cutting at least one thread on one cone. This became known as the British Standard pipe thread (BSP by its acronym in English - conical or parallel BSP). The Whitworth thread is now used internationally as a standard thread for joing light pipes (of low weight)  made of carbon steel.

BSP thread, is a family of standard thread types that has been adopted for interconnecting and for sealing pipe ends by combining an external (male) with an internal (female).



a)     BSPP (parallel): The diameter of this type of thread is constant along the length of the thread.BSPP


b)      BSPT (conical): The diameter of this type of thread increases or decreases along the length of the thread.         BSPT


The thread is the same in both cases, the difference is that one is conical and the other one is parallel.


Moreover, in 1864 in North America, William Sellers set the standard for nuts, rods and screws that became the National Pipe Tapered Thread (NPT by its acronym in English).

NPT (National Pipe American Thread, Conical): It refers to an American standard. This connection is the best known and widely used where the pipe thread provides both mechanical union and hydraulic seal union. NPT has a tapered male and female that seal with the Teflon tape or a joint preparation.

The NPS thread and NPT is the same, the difference is, as we have already said, that NPT is tapered and NPS is parallel.


Attention! The proper functioning of the threading  is obtained when the male and female are both of the same type, either NPT with  NPT, BSPT with BSPT, and BSPP with BSPP. Whenever the thread is well done, this combination will not cause any leaks.


c) TOOLS FOR STEEL PIPE THREADING


a. Tools for threading

The threading can be done with hand tools or machine tools as threading machines. For hand threading we normally use the dies, which are cutting tools used to create the threads of bolts and nuts.


  1. i. Hand Tools


1)      Adjustable threaders:


The adjustable threaders are those that through a mechanical system can regulate the dies distances  in order to get the different measures of the threads.

Those shown below, allows to thread the pipes from 1/4 "to 4" with a single tool by changing the dies: the automatic positioning allows the adjustment of the dies easily and quickly.


                                    TerrajaTerraja




2) Ratchet Drop Head Threaders:




Carraca

The rachet Drop Heat threaders are constituted by a steel head of cylindrical shape, inside which a set of four dies are mounted fixed.

                                                               Cabeza                  Peines

                                                                 HEAD                       DIES


The adjustable threaders and ratchet DHT can thread BSPT, NPT and BSPP.

As for the dies, it is essential  to differentiate the 3 types of threads (the deference of a type of thread to another is the separation of the obtained threads. As some are widers  than others).
 

  1. i. Machine tools

There are several types of the machine tools for cutting and threading, that  also can ream the pipes with a compact structure,  easy to use and highly effective in its  work, such as:




Mastermatic - 2



 

Mastermatic - 2    

Thread types: BSPT, NPY, BSPT, NPT                                                         
                                             



 Roscamatic - 4

Roscamatic - 4
Thread types: BSPT, BSPT (HSS), NPT, NPT (HSS), BSPP, BSPP(HSS), BSW /HSS), PG, UNC




VIDEO

CUTTING AND REAMING:






THREADING:





IMPORTANT: For threading, cutting, or reaming stainless steel tubes is necessary to use dies HSS (high speed steel (HSS or HS)).