Understanding About Tubing Threads

Threads

British Standard Pipe (BSP) threads

These “Gas” profile threads are of two types:

• Parallel: they fit into the same parallel internal thread. The sealing is provided byan incorporated annular seal (or by a sealing washer).
• Taper: they fit into the same parallel or tapered internal thread. The sealing is provided by a precoating on the thread.

Thread designations

• BSP Parallel (BSPP): G followed by the denomination, as per the ISO 228-1. standard. Example: a1/8 BSP parallel thread is written G1/8
• BSP Taper (BSPT): R followed by the denomination, as per the ISO 7-1 standard. Example: a1/8 BSP taper thread is written R1/8

Internal threads:

• BSP parallel: G followed by the designation
• BSP taper: Rc followed by the designation

Metric threads

These ISO profile threads are parallel threads. They fit into the
same parallel internal thread. The sealing is provided by an
incorporated annular seal (or by a sealing washer).

Thread designations: M followed by the diameter and pitch
values in millimetres, separated by the multiplication sign, as
per the ISO 68-1 and ISO 965-1 standards. Example: M7x1

National Pipe Thread (NPT) threads

This is an American standard with taper threads. They fit into
the same tapered internal thread. The sealing is provided by
a precoating on the thread.

Definition of thread cutting using a threading die

Thread cutting is a manual machining process which involves the removal of metal chips.
Its consists of cutting helical-shaped grooves in a previously calibrated cylindrical
workpiece. The part at the end of the operation is a screw.

Die (also called “threading die”)

Different types of dies:

The die is in the form of a nut in which teeth are cut. The types of dies depend on:

Pitch direction

• right-hand dies
• left-hand dies

Pitch type

• fine pitch dies
• normal pitch dies

Die stock

The die stock is a cage which holds the die. It has handles which facilitate its use.

Thread characteristics

A thread is characterised by:

• Profile (triangular, trapezoidal, round, gas)

• Pitch

• Thread direction

• Length

Standardised thread designation

The standardised thread designation consists of:

• System or profile

• Nominal diameter

• Pitch (distance between two consecutive thread crests)

Example: M 10 X 1.5

Procedure for making a thread

1. Engage the die.
2. Ensure that the die is the correct way round and that it is centred on the workpiece.
3. Lubricate with cutting oil.
4. Turn the die in the direction of the cutting (from left to right) to make the thread.
5. Reverse the movement from time to time to break the chips.