Internal thread – Female thread - chart, how to make, how to measure

What is an internal thread?

Internal thread is a thread that is inside the hole, e.g., within the nut. The thread coil is in the centre of what you screw a male thread (which is usually a screw) into.

However, we can list more examples of this type of thread. In addition to nuts, these are:

• internal threads in pipes, 
• pipe fittings, 
• threads drilled into a metal or wooden structure using a machine tap,

Internal threads are also often referred to as the female variety of thread.

How do you make an internal thread?

There are three methods of making internal threads:

• tapping,
• kneading,
• milling.

Tapping is the cheapest and easiest option, which is why it is the most popular. To perform it, you need a tapping tool, i.e. a special drill for thread-cutting.

Unfortunately, this method is characterised by a lower surface quality and has problems with hard-to-machine materials.

As far as kneading is concerned, this is chipless machining. Threads are formed by progressively deforming the surface with a forming tap. This method does not produce chips and has a higher quality of threads produced, but is not suitable for materials not susceptible to plastic deformation.

We are still left with milling, which offers the greatest quality and multifunctionality, but is also the most costly (tool price) and complex.

With milling cutters you will produce very deep threads of various sizes even in hardened materials.

 In addition, they can also be used to produce external threads.

What are the dimensions of internal threads? Internal threads dimension chart

The chart below indicates all the necessary dimensions for standard internal threads that you will find on the market:

How to measure an internal thread with a calliper?

You will measure the internal threads with the calliper using its internal jaws, Simply insert them into the threaded hole, expand to the maximum and then read the result.

Then you can use the chart above to find the type of thread you are dealing with.

As it gives the dimensions in millimetres, you can easily check which thread you are dealing with (even if it is, for example, a 1 inch internal thread).

How to fix an internal thread?

You can easily repair a machined or damaged internal thread with a machine tap. However, use this tool with your head, as its use will lead to a widening of the hole and its coiling.

Unfortunately, this cannot be avoided.

There is another solution - special springs made of spring steel. They function in such a way that once you have widened the hole with the machine tap, you reach for the screwdriver and screw the corresponding spring into the centre.

Despite appearances, reconditioning threads does not make them less durable. In fact, in some cases, the opposite may be true - the process will make them even more resistant and durable.