Thread drill diameter: cutting features, tables

To cut an internal thread on a part, you must first drill a hole. Its size is not equal to the diameter of the thread, but should be slightly smaller. You can find the diameter of the drill for a thread in a special table, but for this you also need to know the type of thread. 

The main parameters

Any thread is characterized by two parameters:

• diameter (D);
• step (P) – the distance from one turn to another.

They are determined by GOST 1973257-73. A large step is considered normal, but several smaller ones correspond to it. A small pitch is used when applied to thin-walled products (thin-walled pipes). They also make a small turn if the applied thread is a way to adjust any parameters. Also, a small step between the turns is done to increase the tightness of the connection and to overcome the phenomenon of self-unscrewing of the part. In other cases, a standard (large) step is cut.

There are many types of threads, since each has its own formation characteristics, the diameter of the threaded hole in each case is different. All of them are spelled out in GOSTs, but most often they use triangular metric and conical metric threads. We will talk about them further.

We usually see triangular threads on bolts and other similar fasteners, conical threads on most plumbing products that require a detachable connection.

Devices

For threading with your own hands, small devices are used:

• dies (they are also called lerks) for applying coils from the outside (usually on a pipe or a metal rod (pin);
• taps – for the inside (here you first need to make a hole under them).
  

All these devices are made of alloys, which are characterized by increased strength and resistance to abrasion. Grooves and grooves are applied to their surface, with the help of which their mirror image is obtained on the workpiece.

Any tap or die is marked – they have an inscription indicating the type of thread that this device cuts – diameter and pitch. They are inserted into holders – knobs and die holders – fixed there with screws. Clamping the threading tool in the holder, put it on / insert it into the place where you want to make a detachable connection. By scrolling the device, turns are formed. It depends on how correctly the device is set at the beginning of work whether the turns “lie down” evenly. Therefore, make the first turns trying to keep the structure even, avoiding shifts and distortions. After a few turns have been made, the process will become easier.

You can cut threads of small or medium diameter manually. Complex types (two- and three-way) or working with large diameters by hand is not possible – too much effort is required. For these purposes, special mechanized equipment is used – on lathes with taps and dies attached to them.

How to cut correctly

You can thread almost any metal and their alloys – steel, copper, aluminum, cast iron, bronze, brass, etc. It is not recommended to do it on a red-hot iron – it is too rigid, it will crumble during operation and it will not be possible to achieve high-quality turns, which means that the connection will be unreliable.

Prepare

It is necessary to work on clean metal – remove rust, sand and other contaminants. Then the place where the thread will be applied must be lubricated (except for cast iron and bronze – they must be worked “dry”). There is a special emulsion for lubrication, but if it is not available, soaked soap can be used. You can also use other lubricants:

• linseed oil for steel and brass;
• turpentine for copper;
• kerosene – for aluminum.
  

You can often hear advice to use machine or mineral oil or even lard when threading. They work well, but experts say that it is better not to do this – the chips will stick to a viscous substance, which will lead to rapid wear of the tap or die.

Slicing process

When cutting an external thread, the die is placed strictly perpendicular to the surface of the pipe or rod. During operation, it should not wag, otherwise the turns will turn out to be uneven and the connection will be ugly and unreliable. The first turns are especially important. It depends on how they “lie down” whether the connection will then be skewed.

By applying an internal thread, the part is fixed motionless. If it is a small piece, it can be clamped in a vise. If the plate is large, ensure its immobility by available methods, for example, by fixing it with bars. M

The tap is inserted into the hole so that its axis is parallel to the axis of the hole. With a little effort, little by little, they begin to twist in a given direction. As soon as you feel that the resistance has increased, unscrew the tap back and clean it of chips. After cleaning, the process continues.

When threading in a blind hole, its depth should be slightly more than required – this excess should include the tip of the tap. If this is structurally impossible, the tip is cut off from the tap. At the same time, it is not suitable for further operation, but there is no other way out.

In order for the turns to turn out to be of high quality, two taps or dies are used – roughing and finishing. The first pass is a rough pass, the second is a finish pass. There are also combined threading devices. They allow you to do everything in one pass.

Another practical tip: to keep the chips out of the work area, when cutting, make one full turn clockwise, then half a turn counterclockwise. After that, the tool is returned to the place where it stopped and again make one revolution. So continue to the required length.

Tapping drill diameter selection tables

When making an internal thread, a hole is pre-drilled under it. It is not equal to the diameter of the thread, since during cutting, part of the material is not removed in the form of chips, but is squeezed out, increasing the size of the protrusions. Therefore, before applying, it is necessary to select the diameter of the drill for the thread. This can be done in tables. They are for each type of thread, but we will give the most popular ones – metric, inch, pipe.

Once again, we draw your attention to the fact that the diameter of the drill for threading is given for large (standard threads).

Table of rod diameters for external thread

When working in an external thread, the situation is very similar – part of the metal is squeezed out, and not cut off. Therefore, the diameter of the rod or pipe on which the thread is applied should be slightly smaller. How accurate – see the table below.