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

Main settings

• diameter (D);
• pitch (P) - the distance from one turn to another.

They are determined by GOST 1973257-73. A large step is considered normal, but it corresponds to several smaller ones. A small pitch is used when applying to thin-walled products (pipes with a thin wall). They also make a small turn if the applied thread is a way to adjust any parameters. Also, a small step between turns is made 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 hole for the thread is different in each case. All of them are prescribed in GOSTs, but most often they use triangular metric and conical metric thread. 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.

Adaptations

To apply carvings with your own hands, use small devices:

All these devices are made of alloys characterized by increased strength and abrasion resistance. Grooves and grooves are applied to their surfaces, 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 - collars and die holders - and secured there with screws. Having clamped the thread cutting device in the holder, it is put on/inserted into the place where you want to make a detachable connection. By turning the device, turns are formed. How correctly the device is positioned at the beginning of work determines whether the turns will “lay down” evenly. Therefore, make the first revolutions, trying to keep the structure level, avoiding shifts and distortions. After a few turns, the process will become easier.

You can cut small or medium diameter threads by hand. Complex types (two- and three-way) or working with large diameters by hand are impossible - too much effort is required. For these purposes, special mechanized equipment is used - lathes with taps and dies attached to them.

How to cut correctly

Threads can be applied to almost any metals and their alloys - steel, copper, aluminum, cast iron, bronze, brass, etc. It is not recommended to do it on hot iron - it is too hard, it will crumble during operation and it will not be possible to achieve high-quality turns, which means the connection will be unreliable.

Tool for the job

Preparation

You need 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 there, you can use soaked soap. You can also use other lubricants:

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

Slicing process

When cutting external threads, the die is placed strictly perpendicular to the surface of the pipe or rod. During operation, it should not wiggle, otherwise the turns will turn out uneven and the connection will be ugly and unreliable. The first turns are especially important. How they “lay down” determines whether the connection will then be skewed.

By applying the internal thread, the part is fixed motionless. If it is a small piece, you can clamp it in a vice. If the plate is large, ensure that it does not move. available methods, for example, fixed with bars. M

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

Photo cutting process

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

In order for the turns to be of high quality, two taps or dies are used - rough and finishing. The first pass is done as a rough pass, the second as a finishing pass. There are also combined devices for applying threads. They allow you to do everything in one pass.

Another practical advice: to prevent chips from falling into work area, when cutting, make one full turn clockwise, then half a turn counterclockwise. After this, return the tool to the place where you stopped and make one revolution again. Continue this way until the required length.

Tables for selecting the diameter of a drill for threading

By doing internal thread a hole is pre-drilled for it. It is not equal to the diameter of the thread, since when 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 application, it is necessary to select the diameter of the drill bit for the thread. This can be done using tables. They are available for every type of thread, but here are the most popular ones - metric, inch, pipe.

Once again, please note that the diameter of the drill bit for the thread is given for large (standard thread).

Table of rod diameters for external threads

When working with external threads, the situation is very similar - part of the metal is extruded rather than 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.

TAPPING HOLES
PIPE TAPER THREAD

DIAMETERS

GOST 21350-75

STATE COMMITTEE OF STANDARDS
COUNCIL OF MINISTERS OF THE USSR

Moscow

DEVELOPED, INTRODUCED AND PREPARED FOR APPROVAL by the All-Union Scientific Research Institute for Normalization in Mechanical Engineering (VNIINMASH)

And about. directors Gerasimov N.N.

Topic leader and performer Zaroslova M.P.

APPROVED AND ENTERED INTO EFFECT by Resolution of the State Committee of Standards of the Council of Ministers of the USSR dated December 12, 1975 No. 3877

STATE STANDARD OF THE USSR UNION

By Decree of the State Committee of Standards of the Council of Ministers of the USSR dated December 12, 1975 No. 3877, the validity period was established

from 01.01.77

1. This standard establishes the diameters of holes for cutting conical pipe threads in accordance with GOST 6211-69 in steel products in accordance with GOST 380-71, GOST 4543-71, GOST 1050-74, GOST 5058-75 and GOST 5632-72 (except for alloys on nickel base) and copper according to GOST 859-66.

2. Diameters of holes with reaming to a cone and their maximum deviations must correspond to those indicated in the drawing. 1 and in table. 1.

Table 1

Dimensions in mm

Note. For threads with nominal size above 2² nominal hole diametersd o and their maximum deviations must be equal to those established by GOST 6211-69 for the internal diameter of the thread.

3. The diameters of the holes without turning to a cone and their maximum deviations must correspond to those indicated in the drawing. 2 and in table. 2.

4. Allowed for pipe cutting tapered thread use holes of other diameters obtained on the basis of experimental data.

5. The diameters of drills for holes for tapping are indicated in the recommended one.

table 2

Dimensions in mm

Making internal threads is usually not difficult. But there are some features of using the tool and selecting holes for cutting metric threads.

Types of thread

They differ in their main characteristics:

• diameter calculation system (inch, metric, others);
• number of passes (two-, three- or single-pass);
• profile shape (rectangular, trapezoidal, triangular, round);
• direction of rotation of the screw (left or right);
• placement on the part (internal or external);
• shape of the part (cone or cylinder);
• purpose (running, fastening and sealing or fastening).

According to the listed characteristics, they distinguish the following types:

• cylindrical (MJ);
• metric and conical (M, MK);
• pipe (G, R);
• Edison round (E);
• trapezoidal (Tr);
• round for plumbing fasteners (Kp);
• persistent (S, S45);
• inch, including cylindrical and conical (BSW, UTS, NPT);
• oil range.

Tools for internal threading

To make internal cutting, you need a tap - a screw-shaped tool with sharp grooves. The rod can be shaped like a cone or a cylinder. The grooves run lengthwise and break the thread into sections called combs. It is the edges of the combs that are the working surfaces.

To ensure a clean groove, the metal is removed gradually, in layers. This requires one very long tool or set.

Single taps are also found on sale; they are often used to correct broken threads. To cut a new one, buy a kit. Therefore, taps are usually sold in pairs: for roughing and for finishing work. The first one cuts a shallow groove, the second one cleans and deepens it. There are also three-pass tools. Thin taps, up to 3 millimeters, are sold in twos, wider ones - in threes. Three-pass taps are inserted into the gates. The design of the knobs is different, but their size must match the size of the cutter.

The tools in the set are distinguished by the marks marked on the tail end. If you look closely, you will notice differences in shape:

• the first tap has heavily cut tooth tips, the outer diameter is slightly smaller than the other tools in the set;
• second tap with shorter fence segment, longer ridges. Its diameter is slightly larger than that of the first;
• the third tap has full ridges of teeth, and its diameter should match the dimensions of the future thread.

Taps are divided into pipe taps (marked “G”) for cutting threads inside the pipe, and metric taps, which are more common.

The quality directly depends on the properties of the tap: it must be made of good metal and spicy. To extend the life of the tool and improve the quality of the thread, lubricant is used. Usually, to acquire a stable cutting skill, you need to make 3 - 5 attempts.

Cutting process

Before you start cutting, you should use drills to make a hole in the workpiece. The diameter of the hole from the drill must match internal size threads. When the size of the hole made with drills is chosen incorrectly, the tool can break or the grooves will turn out to be of poor quality.

During cutting, part of the metal does not fall out with chips, but is pressed along the working surfaces of the tap, forming a groove profile on the workpiece. Taking this feature into account, the size of the drill used to make the hole for the thread is selected slightly smaller than the nominal diameter of the future thread.

For example, when cutting M5 (groove diameter is 5 mm), you should choose a drill for a 4.2 mm hole. To cut M4, the diameter of the drill must be 3.3 millimeters, and before working with an M6 tap, a hole is first made with a 5 mm drill. This indicator is calculated taking into account the thread pitch. The pitch can be calculated mathematically, but in practice they resort to correspondence tables, where for an M5 tap the pitch is 0.8, for M4 this figure is 0.7, for M6 - 1. We subtract the pitch index from the diameter and get required diameter drill. When working with brittle metals, such as cast iron, the drill diameter should be reduced by 0.1 mm compared to the size recommended in the table.

The formula for calculating the hole diameter when working with three-pass taps:

Up=Dm * 0.8;

here: Dm - tap diameter.

Table 1. Correspondence between thread diameters and preparation hole

Before starting work, the tap is inserted into a square shank - a knob. The collars can be regular or ratchet. The carving is done carefully, the first pass is made with a No. 1 tap to the end. Special attention it is necessary to pay attention to the direction of movement: clockwise only, and some force must be applied. It is done like this: 1/2 turn along the stroke alternates with 1/4 turn against the screw stroke to destroy the chips.

Table 2. Hole diameters for inch threads

A couple of drops of lubricant makes working on deaf people much easier. threaded holes. Although machine oil is sometimes used as a lubricant, drying oil is optimal for working with steel. WITH aluminum alloys It is preferable to use kerosene, alcohol or turpentine. Technical oil can also be used, but with less effect.

Homemade tap for aluminum alloys

To create internal threads in brass or light alloy parts, you can use homemade instrument and drills from the usual set. Calibrated steel wire will do. Using a die, an external thread is cut on it, after which the workpiece is hardened. After hardening, it is necessary to release the part to the color of ripe straw. The cutting edges are sharpened using a whetstone or sharpener, after first clamping the part into a collet chuck.

Video on how internal threads are cut:

Despite the fact that cutting internal threads is not a complex technological operation, there are some features of preparation for this procedure. Thus, it is necessary to accurately determine the dimensions of the preparation hole for threading, and also select the right tool, for which special tables of drill diameters for threads are used. For each type of thread, it is necessary to use the appropriate tool and calculate the diameter of the preparation hole.

Types and parameters of thread

The parameters by which threads are divided into Various types, are:

• units of diameter (metric, inch, etc.);
• number of thread starts (one-, two- or three-thread);
• the shape in which the profile elements are made (triangular, rectangular, round, trapezoidal);
• direction of rise of turns (right or left);
• location on the product (external or internal);
• surface shape (cylindrical or conical);
• purpose (fastening, fastening and sealing, chassis).

Depending on the above parameters, the following types of thread are distinguished:

• cylindrical, which is designated by the letters MJ;
• metric and conical, designated M and MK respectively;
• pipe, designated by the letters G and R;
• with a round profile, named after Edison and marked with the letter E;
• trapezoidal, designated Tr;
• round, used for installation of sanitary fittings, – Kr;
• thrust and thrust reinforced, marked as S and S45, respectively;
• inch thread, which can also be cylindrical and conical - BSW, UTS, NPT;
• used to connect pipes installed in oil wells.

Application of the tap

Before you start threading, you need to determine the diameter of the preparation hole and drill it. To facilitate this task, a corresponding GOST was developed, which contains tables that allow you to accurately determine the diameter of the threaded hole. This information makes it easy to select the drill size.

To cut metric threads on the inner walls of a hole made with a drill, a tap is used - a screw-shaped tool with cutting grooves, made in the form of a rod, which can have a cylindrical or conical shape. On its side surface there are special grooves located along its axis and dividing the working part into separate segments, which are called combs. The sharp edges of the combs are precisely the working surfaces of the tap.

In order for the turns of the internal thread to be clean and neat, and for its geometric parameters to correspond to the required values, it must be cut gradually, by gradually removing thin layers of metal from the surface being treated. That is why for this purpose they use either taps, the working part of which is divided along the length into sections with different geometric parameters, or sets of such tools. Single taps, the working part of which has the same geometric parameters along its entire length, are needed in cases where it is necessary to restore the parameters of an existing thread.

The minimum set with which you can sufficiently perform machining of threaded holes is a set consisting of two taps - rough and finishing. The first one cuts a thin layer of metal from the walls of the hole for cutting metric threads and forms a shallow groove on them, the second one not only deepens the formed groove, but also cleans it.

Combination two-pass taps or sets consisting of two tools are used for tapping small diameter holes (up to 3 mm). To machine holes for larger metric threads, you must use a combination three-pass tool or a set of three taps.

To manipulate the tap, a special device is used - a wrench. The main parameter of such devices, which can have different design, is the size of the mounting hole, which must exactly match the size of the tool shank.

When using a set of three taps, differing both in their design and geometric parameters, the sequence of their use must be strictly observed. They can be distinguished from each other both by special marks applied to the shanks and by design features.

1. The tap with which the hole for cutting metric threads is processed first is different minimum diameter among all set tools and cutting teeth, top part which are heavily cropped.
2. The second tap has a shorter fence and longer combs. Its working diameter is intermediate between the diameters of the other tools in the set.
3. The third tap, with which the hole for cutting metric threads is processed last, is characterized by full ridges of cutting teeth and a diameter that must exactly match the size of the thread being formed.

Taps are used primarily for cutting metric threads. Much less often than metric ones, taps designed for processing the internal walls of pipes are used. In accordance with their purpose, they are called pipe, and they can be distinguished by the letter G present in their markings.

Internal thread cutting technology

As mentioned above, before starting work, you need to drill a hole, the diameter of which must exactly fit a thread of a certain size. It should be borne in mind: if the diameters of the holes intended for cutting metric threads are chosen incorrectly, this can lead not only to poor quality execution, but also to breakage of the tap.

Considering the fact that the tap, when forming threaded grooves, not only cuts the metal, but also pushes it, the diameter of the drill for making threads should be slightly smaller than its nominal diameter. For example, a drill for making M3 threads should have a diameter of 2.5 mm, for M4 - 3.3 mm, for M5 you should choose a drill with a diameter of 4.2 mm, for M6 threads - 5 mm, M8 - 6.7 mm, M10 - 8.5 mm, and for M12 - 10.2.

Table 1. Main diameters of holes for metric threads

All diameters of drills for GOST threads are given in special tables. Such tables indicate the diameters of drills for making threads with both standard and reduced pitches, but it should be borne in mind that holes of different diameters are drilled for these purposes. In addition, if threads are cut in products made of brittle metals (such as cast iron), the diameter of the thread drill obtained from the table must be reduced by one tenth of a millimeter.

You can familiarize yourself with the provisions of GOST regulating the cutting of metric threads by downloading the document in pdf format from the link below.

The diameters of drills for metric threads can be calculated independently. From the diameter of the thread that needs to be cut, it is necessary to subtract the value of its pitch. The thread pitch itself, the size of which is used when performing such calculations, can be found out from special correspondence tables. In order to determine what diameter the hole needs to be made using a drill if a three-start tap is used for threading, you must use the following formula:

D o = D m x 0.8, Where:

Before- this is the diameter of the hole that must be made using a drill,

D m– the diameter of the tap that will be used to process the drilled element.