Cutting external thread. Diameters of rods for threading when cutting with dies.

Before cutting a thread, it is necessary to select the diameter of the workpiece for this thread.

slicing die thread, it must be borne in mind that when forming a thread profile metal products, especially steel, copper, etc., stretches and the product increases. As a result, the pressure on the surface of the die increases, which leads to heating and adhesion of metal particles, so the thread may turn out to be torn.

The same considerations should be followed when choosing a shank diameter for external threads as when choosing holes for internal threads. Slicing practice external threads shows that the best quality threads can be obtained if the diameter of the rod is slightly smaller than the outer diameter of the thread being cut. If the diameter of the rod is less than required, then the thread will be incomplete; if more, then the die will either not be able to be screwed onto the rod and the end of the rod will be damaged, or during operation, the teeth of the die may break due to overload, and the thread will be torn off.

In table. 27 shows the diameters of the rods used when cutting threads with dies.

Table 27 Diameters of rods for threading when cutting with dies

The diameter of the workpiece should be 0.3-0.4 mm less than the outer diameter of the thread.

When threading with a die, the rod is fixed in a vice so that the end of the vise protruding above the level of the jaws is 20-25 mm longer than the length of the part to be cut. To ensure plunging, a chamfer is sawn at the upper end of the rod. Then, a die fixed in the die is applied to the rod and the die is rotated with a little pressure so that the die cuts by about 0.2-0.5 mm. After that, the cut part of the rod is lubricated with oil and the screw is rotated in exactly the same way as when working with a tap, that is, one or two turns to the right and half a turn to the left (Fig. 152, b).

Rice. 152. Reception of threading with a die (b)

To prevent marriage and breakage of the teeth, it is necessary that the die enters the rod without distortion.

Checking the cut internal threads is carried out with threaded plug gauges, and the outer thread is checked with threaded micrometers or threaded ring gauges.

Despite the fact that the cutting of internal threads is not a complex technological operation, there are some features of the preparation for this procedure. So, you should accurately determine the dimensions of the preparatory hole for threading, as well as choose the right tool, for which special tables of drill diameters for threading are used. For each type of thread, you must use the appropriate tool and calculate the diameter of the preparation hole.

Varieties and parameters of thread

The parameters by which the thread is divided into Various types, are:

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

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

• cylindrical, which is indicated by the letters MJ;
• metric and conical, denoted respectively by M and MK;
• pipe, for which the letters G and R are used;
• with a round profile, named after Edison and marked with the letter E;
• trapezoidal, designated Tr;
• round, used for the installation of sanitary fittings, - Kr;
• persistent and persistent 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 preparatory hole and drill it. To facilitate this task, the 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 choose the size of the drill.

To cut metric threads on the inner walls of a hole made with a drill, a tap is used - a helical tool with cutting grooves, made in the form of a rod, which can have a cylindrical or conical shape. On its lateral 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 turn out to be clean and neat, and 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 machined. That is why, for this purpose, either taps are used, 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 perform the processing of threaded holes with sufficient quality, is a set consisting of two taps - roughing and finishing. The first cuts holes for cutting metric threads from the walls thin layer metal and forms a shallow groove on them, the second not only deepens the formed groove, but also cleans it.

Combination two-pass taps or sets consisting of two tools are used for threading in holes with small diameters (up to 3 mm). For drilling holes metric thread larger diameters, use a combination 3-pass tool or a set of 3 taps.

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

When using a set of three taps that differ both in their design and geometric parameters, the sequence of their application should be strictly observed. You can distinguish them from each other both by special risks applied to the shanks, and by design features.

1. The tap with which a hole for cutting a metric thread is machined first is different minimum diameter among all set tools and cutting teeth, top part which are heavily cut off.
2. The second tap has a shorter chamfer and longer combs. Its working diameter occupies an intermediate value between the diameters of other tools from the set.
3. The third tap, with which the hole for cutting metric threads is finished 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 threading metric threads. Much less frequently than metric taps, taps are used for processing the inner walls of pipes. They are called pipe pipes in accordance with their purpose, and they can be distinguished by the letter G present in their marking.

Thread cutting technology

As mentioned above, before starting work, it is necessary to drill a hole, the diameter of which must exactly fit the 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 its poor quality, but also to breakage of the tap.

Taking into account the fact that the tap, forming threaded grooves, not only cuts off the metal, but also pushes it through, the diameter of the drill for threading should be somewhat smaller than its nominal diameter. For example, a drill for threading M3 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 thread - 5 mm, M8 - 6.7 mm, M10 - 8.5 mm, and for M12 - 10.2.

Table 1. Main hole diameters for metric threads

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

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

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

D o \u003d D m x 0.8, where:

Before- this is the diameter of the hole to be made with a drill,

D m- the diameter of the tap with which the drilled element will be processed.

The strength of the fastening of the parts to each other is ensured by screwing the carrier of the external thread into the internal of the second product. It is important that their parameters are maintained in accordance with the standards, then such a connection will not be broken during operation and will provide the necessary tightness. Therefore, there are standards for the execution of threads and its individual elements.

Before cutting, a threaded hole is made inside the part, the diameter of which should not exceed its inner diameter. This is done using metal drills, the dimensions of which are given in the reference tables.

Hole parameters

There are the following thread parameters:

• diameters (internal, external, etc.);
• profile shape, height and angle;
• step and entry;
• others.

The condition for connecting the parts to each other is the complete coincidence of the indicators of the external and internal threads. If any of them is performed without compliance with the requirements, then the fastening will be unreliable.

Fastening can be bolted or studded, which, in addition to the main parts, include nuts and washers. Holes are formed in the parts to be fastened before joining, and then cutting is carried out.

To perform it with maximum accuracy, it is necessary to pre-form a hole by drilling, equal to the value of the inner diameter, that is, formed by the tops of the protrusions.

With through execution, the diameter of the hole should be 5-10% over size bolt or stud, then the condition is fulfilled:

d resp = (1.05..1.10)×d, (1),

where d is the nominal diameter of the bolt or stud, mm.

To determine the hole size of the second part, the calculation is carried out as follows: the step value (P) is subtracted from the value of the nominal diameter (d) - the result obtained is the desired value:

d resp = d - P, (2).

The calculation results are clearly demonstrated by the table of diameters of holes for threads, compiled according to GOST 19257-73, for sizes 1-1.8 mm with small and main steps.

Nominal diameter, mm	Pitch, mm	Hole size, mm
1	0,2	0,8
1	0,25	0,75
1,1	0,2	0,9
1,1	0,25	0,85
1,2	0,2	1
1,2	0,25	0,95
1,4	0,2	1,2
1,4	0,3	1,1
1,6	0,2	1,4
1,6	0,35	1,25
1,8	0,2	1,6
1,8	0,35	1,45

An important parameter is the drilling depth, which is calculated from the sum of such indicators:

• screwing depth;
• stock of external thread of the screwed-in part;
• her undercut;
• chamfers.

In this case, the last 3 parameters are reference, and the first is calculated through the accounting factors for the material of the product, which are equal for products from:

• steel, brass, bronze, titanium - 1;
• gray and malleable cast iron - 1.25;
• light alloys - 2.

Thus, the screw-in depth is the product of the material factor and the nominal diameter, and is expressed in millimeters.

Download GOST 19257-73

Thread types

Threads according to the measurement system are divided into metric, expressed in millimeters, and inch, measured in the corresponding units. Both of these types can be made in both cylindrical and conical shapes.

They may have profiles various forms: triangular, trapezoidal, round; divided according to the application: for fasteners, plumbing elements, pipe and others.

The diameters of the preparatory holes for threading depend on its type: metric, inch or pipe - this is standardized by the relevant documents.

Holes in pipe connections, expressed in inches, are prescribed in GOST 21348-75 for a cylindrical shape and GOST 21350-75 for a conical one. The data is valid for copper and nickel-free steel alloys. Cutting is carried out inside the auxiliary parts into which pipes will be screwed - slates, clamps and others.

GOST 19257-73 shows the diameters of holes for cutting metric threads, where the tables show the size ranges of nominal diameters and pitches, as well as the parameters of holes for metric threads, taking into account the values ​​of limit deviations.

The data given in the GOST19257-73 table confirm the calculation given above, in which the parameters of holes for metric types are calculated from the nominal diameter and pitch.

GOST 6111-52 normalizes the diameters of holes for inch conical threads. The document indicates two diameters with a divergence by a cone and one without reaming, as well as drilling depths, all values ​​except the nominal value are expressed in millimeters.

fixtures

manual or automatic ways slicing deliver results various classes precision and roughness. So, the main tool remains a tap, which is a rod with cutting edges.

Markers are:

• manual, to perform metric (M1-M68), inch - ¼-2 ʺ, pipe - 1/8-2 ʺ;
• machine-manual - nozzles for drilling and other machines are used for the same sizes as manual ones;
• nut, which allow you to cut a through version for thin parts, with nominal sizes of 2-33 mm.
• For cutting metric threads use a set of rods - taps:
• draft, having an elongated intake part, consisting of 6-8 turns, and marked with one risk at the base of the shank;
• medium - with an intake part of an average length of 3.5-5 turns, and marking in the form of two marks;
• the finishing one has an intake part of only 2-3 turns, without marks.

When manually cutting, if the pitch exceeds 3 mm, then 3 taps are used. If the step of the product is less than 3 mm, two are enough: roughing and finishing.

Taps used for small metric threads (M1-M6) have 3 flutes for chip removal and a reinforced shank. In the design of the rest - 4 grooves, and the shank is through.

The diameters of all three rods for metric threads increase from rough to finish. The last threaded rod must have a diameter equal to its nominal diameter.

Taps are attached to special devices– tool holder (if it is small size) or collar. With the help of them, the cutting rod is screwed into the hole.

Preparation of holes for cutting is carried out using drills, countersinks and lathes. It is formed by drilling, and by countersinking and boring, its width is increased and the surface quality is improved. Fixtures are used for cylindrical and conical shapes.

The drill is a metal rod consisting of a cylindrical shank and a screw cutting edge. to their main geometric parameters relate:

• helix angle, typically 27°;
• taper angle, which can be 118° or 135°.

Drills are rolled, dark blued, and shiny - polished.

Countersinks for cylindrical shapes are called twigs. They are metal rods with two cutters twisted into a spiral and a fixed guide pin to insert a countersink into the cavity.

cutting technique

You can cut with a hand tap following the following steps:

• drill a hole for the thread of the appropriate diameter and depth;
• carry out its countersinking;
• fix the tap in the holder or collar;
• set it perpendicular to the working cavity in which cutting will be carried out;
• screw the tap with light pressure clockwise into the hole prepared in advance for threading;
• turn the tap back every half turn to cut chips.

For cooling and lubricating surfaces during the cutting process, it is important to use lubricants: machine oil, drying oil, kerosene and the like. Incorrectly selected lubricant can lead to poor cutting results.

Drill size selection

The drill diameter for a hole for a metric thread is also determined by formula (2), taking into account its main parameters.

It should be noted that when cutting in ductile materials such as steel or brass, the turns increase, therefore it is necessary to choose a larger drill diameter for threading than for brittle materials such as cast iron or bronze.

In practice, drill sizes are usually slightly smaller than required hole. So, table 2 shows the ratio of nominal and external threaded diameters, pitch, diameters of the hole and drill for it for cutting metric threads.

Table 2. The ratio of the main parameters of the metric thread with a normal pitch and the diameters of the hole and drill

Nominal diameter, mm	Outer diameter, mm	Pitch, mm	Largest hole diameter, mm	Drill diameter, mm
1	0,97	0,25	0,785	0,75
2	1,94	0,4	1,679	1,60
3	2,92	0,5	2,559	2,50
4	3,91	0,7	3,422	3,30
5	4,9	0,8	4,334	4,20
6	5,88	1,0	5,153	5,00
7	6,88	1,0	6,153	6,00
8	7,87	1,25	6,912	6,80
9	8,87	1,25	7,912	7,80
10	9,95	1,5	8,676	8,50

As can be seen from the table, there is a certain dimensional limit, which is calculated taking into account thread tolerances.

The size of the drill is much smaller than the hole. So, for example, for an M6 thread, the outer diameter of which is 5.88 mm, and its highest value holes should not exceed 5.153 mm, it is worth using a 5 mm drill.

An M8 threaded hole with an outer diameter of 7.87 mm will only be 6.912 mm, which means that the drill for it will be 6.8 mm.

The quality of a thread depends on many factors when cutting it: from the choice of tool to a correctly calculated and prepared hole. Too little will lead to increased roughness and even breakage of the tap. High forces applied to the tap contribute to non-compliance with tolerances and, as a result, dimensions are not maintained.

This table will help you understand the cutting of metric threads and possibly reduce waste. Tabular values ​​​​can be useful to machine operators, shop foremen, engineers.

The diameters of the rods for cutting metric threads are regulated by GOST 16093-2004.

The standard metric thread pitch is indicated(*)

Pipe thread

Pipe thread is a group of standards designed for connection and sealing various kinds structural elements through pipe threads. The quality of work when cutting grooves has big influence on the reliability of the connection and the structure obtained in this way. Particular attention should be paid to the correlation of the thread with the axis of the pipe on which it is applied.

When threading by hand using a die, the concentricity is far from ideal, which can affect the reliability and quality of the connection. As for the use of tools such as a lathe or thread-cutting machine, the application threading heads with precise threading knife, then the indicators of the applied thread are comparable with the theoretical values.

The ROTHENBERGER concern manufactures threading machines, threading die cutters, heads, knives, which ensure the performance of work with high precision. All equipment fully complies with international standards in this area.

Pipe thread cylindrical, G (BSPP)

Also known as the Whitward carving ( BSW (British Standard Whitworth)). Applies this species for organizing cylindrical threaded connections. Also used in cases where an internal cylindrical thread is connected to an external conical thread (GOST 6211-81).

• GOST 6357-81: Basic norms of interchangeability. The thread is pipe cylindrical.
• ISO R228
• EN 10226
• DIN 259
• BS2779
• JIS B 0202

Thread Options

• theoretical profile height (H) - 960491Р;
• designation according to the shape of the profile - inch thread (profile in the form of an isosceles triangle with an angle at the top of 55 degrees);
• maximum diameter pipes - 6 inches (on pipes with a diameter of more than 6, a welded connection is used).

Symbol example:

G - designation of the profile shape (cylindrical pipe thread);

G1 1 / 2 - conditional pass(measured in inches);

A - accuracy class (may be A or B).

To designate a left-hand thread, the LH index is used (example: G1 1 / 2 LH-B-40 - cylindrical pipe thread, 1 1 / 2 - nominal bore in inches, accuracy class B, make-up length 40 millimeters).

The thread pitch can have one of four values:

Table 1

The main dimensions of cylindrical pipe threads are determined by GOST 6357-81 (BSP). It should be remembered that the size of the thread in this case conditionally characterizes the lumen of the pipe, despite the fact that in fact the outer diameter is much larger.

table 2

d - outer diameter of the outer thread (pipe);

D - outer diameter of the internal thread (coupling);

D1 - internal diameter of the internal thread;

d1 - internal diameter of the external thread;

D2 - the average diameter of the internal thread;

d2 is the average diameter of the external thread.

Conical pipe thread, R (BSPT)

It is used for organizing conical pipe connections, as well as for connecting cylindrical and external conical threads (GOST 6357-81). Based on BSW, compatible with BSP.

The sealing function in connections using BSPT is performed by the thread itself (due to its crushing at the junction when the fitting is screwed in). Therefore, the application of BSPT should always be accompanied by the use of a sealant.

This type of thread is characterized by the following parameters:

• GOST 6211-81 -Basic norms of interchangeability. The thread is pipe conical.
• ISO R7
• DIN 2999
• BS21
• JIS B 0203

designation according to the shape of the profile - inch thread with a taper (profile in the form of an isosceles triangle with an angle at the apex of 55 degrees, cone angle φ=3°34′48").

When designating, a letter index of the thread type is used (R for external and Rc for internal) and a numerical indicator of the nominal diameter (for example, R1 1/4 - conical pipe thread with a nominal diameter of 1 1/4). The index LH is used to designate a left-hand thread.

Thread Options

Inch thread with taper 1:16 (taper angle φ=3°34′48"). Profile angle at apex 55°.

Symbol: letter R for external thread and Rc for internal ( GOST 6211-81- Basic norms of interchangeability. Pipe thread conical.), numerical value of the nominal diameter of the thread in inches (inch), the letters LH for the left thread. For example, a thread with a nominal diameter of 1.1/4 is referred to as R 1.1/4.

Table 3

Thread size designation, pitches and nominal values ​​of the outer,
average and internal diameters of pipe taper thread (R), mm