Despite the fact that tapping of internal threads is not a complex technological operation, there are some peculiarities of preparation for this procedure. So, you should accurately determine the dimensions of the preparatory hole for threading, and also choose the right tool, for which special tables of diameters of drills for thread are used. For each type of thread, 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 different types are:

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

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

• cylindrical, which is designated by the letters MJ;
• metric and conical, denoted respectively 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, denoted by Tr;
• round, used for the installation of sanitary fittings, - Kr;
• resistant and resistant 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.

Tap application

Before proceeding with threading, it is necessary to determine the diameter of the preparation 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 hole for the thread. This information makes it easy to select the drill size.

To cut a metric type thread 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 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 dies are precisely the working surfaces of the tap.

In order for the threads of the internal thread to be clean and neat, and its geometric parameters correspond to the required values, it must be cut gradually, by gradually removing thin layers of metal from the surface to be treated. That is why, for this purpose, either taps are used, the working part of which is divided in 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 the help of which it is possible to perform sufficiently high quality machining of holes for thread, is a set consisting of two taps - roughing and finishing. The first cuts off a thin layer of metal from the walls of the hole for cutting metric threads and forms a shallow groove on them, the second not only deepens the formed groove, but also cleans it.

Combined two-way taps or sets of two tools are used for tapping small holes (up to 3 mm). For larger metric holes, use a combination 3-pass tool or a set of 3 taps.

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

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

1. The tap, with which the hole for tapping metric threads is first processed, has the smallest diameter among all the tools in the set and cutting teeth, the top of which is heavily cut.
2. The second tap has a shorter nose and longer flanges. Its working diameter is intermediate between the diameters of the rest of the tools in the set.
3. The third tap, with which the metric hole is machined last, is characterized by full ridges of the cutting teeth and a diameter that must exactly match the size of the thread being formed.

Taps are mainly used for tapping metric type threads. Significantly less often than metric taps are used for processing the inner walls of pipes. In accordance with their purpose, they are called pipe, and they can be distinguished by the letter G, which is present in their marking.

Internal threading technology

As mentioned above, before starting work, you need 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 poor performance, but also to the breakage of the tap.

Taking into account the fact that the tap, while forming the threaded grooves, not only cuts the metal, but also pushes it, the diameter of the drill for making the thread must be slightly 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 - 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. In such tables, the diameters of the drills for making threads with both standard and reduced pitch are indicated, while it should be borne in mind that holes of different diameters are drilled for these purposes. In addition, if the thread is 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 get acquainted with the GOST provisions governing the cutting of metric threads by downloading the pdf document from the link below.

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

D o \u003d D m x 0.8, Where:

Before Is the diameter of the hole to be made with the drill,

D m - the diameter of the tap used to machine the drilled element.

The strength of fastening the parts to each other is ensured by screwing the carrier of the external thread into the internal one 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 implementation of the thread and its individual elements.

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

Hole parameters

The following thread parameters are distinguished:

• diameters (inner, outer, and so on);
• profile shape, height and angle;
• step and entry;
• others.

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

The 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, you must first form a hole by drilling, equal to the size of the inner diameter, that is, formed by the tops of the protrusions.

With a through execution, the hole diameter should be 5-10% larger than the size of the bolt or stud, then the condition is met:

d hole \u003d (1.05..1.10) × d, (1),

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

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

d hole \u003d d - P, (2).

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

Nominal diameter, mm	Step, 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:

• screw-in depth;
• stock of external thread of the screwed-in part;
• its undercut;
• chamfers.

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

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

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

Download GOST 19257-73

Varieties of thread

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

They can have profiles of various shapes: triangular, trapezoidal, round; divided by 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 normalized 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 shape. The data is valid when copper and nickel-free steel alloys are used. Cutting is carried out inside auxiliary parts, into which pipes will be screwed - shale, 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 \u200b\u200bof the maximum deviations.

The data given in the table GOST19257-73 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 standardizes the diameters of the holes for the inch tapered thread. The document indicates two diameters with a divergence per taper and one without reaming, as well as drilling depths, all values, except for the nominal value, are expressed in millimeters.

Gadgets

Manual or automatic cutting methods provide results in different accuracy and roughness classes. So, the main tool remains a tap, which is a rod with cutting edges.

Taps are:

• manual, for metric (M1-M68), inch - ¼-2 ʺ, pipe - 1 / 8-2 ʺ;
• machine-hand - nozzles for drilling and other machines, used for the same sizes as manual;
• nut, which allow you to cut a through version for thin parts, with a nominal size of 2-33 mm.
• A set of tap rods is used for cutting metric threads:
• rough, having an elongated intake part, consisting of 6-8 turns, and marked with one line 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 lines;
• finishing has an intake part of only 2-3 turns, without risks.

For manual cutting, if the pitch exceeds 3 mm, then use 3 taps. If the step of the product is less than 3 mm, two are enough: rough and finish.

The taps used for small metric threads (M1-M6) have 3 flutes, along which the chips are removed, and a reinforced shank. In the design of the rest there are 4 grooves, and the shank is through.

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

The taps are attached to special devices - a tool holder (if it is small) or a knob. With the help of them, the cutting rod is screwed into the hole.

Holes are prepared for cutting using drills, countersinks and lathes. It is formed by drilling, and by countersinking and boring, it increases its width and improves the surface quality. Fixtures are used for cylindrical and conical shapes.

The drill is a metal rod consisting of a cylindrical shank and a helical cutting edge. Their main geometric parameters include:

• a helical lift angle, typically 27 °;
• taper angle, which can be 118 ° or 135 °.

Drills are rolled, blued, dark color, and shiny - ground.

Countersinks for cylindrical shapes are called counterbores. They are metal rods with two helical cutters and a fixed guide pin to guide the countersink into the cavity.

Cutting technique

Using a hand tap, you can perform cutting by observing the following steps:

• drill an opening for the thread of the corresponding diameter and depth;
• to carry out its countersinking;
• fix the tap in the holder or wrench;
• 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 the chips.

To cool and lubricate the surfaces during the cutting process, it is important to use lubricants: engine oil, drying oil, kerosene, and the like. Improperly selected lubricant can lead to poor quality cutting results.

Drill size selection

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

It is worth noting that when cutting in plastic materials, such as steel or brass, an increase in turns occurs, therefore it is necessary to choose a larger drill diameter for the thread than for brittle materials such as cast iron or bronze.

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

Table 2. Correlation of the main parameters of metric threads with normal pitch and diameters of the hole and drill

Nominal diameter, mm	Outside diameter, mm	Step, 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 you can see from the table, there is a certain dimensional limit, which is calculated taking into account the tolerances for the thread.

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

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

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

External thread cutting. 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.

When cutting a thread with a die, it must be borne in mind that when the thread profile is formed, the metal of the product, 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 can turn out to be torn.

When choosing a rod diameter for an external thread, the same considerations should be followed as when choosing holes for an internal thread. The practice of cutting external threads shows that the best thread quality can be obtained if the diameter of the rod is slightly less than the outer diameter of the thread to be cut. If the diameter of the rod is less than required, the thread will be incomplete; if more, then the die either cannot 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.

Table 27 shows the diameters of the rods used when threading with dies.

Table 27 Diameters of rods for thread 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 vise so that the end of the vise protruding above the level of the jaws is 20-25 mm longer than the length of the cut part. A chamfer is cut at the upper end of the bar to ensure plunging. Then a die fixed in the die is placed on the rod and the die is rotated with slight pressure so that the die cuts about 0.2-0.5 mm. After that, the cut part of the rod is lubricated with oil and the die 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).

Figure: 152. Reception of thread cutting with a die (b)

To prevent scrap and breakage of the teeth, it is necessary that the die goes onto the rod without skewing.

Checking the cut internal thread is carried out with threaded plug gauges, and the external one - with threaded micrometers or threaded ring gauges.

This table will help you understand how to cut metric threads and possibly reduce scrap. Table values \u200b\u200bcan be useful for machine operators, shop foremen, engineers.

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

The standard metric lead is indicated by(*)

Pipe thread

Trumpet thread is a group of standards designed for the connection and sealing of various types of structural elements using pipe threads. The quality of the grooving work has a great influence on the reliability of the joint and the resulting structure. Particular attention should be paid to the correlation of the thread with the axis of the pipe on which it is applied.

When threading manually using a die, the alignment 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 threading machine, applications die heads with precise die cutter, then here the indicators of the applied thread are comparable with the theoretical values.

Concern ROTHENBERGER manufactures threading machines, threading dies, heads, knives, ensuring the performance of work with high precision. All equipment fully complies with international standards in this area.

Cylindrical pipe thread, G (BSPP)

Also known as Whitward carving ( BSW (BritishStandardWhitworth)). This type is used to organize cylindrical threaded connections. Also used when connecting cylindrical internal threads with external tapered threads (GOST 6211-81).

• GOST 6357-81: Basic standards of interchangeability. Cylindrical pipe thread.
• ISO R228
• EN 10226
• DIN 259
• BS 2779
• JIS B 0202

Thread parameters

• theoretical height of the profile (H) - 960491Р;
• profile designation - inch thread (profile in the form of an isosceles triangle with an apex angle of 55 degrees);
• the maximum pipe diameter is 6 inches (welded joint is used on pipes with a diameter of over 6).

Example of a symbol:

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

G1 1/2 - nominal bore (measured in inches);

A - accuracy class (can 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 mm).

The thread pitch can have one of four values:

Table 1

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

table 2

d is the outer diameter of the external thread (pipe);

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

D1 - internal diameter of an internal thread;

d1 - inner diameter of the outer thread;

D2 is 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 pipe conical connections, as well as for connecting internal cylindrical and external tapered threads (GOST 6357-81). Based on BSW, has compatibility with BSP.

The sealing function in connections using BSPT is performed by the thread itself (due to its crushing at the connection when screwing in the fitting). Therefore the use of BSPT must always be accompanied by the use of a sealant.

This type of thread is characterized by the following parameters:

• GOST 6211-81 - Basic standards of interchangeability. Conical pipe thread.
• ISO R7
• DIN 2999
• BS 21
• JIS B 0203

designation by the shape of the profile - inch thread with a taper (profile in the form of an isosceles triangle with an apex angle of 55 degrees, taper angle φ \u003d 3 ° 34′48 ").

The designation uses the letter code of the thread type (R for external and Rc for internal) and a numeric indicator of the nominal diameter (for example, R1 1/4 - conical pipe thread with a nominal diameter of 1 1/4). For left-hand threads, the LH index is used.

Thread parameters

Inch thread with a taper of 1:16 (taper angle φ \u003d 3 ° 34'48 "). Profile angle at top 55 °.

Designation: letter R for external thread and Rc for internal ( GOST 6211-81 - Basic norms of interchangeability. Conical pipe thread.), Numerical value of nominal thread diameter in inches (inch), letters LH for left-hand thread. For example, a thread with a nominal diameter of 1.1 / 4 is designated as R 1.1 / 4.

Table 3

Thread size designation, steps and nominal values \u200b\u200bof the outer,
average and inner diameters of the pipe conical thread (R), mm

Screws, bolts, and studs are the most common male threads. Most often, they fall into the hands of a home craftsman ready-made. But it happens that you need to make some clever bolt or non-standard stud. A blank for such a part is a rod, the diameter of which must correspond to the thread to be cut.

Male thread shank diameter depends on nominal thread diameter and thread pitch. All this information is usually indicated in the drawing of the part in the form of the designation M10 × 1.5. The letter “M” denotes a metric thread, the number after the letter is the nominal diameter, the number after the “x” sign is the thread pitch. When using the main (large) step, it may not be specified. Basic thread pitch is defined by the standard and is most preferred.

When choosing the diameter of a rod for an external thread, the same principles are followed as when choosing holes for an internal thread. It has been found that better thread quality is obtained if the diameter of the shank is slightly less than the nominal diameter of the thread to be cut. The metal is squeezed out a little when cutting and the thread profile is complete.

If the diameter of the rod is much less than the required one, then the tops of the threads will be cut, if more, then the die simply will not screw onto the rod or break during operation.

For every combination of diameter and pitch, there is a optimal rod diameter... The easiest way to determine this diameter is from the table, which shows the most common threads that a home craftsman can meet. The main thread pitch for each nominal diameter is shown in bold in the table.

The main tool for cutting external threads is a die. The most commonly used round continuous dies in the form of a hardened steel nut.

To form the cutting edges, the threads of the dies intersect through longitudinal holes, which also provide the exit of chips. To facilitate lead-in, the extreme threads have an incomplete profile. To rotate the plate use ram holder - a tool with a socket for a die and long handles. There are also split and sliding (klupp) dies, but this is a rarity in the home workshop.

To reduce friction and obtain clean threads on steel rods, a lubricant is used - mineral oil or kerosene, on copper - turpentine. At the end of the rod, to facilitate entry, a chamfer with a width of at least the size of the thread pitch must be made.