What kind of drill to use to cut threads for 6

How to cut a thread manually with a tap: learning the main thing

There are many ways to connect parts, but the most common and accessible is still screw (bolt) fasteners. It is this type of fastener that we will talk about. More precisely, we will look at the types of taps, the technology for cutting threads in holes for screws and bolts of different diameters.

Classification of taps and their scope of application

Lathes or drilling machines have been used for cutting threads for quite a long period. They are designed to rotate a workpiece or tool. In almost all cases, the working tool is a tap.

The classification of such a tool is carried out according to a number of different characteristics:

1. Depending on the method of driving them into rotation, they can be divided into machine or machine-hand taps. A special feature of the machine-manual variety of the tool is the presence of a square shank, which is suitable for a holder with two handles. Thanks to such a holder, you can rotate the tool and cut threads. As for the machine variety of the tool in question, it is installed in the chuck of various machines and can have a shank of various shapes and sizes.
2. There are universal and complete taps based on characteristics related to the specific application. The universal version has a working part, which is divided into several sections. They differ from each other in geometric shape, the first is intended for roughing, the second and subsequent ones are for bringing the surface to a suitable state. The complete version is designed for the phased use of several taps. As a rule, a set for cutting threads in ordinary metal is represented by three tools, and for processing hard alloys - five.
3. Taps are classified based on the type of hole being machined: blind or through. To work with through holes, a tool with an elongated conical tip is used, which is a continuation of the working part. Universal taps often have this type of shape. If you need to cut a thread in a blind hole, then use a tool that has a cut tip resembling a milling cutter. Due to this shape, it is possible to cut threads along the entire length of the hole.
4. Thread cutting tools are also classified according to the type of working surface. Examples include helical, straight or shortened grooves. On sale you can find a version that also has a combined surface. It is suitable for processing metals with low hardness. For hard alloys, a tool with cutting elements arranged in a checkerboard pattern is used.

As a rule, high-quality tool steel is used in the manufacture of tools. It can withstand long-term use and does not heat up during operation. A high quality tap has a strict geometric shape and looks neat. A similar tool is often used for cutting metric threads, but there are versions for producing inch threads. The shape of the base can be cylindrical or conical.

: Gray color in the interior - let's analyze it thoroughly

Drill selection

An electric drill is necessary to perform internal threading work.

Before cutting a thread with a tap, you must first drill a hole for it. It is absolutely clear that the diameter of the hole should be smaller than the size of the tap, but how much? There is a universal way to calculate this difference. Any thread is characterized by two parameters:

• diameter, indicated on instruments with a capital letter M in front, for example, M6;
• The pitch of turns is written as a number after the first parameter, for example, M6x1.

By looking at the markings on the tap, it is easy to select the drill for the job. It is necessary to subtract the thread pitch from the diameter value; in the given example, M6x1, you will need a hole measuring 6 - 1 = 5 mm. The drill should be a regular cylindrical one; for metal processing, these are made from high-speed steel R6M5. In order to facilitate the selection of sizes, Table 1 presents the recommended diameters for the holes of the most common metric profile threads.

Table 1

Marking	M3	M4	M5	M6	M8	M10
Hole D, mm	2,5	3,3	4,2	5	6,75	8,5

Since we are talking about manual metal processing, one feature should be taken into account when selecting drills.

When working with an electric drill, no matter how hard you try to keep it level and motionless, the hole will break and its size will be slightly larger than necessary. As a result, after passing with a tap, the thread turns will have a cut profile.

When the bolt is screwed in, there will be some play, and if the tightening torque is high, the coils may simply become deformed. The size of the hole depends on two factors:

Countersinks are needed for chamfering when cutting internal threads.

1. The larger the diameter, the more the hole unfolds. For example, with D = 5 mm there is an increase of 0.08 mm, and with D = 10 mm there is already an increase of 0.12 mm.
2. Working conditions. The mechanic may be in an awkward position or at a height, this will cause increased hole breakdown, which will then affect the quality of the connection.

The problem is solved as follows: to make the internal thread full-profile, you must initially take a drill whose diameter is 0.1 mm less than the required one. It is possible that in some cases it will be difficult to rotate the tap, but usually no difficulties arise.

Preparation of tools

To perform internal thread cutting you will need:

• marking tools (tape measure, ruler, pencil);
• electric drill with a set of drills;
• countersink for chamfering;
• set of taps with hand crank;
• hammer;
• core

Drills should be threaded (sharpened) in accordance with the hardness of the material being processed. The harder it is, the greater the sharpening angle needs to be maintained. During drilling and cutting, the working parts of drills and taps must be lubricated. The choice of lubricant type depends on the viscosity of the steel. For regular and carbon steel, old lard or grease is suitable, for stainless steel - machine oil.

Marking of taps

Taps for cutting internal metric threads are marked with the letter “M”. In this case, the letter “M” is followed by the value of the diameter of the thread being cut. In addition to the diameter, the thread pitch is also indicated on the tool.

For example, the marking M4 × 1 means that the tool can make threads in a hole with a diameter of 4 mm in increments of 1 mm. In the case of a cutting tool for left-hand threads, the tool is marked LH.

The diameter and pitch of the thread are indicated in a manner similar to that described above.

Preparing to cut internal threads

In order for the process of cutting internal threads using a tap to not cause any particular difficulties and result in a high-quality result, it is necessary to properly prepare for this technological operation.

All methods of cutting threads using a tap assume that a hole with the appropriate diameter has already been made in the workpiece.

If the internal thread that needs to be cut has a standard size, then a special table with data in accordance with GOST can be used to determine the diameter of the preparation hole.

Table 1. Diameters of holes drilled for standard metric threads

If the thread that needs to be cut does not belong to the standard category, you can calculate the diameter of the hole to make it using a universal formula. First of all, it is necessary to study the marking of the tap, which must indicate the type of thread being cut, its diameter and pitch, measured in millimeters (for metric).

Then, to determine the cross-sectional size of the hole that needs to be drilled for the thread, it is enough to subtract the pitch from its diameter. For example, if a tool marked M6x0.75 is used to cut a non-standard internal thread, then the diameter of the preparation hole is calculated as follows: 6 – 0.75 = 5.25 mm.

For standard threads belonging to the inch category, there is also a table that allows you to choose the right drill with which to carry out the preparatory work.

Table 2. Diameters of holes drilled for inch threads

An important question to obtain a high-quality result is not only the question of what is used to cut the thread, but also what drill to use to make the preparation hole. When choosing a drill, you need to pay attention to the parameters and quality of its sharpening, as well as to ensure that it rotates in the chuck of the equipment used without runout.

The sharpening angle of the cutting part is selected depending on the hardness of the material that needs to be drilled. The higher the hardness of the material, the greater the sharpening angle of the drill should be, but this value should not exceed 140°.

When drilling blind holes, control the drilling depth

How to cut threads correctly? First you need to select tools and consumables:

1. an electric drill or drilling machine capable of operating at low speeds;
2. a drill whose diameter is calculated or selected using reference tables;
3. a drill or countersink, with the help of which a chamfer will be removed from the edge of the prepared hole;
4. a set of taps of the appropriate size;
5. manual holder for taps (drives);
6. bench vice (if the product in which the thread needs to be cut needs to be fixed);
7. core;
8. hammer;
9. machine oil or other composition, which during the processing process must be used to lubricate both the tap and the thread section being cut by it;
10. rags.

Diagram of thread cutting with a tap

How to cut a thread with a tap?

Tapping internal threads is a precise technological process that is carried out in a certain sequence.

Preparation

Initially, you need to prepare tools and additional devices for the work:

1. Small vice.
2. Electric drill with speed controller, metal drills.
3. Set of taps.
4. Metal brush.
5. Core with a hammer.

Thread cutting with a tap is carried out manually at low speeds of the power tool.

Tapping internal threads

Slicing process

Step-by-step thread cutting instructions:

1. Drill the hole with a core and a hammer.
2. Drill a hole with an electric drill. Hold the equipment exactly perpendicular to the surface being processed. Additionally, lubricate the drill with special oil.
3. Chamfer 1 mm in depth. To do this, use a drill of a larger diameter.
4. Secure the equipment in the collar. Perform two movements forward, one movement back. This way the metal shavings will come out of the hole and there will be no problems with cutting. Movements should be smooth.

During manual cutting, you must not apply pressure or continue to rotate the tool after it gets stuck. If it breaks, you need to drill out the tap using a drill or various metal drills. Another option for removing the fragment is to grind the area around the hole using a grinding machine. Then you need to remove it with pliers.

To create threaded connections, you need to buy the appropriate documents and learn how to work with them. By carrying out the work carefully, without haste, you can get a reliable connection in a short period of time.

Source: https://arbolit.org/instruktsii/kak-pravil-no-narezat-rez-bu-metchikom-algoritm-rekomendacii-tablicy-razmerov.html

Threaded hole diameter: size table according to GOST and calculation

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

Threaded hole diameter

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

Hole parameters

The following thread parameters are distinguished:

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

The condition for connecting parts to each other is the complete coincidence of the external and internal threads. If any of them are not performed in accordance with the requirements, the fastening will be unreliable.

External thread

The fastening can be bolted or stud, which, in addition to the main parts, includes nuts and washers. Before joining, holes are formed in the parts to be fastened, and then cutting is carried out.

To perform it with maximum accuracy, you should first form a hole by drilling, equal to the size of the internal diameter, that is, formed by the tops of the protrusions.

When performing a through design, the diameter of the hole must be 5-10% larger than the size of the bolt or stud, then the following condition is met:

dresponse = (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 pitch value (P) is subtracted from the value of the nominal diameter (d) - the resulting result is the desired value:

dresponse = d - P, (2).

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

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 the following indicators:

• screw-in depth;
• reserve of external thread of the screwed-in part;
• her undercut;
• chamfers.

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

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

Internal thread on barrel coupling

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

Types of carving

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

Classification of threads

They can have profiles of various shapes: triangular, trapezoidal, round; divided according to application: for fasteners, plumbing elements, pipes and others.

The diameters of the preparation 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 specified in GOST 21348-75 for cylindrical shapes and GOST 21350-75 for conical shapes. The data is valid when using copper and nickel-free steel alloys. The cutting is carried out inside the auxiliary parts into which the 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 maximum deviations.

Thread types according to GOST

The data given in the GOST 19257-73 table confirms 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 holes for inch tapered threads. The document indicates two diameters with a taper and one without a taper, as well as drilling depths; all values, except the nominal value, are expressed in millimeters.

Adaptations

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

Tap device

Taps are:

• manual, for metric (M1-M68), inch – ¼-2ʺ, pipe – 1/8-2ʺ;
• machine-manual - attachments for drilling and other machines, used for the same sizes as manual ones;
• nuts, 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:
• rough, having an elongated intake part, consisting of 6-8 turns, and marked with one mark at the base of the shank;
• medium - with a fence of average length of 3.5-5 turns, and markings in the form of two marks;
• the finishing part has a fence of only 2-3 turns, without marks.

Tolerance control of metric thread placement

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

Taps used for small metric threads (M1-M6) have 3 grooves that carry chips and a reinforced shank. The design of the others has 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.

The taps are attached to special devices - a tool holder (if it is small) or a crank. They are used to screw the cutting rod into the hole.

Preparing holes for cutting is carried out using drills, countersinks and lathes. It is formed by drilling, and by countersinking and boring it is increased in width and improves the quality of the surface. The fixtures are used for cylindrical and conical shapes.

Thread pitch

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

• the helical lift angle is usually 27°;
• point angle, which can be 118° or 135°.

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

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

Cutting technique

Using a hand tap, cutting can be carried out following the following steps:

• drill an opening for a thread of the appropriate diameter and depth;
• countersink it;
• secure the tap in the holder or driver;
• align 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 off the chips.

Thread system

To cool and lubricate 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.

Selecting drill size

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

Drill parametersDiameter of the thread hole and its other characteristics

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

In practice, drill sizes are usually slightly smaller than the required hole. Thus, Table 2 shows the ratio of the nominal and outer thread diameters, the pitch, the diameters of the hole and the drill for cutting metric threads.

Table 2. The relationship between the main parameters of metric threads with 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 largest hole value should not exceed 5.153 mm, you should use a 5 mm drill.

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

The quality of the thread depends on many factors when cutting it: from the choice of tool to the correctly calculated and prepared hole. Too little 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.

Source: https://stankiexpert.ru/tehnologii/diametr-otverstiya-pod-rezbu.html

Drill diameter for thread: size table according to GOST | Construction Bulletin

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.

The thread diameter and through hole must comply with the standards, otherwise the grooves will come out too small and the threaded connection will be unreliable

Types and parameters of thread

The parameters by which threads are divided into different 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).

Metric thread parameters

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.

Thread types according to GOST

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.

Tap: design and parameters

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.

Types of thread taps and their differences

Minimum set of taps

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 designs, is the size of the mounting hole, which must exactly match the size of the tool shank.

Some types of tap drivers

When using a set of three taps that differ 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, has the smallest diameter among all the tools in the set and cutting teeth, the upper part of which is heavily cut off.
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.

Set of three taps

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

Table 2. Diameters of holes for inch 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.

GOST 19257-73 Holes for cutting metric threads. DiametersDownload

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:

To = Dm x 0.8, where:

Do is the diameter of the hole that must be made using a drill,

Dm is the diameter of the tap that will be used to process the drilled element.

Scheme of cutting internal threads with a tap

The drivers into which the threaded tap is inserted can have a simple design or be equipped with a ratchet. You should work with such devices with tools fixed in them very carefully. To obtain high-quality and clean threads, rotating the tap clockwise, performed half a turn, must be alternated with turning it one-fourth of a turn against the thread.

The thread will be cut much easier if you use a lubricant during this procedure. The role of such a lubricant when cutting threads in steel products can be played by drying oil, and when processing aluminum alloys - alcohol, turpentine or kerosene. If such technical fluids are not at hand, then ordinary machine oil can be used to lubricate the tap and the thread being cut (however, it has less effect than the substances listed above).

Source: https://svestnik.kz/diametr-sverla-pod-rezbu-tablica-raz/

What should be the diameter of the drill bit for threading table

When choosing the diameter of a drill for threading, a table with the corresponding dimensions will become your indispensable auxiliary tool. With its help, you can significantly simplify the calculations preceding the work and obtain a correctly cut thread that securely holds the fastener.

Classification of threads.

High-quality thread is the key to stable connection of elements of any structure. The final result of your efforts largely depends on the choice of tool in accordance with the material being processed and working conditions.

How to choose drill diameter?

Types of drills

The characteristics of the future thread largely depend on the properties of the metal in which it will be cut. Not all materials hold internal or external threads equally well. Soft metals such as copper and aluminum can be easily machined with a drill and tap, but the threads will deform under the slightest load.

All types of threads are characterized by two main parameters: diameter and pitch.

Diameter is the maximum distance between two opposite points on the circle delimiting the thread hole. You need to understand that the thread has a relief surface in cross-section, so its diameter will differ depending on at what point in the hole you take measurements.

The thread pitch is the range between adjacent turns of a groove cut into metal. The deeper and more prominent the carving, the larger the pitch. This parameter depends not only on the diameter of the screw that will be attached to the hole, but also on the conditions of use of the finished product: for parts subject to high loads, a deep, sparse thread is cut.

The selection of a drill for the thread is carried out based on the above parameters. In order to determine the most suitable tool, use a simple formula:

Figure 1. Table for selecting the diameter of a drill for threads.

D(s) = D(r)-SH, where:

• D(s) – drill diameter;
• D(r) – planned diameter of the future thread;
• Ш – thread pitch.

This formula makes it possible, even in field conditions, to determine the optimal parameters of the hole, which will ensure the most reliable fixation of the elements being connected.

Experienced craftsmen recommend using specially designed tables in their work, which allow you to determine the diameter of the drill and tap with an accuracy of tenths of a millimeter.

If you do thread cutting work quite often, then such a table, printed and laminated, will always be at hand (Fig. 1). The first column indicates the thread diameter (from 2 to 48 mm), and the following columns indicate the options for pitch and drill diameter.

The values ​​cover a wide range of data and will be useful for both the home master and a highly qualified specialist.

Tips for cutting threads

Thread cutting is carried out in one of two ways. The first of these involves making a hole in the metal at the same time as creating spiral grooves. This method is not suitable in all situations and requires powerful equipment: a drilling machine that can do all the work in one pass.

The second method is used at home. In order to make a thread using this method, a hole is first drilled in the metal, and only after that the thread is cut with a tap.

Recently, in construction stores you can find combined drills for threads, with their help they can simultaneously make a hole and cut grooves.

In order for the threads cut in metal to be reliable, high-quality and even, experts recommend following a number of rules when working:

Tap device for cutting threads.

1. Use only sharp tools, as a rusted tap or drill easily breaks and working with them is difficult and takes a long time. Removing such a tool from the hole will be much more difficult than purchasing a new one. Also, do not tempt yourself with the low price of drills and taps from unknown manufacturers, since by purchasing them you will not only damage the product, but also risk causing injury to yourself.
2. Do not forget to periodically lubricate the tool while working. This can be done with a few drops of machine oil or kerosene. Lubrication plays a particularly important role when working with viscous metals, such as aluminum. It prevents chips from sticking to the working surface and cools the drill.
3. The key rule is the optimal selection of hole diameter. If the selected diameter is too large, the fastening element will dangle at the installation site, which will lead to unnecessary vibrations of the product. It is difficult to screw a tap into a narrow hole, which greatly complicates an already difficult job.
4. The main parameters of the hole should depend on its functional purpose. If no excessive load is applied to the connection point, then it is allowed to make it only 5% smaller than the diameter of the future thread. This will make it easier to cut grooves without sacrificing too much joint quality.

The process of creating carvings is quite simple. After the hole in the metal is ready, insert a tap into it, clamp it into the driver and screw it clockwise with force. After each full turn, it is necessary to make half a turn in the opposite direction. This action breaks off metal shavings, resulting in cleaner cuts.

Characteristics of thread drills

Figure 2. Tapping a thread.

Selecting a good drill for metal is not as easy a task as it might seem at first glance. Modern industry produces a huge range of similar products, and it can be very difficult to understand such a variety in the absence of proper experience.

The easiest way to choose a high-quality drill is to focus on its visual characteristics. Tools processed at the final stage of production with superheated steam are dark, even black in color. Such drills are highly resistant to wear.

Drills with gold coating belong to the class of high-quality and rather expensive tools. They owe their color to a thin layer of titanium nitride, which increases strength and reduces friction.

Don’t forget about drills of the usual gray color, since they do not undergo finishing treatment, as a result of which their price remains quite affordable. Such tools are enough for several dozen holes, so if the scale of work is small, a steel drill will not let you down.

It is worth paying attention to the markings of the tools, the shape of their cutting edge, the name of the manufacturer and the price. Knowing the basic parameters of a good drill, it will be very easy for you not to get confused in the store.

Working with a tap

A tap is a special tool whose main purpose is to apply straight internal threads to a pipe or hole. The tap consists of a metal rod in which straight or helical flutes are cut. The sharp top of the grooves forms the cutting edge.

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The tail part of the tap is made in the shape of a square or backed (non-round). During operation, a double-sided lever is attached to it - a knob, during rotation of which the tap is immersed in the metal.

If you need to cut large diameter threads, it is quite difficult to get by with just one tap. In such cases, sets consisting of 2 (rough and finishing) or 3 (roughing, medium and finishing) taps are used. When you have to work with viscous metals that tend to stick to the tool, the number of taps can increase to 5. Each subsequent one corrects the errors and shortcomings of the previous one so that the output is an even thread.

Taps are made from high-speed steel or hard tungsten alloy. They work with them using drilling machines, clamping its back part in a specially shaped chuck. For small scale work, or in cases where the insertion site is difficult to reach for a large tool, manual labor is used: the tap is clamped into the driver and the tool is screwed in clockwise (Fig. 2).

Conclusion on the topic

Threaded connection of parts is one of the most reliable ways of assembling individual elements into a single structure.

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Having a wealth of knowledge about the main features of the thread cutting process, choosing a tool and the diameter of the hole, you can greatly facilitate the upcoming work and achieve the desired result.

Source: https://masterinstrumenta.ru/dreli/diametr-sverla-pod-rezbu-tablica.html

What drill to use for M6 threads?

To determine the required hole diameter for tapping, a table is the most convenient tool. It is possible to make mathematical calculations for this purpose, but if the table is compiled in accordance with GOST, there is no need for unnecessary manipulations.

Tapping Hole Diameter Table

• The threaded hole is used to create detachable connections. But if there are no problems with external threading, since any necessary fasteners made according to GOST can be purchased in stores, then internal threading requires a more careful approach;
• To cut internal metric threads of sizes M4, M5, M6, M8, M10, M12, hand tools or specialized machines are used;
• To obtain a metric internal thread, it is necessary to make a corresponding hole for the future thread in the workpiece;
• The holes made for metric cutting must correspond to the diameter of the tool used for cutting;
• Each hole made requires the use of a tool of a certain size;
• For these purposes, the master must correctly select the diameter of the drill for thread cutting;
• Specialized machines are used to cut large holes of large diameter. The drills in them have a conical shank type;
• Conventional drills are equipped with a cylindrical shank;
• The size of the cone directly depends on the size of the drill. The larger the diameter of the drill, the larger its cone;
• If it is necessary to cut internal threads of types M4, M5, M6, M8, M10, M12, the diameter of the required hole is selected based on the thread diameter and thread pitch;
• To determine what size the diameter should be, you need to determine the size of the holes. To simplify this task, a special table compiled in accordance with GOST is used. This means that the table provides standardized operations. Due to this, it is possible to obtain the same type of product, which is suitable for various types of threaded connections.

Based on the table and strictly following its parameters, you can make internal threads of the required diameter with your own hands, and purchase hardware and bolts in a specialized store. In this case, the connection will be perfectly matched.

Photo table of hole diameters for thread cutting

The thread looks like helical grooves with a constant cross-section. In this case, the thread is divided into two types:

The threaded connection has a wide range of applications. It is difficult to find places where it is not used. Due to the thread, the parts are given rotation, translational movement, and fastening. Almost all mechanisms, machines and components necessarily include a threaded connection.

In this case, cutting is divided into two large groups:

• Single pass cutting;
• Multi-start threaded connection.

Depending on the direction of movement of the screws along the thread, it is divided into:

• Left-handed;
• Right-handed.

If we take into account the size systems standardized according to GOST, cutting can be divided into three more types.

1. Pipe. Its distinctive feature is the initial diameter, which is based on the size of the pipe hole. In this situation, only external cutting is carried out.
2. Metric. It is characterized by a profile made in the form of a triangle. The profile angle can be 60 degrees. Dimensions are determined in millimeters. If the step is large, the markings include numbers and letter values. The number is the outer diameter. If a metric thread has a non-standard small pitch, then a number is added to the letter designation of the type M4, M5, M6, M8, M12, which indicates the pitch. In this situation, the marking is presented as M6x0.6.
3. Inch. For inch threads, a distinctive feature is the size of the profile angle. For inch types it is usually up to 55 degrees. The size of the hole is indicated in inches, and the pitch depends on the number of threads.

Taps

• A tap is the main tool used for cutting threads. In this case, taps can be manual or machine-made. This characteristic determines whether the tap is used on a hand tool or used as a working tool on a specialized machine;
• Manual type taps are produced in sets;
• Typically the kit includes three taps;
• The first two taps from the set carry out preliminary measures, and the last one brings the thread to the required finishing condition;
• With the help of a finishing tap, it is possible to obtain cuttings that are optimal in size and shape;
• Each tap has its own number indicated on the shank;
• In some cases, there are only two taps in the set, where the first is preliminary, and the second is finishing;
• Taps are made on the basis of high-alloy steel, which allows them to work with hard grades of metal;
• To make external threads, a special die is used;
• The die can be round or prismatic.

Internal thread

1. To equip a workpiece with a hole for an internal thread, the first step is to correctly select the size of the drill for M4 or another thread size according to GOST.
2. Having chosen a drill, the master begins to make holes for threading.
3. To proceed to using a special tap for cutting M4, first the hole made is processed with a tool - a countersink. This allows you to achieve the desired cleanliness of the hole.
4. During the process of cutting grooves in a metal workpiece, the material is slightly squeezed out. Because of this, it is important to choose the correct drill size.

   The drill used is slightly larger than the internal threaded diameter of the tool being used.

5. When working with metal of increased hardness, it will not be possible to significantly change the size of the hole when forming grooves due to the characteristics of the material.
6. If metal of medium or small hardness is processed, the dimensions of the hole made change greatly.

   If the size of the hole matches the internal threaded diameter, active extrusion will begin during the processing process. Excess will have a negative impact on the tap. The tool will begin to overheat and chips will stick to the internal surfaces. You'll end up with threads with the wrong flutes, plus you run the risk of breaking your tap.

7. To determine the diameter of a suitable drill, a specialized table is used. It clearly indicates all the main parameters, which allows a beginner to quickly understand all the nuances.

Table

The table consists of several main columns.

• The first indicates the dimensions of the metric thread. It ranges from M2 to a maximum value of M48;
• The second column is where the coarse step is indicated. It is used for every metric thread size. Moreover, depending on the diameter, the pitch differs, gradually increasing as the diameter of the thread increases. When you cut grooves, focus on these parameters;
• The third column indicates the diameter of the drill (hole) for metric cutting with a large pitch.

Sometimes you may need to use a fine step, which is quite rare. But for this case, the table provides a separate column, the indicators of which should be guided in such situations.

Source: https://respect-kovka.com/kakim-sverlom-sverlit-pod-rezbu-m6/

Diameter of holes for metric threads table

       The main thread pitch is highlighted in color

Metric thread, M
Tap	Drill	Tap	Drill
M1.4x0.3	1.1	M14x1.25	12.8
M1.6x0.35	1.25	M14x1	13
M2x0.4	1.6	M16x2	14
M2.5x0.45	2.05	M16x1.5	14.5
M3x0.5	2.5	M16x1.25	14.8
M3.5x0.6	2.9	M16x1	15
M4x0.7	3.3	M18x2.5	15.5
M4x0.5	3.5	M18x2	16
M5x0.8	4.2	M18x1.5	16.5
M5x0.5	4.5	M18x1	17
M6x1	5	M20x2.5	17.5
M6x0.5	5.5	M20x2	18
M7x1	6	M20x1.5	18.5
M7x0.5	6.5	M22x2.5	19.5
M8x1.25	6.8	M22x2	20
M8x1	7	M22x1.5	20.5
M8x0.75	7.2	M24x3	21
M8x0.5	7.5	M24x2	22
M9x1.25	7.8	M24x1.5	22.5
M9x1	8	M27x3	24
M10x1.5	8.5	M27x2	25
M10x1.25	8.8	M27x1.5	25.5
M10x1	9	M30x3.5	26.5
M11x1.5	9.5	M30x3	27
M11x1.25	9.8	M30x2	28
M11x1	10	M30x1.5	28.5
M12x1.75	10.2	M33x3.5	29.5
M12x1.5	10.5	M33x3	30
M12x1.25	10.8	M33x2	31
M12x1	11	M33x1.5	31.5
M14x2	12	M36x4	32
M14x1.5	12.5	M42x4.5	37.5

Pipe thread, G
Tap	Drill	Tap	Drill
G 1/8	8.8	G 1 1/4	39.5
G 1/4	11.8	G 1 3/8	42
G 3/8	15.25	G 1 1/2	45.5
G 1/2	19	G 2	57.5
G 5/8	21	G 2 1/4	63.5
G 3/4	24.5	G 2 1/2	72.5
G 7/8	28.25	G 2 3/4	78.5
G 1	30.75	G 3	85.5
G 1 1/8	35.5

Inch UNC thread
1/4x20	5.1	7/8x9	19.5
5/16x18	6.5	1x8	22.25
3/8x16	8	1 1/8x7	25
7/16x14	9.4	1 1/4x7	28.25
1/2x13	10.8	1 3/8x6	30.75
9/16x12	12.2	1 1/2x6	34
5/8x11	13.6	1 3/4x5	39.5
3/4x10	16.5	2x4	45

Inch fine thread UNF
1/4x28	5.5	3/4x16	17.5
5/16x24	6.9	7/8x14	20.5
3/8x24	8.5	1x12	23.25
7/16x20	9.9	1 1/8x12	26.5
1/2x20	11.5	1 1/4x12	29.75
9/16x18	12.9	1 3/8x12	33
5/8x18	14.5	1 1/2x12	36

Trapezoidal thread Tr
Tr 8x1.5	6.6	Tr 14x3	11.25
Tr 10x1.5	8.6	Tr 14x4	10.25
Tr 10x2	8.2	Tr 16x4	12.25
Tr 10x3	7.5	Tr 18x4	14.25
Tr 12x2	10.2	Tr 20x4	16.25
Tr 12x3	9.25	Tr 22x3	19.25
Tr 14x2	12.2	Tr 22x5	17.25

To cut internal threads with a tap, you first need to drill a hole of a certain diameter. The table shows the most popular threads and drill diameters required for drilling holes for threads.

How to find which drill bit for thread without a table?

Suppose you need to cut an M10x1.5 thread.

1. Let's find out the nominal thread diameter: 10 mm.
2. Find out the nominal thread pitch: 1.5 mm
3. Subtract the second value from the first: 10-1.5 = 8.5 mm
4. The result is 8.5 mm!

Having mastered this method, you can easily and simply select the right drill bit for cutting threads.

How to use the filter on the website to select a drill bit for threading?

For metal drills, a filter with the “For thread” parameter has been implemented, thanks to which you can find the desired drill in literally 2 mouse clicks:

1. Choosing a thread.
2. Click “Apply”.
3. We get suitable drills.

For which materials are the values ​​​​indicated in the table?

The indicated hole diameters provide cutting of metric threads in:

• gray cast iron
• carbon steels of ordinary quality
• quality structural steels
• alloyed structural steels
• high alloy steels
• corrosion-resistant, heat-resistant and heat-resistant alloys
• heat-resistant steels
• aluminum casting alloys
• copper 

A complete list of threads and diameters can be found in GOST 19257-73 - an official technical document with all the nuances and details.

Before you get started, we recommend that you read the practical tips in the article How to properly cut a thread with a tap. And there you can find suitable coolant for different materials.

Source: http://ooo-asteko.ru/diametr-otverstiy-pod-metricheskuyu-rezbu-tablitsa/

How to cut threads by hand

Threaded connections are widely used in various mechanisms and machines. Bolts, studs, screws, nuts are universal, interchangeable fasteners. However, there are times when it is necessary to cut threads by hand. A specialized tool will help you do this job efficiently.

• External thread cutting with a die
• Tapping internal threads

In mechanical engineering, there are three main systems of fastening threads: metric, inch and pipe.

Metric thread has become the most widespread. It has a triangular profile with an angle of 60˚. Its main parameters, diameter and pitch, are expressed in millimeters. Designation example: M16. This means that the thread is metric, has a diameter of 16 mm with a coarse pitch of 2.0 mm. If the step is small, then its value is indicated, for example, M16 * 1.5.

The diameters of inch and pipe threads are expressed in inches. The pitch is characterized by the number of threads per inch. The specified parameters are standardized, so it is always possible to select the necessary tool.

External thread cutting with a die

To cut external threads, you will need the following tools and materials: a die or pipe clamp, a die holder, a file, a vice, a caliper, and machine oil.

The most widespread are round dies (lerks). They are either solid or split. The diameters of solid round dies are standardized. This allows you to choose the appropriate option from a large range of sizes, for example, M10, M12, M14, M16.

A special feature of split dies is the ability to adjust the diameter of the cut thread within 0.10.25 mm. However, they have reduced rigidity, which affects the accuracy of the resulting profile.

Operating procedure

The die is installed in a die holder of a suitable size. After this, it is secured with screws. In the case of external pipe threads, die holders with a ratchet are often used. They provide convenience for working in hard-to-reach places, such as near a wall.

The thickness of the rod is chosen to be 0.10.25 mm less than the diameter of the external thread. For example, for M6 with a large pitch it is 5.805.90 mm; M8 – 7.807.90 mm; M10 – 9.759.85 mm. Measurements are taken using a caliper. The diameters of rods for cutting metric threads of average accuracy class 6g are presented in the table.

To ensure better insertion of the die, a chamfer is filed at the end of the rod. Its width should be 1 - 1.5 mm for M6 M18. The workpiece is lubricated with machine oil, which makes subsequent work easier and allows you to obtain a better surface.

The die is placed on the end of the rod so that its plane is perpendicular to the axis of the bolt being cut. Next, with slight pressure, rotate the die holder clockwise (if the thread is left-handed, then counterclockwise). When the die cuts into the rod by one or two threads, it should be turned back half a turn for better removal of chips. After this, they again make 1-2 turns along the thread and 0.5 in the opposite direction. Using this scheme, the bolt is cut to the required length.

The diameter of the external thread is checked with a regular nut or ring gauge. If necessary, the pitch is controlled with a thread gauge.

Tapping internal threads

To form an internal thread, the following tools and materials are required:

• hammer, center punch, drill, drill bits;
• a set of taps, a driver, a bench vice;
• machine oil.

Tapping technology

The first step is to mark the workpiece and core the center of the future hole. Select a drill corresponding to the required thread diameter. This can be done using lookup tables or approximately using the formula d = D – P. Here D is the thread diameter, P is its pitch, d is the drill diameter. For example, for M10 d = 10 – 1.5 = 8.5 mm.

Step, P

2	0,4	1,6
3	0,5	2,5
3,5	0,6	2,9
4	0,7	3,3
5	0,8	4,2
6	1	5,0
0,75	5,25
0,5	5,5
8	1,25	6,8
1	7,0
0,75	7,25
0,5	7,5
10	1,5	8,5
1,25	8,8
1	9,0
0,75	9,25
0,5	9,5
12	1,75	10,2
1,5	10,5
1,25	10,8
1	11
0,75	11,25
0,5	11,5
14	2	12,0
1,5	12,5
1,25	12,8
1	13,0
0,75	13,25
0,5	13,5
16	2	14,0
1,5	14,5
1	15,0
0,75	15,25
0,5	15,5
18	2,5	15,5
2	16,0
1,5	16,5
1	17,0
0,75	17,25
0,5	17,5
20	2,5	17,5
22	2,5	19,5
24	3	21
27	3	24
30	3,5	26,5

A hole is drilled into the part to the required depth, which must exceed the length of the part being cut. Using a drill with a diameter larger than d, a chamfer is made on the edge of the hole. It serves for centering and better entry of the tap.

The cutting tool is selected based on the main parameters of the thread - diameter and pitch. As a rule, a set of two taps is used. One of them is rough, the other is finishing. The driver is selected according to the size of the square of the tail part of the taps.

The part is securely secured in a vice. The rough tap and hole are lubricated with machine oil. After this, install the tap strictly perpendicular to the surface of the part and, pressing along its axis, rotate the knob by the handles.

After cutting one or two threads, make a quarter turn in the opposite direction. This promotes chip crushing and removal and prevents tool jamming. The work continues, carrying out alternating rotation: ½ turn forward, ¼ turn back. In this case, it is necessary to ensure that there is no skew of the tap. You should also not apply excessive force to it. To prevent jamming, the cutting tool is periodically removed and the hole is cleared of chips.

After cutting the internal thread to the required depth, a finishing tap is installed in the hole. When it goes in the given direction, a crank is put on it and work continues. Add lubricant periodically.

The thread is checked using a plug gauge or bolt. It should screw in effortlessly and not wobble. If necessary, make an additional pass with a finishing tap.

Source: https://tehnouzel.ru/tehnologii-remonta/kak-narezat-rezbu.html

What hole diameter should I drill for metric threads? — Homo habilis. Magazine for skilled people

Yuri Makarov Category: Workshop: March 30, 2014 56205

:  5 / 5

Sam Schmidt, flickr.com CC BY

Threaded connections are one of the most common types of detachable connections. But if fasteners with external threads (screws, studs) are most often purchased ready-made, then the internal threads have to be cut by the craftsman himself when manufacturing the part. To do this, a hole with a certain diameter is drilled into the part.

The diameter of the hole for the internal thread depends on the nominal diameter of the thread and the pitch of the thread. This is usually indicated on the drawing as M8x1. The letter “M” denotes a metric thread, the number after the letter is the nominal diameter, the number after the sign “x” is the thread pitch. If a step is not specified, then the main (major) step is assumed. The main thread pitch is preferred and is specified by the standard for each size.

For each combination of diameter and thread pitch, there is an optimal hole diameter. The easiest way to determine this diameter is from the table, which shows the most common sizes that a home craftsman may encounter. in bold in the table. The table is applicable for steels, cast irons, aluminum and its alloys, and copper.

Thread	Thread pitch	Nominal hole diameter (maximum)
M2	0,25	1,75–1,77 (1,81)
0,4	1,60–1,62 (1,68)
M2.5	0,35	2,15–2,17 (2,22)
0,45	2,05–2,07 (2,13)
M3	0,35	2,65–2,67 (2,72)
0,5	2,50–2,52 (2,58)
M4	0,5	3,50–3,52 (3,64)
0,7	3,30–3,33 (3,46)
M5	0,5	4,50–4,52 (4,64)
0,8	4,2–4,23 (4,42)
M6	0,5	5,50–5,52 (5,64)
0,75	5,20–5,23 (5,42)
1,0	4,95–5,00 (5,21)
M7	0,5	6,50–6,52 (6,64)
0,75	6,20–6,23 (6,42)
1,0	5,95–6,00 (6,26)
M8	0,5	7,50–7,52 (7,64)
0,75	7,20–7,23 (7,42)
1	6,95–7,00 (7,21)
1,25	6,70–6,75 (6,96)
M9	0,5	8,50–8,52 (8,64)
0,75	8,20–8,23 (8,42)
1	7,95–8,00 (8,21)
1,25	7,70–7,75 (7,96)
M10	0,5	9,50–9,52 (9,64)
0,75	9,20–9,23 (9,42)
1	8,95–9,00 (9,21)
1,25	8,70–8,75 (8,96)
1,5	8,43–8,50 (8,73)
M11	0,5	10,50-10–52 (10,64)
0,75	10,20–10,23 (10,42)
1	9,95–10,00 (10,21)
1,5	9,43–9,50 (9,73)
M12	0,5	11,50–11,52 (11,64)
0,75	11,20–11,23 (11,42)
1	10,95–11,00 (11,21)
1,25	10,70–10,75 (10,96)
1,5	10,43–10,50 (10,73)
1,75	10,20–10,25 (10,56)
M14	0,5	13,50–13,52 (13,64)
0,75	13,20–13,23 (13,42)
1	12,95–13,00 (13,21)
1,25	12,70–12,75 (12,96)
1,5	12,43–12,50 (12,73)
2	11,90–11,95 (12,30)
M16	0,5	15,50–15,52 (15,64)
0,75	15,20–15,23 (15,42)
1	14,95–15,00 (15,26)
1,5	14,43–14,50 (14,73)
2	13,90–13,95 (14,30)
M18	0,5	17,50–17,52 (17,64)
0,75	17,20–17,23 (17,42)
1	16,95–17,00 (17,21)
1,5	16,43–16,50 (16,73)
2	15,90–15,95 (16,30)
2,5	15,35–15,40 (15,88)
M20	0,5	19,50–19,52 (19,64)
0,75	19,20–19,23 (19,42)
1	18,95–19,00 (19,21)
1,5	18,43–18,50 (18,73)
2	17,90–17,95 (18,3)
2,5	17,35–17,40 (17,88)

Reducing the hole diameter makes threading more difficult and can cause the tap to break , especially in hard materials. Increasing the hole diameter will not allow cutting the full profile; the thread will be weakened.

https://www.youtube.com/watch?v=u-adRBIR6Ys

The drill for drilling holes is selected from the available set as close as possible to the recommended diameter, or, in extreme cases, to the limit. It should be borne in mind that when drilling holes with a hand drill, the hole will inevitably break, so it is better to take a drill closer to the minimum recommended diameter. It is convenient to use sets that contain drills specifically for holes for metric threads.

In most cases, a home craftsman rarely produces critical, highly loaded parts, so sometimes it is possible to allow a slight increase in the diameter of the hole relative to the maximum size.

If the part performs purely decorative functions, does not experience heavy loads and under no circumstances can create a danger to humans, the margin for threads can be up to 1/3 - 1/5 of a pitch or even less. For example, for a M10x1.5 thread, you can increase the hole to 9.5-9.7 mm. In this case, the screw or pin must fit into the part by at least 8-10 full threads.

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Source: https://homo-habilis.ru/masterskaya/231-kakoj-diametr-otverstiya-sverlit-pod-metricheskuyu-rezbu