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External thread cutting. Diameters of threaded rods 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, you must keep 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 become torn.
When choosing the diameter of a rod for external threads, you should be guided by the same considerations as when choosing holes for internal thread. The practice of cutting external threads shows that 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, the thread will be incomplete; if it is 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.
In table Figure 27 shows the diameters of the rods used when cutting threads with dies.
Table 27 Diameters of threaded rods 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 cutting a thread with a die, the rod is secured in a vice so that the end of the vice protruding above the level of the jaws is 20-25 mm longer than the length of the part being cut. To ensure penetration, a chamfer is filed at the upper end of the rod. Then a die attached to the die is placed on the rod and with slight pressure the die is rotated so that the die cuts in approximately 0.2-0.5 mm. After this, 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, i.e. one or two turns to the right and half a turn to the left (Fig. 152, b).
Rice. 152. Technique for cutting threads with a die (b)
To prevent defects and breakage of teeth, it is necessary that the die fits onto the rod without distortion.
Checking the cut internal threads is done with thread plug gauges, and external threads are checked with thread micrometers or thread ring gauges.
Screws, bolts and studs are the most common externally threaded items. Most often, they fall into the hands of a home craftsman ready-made. But it happens that you need to make some tricky bolt or non-standard pin. The blank for such a part is a rod, the diameter of which must correspond to the thread being cut.
The diameter of the rod for an external thread depends on the nominal diameter of the thread and the size of the thread pitch. All this information is usually indicated on 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 sign “x” is the thread pitch. When using the main (large) step, it may not be indicated. Basic thread pitch defined by the standard and is the most preferred.
When choosing the diameter of a rod for external threads, they are guided by the same principles as when choosing holes for internal threads. It has been established that the best thread quality is obtained if the diameter of the rod is slightly smaller than the nominal diameter of the thread being cut. When cutting, the metal is slightly squeezed out and the thread profile is complete.
If the diameter of the rod is much smaller than the required one, then the tops of the threads will be cut off; if it is larger, then the die simply will not screw onto the rod or will break during operation.
For each combination of diameter and thread pitch there is optimal diameter rod. The easiest way to determine this diameter is from the table, which shows the most common threads that can be encountered House master. The main thread pitch for each nominal diameter is highlighted in bold in the table.
| Thread | Thread pitch | Rod diameter nominal (ultimate) |
| M2 | 0,4 | 1,93-1,95 (1,88) |
| 0,25 | 1,95-1,97 (1,91) | |
| M2.5 | 0,45 | 2,43-2,45 (2,37) |
| 0,35 | 2,45-2,47 (2,39) | |
| M3 | 0,5 | 2,89-2,94 (2,83) |
| 0,35 | 2,93-2,95 (2,89) | |
| M4 | 0,7 | 3,89-3,94 (3,81) |
| 0,5 | 3,89-3,94 (3,83) | |
| M5 | 0,8 | 4,88-4,94 (4,78) |
| 0,5 | 4,89-4,94 (4,83) | |
| M6 | 1 | 5,86-5,92 (5,76) |
| 0,75 | 5,88-5,94 (5,79) | |
| 0,5 | 5,89-5,94 (5,83) | |
| M8 | 1,25 | 7,84-7,90 (7,73) |
| 1 | 7,86-7,92 (7,76) | |
| 0,75 | 7,88-7,94 (7,79) | |
| 0,5 | 7,89-7,94 (7,83) | |
| M10 | 1,5 | 9,81-9,88 (9,69) |
| 1 | 9,86-9,92 (9,76) | |
| 0,5 | 9,89-9,94 (9,83) | |
| 0,75 | 9,88-9,94 (9,79) | |
| M12 | 1,75 | 11,80-11,86 (11,67) |
| 1,5 | 11,81-11,88 (11,69) | |
| 1,25 | 11,84-11,90 (11,73) | |
| 1 | 11,86-11,92 (11,76) | |
| 0,75 | 11,88-11,94 (11,79) | |
| 0,5 | 11,89-11,94 (11,83) | |
| M14 | 2 | 13,77-13,84 (13,64) |
| 1,5 | 13,81-13,88 (13,69) | |
| 1 | 13,86-13,92 (13,76) | |
| 0,75 | 13,88-13,94 (13,79) | |
| 0,5 | 13,89-13,94 (13,83) | |
| M16 | 2 | 15,77-15,84 (15,64) |
| 1,5 | 15,81-15,88 (15,69) | |
| 1 | 15,86-15,92 (15,76) | |
| 0,75 | 15,88-15,94 (15,79) | |
| 0,5 | 15,89-15,94 (15,83) | |
| M18 | 2 | 17,77-17,84 (17,64) |
| 1,5 | 17,81-17,88 (17,69) | |
| 1 | 17,86-17,92 (17,76) | |
| 0,75 | 17,92-17,94 (17,86) | |
| M20 | 2,5 | 19,76-19,84 (19,58) |
| 1,5 | 19,81-19,88 (19,69) | |
| 1 | 19,86-19,92 (19,76) | |
| 0,75 | 19,88-19,94 (19,79) | |
| 0,5 | 19,89-19,94 (19,83) |
The main tool for cutting external threads is a die. Most often, round continuous dies in the form of a hardened steel nut are used.
To form cutting edges The die threads are crossed by through longitudinal holes, which also provide chip exit. To facilitate entry, the outer threads of the thread have an incomplete profile. To rotate the dies use die holder- a tool with a socket for a die and long handles. There are also split and sliding (clump) dies, but these are rare in the home workshop.
To reduce friction and obtain clean threads, a lubricant is used on steel rods - mineral oil or kerosene, and on copper rods - turpentine. At the end of the rod, to facilitate entry, a chamfer must be made with a width of at least the size of the thread pitch.
Metric threads. Diameters of rods and tolerances on them for metric threads M3-M50, made with dies. Drill diameters M1-M10 for drilling holes for metric threads. Threading p
Metric threads. Diameters of rods and tolerances on them for metric threads M3-M50, made with dies. Drill diameters M1-M10 for drilling holes for metric threads. Cutting threads with dies and taps.
- External thread: The die is clamped in the collar with screws located along its contour.
- At the end of the rod on which the thread needs to be cut, sharpening machine chamfer at an angle<60 о до диаметра, равного 80% диаметра резьбы. Затем плашку смазывают густым маслом (напр. солидол), животным жиром (салом) или растительным маслом — жидкое моторное масло лучше не использовать, так как оно зачастую портит резьбу.
- At the end of a rod firmly clamped in a vice with a chamfer in the form of a truncated cone, install a crank with a die exactly in a horizontal plane and rotate the crank clockwise with both hands (looking from above), if the thread is right-handed, with slight pressure on the die. Sometimes it is recommended to smoothly rotate the knob clockwise, sometimes after each half-turn, turn it back a little to break the chips. The main thing is to lubricate all the working blades well so that the threads do not break and the die does not become dull.
- The diameter of the rods for external metric threads should be selected according to Table 1.
Table 1. Diameters of rods for metric threads made with dies
| Diameters | Tolerances for rod diameter |
Diameters | Tolerances for rod diameter |
||
| threads | rod | threads | rod | ||
| Coarse pitch thread | |||||
| 3 | 2,94 | -0,06 | 12 | 11,88 | -0,12 |
| 3,5 | 3,42 | -0,08 | 16 | 15,88 | -0,12 |
| 4 | 3,92 | -0,08 | 18 | 17,88 | -0,12 |
| 4,5 | 4,42 | -0,08 | 20 | 19,86 | -0,14 |
| 5 | 4,92 | -0,08 | 22 | 21,86 | -0,14 |
| 6 | 5,92 | -0,08 | 24 | 23,86 | -0,14 |
| 7 | 6,90 | -0,10 | 27 | 26,86 | -0,14 |
| 8 | 7,90 | -0,10 | 30 | 29,86 | -0,14 |
| 9 | 8,90 | -0,10 | 33 | 32,83 | -0,17 |
| 10 | 9,90 | -0,10 | 36 | 35,83 | -0,17 |
| 11 | 10,88 | -0,12 | 39 | 38,83 | -0,17 |
| Fine pitch thread | |||||
| 4 | 3,96 | -0,08 | 24 | 23,93 | -0,14 |
| 4,5 | 4,46 | -0,08 | 25 | 24,93 | -0,14 |
| 5 | 4,96 | -0,08 | 26 | 25,93 | -0,14 |
| 6 | 5,96 | -0,08 | 27 | 26,93 | -0,14 |
| 7 | 6,95 | -0,10 | 28 | 27,93 | -0,14 |
| 8 | 7,95 | -0,10 | 30 | 29,93 | -0,14 |
| 9 | 8,95 | -0,10 | 32 | 31,92 | -0,17 |
| 10 | 9,95 | -0,10 | 33 | 32,92 | -0,17 |
| 11 | 10,94 | -0,12 | 35 | 34,92 | -0,17 |
| 12 | 11,94 | -0,12 | 36 | 35,92 | -0,17 |
| 14 | 13,94 | -0,12 | 38 | 37,92 | -0,17 |
| 15 | 14,94 | -0,12 | 39 | 38,92 | -0,17 |
| 16 | 15,94 | -0,12 | 40 | 39,92 | -0,17 |
| 17 | 16,94 | -0,12 | 42 | 41,92 | -0,17 |
| 18 | 17,94 | -0,12 | 45 | 44,92 | -0,17 |
| 20 | 19,93 | -0,14 | 48 | 47,92 | -0,17 |
| 22 | 21,93 | -0,14 | 50 | 49,92 | -0,17 |
- Internal thread: cut using taps. A tap is a metal-cutting tool for cutting internal threads in pre-drilled holes. There are manual (rotated using a crank) and machine, nut and tool (master and die). When cutting deep threads, a set of three taps is usually used: the first tap (designation - one notch) is preliminary, the second (two notches) cuts the thread and the third (three marks or without bottom) calibrates it. Nut taps are suitable for cutting short threads (as in a nut) and have sequential cutting edges; after passing the entire length, a full thread is obtained.
- The correct choice of hole diameters is of great importance. If the diameter is larger than it should be, the internal threads will not have a full profile and the result will be a weak connection. With a smaller hole diameter, it is difficult for the tap to enter it, which leads to the breaking of the first turns of the thread or to jamming and breakage of the tap. The diameter of the hole for a metric thread can be approximately determined by multiplying the thread size by 0.8 (for example, for an M2 thread, the drill should have a diameter of 1.6 mm, for M3 - 2.4-2.5 mm, etc. (see. table).
- It is necessary to lubricate the cutting part of the tap with thick oil (for example, grease), animal fat (lard) or vegetable oil - it is better not to use liquid motor oil, as it often spoils the thread - and insert it into the hole.
- Then you need to carefully ensure that the tap runs exactly along the axis of the hole to avoid breakage. After cutting 4-5 turns, the tap is removed from the hole and cleared of chips. After this, it is lubricated again and screwed into the hole again, another 4-5 turns are cut, continuing the operation until it stops (for a blind hole or until the tap comes out (for a through hole).
- Then they clean the first tap, put it in place and take a tap with two marks, lubricate it, manually screw it into the hole and, as soon as it starts to cut into the metal, put a driver on it. After cutting every 5-6 turns, the tap is cleaned of chips and lubricated until the hole passes completely.
- Then clean the second tap, put it in place, take the last tap with three notches, also lubricate it with grease, screw it into the hole by hand until it engages, put on the driver and carefully calibrate the thread. Cleaning of chips and lubrication is repeated as before.
- Inch taps threads are cut in the same way as metric ones. To cut threads on pipes, clamps are used, usually with adjustable cutting elements in a range of threads for pipes with an internal diameter of 1/4 to 4 inches. Threads on large diameter pipes and stubbles are best cut on screw-cutting lathes.
- The diameter of the drill bits for drilling holes for metric threads should be selected according to Table 2.
Table 2. Drill diameters for drilling holes for metric threads
| Outside diameter thread, mm |
Drill diameter (mm) for | |
| Cast iron, bronze | Steel, brass | |
| 1 | 0,75 | 0,75 |
| 1,2 | 0,95 | 0,95 |
| 1,6 | 1,3 | 1,3 |
| 2 | 1,6 | 1,6 |
| 2,5 | 2,2 | 2,2 |
| 3 | 2,5 | 2,5 |
| 3,5 | 2,9 | 2,9 |
| 4 | 3,3 | 3,3 |
| 5 | 4,1 | 4,2 |
| 6 | 4,9 | 5 |
| 7 | 5,9 | 6 |
| 8 | 6,6 | 6,7 |
| 9 | 7,7 | 7,7 |
| 10 | 8,3 | 8,4 |
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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.
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.
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:
d answer = (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:
d answer = 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.
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.
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.
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.
Taps are:
- manual, for metric (M1-M68), inch - ¼-2 ʺ, pipe - 1/8-2 ʺ;
- 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.
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.
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.
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.
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.
Despite the fact that cutting internal threads is not a complex technological operation, there are some features of preparation for this procedure. Thus, it is necessary to accurately determine the dimensions of the preparation hole for threading, and also select the right tool, for which special tables of drill diameters for threads are used. For each type of thread, it is necessary to use the appropriate tool and calculate the diameter of the preparation hole.
Types and parameters of thread
The parameters by which threads are divided into 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).
Depending on the above parameters, the following types of thread are distinguished:
- cylindrical, which is designated by the letters MJ;
- metric and conical, designated M and MK respectively;
- pipe, designated by the letters G and R;
- with a round profile, named after Edison and marked with the letter E;
- trapezoidal, designated Tr;
- round, used for installation of sanitary fittings, – Kr;
- thrust and thrust reinforced, marked as S and S45, respectively;
- inch thread, which can also be cylindrical and conical - BSW, UTS, NPT;
- used to connect pipes installed in oil wells.
Application of the tap
Before you start threading, you need to determine the diameter of the preparation hole and drill it. To facilitate this task, a corresponding GOST was developed, which contains tables that allow you to accurately determine the diameter of the threaded hole. This information makes it easy to select the drill size.
To cut metric threads on the inner walls of a hole made with a drill, a tap is used - a screw-shaped tool with cutting grooves, made in the form of a rod, which can have a cylindrical or conical shape. On its side surface there are special grooves located along its axis and dividing the working part into separate segments, which are called combs. The sharp edges of the combs are precisely the working surfaces of the tap.
In order for the turns of the internal thread to be clean and neat, and its geometric parameters to correspond to the required values, it must be cut gradually, by gradually removing thin layers of metal from the surface being treated. That is why for this purpose they use either taps, the working part of which is divided along the length into sections with different geometric parameters, or sets of such tools. Single taps, the working part of which has the same geometric parameters along its entire length, are needed in cases where it is necessary to restore the parameters of an existing thread.
The minimum set with which you can sufficiently perform machining of threaded holes is a set consisting of two taps - rough and finishing. The first one cuts a thin layer of metal from the walls of the hole for cutting metric threads and forms a shallow groove on them, the second one not only deepens the formed groove, but also cleans it.
Combination two-pass taps or sets consisting of two tools are used for tapping small diameter holes (up to 3 mm). To machine holes for larger metric threads, you must use a combination three-pass tool or a set of three taps.
To manipulate the tap, a special device is used - a wrench. The main parameter of such devices, which can have different designs, is the size of the mounting hole, which must exactly match the size of the tool shank.
When using a set of three taps, differing both in their design and geometric parameters, the sequence of their use must be strictly observed. They can be distinguished from each other both by special marks applied to the shanks and by design features.
- The tap, which is used to process a hole for cutting metric threads first, has the smallest diameter among all the tools in the set and cutting teeth, the upper part of which is heavily cut off.
- 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.
- The third tap, with which the hole for cutting metric threads is processed last, is characterized by full ridges of cutting teeth and a diameter that must exactly match the size of the thread being formed.
Taps are used primarily for cutting metric threads. Much less often than metric ones, taps designed for processing the internal walls of pipes are used. In accordance with their purpose, they are called pipe, and they can be distinguished by the letter G present in their markings.
Internal thread cutting technology
As mentioned above, before starting work, you need to drill a hole, the diameter of which must exactly fit a thread of a certain size. It should be borne in mind: if the diameters of the holes intended for cutting metric threads are chosen incorrectly, this can lead not only to poor quality execution, but also to breakage of the tap.
Considering the fact that the tap, when forming threaded grooves, not only cuts the metal, but also pushes it, the diameter of the drill for making threads should be slightly smaller than its nominal diameter. For example, a drill for making M3 threads should have a diameter of 2.5 mm, for M4 - 3.3 mm, for M5 you should choose a drill with a diameter of 4.2 mm, for M6 threads - 5 mm, M8 - 6.7 mm, M10 - 8.5 mm, and for M12 - 10.2.
Table 1. Main diameters of holes for metric threads
All diameters of drills for GOST threads are given in special tables. Such tables indicate the diameters of drills for making threads with both standard and reduced pitches, but it should be borne in mind that holes of different diameters are drilled for these purposes. In addition, if threads are cut in products made of brittle metals (such as cast iron), the diameter of the thread drill obtained from the table must be reduced by one tenth of a millimeter.
You can familiarize yourself with the provisions of GOST regulating the cutting of metric threads by downloading the document in pdf format from the link below.
The diameters of drills for metric threads can be calculated independently. From the diameter of the thread that needs to be cut, it is necessary to subtract the value of its pitch. The thread pitch itself, the size of which is used when performing such calculations, can be found out from special correspondence tables. In order to determine what diameter the hole needs to be made using a drill if a three-start tap is used for threading, you must use the following formula:
D o = D m x 0.8, Where:
Before- this is the diameter of the hole that must be made using a drill,
D m– the diameter of the tap that will be used to process the drilled element.
