Despite the fact that cutting internal threads is not a complex technological operation, there are some features of preparation for this procedure. Thus, it is necessary to accurately determine the dimensions of the preparation hole for threading, and also select the right tool, for which special tables of drill diameters for threads are used. For each type of thread, it is necessary to use the appropriate tool and calculate the diameter of the preparation hole.
Types and parameters of thread
The parameters by which threads are divided into Various types, are:
- units of diameter (metric, inch, etc.);
- number of thread starts (one-, two- or three-thread);
- the shape in which the profile elements are made (triangular, rectangular, round, trapezoidal);
- direction of rise of turns (right or left);
- location on the product (external or internal);
- surface shape (cylindrical or conical);
- purpose (fastening, fastening and sealing, chassis).
Depending on the above parameters, the following types of thread are distinguished:
- cylindrical, which is designated by the letters MJ;
- metric and conical, designated M and MK respectively;
- pipe, designated by the letters G and R;
- with a round profile, named after Edison and marked with the letter E;
- trapezoidal, designated Tr;
- round, used for installation of sanitary fittings, – Kr;
- thrust and thrust reinforced, marked as S and S45, respectively;
- inch thread, which can also be cylindrical and conical - BSW, UTS, NPT;
- used to connect pipes installed in oil wells.
Application of the tap
Before you start threading, you need to determine the diameter of the preparation hole and drill it. To facilitate this task, a corresponding GOST was developed, which contains tables that allow you to accurately determine the diameter of the threaded hole. This information makes it easy to select the drill size.
To cut metric threads on the inner walls of a hole made with a drill, a tap is used - a screw-shaped tool with cutting grooves, made in the form of a rod, which can have a cylindrical or conical shape. On its side surface there are special grooves located along its axis and dividing the working part into separate segments, which are called combs. The sharp edges of the combs are precisely the working surfaces of the tap.
In order for the turns of the internal thread to be clean and neat, and for its geometric parameters to correspond to the required values, it must be cut gradually, by gradually removing thin layers of metal from the surface being treated. That is why for this purpose they use either taps, working part which along their length is divided 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). For machining holes for metric threads larger diameter you must use a combination three-pass tool or a set of three taps.
Used for manipulating the tap special device- collar. 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, top part which are heavily cropped.
- 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, it should be borne in mind that holes are drilled for these purposes different diameters. 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 follow the link below.
Drill diameters for metric threads you can calculate it yourself. 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.
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 they clean the second tap, put it in place, take the last tap with three marks, 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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Screws, bolts and studs are the most common externally threaded components. 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 rod diameter. The easiest way to determine this diameter is from the table, which shows the most common threads that a home craftsman may encounter. 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 the 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.
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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 a 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 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 threads. The practice of cutting external threads shows that the best thread quality 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.