Even though cutting internal thread does not apply to complex technological operations, there are some features of preparation for this procedure. So, you should accurately determine the dimensions of the preparatory hole for threading, as well as choose the right tool, for which special tables of drill diameters for threading are used. For each type of thread, you must use the appropriate tool and calculate the diameter of the preparation hole.
Varieties and parameters of thread
The parameters by which the thread is divided into different types, are:
- diameter units (metric, inch, etc.);
- number of thread entries (one-, two- or three-way);
- the shape in which the profile elements are made (triangular, rectangular, round, trapezoidal);
- the direction of the rise of the turns (right or left);
- location on the product (external or internal);
- surface shape (cylindrical or conical);
- purpose (fixing, fixing and sealing, running).
Depending on the above parameters, the following types of thread are distinguished:
- cylindrical, which is indicated by the letters MJ;
- metric and conical, denoted respectively by M and MK;
- pipe, for which the letters G and R are used;
- with a round profile, named after Edison and marked with the letter E;
- trapezoidal, designated Tr;
- round, used for mounting sanitary fittings, – Kr;
- persistent and persistent reinforced, marked as S and S45, respectively;
- inch thread, which can also be cylindrical and conical - BSW, UTS, NPT;
- used to connect pipes installed in oil wells.
Application of the tap
Before you start threading, you need to determine the diameter of the preparatory hole and drill it. To facilitate this task, the corresponding GOST was developed, which contains tables that allow you to accurately determine the diameter of the threaded hole. This information makes it easy to choose the size of the drill.
To cut metric threads on the inner walls of a hole made with a drill, a tap is used - a helical tool with cutting grooves, made in the form of a rod, which can have a cylindrical or conical shape. On its lateral surface there are special grooves located along its axis and dividing the working part into separate segments, which are called combs. The sharp edges of the combs are precisely the working surfaces of the tap.
In order for the turns of the internal thread to turn out to be clean and neat, and its geometric parameters to correspond to the required values, it must be cut gradually, by gradually removing thin layers of metal from the surface being machined. That is why, for this purpose, either taps are used, the working part of which is divided along the length into sections with different geometric parameters, or sets of such tools. Single taps, the working part of which has the same geometric parameters along its entire length, are needed in cases where it is necessary to restore the parameters of an existing thread.
The minimum set, with which you can perform the processing of threaded holes with sufficient quality, is a set consisting of two taps - roughing and finishing. The first cuts off a thin layer of metal from the walls of the hole for cutting metric threads and forms a shallow groove on them, the second not only deepens the formed groove, but also cleans it.
Combination two-pass taps or sets consisting of two tools are used for threading in holes with small diameters (up to 3 mm). For drilling holes metric thread larger diameters, use a combination 3-pass tool or a set of 3 taps.
To manipulate the tap, a special device is used - a crank. The main parameter of such devices, which may have different design, is the size of the mounting hole, which must exactly match the size of the tool shank.
When using a set of three taps that differ both in their design and geometric parameters, the sequence of their application should be strictly observed. You can distinguish them from each other both by special risks applied to the shanks, and by design features.
- The tap, with which the hole for cutting metric threads is processed first of all, is distinguished by the smallest diameter among all tools in the set and cutting teeth, top part which are heavily cut off.
- The second tap has a shorter chamfer and longer combs. Its working diameter occupies an intermediate value between the diameters of other tools from the set.
- The third tap, with which the hole for cutting metric threads is finished last, is characterized by full ridges of cutting teeth and a diameter that must exactly match the size of the thread being formed.
Taps are used primarily for threading metric threads. Much less frequently than metric taps are used for processing the inner walls of pipes. They are called pipe pipes in accordance with their purpose, and they can be distinguished by the letter G present in their marking.
Thread cutting technology
As mentioned above, before starting work, it is necessary to drill a hole, the diameter of which must exactly fit the thread certain size. It should be borne in mind: if the diameters of the holes intended for cutting metric threads are chosen incorrectly, this can lead not only to its poor quality, but also to breakage of the tap.
Given the fact that the tap, forming threaded grooves, not only cuts off the metal, but also pushes it through, the diameter of the drill for threading should be somewhat smaller than its nominal diameter. For example, a drill for threading M3 should have a diameter of 2.5 mm, for M4 - 3.3 mm, for M5 you should choose a drill with a diameter of 4.2 mm, for M6 thread - 5 mm, M8 - 6.7 mm, M10 - 8.5 mm, and for M12 - 10.2.
Table 1. Main hole diameters for metric threads
All diameters of drills for GOST thread are given in special tables. Such tables indicate the diameters of drills for making threads with both standard and reduced pitch, while it should be borne in mind that holes are drilled for these purposes. different diameters. In addition, if threads are cut in brittle metals (such as cast iron), the diameter of the drill bit obtained from the table must be reduced by one tenth of a millimeter.
You can familiarize yourself with the provisions of GOST governing the cutting of metric threads by downloading the document in pdf format from the link below.
Drill diameters for metric threads can be calculated independently. From the diameter of the thread to be cut, subtract the value of its pitch. The thread pitch itself, the size of which is used when performing such calculations, can be found in special correspondence tables. In order to determine what diameter the hole must be made with a drill if a three-start tap is used for threading, the following formula must be used:
D o \u003d D m x 0.8, where:
Before- this is the diameter of the hole to be made with a drill,
D m- the diameter of the tap with which the drilled element will be processed.
Metric threads. Rod diameters and tolerances for metric thread M3-M50, performed by dies. Drill diameters M1-M10 for drilling holes for metric threads. Threading
Metric threads. Rod diameters and tolerances for metric thread M3-M50, performed by 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 is to be cut, on grinding 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, a wrench with a die is installed exactly in the horizontal plane and the wrench is rotated clockwise with both hands (looking from above), if the thread is right-handed, with a 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 thread does not break and the die does not become dull.
- The diameter of the rods for external metric thread 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 |
||
| carving | rod | carving | rod | ||
| Coarse 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 |
| Thread with fine pitch | |||||
| 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 with cutters. A tap is a metal-cutting tool for cutting internal threads in pre-drilled holes. There are manual (rotated with a knob) and machine, nut and tool (uterine and ram). When cutting deep threads, a set of three taps is usually used: the first tap (designation - one risk) is preliminary, the second (two risks) cuts the thread and the third (three risks or no bottom) calibrates it. Nut taps are suitable for cutting short threads (as in a nut) and have successive 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, then the internal thread will not have a full profile and a weak connection will result. With a smaller hole diameter, the entry of the tap into it is difficult, which leads to the breakdown of the first turns of the thread or to jamming and breakage of the tap. The diameter of a 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 (eg grease), animal fat (lard) or vegetable oil - it is better not to use liquid engine oil, as it often spoils the thread - and insert it into the hole.
- Then you need to carefully monitor that the tap goes exactly along the axis of the hole in order to avoid breakage. After cutting 4-5 turns, the tap is removed from the hole and cleaned of chips. After that, it is lubricated again and screwed into the hole again, another 4-5 turns are cut, continuing the operation until it stops (with a blind hole or until the tap exits (with a through hole).
- Then they clean the first tap, put it in place and take a tap with two risks, lubricate it, manually screw it into the hole and, as soon as it starts to cut into the metal, put a collar on it. After cutting every 5-6 turns, the tap is cleaned of chips and lubricated until the hole is completely passed.
- Then they clean the second tap, put it in place, take the last tap with three risks, also grease it, screw it into the hole by hand until it engages, put on the knob and carefully calibrate the thread. Chip cleaning and lubrication are repeated as before.
- Inch taps thread is cut in the same way as metric. For threading pipes, die cutters are used, usually with adjustable cutting elements in the thread range for pipes with an internal diameter of 1/4 to 4 inches. Threads on pipes and stubbles of large diameter 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 threads, 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 external threads. Most often, they fall into the hands of a home master ready-made. But it happens that you need to make some tricky bolt or non-standard stud. 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 external threading depends on the nominal diameter of the thread and the size of the thread pitch. All this information is usually indicated on the detail drawing as 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. Main thread pitch defined by the standard and is the most preferred.
When choosing the diameter of the rod for external threads, the same principles are followed as when choosing holes for internal threads. It has been established that the best quality of the thread is obtained if the diameter of the rod is slightly less than the nominal diameter of the thread being cut. When cutting, the metal is squeezed out a little and the thread profile is complete.
If the diameter of the rod is much less than required, 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 optimum rod diameter. The easiest way to determine this diameter is from the table, which shows the most common threads that a home master can meet. Bold type in the table indicates the main thread pitch for each nominal diameter.
| Thread | thread pitch | Rod diameter nominal (limiting) |
| 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. The most commonly used round solid dies in the form of a hardened steel nut.
To form cutting edges, the thread of the die is crossed by through longitudinal holes, which also provide chip exit. To facilitate entry, the extreme threads of the thread have an incomplete profile. To rotate the plate, use die holder- a tool with a socket for a die and long handles. There are also split and sliding (klupp) dies, but this is rare in the home workshop.
To reduce friction and obtain a clean thread on steel rods, a lubricant is used - mineral oil or kerosene, on copper - turpentine. At the end of the rod, to facilitate entry, a chamfer must be made with a width not less than the thread pitch.
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External thread cutting. Diameters of rods for threading when cutting with dies.
Before cutting a thread, it is necessary to select the diameter of the workpiece for this thread.
When cutting a thread with a die, it must be borne in mind that when 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 turn out to be torn.
The same considerations should be followed when choosing a shank diameter for external threads as when choosing holes for internal threads. The practice of cutting external threads shows that the best thread quality can be obtained if the diameter of the rod is somewhat smaller than the outer diameter of the thread being cut. If the diameter of the rod is less than required, then the thread will be incomplete; if more, then the die will either not be able to be screwed onto the rod and the end of the rod will be damaged, or during operation, the teeth of the die may break due to overload, and the thread will be torn off.
In table. 27 shows the diameters of the rods used when cutting threads with dies.
Table 27 Diameters of rods for threading when cutting with dies
The diameter of the workpiece should be 0.3-0.4 mm less than the outer diameter of the thread.
When threading with a die, the rod is fixed in a vice so that the end of the vise protruding above the level of the jaws is 20-25 mm longer than the length of the part to be cut. To ensure plunging, a chamfer is sawn at the upper end of the rod. Then, a die fixed in the die is placed on the rod and the die is rotated with a little pressure so that the die cuts by about 0.2-0.5 mm. After that, the cut part of the rod is lubricated with oil and the screw is rotated in exactly the same way as when working with a tap, that is, one or two turns to the right and half a turn to the left (Fig. 152, b).
Rice. 152. Reception of threading with a die (b)
To prevent marriage and breakage of the teeth, it is necessary that the die enters the rod without distortion.
Checking the cut internal threads is carried out with threaded plug gauges, and the outer thread - with threaded micrometers or threaded ring gauges.
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