Chapter XIX

THREAD CUTTING

§ 77. ELEMENTS OF THREAD. thread profiles. Triangular, rectangular, trapezoidal, thrust, round thread.

Threading is one of the most common operations in mechanical engineering. Threaded parts make up a significant part of all parts included in machines, mechanisms and various devices.

The thread is cut into machine tools(screw-cutting lathe, etc.), however, a significant part is also cut manually using a special thread-cutting tool - taps and dies.

Depending on where the threads are cut - on the surface of the rod or inside the hole, the thread is external (on the rod) and internal (in hole). A part with an external thread is called a screw, a part with an internal thread is called a nut. The screw and nut are a screw pair.

In any thread, the following main elements are distinguished: profile, profile angle, pitch, depth, outer, middle and inner diameters.

The thread profile is the outline of depressions and protrusions obtained by a longitudinal section of the thread (Fig. 144).

Rice. 144. Thread profiles:

a - triangular, b - rectangular, c - trapezoidal, d - persistent, d - round

A thread (coil) is a part of a thread formed during one complete revolution of the profile.

Thread pitch s is the distance (mm) between the corresponding tops of two adjacent turns, measured parallel to the axis.

The thread profile angle α is the angle enclosed between the sides of the thread profile, measured in a plane passing through the axis of the bolt.

The top of the thread is the section of the thread profile that is at the greatest distance from the axis of the bolt.

The base of the thread (depression) is the section of the thread profile located at the smallest distance from the axis.

The thread depth t is the distance from the top of the thread to its base, i.e. the height of the ledge.

The outer diameter d 0 of the thread is the largest diameter measured along the top of the thread, perpendicular to its axis.

The average diameter d cp is the distance between two lines drawn in the middle of the thread profile between the bottom of the cavity and the top of the thread parallel to the axis of the screw.

The internal thread diameter d 1 is the smallest distance between opposite thread bases, measured in a direction perpendicular to the bolt axis.

The thread profile depends on the shape of the cutting part of the tool with which the thread is cut. The most commonly used thread triangular profile(Fig. 144, a). In addition, in practice there are threads with other profiles. The most widespread is cylindrical triangular thread, which is usually called a fastener, since such a thread is cut on fasteners, for example, on studs, bolts and nuts.

In addition to cylindrical triangular threads, there are tapered triangular threads, which make it possible to obtain a tight connection; such threads are found on conical plugs, in fittings, and sometimes in oilers.

Rectangular(Fig. 144, b) and trapezoidal(Fig. 144, c) carving they are used in parts intended for transmitting motion, for example, in lead screws, in support screws for metal-cutting machines, cargo screws, in metalwork vise, etc.

Thrust thread(Fig. 144, d) is used in mechanisms operating with high pressure, acting in one direction, for example, in hydraulic and mechanical presses, jacks, etc.

round thread(Fig. 144, e) has a long service life even when working in a polluted environment. It is cut into wagon couplings, plumbing fixtures, etc.

In industry, detachable connections made using threads are widely used. Such connections are called threaded connections.

Threaded connections can be made:

• on a thread (connections of the "bolt-nut" or "pipe-coupling" type);
• fasteners (bolt, stud, screw).

These connections are used to fasten parts, to ensure accurate movement of elements of measuring systems, as well as to connect pipes.

Advantages and disadvantages of a threaded connection

Advantages:

• multiple assembly and disassembly of the assembly;
• reliability;
• simplicity of design;
• manufacturability.

Disadvantages:

• increased stress in thread cavities;
• low resistance to vibration loads (unscrewing).

The main parameters include:

• pitch (distance between 2 adjacent threads);
• external diameter (diameter of the fastener element, taking into account the protruding thread);
• inner diameter;
• the angle at the top of the threads.

Thread types

Threads can be divided according to the following criteria:

• purpose (fastening, fastening and sealing, running or special thread);
• type of profile (triangular, trapezoidal, persistent, rectangular or round thread);
• step size;
• direction (right and left);
• system for measuring thread parameters (metric and inch);
• location on the part (internal and external);
• surface type;
• number of visits (single-pass and multi-pass).

Properties of various types of threads

A fastening thread is used to connect parts. Fastening and sealing thread is used for such connections, in which not only strength, but also tightness of the connection is important. Leading thread is used to ensure the movement of one part relative to another. A special type of thread is used in watch movements or in eyepieces.

The purpose of the thread usually affects other characteristics of the thread being used. So for fasteners, a triangular metric or inch right-hand single-thread thread is most often used. At the same time, metric thread parameters are standardized for various applications. A triangular thread profile is an equilateral triangle whose apex is cut off. The depressions between the threads are blunt, which is required to reduce stress.

Metric threads can be coarse pitch or fine pitch. In accordance with the standard, for example, a metric M20 thread can have a large pitch of 2.5 mm and 5 smaller pitches of 0.5 to 2 mm. Coupling with fine pitch is used in cases where it is necessary to connect thin-walled parts, as well as to provide braking.

In some cases, multiple threads are used to increase the strength of the connection. This connection option is important in cases where the screw diameter is relatively small. When using a multi-start thread, the pitch, height, and inside diameter will match a single-start thread, and the travel (i.e., movement of the nut) will be much larger. It should be borne in mind that the technology for cutting multiple threads is complex, and, consequently, the cost of performing such an operation is quite high.

The fastening inch thread profile is a triangle with an angle of 55 °. All inch thread parameters are specified in inches. This thread is used in products designed in Western countries, and in Russia it is used only for the repair of imported equipment. However, fixing and sealing inch threads with angles of 55° and 60° are standardized and used in pipelines.

The trapezoidal thread profile is a trapezoid with 30° angles, and the thrust thread is a trapezoid with 30° and 3° angles. Both types of thread are running and are used to transmit movement. For example, a trapezoidal thread is used for reverse gearing in lathes, and a thrust thread is used for transferring one-sided loads in jacks and presses.

Rectangular threads are used to a limited extent in motion transmissions. It has high efficiency, but low strength. In addition, technological difficulties arise in the manufacture of such a thread.

Round thread is used for plumbing fittings, for mechanisms operating in an aggressive environment. The profile of such a thread is formed by arcs and straight lines.

Threading

This operation is performed in the following ways:

• incisors or incisor combs;
• rolling with round threaded dies;
• milling;
• grinding;
• dies and taps.

Threading with cutters is performed on the machine. This method of threading is used when it is necessary to obtain accurate lead screws or calibers. The performance of this method is low, so it is rarely used.

The main method for obtaining threads in industry is the rolling method. In this case, the part is clamped in the machine support and rolled between rollers having a thread profile. As a result, a threaded profile is extruded on the rod.

When milling threads on machines, a comb cutter is used. In this case, the cutter cuts into the body of the part and forms a thread on it. Periodically, the cutter moves to the thread pitch.

To obtain an accurate thread on short parts (gauges, thread rollers), grinding wheels are used.

The most common threading method is the use of dies and taps. At the same time, there are round and sliding dies (klupp).

When cutting external threads on the machine, the die is installed and fastened in a special fixture. When cutting internal threads, a set of machine taps is used.

Manual threading

Often threads need to be cut at home.

In order to cut an external thread, you must do the following operations:

1. Clamp the threading rod in a vise. The shank diameter must be equal to the outer diameter of the selected thread.
2. Pick up a die and install it in the die holder.
3. At the end of the rod, chamfer with a file and lubricate the workpiece with oil.
4. Carefully put the die on the end of the workpiece.
5. Carefully screw the die onto the rod without distortions.
6. Drive the die to the end of the thread.

To obtain an internal thread, you need:

1. According to the table, select the drill diameter required for this thread. In the absence of a table, the diameter of the drill can be approximately estimated by subtracting the thread pitch from its diameter. For example, for a metric M10 thread, the pitch is 1.5 mm. In this case, the diameter of the required drill is 8.5 mm.
2. Mark a recess on the workpiece with a core, clamp the part in a vise and use a drill to drill a hole for the thread. The drill must always be at an angle of 90 ° to the surface of the part. To take into account the taper of the tap, the blind hole must have some margin in depth.
3. Install a countersink into the drill chuck and make a chamfer with a depth of at least 1 mm. Chamfer skew is not allowed.
4. Insert the tap shank number 1 (marked with the 1st mark) into the knob and lubricate the working part of the tap with oil.
5. Rotate the tap wrench. To reduce the load on the tool and release chips for every 2 turns forward, make one turn back.
6. After running the thread with tap No. 1, repeat the operation with tap No. 2, which is marked with two risks and fine tap No. 3 (with three risks).

1 - Taps. 2 - Dies. 3 - collars

When cutting threads, it is advisable to follow the following recommendations:

1. When cutting an external thread on a rod, it is necessary to chamfer, install a die without distortions, and lubricate the workpiece with oil.
2. If the rod is skewed, cut off the damaged piece of metal and start threading again.
3. When cutting an internal thread, the hole must be drilled perpendicular to the plane of the part, a chamfer must be selected, and the rough tap must be lubricated with oil.
4. When choosing taps, give preference to taps made of high-speed steel, since taps made of carbon steel may break.
5. It is also not recommended to use mechanization when cutting internal threads, as this can also lead to breakage of the tap due to difficulties in choosing the force and pressure angle.
6. In the event of a tap breakage, to remove it, you must use a special extractor or try to grind the protruding piece of the tap and unscrew it with pliers. A possible extraction option is to use nitric acid for removing cutting edges tap.

Formation of a helix. If we cut out a right-angled triangle 2 from paper, in which the leg AB is equal to the circumference of the cylinder 7, i.e. AB \u003d pi (Fig. 109, a), and screw it onto the surface of the qi-

Lindra, then the leg AB wraps around the cylinder once, and the hypotenuse AC forms a curve on its surface, called a helix.

The angle a at which the helix rises is called the helix angle.

The helix (thread) can be right and left, depending on the direction of the rise of the turns on cylindrical surface. If the helical line rises from left to right (counterclockwise), then the thread corresponding to it is called right (Fig. 109, a). When a helix is ​​formed in the opposite direction (Fig. 109, b), the thread corresponding to it is called left.

To obtain a thread with a certain helix angle, a helical groove of a certain profile is cut on a cylindrical surface.

The main elements of the thread. Depending on where the thread is cut - on the surface of the rod or inside the hole, there are external threads (on the rod) and internal threads (in the hole).

A rod with an external thread is called a screw, a piece with an internal thread is called a nut. It should be noted that in mechanical engineering, not all rods with helical threads are called screws. Other names have also been adopted.

If the rod is cut to its full length, then such a part is called a screw. If the rod is not cut along the entire length and a smooth surface remains on it, such a part is called a bolt. Fasteners for connecting wooden parts, regardless of the length of the thread, are called wood screws or wood screws.

Parts with an external thread for transmitting movement are usually called screws; e.g. machine lead screw, etc.

In any thread, the following main elements are distinguished: profile, pitch, depth, outer, middle and inner diameters.

The outlines of depressions and protrusions in a longitudinal section passing through the axis of the bolt or nut form a thread profile (Fig. 110). According to the shape of the thread profile, they are divided into triangular with cut or rounded tops (Fig. 110, a); rectangular (Fig. 110, b); trapezoidal (Fig. 110,

C) persistent (sawtooth, Fig. 110, d) and round (Fig. 110, e).

A thread (coil) is a part of a thread formed during one complete revolution of the profile.

Thread pitch 5 is the distance between the parallel sides of two adjacent turns, measured along the axis of the thread. In a triangular thread, the pitch is the distance between the tops of two adjacent turns (Fig. 110, a).

The thread profile angle φ is the angle enclosed between the sides of the thread profile, measured in a plane passing through the axis of the bolt.

The top of the thread is the section of the thread profile located on greatest distance from the axis of the bolt.

The base of the thread (depression) is the section of the thread profile located at the smallest distance from the axis of the bolt.

Thread depth AND is the distance from the top of the thread to the base of the profile, measured perpendicular to the axis of the bolt.

The outside diameter of the thread d0 is the largest diameter measured at the top of the thread in a plane perpendicular to the axis of the bolt.

Average diameter * / cp - the distance between two lines parallel to the axis of the bolt, each of which is at an equal distance from the top of the thread and the bottom of the cavity. The mean diameter is measured in a plane perpendicular to the axis of the bolt or nut.

Thread inner diameter<1 - это наи­меньший диа­метр резьбы, из­меренный по впадинам ее витков в направ­лении, пер­пендикулярном к оси болта.

According to the number of threads in the threaded thread, the threads are divided into single-thread (single-thread), when only one end of the thread is visible at the end of the screw or nut (Fig. 111, a), and multi-thread, in which two (two-thread) or several are visible at the end of the screw or nut ends of the coil (Fig. 111, a, b). In these cases, the thread pitch is the distance along the axis of the screw between the same points of the turn of the same thread.

Types and systems of threads. Thread profiles are obtained depending on the shape of the cutting part of the tool with which the thread is cut. The type or profile of the thread is selected in relation to its purpose.

By appointment, the threads are divided into fastening and special. Fastening threads include triangular threads, special ones - rectangular, trapezoidal, persistent and round (Fig. 110).

The most common is a cylindrical triangular thread, in which the tops of the profile lie on a cylindrical surface. Usually this thread is called a fastening thread, since it is cut on bolts, studs, nuts, etc. To obtain especially tight (ensuring tightness) connections, a triangular thread is cut into conical
plugs, fittings of grease fittings, fittings, etc. For this thread, the tops of the profile lie on a conical surface. Rectangular and trapezoidal threads are cut on parts that form a rotational movement in translational motion, for example, on screw-cutting lead screws, locksmith screws, etc. Thrust threads are cut on parts that experience a lot of pressure in one direction, for example, on screws of powerful presses, jacks, etc. e. Round thread has a high endurance in a polluted environment and therefore is used in fittings, in car couplers, socles and sockets of electric light bulbs, etc.

In mechanical engineering, three thread systems are accepted: metric, inch and pipe.

The metric thread in the profile has the form of an equilateral triangle with an angle at the apex of 60 ° (Fig. 112, a). The tops of the protrusions of the screw and nut are cut flat to prevent seizing during make-up. Metric threads are characterized by the pitch and diameter of the screw, expressed in millimeters. Metric threads are divided into coarse pitch threads and fine pitch threads.

Threads with a large pitch are indicated by the letter M and a number characterizing the diameter, for example: M5, M20, etc. Threads with a fine pitch are indicated by the letter M and numbers showing the diameter and pitch (through the multiplication sign), for example: M4X1.5; Ml2X1 etc.

Inch thread is used only for repair work, in the manufacture of spare parts for old machines (Fig. 112, b). This thread has an isosceles triangle in the profile with an angle at the apex of 55°. The tops of the protrusions of the screw and nut are flat cut, there are gaps along the outer and inner diameters of the thread. Inch thread

It is called the number of threads per 1 inch of its length. The outer thread diameter (bolt diameter) is measured in inches.

The pipe thread has the same profile as the inch thread, but less in pitch (Fig. 112, c). It is measured in inches and is characterized by the number of threads per 1 "This thread covers diameters from 1/8 to 6" with the number of threads per 1" from 28 to 11. The diameter of a pipe thread is conventionally considered the inner diameter of the pipe (hole diameter), and not the outer .Pipe thread is used to connect pipes, pipeline fittings and other thin-walled parts.

Determination of thread sizes. In the practice of metalwork processing, it often becomes necessary to

Determine the dimensions of the thread elements on the finished part.

The outer diameter is determined using a caliper or micrometer, the thread pitch is determined using a millimeter or inch thread gauge (Fig. FROM, a, b).

In the absence of a thread gauge, the thread pitch is measured with a scale ruler or caliper (Fig.

113, c). To do this, a ruler is applied to the thread along its axis so that its zero division coincides with the top of one of the turns, and the number of thread turns that fit along the length of I "(25.4 mm) is counted. Dividing 1" by the resulting number of turns, determine the thread pitch in inches and the number of threads per 1"

Similarly, the pitch of the metric thread is determined. If an integer number of turns does not fit into G ", then the calculation is made on a length of two or three inches.

The pitch of a thread can also be measured by its impression on paper or wood. This technique often has to be resorted to when measuring the pitch of an internal thread of a small diameter. To do this, a thin wooden stick is inserted into the thread hole, pressed against the thread and an impression is obtained, according to which the thread pitch is measured. The pitch of a special thread (rectangular, trapezoidal) is measured with a caliper or by imprinting the thread on paper.

The most common connections for machine parts are threaded. The widespread use of threaded connections in machines and mechanisms is explained by their simplicity and reliability, ease of adjusting the tightening, as well as the possibility of disassembling and reassembling them without replacing the part.

Thread cutting is its formation by removing chips (as well as plastic deformation) on the outer or inner surfaces of workpieces.

The thread is external and internal. A part (rod) with an external thread is called a screw (Fig. 253, a), and with an internal one - a nut (Fig. 253, b). These threads are made on machines and manually. Below we consider the manufacture of threads by the second method.

A helix can be imagined as follows. Take a cylindrical rod with a diameter D and a right triangle cut out of paper or foil abc, whose side AB equal to the circumference of the cylinder pD, those. 3.14 D(Fig. 254). ober-nem triangle ABC around the cylinder so that the side AB aligned with the circumference of the lower base of the cylinder. Then the other side of the triangle sun will be located along the generatrix, and the hypotenuse. 4C forms a helical line on the surface of the cylinder. In this case, the side of the triangle sun will be the pitch of the helix, the side AU is the length of one turn, and the angle CAB- the angle of elevation of the helix (o).

Depending on the direction of the rise of the turns on the cylindrical surface, the helical line (thread) can be right and left. If the helix, when winding a triangle onto a cylinder, moving away from the base, gradually rises from left to right, i.e. counterclockwise (Fig. 254, a, b), then it is called right, respectively, and the thread is called right. If the helix, when winding the triangle onto the cylinder, moving away, gradually rises from right to left, i.e. clockwise (Fig. 254, c, d), then it is called left, respectively, and the thread is called left.

The helical line and the thread corresponding to it are called right because to screw the screw with this thread, the screw (or nut) must be rotated to the right, i.e. clockwise (Fig. 255, a). With a left-hand thread, the screw (or nut) must be rotated to the left to tighten, i.e. counterclockwise (Fig. 255, b). In mechanical engineering, right-hand threads are more often used.

Circular material cross-section remaining intact after threading (cassock. 256) is the internal cross section of the thread, and the diameter of this section is the internal diameter of the thread. The outside diameter of the shank is the nominal diameter of the thread (d), or thread diameter.

MAIN ELEMENTS OF THREAD

For any thread, the following main elements are distinguished: profile; profile angle and height; step; outer, middle and inner thread diameters.

The thread profile (Fig. 257) is considered in a section passing through the axis of the bolt or nut. A thread (coil) is a part of a thread formed during one full turn of the profile.

Angle b profile - the angle between the sides (faces) of the thread profile, measured in a plane passing through the axis of the bolt. In a metric thread, this angle is 60 °, in an inch - 55 °.

Height (depth, threads) H Profile - the distance from the top of the thread to the base of the profile, measured perpendicular to the axis of the bolt.

Step R threads - the distance between parallel sides or tops of two adjacent turns, measured along the axis of the thread. Rice. 257. The main elements of the thread

In a metric thread, the pitch is expressed in millimeters; inch thread is characterized by the number of threads (turns) per inch.

Outside diameter d threads - the diameter of the cylinder circumscribed near the threaded surface. The outer diameter is measured for bolts along the tops of the thread profile, for nuts - along the troughs.

Inner diameter d threads - the diameter of the cylinder inscribed in the threaded surface. The inner diameter is measured for bolts along the hollows, for nuts - along the tops of the thread profile.

Average diameter d threads - the diameter of a cylinder coaxial with a threaded cylinder, the generatrix of which is divided by the sides of the profile into equal segments.

Threaded connections are called connections of parts using threads - alternating protrusions and depressions on the surface of bodies of revolution located along a helix. There are conical and cylindrical threaded connections. Cylindrical, in turn, are divided into connections with fastening and special threads.

Fastening threads include large and small metric threads according to GOST 9150-81, as well as threads with spiral inserts; to special ones - pipe, rectangular, thrust, round, trapezoidal, etc.

A part with an internal thread is called nut, from the outside - bolt(screw, pin).

The thread profile is the contour of a thread section in a plane passing through the axis of the threaded part. GOST 9150-81 and GOST 8724-81 establish a single nominal profile for cylindrical metric threads with a diameter of up to 600 mm, including threads with a diameter of less than 1 mm. The nominal thread profile and its elements are shown in (Fig. 1, a). The cavity of the external thread (Fig. 1, b) can be flat-cut or rounded: R max =0.144P, R min \u003d 0.108P,
where R- cavity radius;
R- thread pitch.

Threads are defined by the following main parameters:

• outer, middle and inner diameters;
• step;
• profile angle;
• angle of inclination of the sides of the profile.

Outer thread diameter d (see Fig. 1, a) - the diameter of the cylinder described with respect to the tops of the external thread (or the troughs of the internal thread).

Inner diameter d1 - the diameter of the cylinder inscribed in the tops of the internal thread (or the trough of the external thread).

Rated values d and d1 for external and internal threads are the same.

Average diameter d2 - the diameter of an imaginary cylinder, the surface of which intersects the threads in such a way that the width of the threads and the width of the troughs are equal.

thread pitch R - the distance between the parallel sides of two adjacent coils, measured along the axis.

GOST 8724-81 sets diameters in the range of 0.25 ... 600 mm and steps of 0.075 ... 6 mm. Metric threads can have a coarse pitch (with diameters of 0.25 ... 68 mm) and a fine pitch (with diameters of 1 ... 600 mm).

Profile angle α - the angle between the sides of the profile, measured in the axial plane. Angle of inclination of the sides of the profile β - the angle between the profile side and the perpendicular to the thread axis. For threads with a symmetrical profile β=0.5α. For threads with an asymmetrical profile, such as thrust or conical threads, the angle of inclination of each side is determined independently.

Height of the original triangle H - the height of the acute-angled profile obtained by continuing the lateral sides of the profile until they intersect. Working profile height H1- the height of a flat-cut theoretical profile, equal to the half-difference of the outer and inner diameters. For metric threads H=0.866025R, H 1 \u003d 0.54126 R.

In reality, the contact height is smaller, since the tolerance system provides for certain clearances, for example, along the inner diameters of the nut and bolt threads.

Working coil height H1- the highest contact height; the smallest contact height is indicated H 1 min. For flat profile threads H1 and H 1 min determine the largest and smallest overlap of threads and nuts.

Helix angle (helix)

For multi-start threads, the numerator of this formula should be substituted for R work n 0 P,
where n 0- number of visits.

Make-up length l(nut height H) - the length (height) of the contact surfaces of the bolt and nut, measured along