The main characteristic of any thread is its profile. This name means a section of a threaded thread by a plane passing along the axis of the product on which the thread is made.

Angle, depression, peak - all these are profile elements. The angle formed by the side faces of the thread is called the profile angle. Its measurement is carried out in the diametrical plane.

As for the top of the profile, it is formed by a line that connects the side edges of the thread at the top point (E), as shown in Figure 1 below.

Figure 1. Profile elements

The profile cavity in Figure 1, a, b is denoted by the letter F. This is a line that also connects the side lines of the coil only at the bottom point. In other words, the trough is the lowest point of the helical groove.

Profile top/bottom shape can be of two types:

1. flat-cut (a);
2. rounded (b).

Step

Pitch - the value of the distance of similar points of adjacent turns. It is measured in a plane parallel to the axis of the thread. The units of this parameter are millimeters, at least in production millimeters are much more common than inches, in which the thread pitch can also be measured.

On machines designed for turning, both modular and pitch threads can be performed.

Diameter

There are three types of thread diameters:

• outer - the diameter of a circle circumscribed around the upper points of the threaded surface;
• middle - the diameter of the circle, whose generatrix intersects the thread profile and at the same time segments equal to half the nominal pitch are formed;
• internal - the diameter of the circle inscribed at the lower points of the thread.

Climb angle

The helix angle is the angle formed by a threaded protrusion and a plane perpendicular to its axis.

Thread classification

The thread can be directed both to the left and to the right, which is shown in Figure 2.

Figure 2. Types of thread

You can determine the lift of the screw thread by hand. If you put a shaft with a threaded surface on the palm of your left hand, where the rise of the screw coincides with your thumb, then the thread is left-handed. Otherwise, the thread is right-handed. The type of thread determines in which direction the nut will be screwed on: in the first case, the nut is screwed counterclockwise, and in the second - clockwise.

The profile sets the type of thread:

• triangular;
• rectangular;
• trapezoidal;
• stubborn.

It should be noted that triangular systems include metric, inch and pipe threads. Each of them will be discussed.

Metric threads are considered the most popular. Moreover, threads are common, characterized by different pitch sizes (measured in mm). Their profile angle is 60°. TO characteristic features This design can be attributed to the presence of play between the profiles of the threaded pair of bolt-nut. Consequently, such a thread is most often found on fasteners such as screws, bolts, studs, etc., the main purpose of which is to connect the elements of mechanisms.

Inch threads are not as common as metric threads. Their profile angle is slightly less than the first, and is 55 °, and the pitch is measured by the number of threads per inch. At the upper and lower points of this thread, play is also provided. Foreign-made machine parts are equipped with such a thread, so they are made as needed (when the original product fails).

The angle of the pipe thread profile coincides with the inch and is 55°. Distinctive feature threads of this type - the rounded shape of the tops and valleys, as well as the absence of gaps at these points. Due to this, this threaded connection is watertight. Such a thread is equipped with gas and water pipes and their connecting elements (for example, couplings).

Trapezoidal threads are characterized by a trapezoid profile and a 30° angle. The profile is formed by straight lines, slightly rounded at the top and bottom where gaps are expected. Its use is relevant on screws that convert the rotation of one part into a rectilinear movement of another.

A rectangular thread is distinguished by a square profile, each face of which is equal to half the pitch. There is no backlash here. Its scope is exactly the same as in the case of trapezoidal threads. At the same time, it is extremely rare to encounter it, since it is not standardized.

When connecting, the thrust thread is in contact with the sides that bear the main load, as well as the upper and lower points of the thread of the connected elements. A gap is provided between other profile elements. A similar thread is made on the couplings that connect the pipelines of compressors and compressed air tanks, and on the screws of hydraulic presses.

Adjustment of equipment for thread cutting

To set up a lathe for threading, you need to know the following: with a complete revolution of the workpiece, the threading tool must move by the amount of pitch (when making a single thread) or stroke (when making a multiple thread).

Upon completion of the operation, which involves deepening the tool into the body of the part with each pass, a helical groove and a ledge are formed, which are threads.

The concept of a helix. If (Fig. 166, a) a right-angled triangle ABC, cut out of paper or thin tin, the side AB of which is equal to the circumference πD of the base of the cylinder E, is screwed onto the cylinder so that the side AB coincides with the base of the cylinder, then the side AC forms on the side surface of its line, called helical.

Screw thread formation. Suppose that a flat figure, for example, a triangle abc (Fig. 166, b), with side ab touches the generatrix of the cylinder E and is located in a plane passing through its axis. Suppose further that this triangle moves, remaining in a plane passing through the axis of the cylinder E, and its top slides along a helical line drawn on the cylinder. When the triangle is moved on the side surface of the cylinder E, a helical protrusion N and a helical groove M are obtained, forming an external screw thread.

If the triangle abc were to move along a helical line drawn on the inner cylindrical surface(on the walls of the hole), an internal screw thread would be formed on this surface.

The helical protrusion of the thread, which is obtained after one complete revolution of the figure forming it, is called a coil.

Thread profile. Screw threads, adopted in practice, are formed by moving along the side surface of the cylinder not only a triangle, but also other flat figures (trapezoids, squares, etc.), selected depending on the conditions in which the thread operates. In accordance with this, the main feature that characterizes the thread is its profile.

The thread profile is the section of its coil by a plane passing through the axis of the cylinder (i.e., the diametral plane) on which the thread is formed.

Rice. 166. Formation of a screw thread

Thread profile elements. The elements of a thread profile are its sides, angle, top and bottom.

The profile angle is the angle between the sides of the coil, measured in the diametrical plane. This angle (Fig. 167, a) is denoted by the letter α.

Rice. 167. Profile elements (a, b) and thread pitch (c)

The top of the profile is the line connecting its sides along the top of the coil (P, Fig. 167, a, b).

The profile cavity is the line that forms the bottom of the helical groove (R, Fig. 167, a, b).

The outlines of the top and bottom can be flat-cut (Fig. 167, a) or rounded (Fig. 167, b).

thread pitch. The next element that characterizes the thread is its pitch.

The thread pitch is the distance between two like (i.e., right or left) points of two adjacent turns, measured parallel to the thread axis.

On fig. 167, at such points are points A and A 1 points B and B 1, points C and C 1, etc. The distance between these points, measured parallel to line 00 (i.e., the thread axis), is the thread pitch, denoted by the letter S.

Almost all threads used in mechanical engineering have a pitch measured in millimeters. There are, however, also threads in which the pitch is expressed as the number of threads per inch of thread length.

In addition to screws, lathe worms are cut with a modular or pitch pitch.

Thread diameters. There are three thread diameters: outer, inner and middle.

The outside diameter of the thread (d) is the diameter of the cylinder circumscribed near the flank of the thread.

For a bolt, the outer diameter corresponds to the diameter along the tops of the profile (Fig. 168, a), measured perpendicular to the axis of the thread, and for a nut, along the profile depressions (Fig. 168, b).

Rice. 168. Thread diameters: external and internal (a, b) and average (c)

The internal diameter of the thread (d 1) is the diameter of the cylinder inscribed in the threaded surface.

For a bolt, the inner diameter corresponds to the diameter along the profile depressions (Fig. 168, a), measured perpendicular to the axis of the thread, and for a nut, along the tops of the profile (Fig. 168, b).

The average thread diameter (d 2) is the diameter of a cylinder coaxial with the thread, the generatrix of which is divided by the sides of the profile into equal segments.

On fig. 168 into this threaded cylinder is shown in dash-dot lines. In the figure, AB \u003d BC \u003d CD, etc., and therefore d 2 is the average diameter.

Thread angle. When cutting threads on a lathe, it is necessary to take into account the angle of its rise.

The angle of elevation is the angle formed by the direction of the threaded protrusion of the thread with a plane perpendicular to its axis.

Right and left threads. In the direction of the coil, right (Fig. 169, b) and left (Fig. 169, a) threads are distinguished.

Rice. 169. Left (a) and right (b) threads

If the rise of the thread of the screw, placed on the palm of the right hand, coincides with the direction of the bent thumb, this thread is right-handed.

The coincidence of the rise of the thread with the direction of the bent thumb of the left hand indicates that this thread is left-handed.

A nut is screwed onto a screw with a right-hand thread when turning to the right (clockwise), onto a screw with a left-hand thread - when turning to the left (counterclockwise).

The main parameters of the thread (Fig. 5.2) are:

1) diameters (screws and nuts) outer ( d, D); middle ( d 2 , D 2); internal ( d 1 , D 1);

2) profile;

3) profile angle ( α );

4) step ( p);

5) elevation angle ( ψ ).

The diameters of the screw, as a male part, are indicated by small letters ( d), the diameters of the nut, as a female part, are large ( D). The nominal values ​​​​of the diameters of the same name are equal; the difference is in the tolerances. On the surface of an imaginary cylinder with a diameter d The 2 widths of the threads and thread cavities are the same.

Thread profile - this is the profile of the protrusion and groove of the thread in the plane of its axial section.

Profile angle (α ) is the angle between adjacent sides of an axial section thread.

Thread profile is also characterized by the following parameters:

· the height of the original triangle threads ( H), i.e. a triangle, the vertices of which are formed by the points of intersection of the extended sides of the thread profile;

· profile working height threads ( H 1), equal to the length of the projections of the section of mutual overlap of the profiles of the mating external and internal threads onto the perpendicular to the axis of the thread.

Rice. 5.2. Thread Options

thread pitch (p) is the distance along a line parallel to the thread axis between the midpoints of the nearest thread flanks of the same name, lying in the same axial plane on one side of the thread axis.

For multi-start threads introduce an additional term - screw stroke » (where is the number of starts) is the translational axial movement of the screw in one revolution in a fixed nut or the relative axial movement of the nut in one revolution. For single start threads:

Climb angle carving (ψ ) - the angle of elevation of the helix sweep along the average diameter (Fig. 5.3):

All geometric parameters of threads and tolerances for their dimensions are standardized.

By appointment threads are divided into the following groups:

Rice. 5.3. Development of a helix along the average diameter

1) fastening threads for fastening parts. They are performed, as a rule, with a triangular profile. It's caused increased friction, providing less risk of loosening of the tightened thread; increased strength threads; ease of manufacture;

2) fastening and sealing threads , serving both for fastening parts and for preventing liquid from escaping (in pipeline connections and fittings). These carvings given reasons they also perform a triangular profile, but without radial gaps and with smooth roundings;

3) threads for transmission of motion used in lead and load propellers. To reduce friction, these threads are trapezoidal with a symmetrical profile and with an asymmetrical profile (thrust), and sometimes with a rectangular profile. Thrust threads are designed to absorb large axial forces acting in one direction.

The given division of threads is not strict. For example, triangular profile threads are sometimes used for high-precision fine pitch lead screws, and thrust threads are used as fasteners.

Threads due to gaps, as a rule, cannot be used as centering elements.

Thread profile selection is determined by many factors, the most important of which are strength, manufacturability and friction forces in the thread. For example, threads of screw mechanisms should be with low friction forces to increase the coefficient of performance (COP) and reduce wear. Strength in many cases is not for them the main criterion that determines the size of the screw pair.

Depending on the profile, the following threads are distinguished:

1) metric;

2) pipe;

3) round;

4) trapezoidal;

5) persistent;

6) conical.

Metric thread with a triangular profile (see Fig. 5.2) is the main fastening thread. Metric threads are divided into coarse and fine pitch threads. The main thread is taken with a large pitch, whose static load-bearing capacity is higher compared to fine pitch threads, the impact on strength of manufacturing errors and wear is less.

Metric fine pitch threads are used in the following cases:

1) for dynamically loaded parts and parts, the diameters of which are mainly determined by bending and torsion stresses (shafts);

2) for hollow thin-walled parts;

3) parts in which the thread is used for adjustment;

4) when the use of fine threads facilitates locking, reduces differences in shaft diameters, etc.

Large-pitch metric threads are denoted by the letter M and a number expressing the diameter of the thread in millimeters, for example, M20. For metric threads with a fine pitch, the pitch in millimeters is additionally indicated, for example, M20 × 1.5.

Pipe thread it also has a triangular profile, but unlike the metric thread, its tops and bottoms are rounded. Threads of these types are used for hermetic connection of pipes and fittings of pipelines in the range of conditional sizes from 1/16 "to 6". Pipe threads are made with fine pitch and no gaps in the ridges and valleys for better sealing. For the main dimension characterizing the thread and indicated in the thread designation, the conditional inner diameter of the pipe is taken.

round thread made with profile angle α = 60°. The profile of this thread consists of arcs connected by short straight sections. Large radii of curvature eliminate significant stress concentration. Contaminants entering the thread are squeezed out into the gaps. Threads of these types are mainly used for screws subjected to high dynamic stresses, as well as often screwed and unscrewed in a contaminated environment (in fire fittings, car ties).

Trapezoidal thread is the main thread for screw-nut gears. It has lower friction losses compared to triangular threads, is convenient to manufacture and is more durable than rectangular threads. If necessary, it allows the selection of gaps by radial approach when the nut is split along the diametrical plane.

Trapezoidal thread has a profile angle of 30°, profile working height H 1 = 0,5p, average diameter d 2 = d – 0,5p, clearance from 0.15 to 1 mm depending on the thread diameter.

Trapezoidal threads are standardized in the diameter range from 8 to 640 mm, the possibility of using threads with small, medium and large pitches is provided.

Thrust thread used for screws with a large one-sided axial load in presses, pressure devices of rolling mills, in cargo hooks, etc. The profile of the turns is asymmetrical trapezoidal. The angle of inclination of the working side of the profile was chosen to be 3° to increase the efficiency (threads with a profile inclination angle of 0° are inconvenient in manufacturing), the angle of inclination of the non-working side of the profile is 30°, and a significant radius of cavity rounding is provided to reduce stress concentration. Working profile height H 1 = 0,75p.

Reinforced thrust threads have an angle of inclination of the non-working side of the profile of 45°, which provides a significant reduction in stress concentration by increasing the collapse stress. Their fatigue resistance is increased by 1.5 times.

Tapered threads provide impermeability without special seals. They are used to connect pipes, install plugs, oilers, etc. Impermeability is achieved by a snug fit of the profiles along the tops. Tapered threads can be tightened to compensate for wear and create the required tightness, these threads provide fast screwing and unscrewing.

Today, various threaded connections are often used in the field of mechanical engineering. Carving is intricate technological process requiring certain skills and abilities. For threading, you must be able to set up the machine, select, sharpen and install the cutting tool and, of course, be able to use threaded measuring instruments. Currently, metric threads are often used (has a triangular profile). Let's start everything in order and first we will analyze the general concepts:

Thread profile

The profile of a metric thread is similar to an equilateral triangle with a 60° angle. Thread tips can be sharp or slightly sheared, depending on the sharpness of the cutter and the depth of cut. It is not recommended to make a metric thread with a different angle, as this will violate the requirements for a metric thread, in particular, for its profile. A modified thread profile affects the threaded connection of parts such as a screw and a nut.

External thread profile fig. left and internal thread fig. on right

Profile of metric and cylindrical threads and its reaming

• d, d1, d2- outer, inner and average diameter of the bolt
• D, D1, D2– outer, inner and average diameter of the nut
• R- step; ω - elevation angle

thread pitch

The thread pitch is the distance between two thread ends measured along an axis.

The helix angle is the angle formed by the direction of the thread shoulder and the plane perpendicular to the thread axis. Determined by the formula:

tgω=p/( π d2)

Thread lead - a single-start thread is equal to its pitch. If the thread has several starts k, then the thread lead is calculated by the formula: in one revolution, the nut will move k times further in the axial direction.

Ph=pk

Thread classification

By type of surface: cylindrical and conical.

According to the location of the thread: external and internal.

In the direction of the helical protrusion of the thread: right and left.

By the number of starts: single-start - formed by one ledge and multi-start - formed by two or more thread ledges.

By profile:

triangular	Trapezoidal	Persistent	Triangular for pipe and cylindrical thread

By appointment: fastening and running

According to the system of dimensions: metricα =60° and inch α =55°.

The initial profile of a metric thread is a triangle of height P with an angle α of 60°. The apex of the triangle is cut off, the thread profile cavity has a flat or rounded shape. The rounded shape is preferred. Metric threads are made with coarse and fine pitch. The diameters and steps of the metric thread are specified in GOST 8724-81, and the profile and its dimensions are GOST 9150-81. Inch thread dimensions are expressed in inches (one inch is equal to 25.4mm), the original inch thread profile is a triangle with an angle at the apexα=55°.

Find the cutting tool for your specific application and
get immediate advice on cutting data.

This section provides formulas and definitions for threading and how to calculate cutting speed, feed, and other parameters for thread turning, thread milling, and tapping operations. You will also find here the symbols for various thread profiles according to international standards.

Formulas and definitions for thread turning

plunge depth

By machining the full depth of infeed in multiple passes, the insert nose radius is not overloaded.

Example: if the infeed depth (radial infeed) per pass is 0.23–0.10 mm, then the total depth ( a p) and profile depth (0.94 mm) for a metric thread with a pitch of 1.5 mm will be machined in 6 passes (nap).

​	1st pass, plunging depth 0.23mm
		​
​	= 0,009"
	​	​
​	​
​	2nd pass, plunging depth 0.42 - 0.23 = 0.19 mm
		​
​	0,017 – 0,009 ​= 0,008"
		​
	​
​	3rd pass, plunging depth 0.59 - 0.42 = 0.17 mm
		​
​	0,023 – 0,017 = 0,006"​
	​
​	4th pass, plunging depth 0.73 - 0.59 = 0.14 mm
		​
​	0,029 – 0,023 = 0,006"​
	​
​	5th pass, plunging depth 0.84 - 0.73 = 0.11 mm
		​
​	0,033 – 0,029 = 0,004"​
	​
​	6th pass, plunging depth 0.94 - 0.84 = 0.10 mm
		​
​	0,037 – 0,033 = 0,004"​

The depth of cut can be calculated using the formula:

Δ a p = radial infeed, depth of cut per pass

X = pass number (consecutively from 1 to nap)

a p = total thread depth + machining allowance

nap= number of passes

Y = 1st pass = 0.3

Pitch 1.5 mm
a p = 0.94 mm
nap = 6

γ 1 \u003d 0.3
γ 2 =1
γ n = x-1

1. depression
   Surface at the base joining two adjacent profile sides
2. Side profile
   The surface of the thread connecting the top and bottom of the profile
3. Vertex
   Surface joining the two sides of the profile at the outer diameter

P = thread pitch in mm or threads per inch

The distance between two corresponding points of adjacent threads, measured parallel to the axis of the thread.

β = thread profile angle

The angle between the sides of the profile, measured in the axial plane.

φ = helix angle of the thread

The angle formed by a tangent to a thread helix at points lying on the mean diameter and a plane perpendicular to the thread axis.

Diameter Options

d = outside diameter of external thread

D = outside diameter of internal thread

d 1 = internal diameter of external thread

D 1 = inside diameter of internal thread

d 2 = average external thread diameter

D 2 = average internal thread diameter

The effective diameter of a screw thread is approximately halfway between the outer and inner diameters.

Thread angle

The helix angle (φ) depends on the diameter and pitch of the thread. This parameter can be represented as a development of a right triangle. The lead angle is calculated using the formula below.