It is well known that threaded is one of the most common detachable joints , allowing assembly and disassembly work without damaging the integrity of structures, machines and mechanisms. The basis of such a connection is a thread applied to two or more surfaces of bodies of revolution, which is mainly divided depending on the indicators described below. The thread classification is presented in the table below.
Metric thread
A screw thread on or in a material having a tooth profile in the form of an isosceles triangle is a metric thread, its dimensions are measured in millimeters. According to the shape of the surface of the application, this thread is cylindrical, but may be conical.
The latter is most popular in use, especially for the following fasteners:
- bolts
- anchors;
- screws;
- hardware;
- hairpins;
- nuts and stuff.
A screw thread applied to a conical base is called a metric conical thread. It is used in places requiring fast locking of joints, without additional sealing and with the cessation of leakage by simple pulling along the axis. Used in the construction of plugs and pipe connections:
It is important to know that tapered and cylindrical threads have the same profile, which allows them to be screwed together. Metric threads are classified by size, direction of rotation, pitch and additional parameters, which are reflected in the marking.
Sizes of metric threads
The scatter of the diameters of this thread in industry has a range from 0.25 to 600 mm, and with a diameter of more than 68 mm the thread will be only small, while up to this value it varies. Large pitch threads are used in joints under high and impact loads. It is also interesting that for large threads, the pitch is always fixed in relation to the diameter, in contrast to small, which can change, which is separately and additionally indicated when marking.
For example, if “M16” is found in technical documents or drawings at the junctions of parts, this means that the letter M means a metric thread. The dimensions of the outer diameter of the turns are 16 mm, and the large pitch of the standard thread is 2 mm, according to the information indicated in the table (thread of the second row is indicated in brackets). So, the thread is metric: main dimensions (GOST 24705-2004).
Fine thread
In the marking, a small step is indicated after the diameter. It looks like this: “M16 × 0.5”, where, as already known, M is a metric thread. The dimensions of the outer diameter are 16 mm, with a step size of 0.5 mm. Interestingly, after a diameter of 2 mm, the difference between the threaded pitch becomes significantly noticeable, leading to separation. Moreover, products of equal diameter have several types of fine thread pitch, as discussed in 16 mm:
- 1.5 mm;
- 1.0 mm;
- 0.75 mm;
- 0.5 mm
As an example, a part of the table is given that makes it possible to understand and visually evaluate the range of small threads, without taking into account the large step discussed earlier.
Metric thread: main dimensions (GOST 24705-2004)
Specified Parameters
In multi-thread threads, the step is indicated separately (in brackets), and in its place the number of entries is indicated. Here is how this and other additional parameters are indicated when marking:
- (P1) - where P is the pitch of 1 mm and the turns are 3 (example: M42 × 3 (P1));
- LH - left-hand thread (example: M40 × 2LH);
- MK - metric conical thread (example: MK24x1.5);
- EG-M or GM, where G denotes a thread on a cylindrical base of a wire insert or fitting (example: EPL 6-GM5);
- g, h, H is the tolerance field, is the tolerance of the average diameter in combination with the diameter of the protrusion (example: M12-6g), and for different tolerances of the inner and outer diameters, both tolerances are indicated in the marking (example: M12-6g / 8H).
Thread diameters
There are indicators indicated in the summary tables, which are important to consider when metric threads are considered - dimensions diameters:
- external (D and d);
- internal (D 1 and d 1 );
- medium (D 2 and d 2 );
- internal along the bottom of the cavity (d 3 ).
With widespread use in a threaded joint of a sliding fit, the average diameter has become very important, and in cases of equality of values, the greatest d 2 bolts and smallest D 2 nuts.
In capital letters D, the diameters of the internal threads are indicated, and parts applied to the external surface are indicated in small letters - d. The numbers indicate the location. The degree of accuracy of tolerance fields is classified by alphabetic characters: E, F, G, H, d, e, f, g, h, and, as in diameters, the location is indicated by the size of the letter.
Ratio of metric to inch threads
In contrast to European and neighboring countries, where after the rule of Napoleon the metric system became widespread, in the countries of the former colonies of Britain and its satellites, all measurements take place in the imperial system. In this system, threads and their connections are measured in inches.
A screw thread having a tooth profile in the form of an isosceles triangle, with a vertex angle of 55 degrees. (60 degrees in the UTS standard for the USA and Canada), is called an inch thread, its dimensions are in inches, and the pitch is in the number of turns per inch (1 "= 24.5 mm). Such fastening threads are made in the range from 3/16 ", when designating, only the outer diameter is indicated.
Sizes of inch and metric threads are measured with a caliper, and if in the case of a metric thread this is enough, then in inch after measurement use a special table. When measuring threads, special templates are used, but there is also a popular way to measure the pitch: if, wrapping a sheet of paper on a thread, scrolling the product several times, a trace will be printed on the paper, making it possible to measure with a ruler. When using a notebook sheet in a box as paper, the need for a ruler disappears - it is enough to count the number of marks in 2 cells (1 cm) and divide by 10.
Hole sizes
Getting threads is due to:
- cold rolling by rollers and heads;
- cutting with cutters, combs or cutters;
- dice or tapping;
- precision casting;
- abrasive or EDM.
For cutting external threads, the workpiece is cylindrical and chamfered, and a bit smaller than the required thread metric (dimensions) hole, but larger than its inner diameter, is drilled under the internal thread. Indeed, when determining the dimensions of holes for metric threads, it must be taken into account that when cutting a notch, a partial extrusion of the material that subsequently participates in the formation of the threaded profile occurs. It is also important to reckon with the properties of the material in which drilling is performed, reducing the size of the drill by 0.1 mm.
Sizes of nuts with metric thread
A nut is one of the components of fasteners having an internal thread. They vary in height with respect to diameter and strength, according to purpose and configuration. The most widely used are turnkey or hex nuts, here is a list of them with state standards:
- GOST 5915-70 - medium sizes;
- GOST 15523-70 - high;
- GOST 22354-77 - increased strength;
- GOST 5916-70 - low nut with a recess;
- GOST 10605-94 - for a thread diameter of more than 48 mm.
There are many nuts and special purposes, here are examples of some and their GOSTs:
- cap type (hex) - GOST 11860-85;
- for manual screwing (wing nuts) - GOST 3032-74;
- slotted castellated - GOST 5919-73;
- rounded with a slot - GOST 11871-88, GOST 10657-80;
- round, with end, radial holes - GOST 6393-73;
- for rigging (eye nuts) - GOST 22355 (DIN580, DIN 582).
The most important thread connection parameter is matching nuts and threads. The values of the more popular threads with large pitch are shown in the table below, where S is the turnkey size, e is the width of the nut, and m is its height.
Correspondence table of threads and nuts (GOST 5915-70 and GOST 10605-94)
Standards
The main threaded dimensions comply with GOST 24705-2004, which modifies the standard - ISO 724: 1993 (international classification). Since July 1, 2005, this GOST has been the state standard of the Russian Federation and takes into account the interests of the economy of another 12 countries formerly part of the USSR that voted for it. The dimensions of metric threads of GOST 9150 for general purposes, as well as the diameters and steps of GOST 8724, fall under its action.
By interchangeability standards, this GOST refers to the following international and national standards systems:
- GOST 8724-2002 (ISO 261-1998);
- GOST 9150-2002 (ISO 68-1: 1998);
- GOST 11708–82;
- GOST 16093-2004 (ISO 965-1: 1998 and ISO 965-3: 1998).
This GOST fixes all the main dimensions, possible tolerances, terminology and formulas for calculating diameters:
- D 2 = D - 2 x 3/8 H = D - 0.6495 P;
- d 2 = d - 2 x 3/8 H = d - 0.6495 P;
- D 1 = D - 2 x 5/8 H = D - 1,0825 P;
- d 1 = d - 2 x 5/8 H = d - 1,0825 P;
- d 3 = d - 2 17/24 H = d - 1.2267 P.
It is difficult to imagine modern life without machines and mechanisms, it is even more difficult to imagine technology without detachable connections that the thread provides. Efficiency, relative ease of manufacture and comfortable use have provided threaded connections with an honorable place in world history.