One of the achievements, which played an important role in accelerating the scientific and technological progress that distinguishes the last 500 years of the history of mankind, is the invention of a threaded connection. Let's take a closer look at its features.
Rivets and wedges
In ancient times, the strength of prefabricated structures was given by such joining methods as wedges (in wooden products) and rivets (in metal products). The main disadvantage of such connections was and remains the inability to reuse fasteners.
Wedges and rivets are disposable, and this requires unnecessary costs. The first threads also had a significant limitation, since they were not made on cutting machines, but using templates that forged the threads on hot metal. The interchangeability of parts having external and internal threads made in this way was not discussed.
Metric thread
In most countries of the world, a metric measurement system is adopted, which is based on the convention that one thousand millimeters is in one meter. This measuring system and the growing production of machines led to the widespread use of metric threads and the simplicity of its designation. For example, the designation M10 (thread) - here the letter "M" indicates its diameter in the metric measurement system .
However, this is not all the parameters that characterize a threaded connection. In this case, in the characteristic "M10 thread", the diameter is indicated by the number 10, it indicates the outer diameter on the bolt . In addition to the diameter of the thread, there is such an indicator as the thread pitch.
Depending on the strength of the joint, the thread pitch may be different. So, a standard M10 thread can have the following pitch values:
- 0.5 mm;
- 0.75 mm;
- 1 mm;
- 1.25 mm.
For the largest standard thread, the step in its designation may not be indicated, since it is taken as the main one. So, if we see the designation M10, and after that there are no additions, then by default this M10 thread has a pitch of 1.25 millimeters.
Internal metric thread
Naturally, in order for the threaded connection to be applied, two threads must be combined, cut in the corresponding places of the parts to be joined:
- external - on a bolt or hairpin;
- internal - in the nut or body of the part.
In the part in which the internal thread M10 will be used, the hole must be drilled taking into account the length and pitch. This diameter is often easy to calculate, having basic knowledge in mathematics and not even having tables with standard values at hand, since the standard for the thread adopted in the metric system has a profile tip angle of 60 degrees.
The rules of an isosceles triangle and the Pythagorean theorem will make it easy to determine that the thread height in increments of 1 millimeter will be equal to 0.866 millimeters. Thus, the hole in which the M10 thread will be created must be at least 8.268 mm:
10 - 0.866 - 0.866 = 0.8268
Since the thread is located on both sides of the drilling axis, the thread height is subtracted from the outer diameter twice.
Why there is a different step
As mentioned earlier, depending on the required strength of the connection, a different thread pitch is used. The most common among those used in engineering and other areas is a major step. However, for increased bond strength, a small thread pitch is also used.
In addition, it is also used where greater accuracy is required. For example, in a measuring tool of higher accuracy than a vernier caliper, a metric thread with a small or very small thread pitch is used. In this case, the translational movement, the size of which must be measured, is distributed over the circumference of the thread, which makes it possible to measure not only tenths, but also hundredths of a millimeter.
Naturally, a large thread pitch will prevent accurate measurement. After all, an M10 thread with a large pitch of 1.25 millimeters will have a measuring accuracy two and a half times less than with a very small pitch of 0.5 millimeters.
It should be noted that due to the more complicated (from the point of view of technology) production of thread with a very small pitch, it is practically not used in mechanical engineering. The main area of its application is a measuring tool.
Although most threaded connections have a right-hand thread (to tighten the bolt, it must be rotated clockwise), if necessary, the left one can be used. For example, a set of pedals for a bicycle has left and right mounting threads.