What is the difference between imperial and metric fasteners? In fact, it is easy to understand. First of all, the imperial system is a measurement system used in English-speaking countries such as the United Kingdom and the United States. Due to various historical reasons, the imperial system is quite complicated. The metric system is also called "meter system" and "meter system". It is an international weights and measures system introduced to China after the signing of the "Sino-French Commercial Charter" in 1858. Founded in France. On April 7, 1795, the French National Assembly decided to promulgate the Mitu system regulations.
Corresponding to this, because of the cultural differences in different regions, the units of measurement represented by the two are different. First, the inch thread is the thread size marked in inch. It is divided into two types: cylinder and cone according to the shape; according to the tooth angle 55°, 60° two kinds. The 1/4, 1/2, and 1/8 marks in the thread refer to the diameter of the thread size in inches. One inch is 8 cents, 1/4 inch is 2 cents, and so on.
The biggest difference between metric threads and inch threads is that the pitch is measured in millimeters. Metric threads include ordinary threads (tooth angle 60°); trapezoidal threads (tooth angle to 30°); zigzag threads (tooth angle to 33°); square threads, etc. The metric thread is represented by the pitch, the metric thread is a 60-degree equilateral tooth type, and the imperial thread is an isosceles 55-degree tooth type.
Knowing this, let's take a look at imperial fasteners and metric fasteners. For imperial and metric fasteners, especially threaded fasteners, it seems that their differences are only caused by different measurement units. In use It is only necessary to perform the necessary calculation conversion between inches and millimeters, but is it really that simple?
The size expression of threaded fasteners, whether it is imperial or metric, is usually the nominal diameter, pitch, plus a length dimension, but the form of expression is different.
In the thread specification of British threaded fasteners, the number of thread teeth is always displayed directly after the thread diameter, such as 1/4-28, where 28 means 28 teeth per inch, that is, the pitch is 1/28 inch. In metric threaded fasteners, there is no such parameter in the thread specification, the pitch is specified as the distance between two adjacent thread crests, such as M10X1.25, the pitch defined here is 1.25mm, and there is no Number of teeth per unit distance.
So there might be some confusion here, because imperial fasteners do not describe the actual crest-to-crest distance, but are defined as the number of threads per inch. If you need to convert between the two, for example, when converting metric to imperial, you first need to change the pitch in the metric size from millimeters to inches, and then find the inverse.
For the length representation of externally threaded fasteners, the imperial and metric measurement methods are the same. For flat head fasteners, the length includes head height. For male head fasteners, the length is calculated from the bottom of the head. The only difference between them is the units, with the metric system using millimeters and the imperial system using inches.
For metric fasteners, the larger the diameter number, the larger the "block head" of the fastener; the larger the pitch number, the thicker the thread, that is, the fewer threads per unit size; the larger the length value, the tightening The longer the piece.
Regarding the comparison between the imperial system and the metric system, another important technical parameter is strength. So is the 8.8 grade of the metric fastener similar to the 8 grade of the imperial fastener?
The answer is definitely no, they look similar but are almost completely different. Section 3.4 of SAE Standard J1199 (Mechanical and Material Requirements for Metric Externally Threaded Steel Fasteners) gives guidance on conversion between metric and imperial strength grades, which covers eight performance classes of steel materials. Mechanical and material requirements for threaded metric fasteners in sizes M1.6 to M36 for many different applications.
Metric fastener strengths are represented by property classes that correspond to strength classes, briefly summarized below:
Metric Grade 4.6 is approximately equivalent to SAEJ429 Grade 1 and ASTM A307 Grade A
Metric grade 5.8 is approximately equivalent to SAEJ429 grade 2
Metric Grade 8.8 is approximately equivalent to SAEJ429 Grade 5 and ASTM A449
Metric Grade 9.8 is about 9% stronger than equivalent SAEJ429 Grade 5 and ASTM A449
Metric Grade 10.9 is approximately equivalent to SAEJ429 Grade 8 and ASTM A354 Grade BD
