Comparison of New and Old National Standard Thread Tolerance With Reference to Location
Threads are an important part of many kinds of industry and plays the role of fastener in machines, vehicles and other construction components. Every thread shall have a certain tolerance which is determined by authority rule or engineer according to work or material situation. According to National Standard, in GB/T1231-1991 and GB/T5765-1995, two standards are mentioned relative to thread tolerance with reference to location. In this article, we will give a comparison of difference between new standard (GB/T 5765-1995) and old standard (GB/T 1231-1991).
A thread shall be composed by two parts: major diameter and thread crest for the purpose of precise location. The new standard sets more precise rules for thread building. According to the new standard, the magnitude of major diameter tolerance should be precise enough to prevent thread from separated from their position when under tension or compression force. As for thread crest form, the rules are also more specific for each thread size in certain material. The magnitude of thread crest tolerance for metric thread should be 0.8mm in length, except for sizes M14 and larger, in which the thread crest tolerance should be 1.2mm in length. Besides, metric standard requires straightness of thread form to guarantee flat position of thread crest.
Besides, the new standard tends to set tighter tolerance compared to old standard. In former standard, minor diameter tolerance is in the range of 0.3mm to 0.5mm, depending on size. According to the new standard, minor diameter tolerance is tighten to 0.3mm or smaller in almost all size of metric thread. In addition to that, the new standard also sets axis angle tolerance, which is the angle between thread axis and base line, in size M8 and smaller, with the maximum angle being 5°30’.
The changes between two standards above also leads to what degrees of tolerance difference. Generally speaking, there will be some differences in manufacturing cost and dimensioning accuracy. As for manufacturing cost, the new standard tends to cost a bit more for tighter tolerance, although there will be a decrease of gap between the parted thread in the new system. As for dimensioning accuracy, the new standard tends to be higher.
To conclude, after comparing between new and old national standard thread tolerance with reference to location, it is obvious that the new standard has some improvements from the old one. Tightened tolerance as well as more specific rules for form make the new standard more suitable for precision engineering. On the other hand, tighter tolerance also require more cost and accuracy in manufacturing and dimensioning. Therefore, it is suggested to choose which standard according to the particular situation when dealing with different threads.
