Screw threads

Screw threads are complex helical forms and their detailed characteristics in terms of such things as angles, root diameter, pitch circle diameter and radii are closely defined by ISO standards. Thus, if the designation 'M8' appears on a drawing it would appear at first sight to be very loosely defined but this is far from the case. Screw threads are closely defined in the standard ISO 6410, parts 1, 2 and 3:1993. The 'M8' designation automatically refers to the ISO 68-1:1998, ISO 6410-1, 2 and 3:1993 standards in which things like the thread helix angle, the vee angles and the critical diameters are fully defined. Thus, as far as screw threads are concerned, there is no need to do a full drawing of a screw thread to show that it is a screw thread. This takes time and costs money. The convention for drawing an engineering thread is shown using a combination of ISO type A and B lines as shown in the drawings in Figures 3.1, 3.2 and 3.3. A screw thread is represented by two sets of lines, one referring to the crest of the thread (type A line) and the other referring to the roots of the thread (type B line). These can be seen for a bolt and a hole in Figures 3.5 and 3.6. This representation can be used irrespective of the exact screw thread. For example, on the vice assembly drawing in Figure 3.1, the screw thread on the bush screw (part number 5) and the jaw clamp screw (part number 6) are very different. In the real vice, the former is a standard vee-type thread whereas the latter is a square thread. Line thicknesses become complicated when a male-threaded bolt is assembled in a female-threaded hole. The thread crest lines of the bolt become the root lines of the hole and vice versa. This means that in an assembly, lines change from being thick to thin and vice versa. This is shown in the vice assembly drawing in Figure 3.1, with respect to the bush screw (part number 5)/jaw clamp screw (part number 6) assembly.

Engineering Drawing for Manufacture
by Brian Griffiths
Publisher: Elsevier Science & Technology Books