REPRESENTATION THREAD IN DRAWING

Representation Thread in Drawing

The true projection of a threaded portion of a part consists of a series of helices and it takes considerable time to draw them. Hence it is the usual practice to follow some conventional methods to represent screw threads. Figure 1 shows the true projection of a screw thread, whereas the conventional representation of external and internal threads as recommended by ISO is shown in Fig. 2.

Figure 1 True Projection of a Screw Thread

Figure 2 Conventional Representation of Thread

The crests of threads are depicted by a continuous thick line in Fig. 2, whereas the roots are indicated by a continuous thin line. Dotted lines represent the crests and roots of hidden screw threads. Hatching should be extended to the line defining the thread crest for threaded elements in section. The interwoven roots are depicted by a piece of a circle drawn with a continuous thin line and measuring approximately three-quarters of the circumference in the view from the side.

Depending on how visible the limit of practical length of screw threads is, it is depicted by a continuous thick line or a dotted line. A run-out is the length to which the unfinished threads are produced beyond the useful limit. Two slanted lines are used to depict it.

Though it saves effort, the simpler representation is ineffective in conveying thread shapes. Figure 3 depicts the schematic depiction that was employed for this purpose. In practice, only visible threads, such as external and internal threads in section, are represented using the schematic form. The crest diameters, both in external and internal threads, are drawn by thick lines in Fig. 3. The crests are represented by thin lines that extend up to the major diameter, while the roots are represented by thick lines that extend up to the minor diameter, both of which are drawn with a slope equal to half the pitch.

Figure 3. Schematic Representation of Threaded Parts–V-Threads

Representation of Thread Part in Assembly

The schematic representation of threads in engagement is shown in Figure 4a, while the traditional representation of threads in engagement is shown in Figures 4b and c.

The internal threaded part is shown in section in Figure 4a, whereas the external threaded part is depicted unsectioned. External threaded parts are shown covering internal threaded parts in Figs. 4b and c, but they should not be shown as buried by them.

Figure 4. External and Internal Threads in Engagement