By | April 28, 2015

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. Screw thread nomenclature figure below shows the true projection of a screw thread, whereas the conventional representation of external and internal threads as recommended by BIS is shown in Conventional representation of threads figure below.

Screw thread nomenclature

It may be noted from Conventional representation of threads figure below, that the crests of threads are indicated by a continuous thick line and the roots, by a continuous thin line. For hidden screw threads, the crests and roots are indicated by dotted lines. For threaded parts in section, hatching should be extended to the line defining the crest of the thread. In the view from side, the threaded roots are represented by a portion of a circle, drawn with a continuous thin line, of length approximately three-quarters of the circumference.

Standard Abbreviations of Draughting

Conventional representation of threads

The limit of useful length of screw threads is represented by a continuous thick line or a dotted line, depending on its visibility. The length upto which the incomplete threads are formed beyond the useful limit, is known as a run-out. It is represented by two inclined lines.

The simplified representation, though it saves time, is not an effective method to convey thread forms. The schematic representation, used for the purpose is shown in Schematic representation of threaded parts–V-threads figure below. In practice, the schematic representation is followed for only visible threads, i.e., for external threads and internal threads in section. From the below figure, it may be observed that the crest diameters, both in external and internal threads, are drawn by thick lines. Further, the crests are represented by thin lines, extending upto the major diameter and the roots by thick lines, extending upto the minor diameter, these lines being drawn inclined with a slope equal to half the pitch.

Use of Scale in Drawing

Schematic representation of threaded parts–V-threads

Schematic representation of threaded parts–Square threads figure below illustrates the schematic representation of square threads.

Schematic representation of threaded parts–Square threads

Representation of Threaded Parts in Assembly

Figure a shows the schematic representation and Figs. b and c, the conventional representation of threads in engagement. Figure a represents the internal threaded part in section; however, the external threaded one is shown unsectioned. In Figs. b and c, the external threaded parts are shown covering the internal threaded parts and should not be shown as hidden by them.

External and internal threads in engagement