Draughting conventions associated with threads

Threads are so regularly used on engineering drawings that recognizable conventions are required to save draughting time. Figure 15.12 shows the convention for a male thread. The thread runout along the shank of the stud is indicated by a line drawn at 30° to the thread axis. The minor diameter of the thread is shown by parallel lines and in the end elevation the projected circle is not continuous. The break in the inside circle distinguishes the end elevation of a male thread from a female thread. Line thicknesses are given for each part of the thread. The actual dimensions of the minor diameter for any particular thread size can be

Threated Bolt Hidden Lines Drafting
Tapping hole
Orthographic Drawing Threaded Hole

Fig. 15.15

approximated at 80% of the major diameter for the purposes of conventional representation.

Figure 15.13 shows the convention for a female thread applied to a blind tapped hole in a sectional view. Note that the minor diameter is drawn as a complete circle in the end elevation, the major diameter is broken and the different line thicknesses also help to distinguish the female from the male thread. The effective length of the thread is again shown by parallel lines and the runout by the taper at 30° to the thread axis. In the sectional elevation, the section lines drawn at 45°, continue through the major to the minor diameter. The included angle left by the tapping drill is 120°. Line thicknesses are indicated in the circles.

Note. the line at 30° indicating the runout was the practice in BS 308:1972. However, in 1984 the revised standard permitted the line to be omitted if there was no functional need. The tapered lines for incomplete threads are shown here for completeness of text.

A female thread through a collar is shown in Fig. 15.14. Note that the projection of the major and minor diameters drawn as hidden detail will be indicated by dashed narrow lines.

Hidden Detail Engineering Drawing

Fig. 15.14

A section through the collar is given in Fig. 15.15. The projection of the major diameter is drawn by a continuous narrow line and the cross hatching extends to the minor diameter.

A section through a sealing cap in Fig. 15.16 illustrates a typical application where an internal thread terminates in an undercut.

Screw threads are produced by cutting or rolling. A cut thread can be made by the use of a tap for female threads, or a die in the case of a male thread. Figure 15.17 shows a male thread cut by a die and terminating in a runout. In this application the bar diameter has been drawn equal in size with the major diameter of the thread.

Fig. 15.15

Basic major dia.

Fig. 15.16

Basic major dia.

Fig. 15.16

Engineering Drawing Thread Symbol

Fig. 15.17

Screwcutting may be undertaken on the lathe and the cutting tool is shaped to match the thread angle. Generally the thread terminates in an undercut and this feature is illustrated in Fig. 15.18. It is normal draughting practice to draw an undercut in line with the minor diameter of the thread. Too narrow an undercut or the demand for perfect or full threads up to shoulders, or to the bottom of blind holes, increases manufacturing costs.

Fig. 15.18

A rolled thread application is indicated in Fig. 15.19. The thread is formed by deformation and runs out to a bar diameter which is approximately equal to the effective diameter of the thread.

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