Screws for bolts and nuts, and for metal work, are now generally made according to Sir J. Whitworth's standard, the same form of thread being used throughout, and the same pitch and depth of thread being always used for screws of the same diameter, so that both screws and nuts are always interchangeable, which is an immense advantage in case of loss or fracture.

Whitworth's Standard Screw Thread is shown in section in Fig. 187. The sides of the thread are inclined at 55° to one another, and the sharp angles at the top and bottom are rounded off, each to a depth of about 1/6 of the depth of the thread - thus the depth of the thread is only 2/3 of what it would be if the angles were left sharp.

The following Table shows the number of threads per inch for screws of different diameters : -

Fig. 187.

Whitworth's Gas Threads

For the screwed ends of wrought-iron gas-tubing and for common metal work a shallower thread is used, as shown in the following Table : -

Diameter in inches . .1/8 1/4 3/8 1/2 3/4 1 1 1/4 1 1/2 1 3/4 2

Number of threads per inch 28 9 19 14 14 11 11 11 11 11

Stove Screws are small screws of the form shown in Fig. 188, used for uniting the different parts of stoves, grate fronts, etc. The heads are sometimes square, or cup-shaped, instead of being circular and flat as shown in Fig. 188.