53. Screw surfaces

Screw include surfaces created by screw movement of a rectilinear generatrix. Ruled helical surfaces are called helicoids.

The straight helicoid is formed by the movement of the rectilinear generatrix i along two guides: the helix line t and its axis i; at the same time, the generator l intersects the helical axis at a right angle (Fig. 102, a). The straight helicoid is used when creating spiral staircases, augers, as well as power threads, in machine tools.

The oblique helicoid is formed by the movement of the generatrix along the helical guide T and its axis i so that the generator l intersects the axis i at a constant angle φ different from the direct one, i.e. equal to 2f (fig. 102, b). Inclined helicoids limit the surface of the threads.

Fig. 102