Elements of descriptive geometry, with applications to isometric projection and othe forms of one-plane projection; a text-book for colleges and ingineering schools by O. E. Randall.

114

DESCRIPTIVE GEOMETRY

along the surface of a right circular cylinder in such a way that
one side of the square always remains on the surface of the cylinder
and the plane of the square always contains the axis of the cylinder.
On account of this motion each apex of the square will generate
a helix (see Section 213).
If the downward movement of the generating square per revolution is equal to twice the side of the square, the thread is called
single threaded, if equal to four times the side of the square it
is called double threaded, and if equal to six times the side of the
square it is called triple threaded.
Fig. 116 represents a double-square-threaded screw. The inner
cylinder is represented in plan by c,-o,-k,,-p, and in elevation by
k'-n'-r'-q. The generating square in its original position, when
its plane is parallel to F, is represented in plan by k,-d, and in
elevation by k'-d'-f'-l.
The helices generated by the four vertices 7I, ), 1', and L of the
square may be represented by Section 214, remembering that the
pitch of each helix is equal to four times the side of the generating
square.
The thread directly below the one just generated is generated
by the square Z- W-X- Y, which moves below the square K-D-F-L
and at a distance equal to the side of the square.
The spaces between the threads of a square-threaded screw, so
far as the drawing is concerned, may be made equal in dimension
to the threads themselves.