From BlenderWiki

Screw

The Screw tool combines a repetitive Spin with a translation, to generate a screw-like, or spiral-shaped, object. Use this tool to create screws, springs, or shell-shaped structures.

Unlike the spin tools, the rotation axis, still passing through the 3D cursor, is aligned with the y-axis of the view (i.e. up-down on the screen).

There are strict conditions about your selection, when you want to use this tool (if there is a problem with the selection, the tool will warn you with the error message: “You have to select a string of connected vertices too”):

• You must have one and only one open line of vertices (e.g. a simple edge, a half circle, …). You need only to ensure that the line has two “free” ends (at a “free” end, a vertex is connected to only one other vertex). The Screw function uses these two points to calculate the translation vector that is added to the “Spin” for each full rotation (see examples below). If these two vertices are at the same location, this creates a normal “Spin”. Otherwise, interesting things happen!
• You may have as much profiles as you like (like circles, squares, and so on – note that all vertices in a profile need not to be in the same plane, even if this is the most common case).
• You may also have other, more complex, selected islands (like closed or opened volumes), the same rules as extrusion applies here too – as long as it does not contain an “opened line” in it.

Note that the open line is always extruded, so if you only use it to “guide” the screw, you will have to delete it after the tool completion (use linked-selection, CtrlL, to select the whole extrusion of the open line).

Here are the options of this tool:

• The point of view will determine around which axis the selection is screwed (remember: here it is around the y-axis of the view…).
• The position of the 3D cursor will be the center of the rotation.
• The Steps numeric field specifies how many extrusion will be done each turn.
• The Turns numeric field specifies how many turns will be executed.
• The Clockwise toggle button control the direction of rotation (as you might have guessed, clockwise when enabled and counter-clockwise otherwise).
• The Keep Original button, when enabled, will keep the original selected elements, as separated islands in the mesh (i.e. unlinked to the result of the screw extrusion).

Examples

The method for using the Screw function is strict:

• Set the 3D window to front view (1 NumPad).
• Place the 3D cursor at the position through which the rotation axis must pass (the rotation axis will be vertical).
• Select all vertices that will participate in the “Screw” (remember: one and only one “open line”).
• Set Steps and Turns to your liking.
• Press Screw.

If the two “free” ends are aligned vertically, the result is as seen above. If they are not, the vertical component of the translation vector remains equal to the vertical component of the vector joining the two “free” vertices, while the horizontal component generates an enlargement (or reduction) of the screw as shown in (Enlarging screw (right) obtained with the profile on the left). In this example the open line serves as the profile as well as defining the translation.