Bridger wrote:

On Wed, 28 Jan 2004 08:06:15 -0500, BRuce BRuce wrote:

I confused, wouldn't the helix angle be the same regardless of the
diameter? threads per inch is threads per inch. the angle doesn't
change but the surface area changes.

the angle decreases as diameter increases.

think of one rotation as a triangle. height is thread pitch- it stays
the same. width is diameter- it changes, so the diagonal must change
too.

True, the thread on a bolt or dowel is a rolled up inclined plane, one of
the basic machines.

The dimensions of the defining triangle is:
Height = distance between peaks (or grooves)
Base = circumference of cylinder (diameter * pi)
Hypotenuse = Length of one turn of the thread.

Thead angle will be arcsine(Height/Hypotenuse)
or arctan(Height/Base)

Increasing the diameter of the cylinder will make the triangle longer and
thus make the angle smaller.

When the thread angle is about 45 degrees or more the screw and nut can
and is used backwards. In other words, instead of using the screw to move
or tighten the nut, the nut is moved to cause the screw to rotate.
Linear motion is turned into rotary motion. A good example would be the
"yankee" screwdriver.

ARM