(57) A fast open compass comprises one handle (1), a joint part (8) and two legs (21,
22) connected to the joint part. There is a screw thread between the legs. The screw
has left spin screw thread and right spin screw thread. There are nuts (41, 42) on
the legs that can be connected to the left spin screw thread and the right spin screw
thread, respectively. Thread of the screw (3) is selected in such manner that the
coupling between screw and nuts is reversible. The angle of the left spin screw thread
and the right spin screw thread of the screw is advantageously 20∼35°. When legs are
been unfold or fold directly, the left spin screw thread (31) and the right spin screw
thread of the screw can be unlocked and moved against each respective nuts. This invention
uses an adjustable screw between legs with a relatively large rise angle. This make
it possible to apply relatively small force to overcome the resistance to pull the
screw out of or push it into the nuts straight way. Therefore, a precise and fast
positioning is possible.
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