Because of the mechanical advantage due to the fine pitch of the screw threads that move the thimble and the right-hand measuring rod, it is easy to use enough force in closing the rods on the object being measured to deform either the rods or the object. In the diagram you can see a Friction Screw, which applies "just enough" torque to rotate the thimble in a gentle manner.


http://www.upscale.utoronto.ca/PVB/Harrison/Micrometer/Micrometer.html