Definition: Invisible strike errors result from a
failure to maintain proper minimum die clearance. Minimum die clearance is the
closest approach the dies make in the absence of a planchet. It is a pre-set
safety margin that prevents the dies from clashing. Normal minimum die clearance
is greater than zero but considerably less than the thickness of a planchet.
This serves to prevent die clash while insuring a strong strike.
Once in awhile, the
dies fall out of adjustment or something prevents them from approximating
normally so that minimum die clearance exceeds the thickness of a planchet. Any
planchet that enters the striking chamber during such a malfunction leaves the
same way it came in - featureless. However, if another planchet, coin, or
foreign object intrudes into the striking chamber, it takes up the excess space
between the dies. This permits a die-struck design to be generated on the
opposite face, directly across from the indent, partial brockage, or
strike-through error created by the intrusive object.